Samacheer Kalvi Class 11 Bio Botany Solutions Chapter 13 Photosynthesis

Part-I

 

Question. Assertion (A): Increase in Proton gradient inside lumen responsible for ATP synthesis.
Reason (R): Oxygen evolving complex of PS I located on the thylakoid membrane facing stroma, releases H+ ions.
(a) Both Assertion and Reason are True.
(b) Assertion is True and Reason is False.
(c) Reason is True and Assertion is False.
(d) Both Assertion and Reason are False.
Answer: (a) Both Assertion and Reason are True.
In simple words: The proton gradient increases inside the thylakoid lumen, which then helps make ATP. This happens because the oxygen-evolving complex, part of Photosystem I, releases hydrogen ions into the lumen.

🎯 Exam Tip: Remember that a proton gradient is crucial for ATP synthesis in both photosynthesis and respiration. Knowing where key complexes are located helps understand the process.

 

Question 2. Which chlorophyll molecule does not have a phytol tail?
(a) Chl - a
(b) Chl - b
(c) Chl - c
(d) Chl - d
Answer: (c) Chl - c
In simple words: Chlorophyll 'c' is different from other chlorophyll types because it does not have a long phytol tail. This tail helps other chlorophylls attach to the thylakoid membrane.

🎯 Exam Tip: Focus on the structural differences between various chlorophyll types, especially the presence or absence of a phytol tail, as this affects their function and location.

 

Question 3. The correct sequence of flow of electrons in the light reaction is:
(a) PS II, plastoquinone, cytochrome, PS I, ferredoxin.
(b) PS I, plastoquinone, cytochrome, PS II ferredoxin.
(c) PS II, ferredoxin, plastoquinone, cytochrome, PS I.
(d) PS I, plastoquinone, cytochrome, PS II, ferredoxin.
Answer: (a) PS II, plastoquinone, cytochrome, PS I, ferredoxin.
In simple words: During the light reaction, electrons move in a specific order: first from Photosystem II, then to plastoquinone, then cytochrome, then Photosystem I, and finally to ferredoxin. This continuous flow creates energy for the plant.

🎯 Exam Tip: Memorize the Z-scheme of electron transport. It helps to visualize the path of electrons from water splitting to the final electron acceptor, NADPH.

 

Question 4. For every \( \text{CO}_2 \) molecule entering the \( \text{C}_3 \) cycle, the number of ATP & NADPH required are:
(a) 2ATP + 2NADPH
(b) 2ATP + 3NADPH
(c) 3ATP + 2NADPH
(d) 3ATP + 3NADPH
Answer: (c) 3ATP + 2NADPH
In simple words: To process just one molecule of carbon dioxide in the C3 cycle, the plant uses up three ATP energy packets and two NADPH molecules. This energy helps turn carbon dioxide into sugar.

🎯 Exam Tip: Remember the energy cost for fixing one CO2 molecule in the C3 cycle. It's a common value tested in exams, indicating the efficiency of the cycle.

 

Question 5. Identify true statement regarding light reaction of photosynthesis?
(a) Splitting of water molecule is associate with PS I.
(b) PS I and PS II involved in the formation of \( \text{NDPH} + \text{H}^+ \).
(c) The reaction center of PS I is Chlorophyll a with absorption peak at 680 nm.
(d) The reaction center of PS II is Chlorophyll a with absorption peak at 700 nm.
Answer: (b) PS I and PS II involved in the formation of \( \text{NDPH} + \text{H}^+ \).
In simple words: Both Photosystem I and Photosystem II work together to create NADPH and hydrogen ions during the light reaction. This process is essential for providing energy for the next steps of photosynthesis.

🎯 Exam Tip: Understand the specific roles of PS I and PS II. PS II primarily splits water and contributes electrons, while PS I is crucial for the final production of NADPH in non-cyclic photophosphorylation.

 

Question 6. Two groups (A&B) of bean plants of similar size and same leaf area were placed in identical conditions. Group A was exposed to light of wavelength 400 – 450 nm & Group B to light of wavelength of 500 – 550nm. Compare the photosynthetic rate of the 2 groups giving reasons.
Answer: Group 'A' plants, exposed to light between 400 – 450 nm (blue region), will have a high rate of photosynthesis. This is because chlorophyll 'a', a key pigment for photosynthesis, absorbs light most effectively at around 450 nm. The blue light is rich in energy, making it very efficient for photosynthesis. In contrast, Group 'B' plants, exposed to light between 500 – 550 nm (green region), will have a low rate of photosynthesis. Chlorophyll pigments do not absorb green light well; instead, they reflect it, which is why plants appear green. This reflection means less light energy is used for making food.

In simple words: Plants in blue light (Group A) will make food faster because they absorb blue light well. Plants in green light (Group B) will make food much slower because they reflect green light instead of absorbing it.

🎯 Exam Tip: Remember that chlorophyll absorbs red and blue light most efficiently and reflects green light. This explains why plants look green and photosynthesize best under red or blue light.

 

Question 7. A tree is believed to be releasing oxygen during nighttime. Do you believe the truthfulness of this statement? Justify your answer by giving reasons?
Answer: Yes, the statement is true; a tree can release \( \text{O}_2 \) during nighttime. This occurs in special plants called CAM (Crassulacean Acid Metabolism) plants. At night, CAM plants take in carbon dioxide and fix it using phosphoenolpyruvic acid. They then convert this into malic acid, similar to the C4 cycle. This process allows them to store \( \text{CO}_2 \) at night and release oxygen during the day when sunlight is available, even if their stomata are closed. Some other plants might also release trace amounts of oxygen due to residual photosynthesis activity or specific metabolic pathways, but CAM plants are the primary example for significant nighttime \( \text{O}_2 \) release under specific conditions.

In simple words: Yes, some trees can release oxygen at night. These are special CAM plants that store carbon dioxide when it's dark and use it later in the day to make food and release oxygen, even without direct sunlight.

🎯 Exam Tip: Differentiate between normal plant respiration (which consumes oxygen) and the unique adaptations like CAM photosynthesis that allow some plants to fix carbon at night and potentially release oxygen. This is a common misconception.

 

Question 8. Grasses have an adaptive mechanism to compensate for photorespiratory losses – Name and describe the mechanism.
Answer: Grasses use a special way called the C4 pathway to reduce energy loss from photorespiration. This pathway has two main steps happening in two different types of cells: mesophyll cells and bundle sheath cells. This arrangement helps them grow well even in hot, dry places.
**Mesophyll cells:**
• At first, carbon dioxide is taken up by a 3-carbon compound called Phosphoenolpyruvate (PEP). This happens with the help of PEP carboxylase and creates a 4-carbon compound called oxaloacetate.
• Oxaloacetate is then changed into malate or aspartate, which are also 4-carbon compounds. These then move into the bundle sheath cells.
**Bundle Sheath:**
• Inside the bundle sheath cells, malate and aspartate release carbon dioxide. This makes a lot of carbon dioxide available for the RUBISCO enzyme.
• Because there is a high concentration of carbon dioxide, RUBISCO works mostly as a carboxylase (fixing \( \text{CO}_2 \)) instead of an oxygenase (causing photorespiration). This stops energy loss.
• The remaining 3-carbon compound, pyruvate, goes back to the mesophyll cells to restart the cycle.

In simple words: Grasses use a special two-step process called the C4 pathway to avoid wasting energy during photosynthesis. They separate where they take in carbon dioxide and where they fix it, which helps them work better in hot weather and keep making sugar efficiently.

🎯 Exam Tip: Remember that C4 plants use a spatial separation of CO2 fixation, involving both mesophyll and bundle sheath cells, to minimize photorespiration. This adaptation is key to their success in warm climates.

 

Question 9. In Botany class, teacher explains, Synthesis of one glucose requires 30 ATPs in C4 plants and only 18 ATPs in C3 plants. The same teacher explains C4 plants are more advantageous than C3 plants. Can you identify the reason for this contradiction?
Answer: The C4 plants need more ATP to make one glucose molecule (30 ATPs) compared to C3 plants (18 ATPs). However, C4 plants are still more helpful in some situations because of these reasons. Their unique structure and process help them grow better in specific environments, even if it costs more energy in the short term.

In simple words: Even though C4 plants use more energy (ATP), they are better in hot and dry places. This is because they can grab carbon dioxide more efficiently and avoid wasting energy through photorespiration, which happens a lot in C3 plants when it's hot.

🎯 Exam Tip: The advantage of C4 plants lies in their ability to minimize photorespiration in hot and dry environments, despite a higher ATP cost per glucose molecule. This makes them more productive under specific stress conditions.

 

Question 10. When there is plenty of light and higher concentration of \( \text{O}_2 \), what kind of pathway does the plant undergo? Analyse the reasons.
Answer: When there is lots of light and a high amount of oxygen, plants might go through photorespiration. This process actually lowers the rate of making food (photosynthesis). This negative effect of oxygen on photosynthesis was first found by Warburg in 1920 when he studied green algae called Chlorella. Photorespiration happens because the enzyme RUBISCO, which usually helps fix carbon dioxide, starts reacting with oxygen instead when oxygen levels are high. This wasteful process does not produce ATP or NADPH, reducing the overall efficiency of photosynthesis.

In simple words: With lots of light and oxygen, plants can start a wasteful process called photorespiration, which slows down how much food they can make. This is because the enzyme that usually takes in carbon dioxide starts taking in oxygen instead.

🎯 Exam Tip: Understand the conditions that favor photorespiration: high oxygen concentration, high temperature, and strong light. It's a key factor limiting C3 plant efficiency in certain environments.

Part-II.

11th Bio Botany Guide Photosynthesis Additional Important Questions and Answers

I. Choose the Correct Answers

 

Question 1. In photosynthesis, the overall reaction is:
(a) endothermic reaction
(b) exothermic reaction
(c) endergonic reaction
(d) exergonic reaction
Answer: (c) endergonic reaction
In simple words: Photosynthesis needs energy to happen, which it gets from sunlight. Because it takes in energy, it is called an endergonic reaction.

🎯 Exam Tip: Remember that "endergonic" means energy-requiring, while "exergonic" means energy-releasing. Photosynthesis requires energy input (light) to build complex molecules.

 

Question 2. The physiological unit of photosynthesis is
(a) 150-250 Chlorophyll molecules
(b) 200-300 chlorophyll molecules
(c) 440-660 chlorophyll molecules
(d) 450- 650 chlorophyll molecules
Answer: (b) 200-300 chlorophyll molecules
In simple words: The basic working part for making food in plants is a group of about 200 to 300 chlorophyll molecules. These molecules work together to capture light.

🎯 Exam Tip: Quantasomes or photosynthetic units are functional groups of pigments. Knowing the approximate number of chlorophyll molecules within these units is important.

 

Question 3. How many million tonnes of dry matter produced annually by photosynthesis?
(a) 1700 million tonnes
(b) 1900 million tonnes
(c) 1400 million tonnes
(d) 2000 million tonnes
Answer: (a) 1700 million tonnes
In simple words: Every year, plants all over the world make about 1700 million tonnes of dry matter through photosynthesis. This massive production is the basis of nearly all life on Earth.

🎯 Exam Tip: Quantify the global impact of photosynthesis; this figure emphasizes its importance for biomass production.

II. Match Correctly & Choose The Right Answer

 

Question 1. Match the following and choose the right answer.
I) Blackmann – A) The importance of Chlorophyll
II) Warburg – B) Law of limiting factor
III) Dustrochet – C) C4 cycle
IV) Hatch & Slack – D) Chlorella

Answer: (c) B-D-A-C
In simple words: We need to match each scientist or concept with their correct finding or associated term. Blackmann is known for the law of limiting factors. Warburg worked with Chlorella. Dustrochet looked into the importance of chlorophyll. Hatch & Slack discovered the C4 cycle.

🎯 Exam Tip: For matching questions, it's often helpful to first match the pairs you are most confident about, then deduce the remaining ones. Knowledge of key scientists and their contributions is essential.

 

Question 5. Thylakoid disc diameter is:
(a) 0.35 to 0.75 microns
(b) 0.25 to 0.8 microns
(c) 0.45 to 0.8 microns
(d) 0.50 to 0.9 microns
Answer: (b) 0.25 to 0.8 microns
In simple words: The tiny disc-shaped structures inside a chloroplast, called thylakoids, have a diameter between 0.25 and 0.8 microns. This small size helps them stack up and create more surface area for light reactions.

🎯 Exam Tip: Knowing the dimensions of organelles like thylakoids helps appreciate the microscopic scale at which photosynthesis occurs. Precise measurements are important details.

 

Question 6. The pigment responsible for the yellowing of leaves during autumn season is
(a) Violaxanthin
(b) Fucoxanthin
(c) Phycobillin
(d) Lycopene
Answer: (d) Lycopene
In simple words: Lycopene is the pigment that makes leaves turn yellow in autumn. As green chlorophyll breaks down, other colors like yellow from lycopene become visible, giving autumn leaves their distinct hues.

🎯 Exam Tip: Understand that leaf color changes in autumn are due to the breakdown of chlorophyll, revealing other pigments like carotenoids (which include lycopene, violaxanthin, and fucoxanthin) that were present but masked by the green.

 

Question 7. The no of quanta of light required for the release of one oxygen molecule
(a) 18 quanta
(b) 8 quanta
(c) 81 quanta
(d) 19 quanta
Answer: (b) 8 quanta
In simple words: To make just one molecule of oxygen during photosynthesis, the plant needs to absorb 8 units of light energy, called quanta. This shows how much energy is needed for the process.

🎯 Exam Tip: This value (8 quanta per \( \text{O}_2 \) molecule) is related to the quantum requirement and quantum yield of photosynthesis. It reflects the energetic efficiency of the light reactions.

 

Question 8. Each pyrrole ring comprises of:
(a) six carbons and one nitrogen atom
(b) three carbons and one nitrogen atom
(c) four carbons and one nitrogen atom
(d) four carbons and two nitrogen atoms
Answer: (c) four carbons and one nitrogen atom
In simple words: A pyrrole ring, which is a key part of chlorophyll, is made up of four carbon atoms and one nitrogen atom. This structure is very important for the chlorophyll molecule to work.

🎯 Exam Tip: Understand the basic chemical structure of chlorophyll, particularly the pyrrole rings, as they are essential for light absorption and electron transfer.

 

Question 9. RUBISCO – Constitute ............. of chloroplast protein
(a) 17%
(b) 20%
(c) 18%
(d) 16%
Answer: (c) 18%
In simple words: RUBISCO is a very common protein found in chloroplasts, making up about 18% of all the protein there. It is the most abundant enzyme on Earth, helping plants take in carbon dioxide.

🎯 Exam Tip: RUBISCO is known as the most abundant enzyme on Earth. Knowing its relative proportion in chloroplast protein highlights its critical role in carbon fixation.

 

Question 10. Pheophytin resembles chlorophyll 'a' except that it lacks:
(a) Fe atom
(b) Mn atom
(c) Mg atom
(d) Cu atom
Answer: (c) Mg atom
In simple words: Pheophytin is like chlorophyll 'a' but it's missing the magnesium atom in its center. This small difference changes how it works in photosynthesis.

🎯 Exam Tip: Focus on the key difference between chlorophyll and pheophytin. The absence of the central magnesium ion in pheophytin is critical for its function as an early electron acceptor in Photosystem II.

 

Question 11. According to Emerson the fall in quantum yield about 680 nm is called
(a) Phoisynthtic drop
(b) Emerson drop
(c) Airburg effect
(d) Red drop
Answer: (d) Red drop
In simple words: Emerson observed that when plants are given only red light around 680 nm, the efficiency of photosynthesis drops sharply. This decrease is called the "red drop" effect.

🎯 Exam Tip: The "red drop" and "Emerson enhancement effect" are important concepts illustrating the cooperation between Photosystem I and Photosystem II. Understand why the yield drops at longer red wavelengths.

 

Question 12. Which one of the photosynthetic pigments is called shield pigment:
(a) carotenes
(b) chlorophyll 'b'
(c) pheophytin
(d) carotenoids
Answer: (d) carotenoids
In simple words: Carotenoids are often called "shield pigments" because they protect chlorophyll from damage caused by too much light. They absorb extra light energy and safely release it as heat.

🎯 Exam Tip: Carotenoids serve as accessory pigments that broaden the spectrum of light absorbed and, crucially, protect chlorophyll from photo-oxidative damage, hence the term "shield pigments."

 

Question 13. Which of the following equation correctly sums up photosynthesis
(a) \( 6\text{CO}_2 + 12\text{H}_2\text{O} \xrightarrow{\text{Light}, \text{Chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 + \text{H}_2\text{O} \)
(b) \( 6\text{CO}_2 + 12\text{H}_2\text{O} \xrightarrow{\text{Light}, \text{Chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{H}_2\text{O} + 6\text{O}_2 \uparrow \)
(c) \( 6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{Light}, \text{Chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \uparrow \)
(d) None of the options
Answer: (b) \( 6\text{CO}_2 + 12\text{H}_2\text{O} \xrightarrow{\text{Light}, \text{Chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{H}_2\text{O} + 6\text{O}_2 \uparrow \)
In simple words: The correct equation shows that plants use six carbon dioxide molecules and twelve water molecules, with the help of light and chlorophyll, to make one glucose sugar, six water molecules, and six oxygen molecules. The arrow pointing up for oxygen means it is released as a gas.

🎯 Exam Tip: Always remember the balanced equation for photosynthesis, especially the number of water molecules consumed and produced. The net equation often simplifies by canceling out water, but the detailed one is crucial.

 

Question 14. Photosynthetic rate of red light (650 nm) is equal to:
(a) 42.5
(b) 10.0
(c) 43.5
(d) 40.8
Answer: (c) 43.5
In simple words: The rate at which plants make food under red light at 650 nm is often measured as 43.5. Red light is very effective for photosynthesis.

🎯 Exam Tip: Different wavelengths of light affect the rate of photosynthesis differently. Red light (600-700 nm) and blue light (400-500 nm) are typically the most effective.

 

Question 15. Which photosystem is found to be located on the outer surface of the thylakoid?
(a) PS I
(b) P.S II
(c) P. 890
(d) Both (a) and (b)
Answer: (a) P.S I
In simple words: Photosystem I is located on the outer part of the thylakoid membrane, which faces the stroma. This position helps it connect with other reactions.

🎯 Exam Tip: Photosystem I is predominantly found in the unstacked regions (stroma lamellae) of the thylakoid membrane, while Photosystem II is mainly in the stacked regions (grana). This spatial separation is functionally important.

 

Question 16. Phosphorylation taking place during respiration is called as:
(a) Photophorylation
(b) Oxidative phosphorylation
(c) Reductive phosphorylation
(d) None of the options
Answer: (b) Oxidative phosphorylation
In simple words: When energy (ATP) is made during breathing (respiration) through adding a phosphate group, it's called oxidative phosphorylation. This process uses oxygen to create a lot of ATP.

🎯 Exam Tip: Distinguish clearly between photophosphorylation (occurs in photosynthesis, driven by light) and oxidative phosphorylation (occurs in respiration, driven by redox reactions).

 

Question 17. The term Quantosome was coined by
(a) Emerson
(b) Liebig
(c) Calvin & Melvin
(d) Park & Biggins
Answer: (d) Park & Biggins
In simple words: Park and Biggins were the scientists who came up with the word "quantosome." They used it to describe the small units in chloroplasts where light energy is captured during photosynthesis.

🎯 Exam Tip: Associate key terms with their discoverers. Quantasomes are structural units of photosynthetic pigments that carry out the primary photochemical reactions.

 

Question 18. In bioenergetics of light reaction, to release one electron from pigment system it requires:
(a) two quanta of light
(b) four quanta of light
(c) one quantum of light
(d) eight quanta of light
Answer: (a) two quanta of light
In simple words: To make just one electron leave a pigment system in the light reaction, it needs two units of light energy, called quanta. This energy pushes the electron to a higher level.

🎯 Exam Tip: The absorption of light energy by pigment molecules is a fundamental step in photosynthesis. Understand that a specific amount of energy (quanta) is needed to excite an electron and initiate its transfer.

 

Question 19. Photosynthesis produces
(a) 1700 million tonnes of dry matter/year by fixing \( 75 \times 10^{12} \) kg of carbon every year
(b) 7100 million tonnes of dry matter/year by fixing \( 75 \times 10^{12} \) kg of carbon every year
(c) 1600 million tonnes of dry matter/week by fixing \( 56 \times 12^{10} \) kg of carbon every week
(d) 6100 million tonnes of dry matter/month by fixing \( 100 \times 10^{12} \) kg of carbon every month
Answer: (a) 1700 million tonnes of dry matter/year by fixing \( 75 \times 10^{12} \) kg of carbon every year
In simple words: Each year, photosynthesis creates 1700 million tonnes of dry material and uses about \( 75 \times 10^{12} \) kilograms of carbon. This shows how important plants are for the Earth's environment and for making food.

🎯 Exam Tip: This question assesses your knowledge of the global scale and impact of photosynthesis, highlighting the massive amount of organic matter produced and carbon fixed annually.

 

Question 20. In C4 plants, how many ATPs and NADPH + \( \text{H}^+ \) are utilised for the release of one oxygen molecule:
(a) 3 ATPs and 2 NADPH + \( \text{H}^+ \)
(b) 4 ATPs and 3 NADPH + \( \text{H}^+ \)
(c) 2 ATPs and 2 NADPH + \( \text{H}^+ \)
(d) 5 ATPs and 2 NADPH + \( \text{H}^+ \)
Answer: (d) 5 ATPs and 2 NADPH + \( \text{H}^+ \)
In simple words: C4 plants need 5 ATPs and 2 NADPH molecules to produce one oxygen molecule during photosynthesis. This higher energy requirement is linked to their special way of fixing carbon dioxide.

🎯 Exam Tip: Remember that C4 plants have a higher ATP requirement compared to C3 plants for the same amount of carbon fixed, due to the additional steps in their carbon fixation pathway.

 

Question 21. Products of light reaction in photosynthesis are
(a) ATP & \( \text{NADPH}_2 \)
(b) Ferredoxin and cytochrome \( \text{b}_6 \)
(c) ADP & NADP
(d) Cytochrome
Answer: (a) ATP & \( \text{NADPH}_2 \)
In simple words: The light reaction of photosynthesis produces ATP (energy packets) and \( \text{NADPH}_2 \) (electron carriers). These two products are then used in the dark reaction to make sugar.

🎯 Exam Tip: The main goal of the light-dependent reactions is to convert light energy into chemical energy in the form of ATP and NADPH, which are crucial for the subsequent carbon fixation in the Calvin cycle.

 

Question 22. In the sugarcane plant, the dicarboxylic acid pathway was first discovered by:
(a) Hatch and Slack
(b) Kortschak, Hart and Burr
(c) Calvin and Benson
(d) Mitchell and Root
Answer: (b) Kortschak, Hart and Burr
In simple words: The dicarboxylic acid pathway, which is also known as the C4 pathway, was first found in sugarcane by scientists Kortschak, Hart, and Burr. This discovery helped explain how some plants are more efficient in hot weather.

🎯 Exam Tip: While Hatch and Slack elucidated the C4 pathway, Kortschak, Hart, and Burr initially observed the formation of 4-carbon acids as the first stable products of carbon fixation in sugarcane.

 

Question 23. Photosynthetic pigments in chloroplasts lie embedded in
(a) Chloroplast envelope
(b) Plastogloblue
(c) matrix
(d) thylakoids
Answer: (d) thylakoids
In simple words: The pigments that capture sunlight for photosynthesis, like chlorophyll, are found inside the thylakoids. These are small, disc-like structures within the chloroplast.

🎯 Exam Tip: Remember that the light-dependent reactions, including the absorption of light by pigments, occur on the thylakoid membranes within chloroplasts.

 

Question 24. Indicate the correct answer:
(a) C4 plants are adapted to only rainy conditions
(b) C4 plants are partially adapted to drought condition
(c) C4 plants are exclusively adapted to desert condition
(d) C4 plants are partially adapted to temperate condition
Answer: (b) C4 plants are partially adapted to drought condition
In simple words: C4 plants are somewhat able to handle dry conditions, but not completely. They have special ways to save water and keep making food when water is scarce.

🎯 Exam Tip: C4 plants are well-adapted to hot and arid conditions because their mechanism minimizes photorespiration and water loss, making them more efficient in such environments. However, they are not exclusively limited to deserts and can thrive in various warm, dry regions.

 

Question 25. Carotenoids and Xanthophylls are also known as
(a) Respiratory pigments
(b) Accessory pigments
(c) Photosynthetic pigments.
(d) Photolytic pigments
Answer: (b) Accessory pigments
In simple words: Carotenoids and Xanthophylls are special pigments that help chlorophyll absorb more light for photosynthesis. They also protect the plant from too much light.

🎯 Exam Tip: Remember that accessory pigments broaden the spectrum of light absorbed for photosynthesis, allowing the plant to capture more energy than chlorophyll alone.

 

Question 26. Which metal ion is a
(a) Iron
(b) cobalt
(c) Magnesium
(d) Zinc
Answer: (c) Magnesium
In simple words: Magnesium is a key metal found at the center of the chlorophyll molecule, which is very important for plants to make food using sunlight. Without magnesium, chlorophyll cannot be formed correctly.

🎯 Exam Tip: Know the role of essential elements in biological molecules; for chlorophyll, magnesium is a central component, and iron is important for its synthesis, though not part of the molecule itself.

 

Question 27. The important external factors affecting photosynthesis are:
(a) light, chlorophyll, temperature
(b) light, stomatal opening, oxygen
(c) light, \( \text{CO}_2 \) and oxygen
(d) light, \( \text{CO}_2 \) and oxygen
Answer: (d) light, \( \text{CO}_2 \) and oxygen
In simple words: The main things outside a plant that change how fast it makes food are how much light it gets, how much carbon dioxide is in the air, and how much oxygen is present. These factors directly influence the rate of photosynthesis.

🎯 Exam Tip: Distinguish between internal factors (like chlorophyll content, leaf structure) and external factors (light, \( \text{CO}_2 \), temperature, water) when discussing photosynthesis.

 

Question 28. The process of photophosphorylation was discovered by
(a) Priestly
(b) Calvin
(c) Arnon
(d) Warburg
Answer: (c) Arnon
In simple words: Daniel Arnon was the scientist who found out about photophosphorylation, which is how plants make ATP energy using sunlight during photosynthesis. This was a significant discovery in understanding how plants convert light into chemical energy.

🎯 Exam Tip: Remembering key scientists and their contributions helps connect historical context with scientific discoveries in biology.

 

Question 29. Which of the following is a C4 plant
(a) Potato
(b) Sugarcane
(c) Pea
(d) Papaya
Answer: (b) Sugarcane
In simple words: Sugarcane is an example of a C4 plant, which means it uses a special way to fix carbon dioxide that helps it grow well in hot and dry places. C4 plants are often more efficient at photosynthesis in such conditions.

🎯 Exam Tip: Identify common C3 and C4 plants, as their photosynthetic pathways are distinct and adapted to different environmental conditions.

 

Question 30. Splitting of water molecule (photolysis) produces:
(a) hydrogen and oxygen
(b) electrons, protons and oxygen
(c) electrons and oxygen
(d) hydrogen, carbon dioxide and oxygen
Answer: (b) electrons, protons and oxygen
In simple words: When water splits apart during photosynthesis, it creates tiny charged particles called electrons and protons, along with oxygen gas. This process is essential for the light reactions.

🎯 Exam Tip: Photolysis of water (also known as the Hill reaction) provides electrons for the electron transport chain, protons for the proton gradient, and releases oxygen as a byproduct.

 

Question 31. Dimorphism in chloroplasts is seen in
(a) C4 plants
(b) C2 plants
(c) CAM – plants
(d) C3 plants
Answer: (a) C4 plants
In simple words: C4 plants have two different types of chloroplasts in their cells, which is called dimorphism. This special structure helps them carry out photosynthesis more efficiently in hot climates.

🎯 Exam Tip: Dimorphic chloroplasts (Kranz anatomy) are a hallmark of C4 plants, allowing for spatial separation of carbon fixation and Calvin cycle processes.

 

Question 32. Energy required for ATP synthesis in PSII comes from
(a) Proton gradient
(b) Electron gradient
(c) Reduction of glucose
(d) Oxidation of glucose
Answer: (a) Proton gradient
In simple words: The energy to make ATP in Photosystem II comes from a buildup of protons on one side of a membrane, creating an energy difference like a dam. This proton gradient is crucial for generating chemical energy.

🎯 Exam Tip: Chemiosmosis, driven by the proton gradient across the thylakoid membrane, is the primary mechanism for ATP synthesis during light reactions.

 

Question 33. The by-product of Photosynthesis is
(a) \( \text{O}_2 \)
(b) \( \text{CO}_2 \)
(c) Carbohydrate
(d) \( \text{H}_2\text{O} \)
Answer: (a) \( \text{O}_2 \)
In simple words: The extra thing that plants make and release into the air when they do photosynthesis is oxygen. This oxygen is what living things breathe.

🎯 Exam Tip: Remember that while plants use \( \text{CO}_2 \) and \( \text{H}_2\text{O} \) as reactants and produce carbohydrates (food), oxygen is the vital byproduct released into the atmosphere.

 

Question 34. Which of the following process is called reverse of Glycolysis?
(a) \( \text{CO}_2 \) reduction
(b) RUBP carboxylation
(c) RUBP regeneration
(d) ATP synthesis
Answer: (a) \( \text{CO}_2 \) reduction
In simple words: The process where carbon dioxide is changed into sugars is similar to doing glycolysis backwards, as both involve converting carbon compounds. This step is a core part of building glucose in photosynthesis.

🎯 Exam Tip: \( \text{CO}_2 \) reduction is part of the Calvin cycle, which is essentially a reversal of some catabolic pathways like glycolysis in terms of carbon flow.

 

Question 35. The Dark reaction of Photosynthesis occurs in
(a) Matrix
(b) Grana
(c) Stroma
(d) Cytoplasm
Answer: (c) Stroma
In simple words: The part of photosynthesis that doesn't need light directly, where sugars are made from carbon dioxide, happens in the fluid-filled space of the chloroplast called the stroma. It's like the main factory floor of the chloroplast.

🎯 Exam Tip: Clearly differentiate between the light-dependent reactions (grana/thylakoids) and the light-independent reactions (stroma) within the chloroplast.

 

Question 36. A granal chloroplasts are characteristics of
(a) Mesophyll of pea leaves
(b) Bundle sheath of Mango leaves
(c) Mesophyll of maize leaves
(d) Bundle sheath of Sugar cane leaves
Answer: (d) Bundle sheath of Sugar cane leaves
In simple words: Granal chloroplasts, which are chloroplasts with stacks of thylakoids (grana), are found in the bundle sheath cells of C4 plants like sugarcane. This specialized structure helps in their efficient carbon fixation.

🎯 Exam Tip: In C4 plants, mesophyll cells typically have granal chloroplasts, while bundle sheath cells can have either granal or agranal (no grana) chloroplasts, depending on the specific C4 type. The question implies the presence of granal chloroplasts as a characteristic, which holds true for many C4 bundle sheath cells that still perform some light reactions.

 

Question 38. The enzyme that is not found in a C3 plant is
(a) RUBP carboxylase
(b) PEP carboxylase
(c) NADP reductase
(d) ATP synthase
Answer: (b) PEP carboxylase
In simple words: C3 plants do not have the enzyme PEP carboxylase because it is mainly used by C4 plants to start fixing carbon dioxide. C3 plants primarily use RuBisCO for this initial step.

🎯 Exam Tip: PEP carboxylase has a high affinity for \( \text{CO}_2 \) and is crucial for the initial carbon fixation in C4 and CAM plants, making it a key distinguishing enzyme.

 

Question 39. Which of the following factors affect the rate of photosynthesis?
I. Light
II. Protoplasmic factor
III. Hormones Codes
IV. Haemoglobin
(a) only III
(b) I and II
(c) only IV
(d) I, II, and III
Answer: (d) I, II, and III
In simple words: Light, internal factors within the cell (protoplasmic factors), and certain hormones all influence how fast photosynthesis happens. Haemoglobin is not involved in plant photosynthesis.

🎯 Exam Tip: External factors like light and \( \text{CO}_2 \) concentration, and internal factors like chlorophyll content, enzyme activity (protoplasmic factors), and plant hormones, are all crucial for regulating photosynthetic rate.

 

Question 40. Match the following columns

(a) B-A-C-D
(b) D-B-C-A
(c) B-D-A-C
(d) D-A-C-B
Answer: (b) D-B-C-A
In simple words: This question matches key components of the Calvin cycle. The 5-carbon sugar is RuBP, the 3-carbon sugar is Glyceraldehyde 3-phosphate, 3-PGA is 3-phosphoglyceric acid, and RuBisCO is the enzyme that combines \( \text{CO}_2 \) with RuBP.

🎯 Exam Tip: Memorize the key molecules and enzymes involved in the Calvin cycle, specifically their carbon numbers, to easily match them in such questions.

 

Question 41. One complete light reaction involves light energy.
(a) 30 quanta
(b) 48 quanta
(c) 40 quanta
(d) 25 quanta
Answer: (c) 40 quanta
In simple words: For a full light reaction to happen, where all the energy is captured and converted, it needs about 40 packets of light energy, called quanta. This energy helps move electrons and make ATP and NADPH.

🎯 Exam Tip: The quantum requirement refers to the number of light quanta needed to evolve one molecule of oxygen; a higher number implies less efficiency in light capture.

 

Question 42. During photosynthesis which of the following event does not take place?
(a) oxidation of \( \text{CO}_2 \)
(b) Reduction of \( \text{CO}_2 \)
(c) oxidation of \( \text{H}_2\text{O} \)
(d) Light absorption
Answer: (a) oxidation of \( \text{CO}_2 \)
In simple words: During photosynthesis, carbon dioxide is reduced (gains electrons), not oxidized. Water is oxidized (loses electrons), and light is absorbed to power the process.

🎯 Exam Tip: Remember the core redox reactions: \( \text{H}_2\text{O} \) is oxidized (electron donor), and \( \text{CO}_2 \) is reduced (electron acceptor) in photosynthesis.

 

Question 43. The site of light trapping in the chloroplast is
(a) Thylakoid membrane
(b) Stroma
(c) Plasma fluid
(d) Stromal lamellae
Answer: (a) Thylakoid membrane
In simple words: The tiny disc-like structures inside chloroplasts, called thylakoid membranes, are where light energy is caught by pigments like chlorophyll. This is the very first step of photosynthesis.

🎯 Exam Tip: The thylakoid membranes contain the photosystems and electron transport chain components necessary for the light-dependent reactions.

 

Question 44. Kranz anatomy is traced in the Leaves of
(a) Wheat
(b) Potato
(c) Mustard
(d) Sugarcane
Answer: (d) Sugarcane
In simple words: Kranz anatomy, a special leaf structure with a ring of large bundle sheath cells around vascular bundles, is typically found in plants like sugarcane. This anatomy is characteristic of C4 plants.

🎯 Exam Tip: Kranz anatomy provides the spatial separation for the C4 pathway, optimizing carbon fixation in hot, dry conditions by minimizing photorespiration.

 

Question 45. The intermediate got from Kreb's cycle that is used for chlorophyll synthesis is
(a) Citric acid
(b) Isocitric acid
(c) Succinic acid
(d) Fumaric acid
Answer: (c) Succinic acid
In simple words: Succinic acid, a molecule made during the Kreb's cycle (also known as the citric acid cycle), is then used as a building block to create chlorophyll, the green pigment in plants. It links respiration to pigment production.

🎯 Exam Tip: Understanding the interconnectivity of metabolic pathways is crucial; intermediates from one cycle often serve as precursors for other biosynthetic processes.

 

Question 46. The existence of light and dark reaction of photosynthesis was proved by
(a) Blackman
(b) Emerson
(c) Warburg
(d) Arnon
Answer: (a) Blackman
In simple words: Frederick Blackman was the scientist who first showed that photosynthesis happens in two main steps: one that needs light and another that doesn't, even though it still relies on the products of the light step. This helped explain how light intensity and temperature affect the process.

🎯 Exam Tip: Blackman's experiments led to the concept of limiting factors, showing that the rate of photosynthesis is controlled by the slowest step in the process.

 

Question 47. Choose the wrong match.
(a) Hatch & Slack – Dicarboxylic acid pathway
(b) Decker – PCO cycle
(c) Ruben, Kamen – CAM cycle
(d) Calvin Benson – PCR cycle
Answer: (c) Ruben, Kamen – CAM cycle
In simple words: Ruben and Kamen are known for their work on the mechanism of photosynthesis using isotopes, not specifically the CAM cycle. The CAM cycle involves a special way plants fix carbon at night.

🎯 Exam Tip: Familiarize yourself with the major discoveries and their associated scientists to avoid common mismatches like these.

 

Question 49. Say True or False with respect to C2 cycle
I) RUBISCO has the most abundant protein on earth.
II) Photorespiration does not yield any free energy in the form of ATP
III) The end product is a 2 - c compound. So the cycle is known as C2 cycle
IV) Under certain conditions 50% of the photosynthetic potential is lost because of photorespiration.

(a) True - False - False - False
(b) True - True - False - True
(c) True - True - False - False
(d) False - False - True - True
Answer: (b) True – True – False – True
In simple words: RuBisCO is indeed the most common protein on Earth. Photorespiration does not make useful energy like ATP. The end product of photorespiration is a 3-carbon compound, not 2-carbon. Also, photorespiration can reduce photosynthetic efficiency by a large amount under certain conditions.

🎯 Exam Tip: Understand the characteristics of photorespiration (C2 cycle), especially its inefficiency compared to regular photosynthesis and why it occurs.

 

Question 50. Say True or False and choose the right option from the given choice.
I) PAR is between – 400 – 700 mil.
II) Heliophytes (Beans) require higher light intensity than scotophytes (oxalis)
III) Red light induces lowest rate of photosynthesis.

(a) True - False - True
(b) True - True - True
(c) True - True - False
(d) False - False - True
Answer: (c) True – True – False
In simple words: Photosynthetically Active Radiation (PAR) is indeed in the 400-700 nanometer range. Sun-loving plants (heliophytes) need more light than shade-loving plants (scotophytes). Red light actually causes a very high rate of photosynthesis, not the lowest.

🎯 Exam Tip: Know the optimal wavelengths for photosynthesis (blue and red light) and understand how different light intensities affect sun-loving versus shade-loving plants.

 

Question 51. Choose the wrongly matched pair
(a) Stephen hales – Father of plant physiology
(b) Lavoisier – Purifying gas oxygen is produced in sun light
(c) Vonmayer – Green plants convert solar energy into chemical energy
(d) Emerson &Arnold – C4 cycle
Answer: (d) Emerson & Arnold - C4 cycle
In simple words: Emerson and Arnold are recognized for their work on quantum yield and light reactions in photosynthesis, not for discovering the C4 cycle. The C4 cycle was discovered by Hatch and Slack.

🎯 Exam Tip: Correctly associate scientists with their major contributions, especially for key processes and pathways in plant physiology.

 

Question 52. Choose the rightly matched pair
(a) Chlorophyll a – Accessory pigments and trap solar energy
(b) Chlorophyll b – Differs from Chlorophyll a in having \( \text{CH}_3 \) instead of CHO – at 3rd C atom
(c) Chlorophyll c – Differs from Chlorophyll a by lacking a phytol tail
(d) Chlorophyll d – It has CHO at 3rd at the 3rd carbon atom at 11 – pyrrole ring
Answer: (c) Chlorophyll c – Differs from chlorophyll a by lacking a phytol tail.
In simple words: Chlorophyll 'c' is different from chlorophyll 'a' because it does not have a long hydrocarbon chain called a phytol tail. This structural difference affects its properties.

🎯 Exam Tip: Understand the key structural differences between various types of chlorophyll (a, b, c, d) and their implications for light absorption and localization.

 

Question 53. Choose the right matched pairs from the given options.
I) Green non sulphur bacteria – Clostridium & Lynbya
II) Green sulphur bacteria – Chlorobacterium & Chlorobium
III) Purple sulphur bacteria – Thiospirillum & Chromatium
IV) Purple non sulphur bacteria – Rhodopseudomonas & Rhodospirillum
(a) I, II, & III
(b) II, III & IV
(c) I, II & IV
(d) I, III & IV
Answer: (b) II, III & IV
In simple words: The correctly matched pairs group different types of photosynthetic bacteria with their representative genera. Green sulphur bacteria include Chlorobacterium and Chlorobium. Purple sulphur bacteria include Thiospirillum and Chromatium. Purple non-sulphur bacteria include Rhodopseudomonas and Rhodospirillum. Green non-sulphur bacteria are not associated with Clostridium or Lynbya.

🎯 Exam Tip: Familiarize yourself with the common examples of various photosynthetic bacterial groups and their characteristics, such as their pigments and electron donors.

II. Assertion (A) & Reason (R)

 

Question 1. Assertion (A): Chlorophyll appears green.
Reason (R): It absorbs light mainly in the region of green part of light spectrum.
Answer: (c) Assertion (A) is False but Reason (R) is true.
In simple words: Chlorophyll looks green because it reflects green light, not because it absorbs it. The reason is true because green light carries lower energy than red or blue light. So, assertion is true, but the reason given is incorrect.

🎯 Exam Tip: When answering Assertion-Reason questions, first evaluate if each statement is individually true or false, then determine if the Reason correctly explains the Assertion.

 

Question 2. Assertion (A): Red of spectrum contains high energy.
Reason (R): Green light of Visible spectrum contain low energy than red light.
Answer: (c) Assertion (A) is False but Reason (R) is true.
In simple words: Red light actually has lower energy compared to blue or violet light because it has a longer wavelength. Green light does indeed have less energy than red light. Therefore, Assertion (A) is false, but Reason (R) is true.

🎯 Exam Tip: Recall the electromagnetic spectrum: shorter wavelengths (like blue/violet) have higher energy, while longer wavelengths (like red) have lower energy.

 

Question 3. Assetion (A): Non cyclic photo phosphorylation occurs in the stroma of chloroplasts
Reason (R): There is a continuous flow of electrons in this process.
Answer: (d) Both Assertion (A) and Reason (R) are false.
In simple words: Non-cyclic photophosphorylation happens on the thylakoid membranes, not in the stroma. While there is an electron flow, the assertion about the location is incorrect. Both statements are false.

🎯 Exam Tip: Non-cyclic photophosphorylation takes place on the thylakoid membranes, and it involves a non-cyclic flow of electrons from water to \( \text{NADP}^+ \).

 

Question 4. Assetion (A): Carotenes and Xanthophylls are soluble in either
Reason (R): These are accessory pigments of photosynthesis
Answer: (b) Both Assertion (A) and Reason (R) are true, Reason is not the correct explanation of Assertion.
In simple words: Carotenes and xanthophylls are indeed fat-soluble. They are also correct to call accessory pigments that help in photosynthesis. However, their solubility is not directly explained by their role as accessory pigments.

🎯 Exam Tip: Though both statements are true, the reason must directly explain the assertion. Here, solubility is a chemical property, and being an accessory pigment is a functional role.

 

Question 5. Assetion (A): Carotenoids are accessory pigments
Reason (R): Absorbed light energy is transferred to reaction centre by carotenoids.
Answer: (a) Assertion (A) and Reason (R) True and Reason is the correct explanation of Assertion.
In simple words: Carotenoids are indeed accessory pigments. The reason they are called accessory pigments is precisely because they absorb light energy and then pass it on to the main reaction center chlorophyll. This transfer of energy is their key function.

🎯 Exam Tip: Accessory pigments broaden the range of light wavelengths captured and protect chlorophyll from photodamage by dissipating excess energy.

III. 2 Marks Questions

 

Question 1. What is the function of the plant in the universe?
Answer: Plants are crucial for life on Earth. They act as the primary producers, meaning they create organic compounds like carbohydrates, lipids, proteins, and nucleic acids, which form the base of the food web. This process involves converting sunlight into chemical energy. Additionally, plants release oxygen into the atmosphere, which is essential for the respiration of most living organisms. They also play a vital role in balancing the carbon cycle by absorbing carbon dioxide.
In simple words: Plants make food for all living things and release oxygen for us to breathe. They also help keep the air clean by using carbon dioxide.

🎯 Exam Tip: When describing the function of plants, always mention their roles as producers in the food chain, oxygen liberators, and regulators of the carbon cycle.

 

Question 2. What is PAR?
Answer: PAR stands for Photosynthetically Active Radiation, which is the range of light wavelengths that plants use for photosynthesis, typically between 400 and 700 nanometers. Within this range, blue and red light are absorbed most effectively, leading to the highest rates of photosynthesis. Green light is largely reflected by chlorophyll, which is why plants appear green, and it generally induces the lowest rate of photosynthesis.
In simple words: PAR is the type of light that plants actually use to make food, which is the colors from blue to red. They don't use green light very much.

🎯 Exam Tip: Define PAR with its wavelength range and explicitly state which colors of light are most and least effective for photosynthesis.

 

Question 3. What is the site of photosynthesis?
Answer: The main site of photosynthesis in plants is the chloroplast, which are specialized organelles found within plant cells. Within the chloroplasts, the light-dependent reactions, where light energy is converted into chemical energy (ATP and NADPH), occur on the thylakoid membranes (which form stacks called grana). The light-independent reactions, also known as the Calvin cycle or dark reactions, where carbon dioxide is fixed into sugars, take place in the stroma, the fluid-filled space surrounding the thylakoids. This division of labor ensures efficient energy conversion and sugar synthesis.
In simple words: Photosynthesis happens inside little parts of plant cells called chloroplasts. The first part, using light, happens on disc-like structures called thylakoids, and the second part, making sugar, happens in the liquid part around them, called the stroma.

🎯 Exam Tip: Be precise in identifying the chloroplast as the overall site and then specifying the thylakoid membrane for light reactions and the stroma for dark reactions.

 

Question 4. Name the photosynthetic pigments of Algae?
Answer: Algae use various photosynthetic pigments, not just chlorophyll 'a' like higher plants. Here are some examples:
• Chlorophyll b – This is found in green algae.
• Chlorophyll c – Present in dinoflagellates, diatoms, and brown algae.
• Chlorophyll d – This type of chlorophyll is found in red algae.
• Xanthophylls and Carotenes are also common accessory pigments in many types of algae, helping them absorb a wider range of light colors.
In simple words: Algae have different kinds of green pigments, like chlorophyll 'b', 'c', and 'd', depending on the type of algae. They also have other colorful pigments that help them catch sunlight.

🎯 Exam Tip: When listing algal pigments, remember that different algal groups possess unique combinations of chlorophylls and accessory pigments that allow them to adapt to various light environments.

 

Question 5. Endosymbiotic hypothesis says that chloroplasts evolved from bacteria. Substantiate the statement.
Answer: The endosymbiotic hypothesis suggests that chloroplasts, like mitochondria, originated from ancient free-living photosynthetic bacteria that were engulfed by ancestral eukaryotic cells. This theory is strongly supported by several pieces of evidence. Chloroplasts contain their own circular DNA, similar to bacterial chromosomes, and their ribosomes are 70S, which is characteristic of bacteria rather than the 80S ribosomes found in the eukaryotic cytoplasm. Furthermore, chloroplasts reproduce by binary fission, a method common in bacteria. These features give chloroplasts a "semi-autonomous" status within the cell, indicating their bacterial lineage.
In simple words: Chloroplasts have their own special small DNA and ribosomes, just like bacteria. They also split in half to make more of themselves, like bacteria do. These facts make scientists think that chloroplasts used to be independent bacteria that came to live inside other cells a long, long time ago.

🎯 Exam Tip: For endosymbiotic theory questions, focus on the key pieces of evidence: presence of circular DNA, 70S ribosomes, binary fission, and double membranes, all pointing to a prokaryotic origin.

 

Question 6. Write down the significance of photorespiration.
Answer: Photorespiration, though often considered inefficient, has some significant roles in plants. It acts as a protective mechanism, especially under conditions of high light intensity and low \( \text{CO}_2 \) concentration, by dissipating excess energy that could otherwise damage the photosynthetic apparatus. During photorespiration, important biomolecules like glycine and serine are synthesized, which can serve as precursors for other metabolic pathways, including the production of chlorophyll, proteins, and nucleotides. It also consumes excess reducing power (NADH + H+), and glycolate produced during the process can protect cells from damage caused by photooxidation.
In simple words: Photorespiration helps plants by using up extra energy when there's too much light and not enough \( \text{CO}_2 \). It also makes some useful building blocks for other parts of the plant and helps protect cells from damage.

🎯 Exam Tip: While photorespiration is energy-costly, remember its protective role under stress and its contribution to the synthesis of important amino acids, which are its primary significances.

 

Question 7. What are the conclusions of Hill's Reaction?
Answer: Hill's reaction, demonstrated by Robert Hill, showed several key aspects of photosynthesis. The main conclusions are:
• During photosynthesis, oxygen is evolved from water, not from \( \text{CO}_2 \). This was a crucial discovery clarifying the source of atmospheric oxygen.
• Electrons needed for the reduction of \( \text{CO}_2 \) are obtained from the splitting of water molecules.
• A reduced substance (like \( \text{NADPH}_2 \)) is produced during the light reactions, which then helps to reduce \( \text{CO}_2 \) into sugars in the dark reactions.
• Hill showed that isolated chloroplasts could produce oxygen in the presence of an artificial electron acceptor (like ferricyanide), confirming that the light-dependent reactions of photosynthesis were distinct from carbon fixation. The general equation can be written as \( 2\text{H}_2\text{O} + 2\text{A} \rightarrow 2\text{AH}_2 + \text{O}_2 \), where A is the electron acceptor.
In simple words: Hill's Reaction proved that plants make oxygen from water, not carbon dioxide. It also showed that water provides the electrons needed to turn \( \text{CO}_2 \) into food, and that light makes a reduced substance that helps this happen.

🎯 Exam Tip: The Hill reaction is fundamental; it proved that water is the source of oxygen and that electron acceptors are reduced during the light-dependent stages of photosynthesis.

 

Question 8. What are Xanthophylls?
Answer: Xanthophylls are a type of carotenoid pigment, which are yellow to orange in color. They are mostly tetraterpenes and play a role in absorbing light, primarily in the blue to violet region of the visible spectrum. Unlike chlorophyll, they contain oxygen. A well-known example of a xanthophyll is lutein, along with violaxanthin and fucoxanthin, which are responsible for the yellow color change observed in leaves during the autumn season. Xanthophylls are crucial accessory pigments that protect chlorophyll from oxidative damage by absorbing and dissipating excess light energy.
In simple words: Xanthophylls are yellow or orange pigments that help plants catch light, especially blue light. They also protect the plant's green chlorophyll from getting damaged by too much sunlight. You see them when leaves turn yellow in autumn.

🎯 Exam Tip: Remember that xanthophylls are oxygen-containing carotenoids, serving both as accessory light-harvesting pigments and as protective agents against photo-oxidative stress.

 

Question 9. Notes on Phycobillins.
Answer:

• Phycobilins are special pigments that contain protein.
• They can dissolve easily in water.
• These pigments do not have magnesium (Mg) or a phytol tail, unlike chlorophyll.
• There are two main types: Phycocyanin and Phycoerythrin.
• Phycocyanin is commonly found in cyanobacteria.
• Phycoerythrin is found in red algae (Rhodophycean Algae). These pigments help absorb light in conditions where chlorophyll might be less effective.

In simple words: Phycobilins are protein-based colors that dissolve in water. They don't have magnesium or a tail like chlorophyll. Phycocyanin is in blue-green algae, and phycoerythrin is in red algae.

🎯 Exam Tip: Remember that phycobilins are unique because they are water-soluble and lack magnesium, key characteristics for distinguishing them from other photosynthetic pigments.

 

Question 10. Define absorption spectrum.
Answer: Pigments are able to take in (absorb) light at different wavelengths. When we plot a graph showing how much light a pigment absorbs at each wavelength, the resulting curve is called its absorption spectrum. This graph helps us understand which colors of light a pigment uses most.In simple words: An absorption spectrum is a graph that shows which colors of light a pigment soaks up the most.

🎯 Exam Tip: When defining absorption spectrum, always mention "plotting the amount of absorption" against "different wavelengths of light" to get full marks.

 

Question 11. Why do we call carotenoids shield pigments?
Answer:

• Carotenoids are yellow to orange pigments, mainly tetraterpenes, which absorb light strongly in the blue-to-violet part of the visible spectrum.
• These pigments help protect chlorophyll from damage caused by too much light or oxygen. They act like a shield, preventing harmful oxidative reactions that can happen during photosynthesis.

In simple words: Carotenoids are yellow-orange colors that take in blue-violet light. They protect the main green chlorophyll from getting damaged by strong light or oxygen, acting like a shield.

🎯 Exam Tip: Highlight their protective role ("shield") and their absorption of high-energy light (blue-violet region) when explaining why carotenoids are important.

 

Question 12. What is known as substrate-level phosphorylation?
Answer: Phosphorylation is the process of adding a phosphate group. During respiration, if ATP (adenosine triphosphate) is directly produced from the energy released by the breakdown of a substrate molecule, it is called substrate-level phosphorylation. This is a direct method of ATP formation.In simple words: Substrate-level phosphorylation is when ATP is made directly from an energy-rich molecule during breathing, without needing a special chain of reactions.

🎯 Exam Tip: Emphasize that in substrate-level phosphorylation, ATP is formed *directly* from a metabolic intermediate, distinguishing it from oxidative phosphorylation.

 

Question 13. What are Quantosomes?
Answer:

• Quantosomes are the basic units for photosynthesis, found within the inner membranes of thylakoids. They are typically about 180 Å by 160 Å in size.
• These structures were named by Park and Robbins in 1964.
• Each quantosome contains approximately 230 chlorophyll molecules.
• They act as a complete photosynthetic unit responsible for producing one oxygen molecule or reducing one carbon dioxide molecule.

In simple words: Quantosomes are small units inside plant cells where photosynthesis happens. They contain chlorophyll and help make oxygen or use carbon dioxide.

🎯 Exam Tip: Key points for quantosomes are their location (thylakoid inner membrane), function (photosynthetic unit), and composition (chlorophyll molecules).

 

Question 14. What is Bioluminescence?
Answer: Bioluminescence is a special ability of some living organisms to produce light. This light is created through certain biochemical reactions within their bodies. For example, fireflies produce light to attract mates.In simple words: Bioluminescence is when living things make their own light, like fireflies glowing in the dark.

🎯 Exam Tip: To score well, define bioluminescence as "emission of light by living organisms" and mention that it's due to "biochemical reactions."

 

Question 15. What is the significance of photorespiration?
Answer: Significance of photorespiration.
In simple words: Photorespiration has certain meanings for plants.

🎯 Exam Tip: While the full answer isn't provided here, understanding that photorespiration generally reduces photosynthetic efficiency is important.

 

Question 16. Explain water oxidizing clock or S state mechanism.
Answer: KoK and his colleagues studied the process of water splitting, also known as the water oxidizing clock or S-state mechanism, in 1970. This mechanism involves a series of five states, labeled \( S_0, S_1, S_2, S_3, S_4 \). Each S-state gains a positive charge when it absorbs a photon of light (hv). After reaching the \( S_4 \) state, it then gives off four positive charges, four electrons, and releases oxygen. This process is crucial because it supplies the electrons needed for the electron transport chain in photosynthesis.
Two molecules of water are processed in this cycle. At the end of water photolysis, \( 4H^+ \), \( 4e^- \), and \( O_2 \) are released from the water.
\( 4H_2O \rightarrow 4H^+ + 4OH^- \)
\( 4OH^- \rightarrow 2H_2O + O_2 + 4e^- \)
\( \implies \) Combining these, the net reaction is: \( 2H_2O \rightarrow 4H^+ + O_2 + 4e^- \)In simple words: The water oxidizing clock explains how water molecules are broken apart in photosynthesis to make oxygen. It's like a small machine with five steps, each step getting energy from light until it releases electrons and oxygen.

🎯 Exam Tip: Focus on the sequential absorption of photons (S-states) and the final release of electrons, protons, and oxygen as key aspects of the water oxidizing clock mechanism.

 

Question 17. Distinguish between photophosphorylation and oxidative phosphorylation.
Answer:

Both processes create ATP, but they use different initial energy sources – light for one and chemical reactions for the other.In simple words: Photophosphorylation uses light to make ATP during photosynthesis, while oxidative phosphorylation uses energy from breaking down food to make ATP during breathing.

🎯 Exam Tip: Clearly state the energy source (light vs. chemical oxidation) and the cellular process (photosynthesis vs. respiration) for each type of phosphorylation to show a clear distinction.

 

Question 18. What are the air pollutants, that affect the rate of photosynthesis?
Answer: Air pollutants can harm how well plants perform photosynthesis. Substances like sulfur dioxide (\( SO_2 \)), nitrogen dioxide (\( NO_2 \)), ozone (\( O_3 \)), and smog can significantly reduce the rate of this vital process. These pollutants can damage plant cells, especially the leaves, where photosynthesis takes place.In simple words: Air pollution from things like sulfur dioxide, nitrogen dioxide, ozone, and smog slows down how fast plants can make food through photosynthesis.

🎯 Exam Tip: List specific pollutants like \( SO_2, NO_2, O_3 \), and smog, and briefly explain their general negative impact on the photosynthetic rate.

 

Question 19. What are the conditions for the occurrence of Non-cyclic photophosphorylation?
Answer:

• Non-cyclic photophosphorylation happens when both photosystem I (PSI) and photosystem II (PSII) are active and working together.
• There must be enough \( NADP^+ \) available for reduction.
• The process requires two water molecules to be split, releasing electrons, protons, and oxygen.

This ensures a continuous flow of electrons and the production of both ATP and NADPH.In simple words: Non-cyclic photophosphorylation needs both Photosystem I and II to be working, enough \( NADP^+ \) to take electrons, and water to split and provide electrons.

🎯 Exam Tip: Mention the involvement of both photosystems (PSI and PSII), the availability of \( NADP^+ \), and the splitting of water molecules as essential conditions for non-cyclic photophosphorylation.

 

Question 20. Name any three photosynthetic bacteria.
Answer: Three photosynthetic bacteria are:

1. Chlorobacterium
2. Thiospirillum
3. Rhodospirillum

These bacteria can carry out photosynthesis but often use different electron donors than water.In simple words: Three bacteria that can make their own food using light are Chlorobacterium, Thiospirillum, and Rhodospirillum.

🎯 Exam Tip: List distinct names of photosynthetic bacteria; avoid common synonyms or general types to score accurately.

 

Question 21. What are the 2 phases of light reaction?
Answer: The light reaction in photosynthesis has two main phases:

1. Photooxidation phase (POP):
   • During this phase, light energy is absorbed.
   • This energy then gets transferred from accessory pigments to the reaction center.
   • Finally, the chlorophyll 'a' molecule at the reaction center becomes excited and activated.
2. Photochemical phase (PCO):
   • This phase involves the splitting of water (photolysis) and the release of oxygen.
   • It also includes electron transport and the creation of assimilatory power (ATP and NADPH). These products are then used in the dark reaction.

In simple words: The light reaction has two parts: first, light energy is taken in and excites chlorophyll (photooxidation), and second, water splits, oxygen is released, and energy is made (photochemical phase).

🎯 Exam Tip: Clearly separate the two phases (Photooxidation and Photochemical) and list key events for each, such as light absorption and chlorophyll activation for POP, and water splitting and ATP/NADPH formation for PCO.

 

Question 22. Greenlight induces lowest rate of photosynthesis justifies.
Answer:

• Green light causes the lowest rate of photosynthesis because chlorophyll, the main pigment, does not absorb green light effectively. Instead, it reflects green light, which is why plants appear green.
• The Photosynthetically Active Radiation (PAR) ranges from 400 to 700 nm, where blue and red light are absorbed most.
• The photosynthetic rate is highest in blue and red light, but very low in green light.

Therefore, green light is the least efficient for photosynthesis.In simple words: Green light makes photosynthesis happen very slowly because plants mostly reflect green light instead of absorbing it. They absorb red and blue light much better.

🎯 Exam Tip: Emphasize that chlorophyll reflects green light, leading to minimal absorption and thus the lowest photosynthetic rate in the green spectrum.

 

Question 24. Give the balance sheet of Calvin or C3 cycle.
Answer: In the Calvin or C3 cycle, one molecule of carbon dioxide (\( CO_2 \)) is fixed in each turn. Therefore, six turns of the cycle are needed to fix six molecules of \( CO_2 \), which is required to form one molecule of glucose (\( C_6H_{12}O_6 \)).

This balance sheet shows the overall inputs and outputs for making one glucose molecule.In simple words: To make one sugar molecule (glucose), the Calvin cycle uses 6 carbon dioxide, 18 ATP, and 12 NADPH. It produces one glucose, 18 ADP, and 12 NADP.

🎯 Exam Tip: For the Calvin cycle balance sheet, remember the exact numbers: 6 \( CO_2 \), 18 ATP, and 12 NADPH are needed to produce one molecule of glucose.

 

IV. 3 Mark Questions

 

Question 1. Mention any three significance of photosynthesis.
Answer: The three important significances of photosynthesis are:

1. Photosynthetic organisms are the main producers of food for all living organisms on Earth, either directly or indirectly. Without them, most life forms could not survive.
2. Photosynthesis is the only natural process that releases oxygen into the atmosphere, which is essential for respiration by many organisms and helps keep the oxygen levels balanced.
3. This process plays a crucial role in balancing the natural cycles of oxygen and carbon in nature. It removes carbon dioxide from the atmosphere and converts it into organic compounds.

In simple words: Photosynthesis makes food for almost all life, puts oxygen in the air for us to breathe, and helps balance carbon dioxide and oxygen in nature.

🎯 Exam Tip: Focus on food production, oxygen release, and carbon cycle balance as the primary significances of photosynthesis for a complete answer.

 

Question 2. What are the properties of light.
Answer: Light has several key properties:

• Light travels as a transverse electromagnetic wave, meaning its vibrations are perpendicular to its direction of movement.
• It is made up of oscillating electric and magnetic fields that are at right angles to each other and to the direction the light is moving.
• Light travels very fast, at a speed of \( 3 \times 10^8 \) meters per second (\( ms^{-1} \)) in a vacuum.
• The wavelength of light is the distance between two consecutive peaks or troughs of its wave.
• Light also behaves like a particle, called a photon. Each photon carries a specific amount of energy, known as a quantum.
• The energy of a photon depends on its frequency. Higher frequency light has more energy.

Light's dual nature (wave and particle) helps explain its behavior in different situations.In simple words: Light moves like a wave and a particle, called a photon. It travels very fast, has electric and magnetic parts, and its energy depends on its color.

🎯 Exam Tip: When describing light properties, mention its dual nature (wave and particle), its speed, and the relationship between photon energy and frequency.

 

Question 3. Distinguish between Absorption spectrum & Action Spectrum.
Answer:

Comparing these two spectra helps scientists understand which pigments are actually involved in photosynthesis.In simple words: An absorption spectrum shows which light colors a pigment soaks up. An action spectrum shows which light colors make photosynthesis work best.

🎯 Exam Tip: Remember that absorption spectrum shows *what* light is absorbed, while action spectrum shows *how effectively* that light absorption leads to photosynthesis.

 

Question 4. Distinguish between fluorescence and phosphorescence.
Answer:

The main difference is the duration of light emission after the exciting light is removed.In simple words: Fluorescence is when something glows right away and stops quickly after the light is turned off. Phosphorescence is when something glows for a longer time, even after the light is turned off.

🎯 Exam Tip: The key distinction lies in the *timing* of light emission: immediate for fluorescence vs. delayed and prolonged for phosphorescence.

 

Question 5. What is meant by the ground state?
Answer: The ground state refers to the most stable, lowest-energy condition of an atom or molecule. When a pigment molecule absorbs a photon of light, it gains energy and moves to an excited state. If the light source is removed, the high-energy electrons return to their normal, low-energy orbitals. This causes the excited molecule to go back to its original, stable, lowest-energy condition, which is called the ground state.In simple words: The ground state is when an atom or molecule is in its most stable, lowest-energy form. After absorbing energy, it returns to this calm state once the energy is gone.

🎯 Exam Tip: Define the ground state as the "most stable, lowest-energy condition" of a molecule, and explain that molecules return to this state after releasing absorbed energy.

 

Question 7. What is DCMU?
Answer: DCMU stands for Dichlorophenyl-dimethylurea. It is a chemical herbicide that specifically targets and inhibits photosynthesis.

• DCMU works by blocking the binding site of plastoquinone in Photosystem II (PSII).
• This action stops the flow of electrons during the light-dependent reactions of photosynthesis.
• By inhibiting electron flow from plastoquinone to cytochrome, DCMU effectively prevents the overall process of photosynthesis.

This shows how targeted chemicals can disrupt essential biological pathways.In simple words: DCMU is a weed killer that stops plants from making food. It does this by blocking how electrons move in Photosystem II during photosynthesis.

🎯 Exam Tip: Remember DCMU's full name, its function as a herbicide, and its specific mechanism: inhibiting electron flow by binding to the plastoquinone site in PSII.

 

Question 8. State black man's law of limiting factor.
Answer: Blackman's Law of Limiting Factors is a modified version of Liebig's Law of the Minimum.
Definition: According to Blackman's law, at any given moment, the rate of a physiological process (like photosynthesis) is limited by the factor that is nearest to its minimum value or in the least supply. This means that if one essential factor is scarce, increasing other factors will not speed up the process until the limiting factor is also increased.
Example: If light intensity is low and carbon dioxide concentration is also low, the factor that is lowest (the limiting factor) will determine the rate of photosynthesis. For instance, if carbon dioxide is more limiting than light, increasing light intensity won't help until carbon dioxide levels are also raised. This highlights how various environmental conditions interact.In simple words: Blackman's law says that a process, like how fast a plant grows, will only go as fast as its slowest step or the thing it has the least of. If a plant needs light and CO2, and there's not much CO2, giving it more light won't help until it gets more CO2.

🎯 Exam Tip: State the core idea: the rate of a process is limited by the factor in least supply. Provide a clear example, such as \( CO_2 \) or light, to illustrate the concept.

 

Question 9. What is meant by dicarboxylic acid pathway?
Answer: The dicarboxylic acid pathway, also known as the C4 pathway, is a two-phase process. The first phase takes place in the mesophyll cells' stroma, where the primary \( CO_2 \) acceptor molecule, phosphoenolpyruvate (PEP), which is a 3-carbon compound, forms a 4-carbon compound called oxaloacetic acid (OAA). Since the first stable product is a 4-carbon compound, this pathway is named the C4 cycle or dicarboxylic acid pathway. This pathway helps plants, especially in hot climates, to efficiently fix carbon dioxide.In simple words: The dicarboxylic acid pathway (C4 pathway) is how some plants first use carbon dioxide to make a 4-carbon acid. This happens in two steps and helps plants in warm places use carbon dioxide better.

🎯 Exam Tip: Define the dicarboxylic acid pathway by mentioning its first stable 4-carbon product (OAA), its initial reactant (PEP), and its occurrence in mesophyll cells.

 

Question 10. State some interesting facts about C4 cycle.
Answer: Here are some interesting facts about the C4 cycle:

• The C4 cycle is an alternative pathway for fixing carbon dioxide, different from the more common C3 cycle.
• It is found in nearly 1000 plant species, including about 300 dicots and mostly 700 monocots, especially tropical and subtropical grasses.
• C4 plants contribute to about 5% of Earth's biomass and are responsible for 1% of its known plant species.
• Approximately 30% of terrestrial carbon fixation on Earth is attributed to C4 plants. If the number of C4 plants increased, it could help avoid severe climate change by improving carbon sequestration.

These facts highlight the ecological importance of C4 plants.In simple words: The C4 cycle is another way plants take in carbon dioxide, found in many tropical grasses. These plants are a small part of Earth's plant life but help a lot with taking carbon dioxide out of the air.

🎯 Exam Tip: When listing facts about the C4 cycle, include its role as an alternative \( CO_2 \) fixation pathway, its prevalence in certain plant types, and its contribution to global carbon fixation.

 

Question 11. What is Kranz Anatomy or what is meant by Dimorphism of Chloroplasts in C4 plants.
Answer:

Kranz anatomy, with its specialized bundle sheath cells, is a key adaptation for efficient photosynthesis in hot and dry environments.In simple words: Kranz anatomy is a special leaf structure in C4 plants, where the vein cells have different types of chloroplasts than other leaf cells. C3 plants don't have this.

🎯 Exam Tip: When explaining Kranz anatomy, emphasize the presence of distinct bundle sheath cells and mesophyll cells, each containing different types of chloroplasts (chloroplast dimorphism).

 

Question 12. what is the significance of the CAM cycle?
Answer: The CAM (Crassulacean Acid Metabolism) cycle is very important, especially for succulent plants growing in dry or arid conditions.

1. It allows succulent plants to get \( CO_2 \) by breaking down malic acid even when their stomata are closed during the day. This is crucial for survival in water-scarce environments.
2. During the daytime, the stomata are closed, which prevents water loss through transpiration. Even with closed stomata, photosynthesis continues because stored \( CO_2 \) from the malic acid breakdown is available.
3. The stomata are also closed during the day to help the plants conserve water and avoid water loss through transpiration. This reverse rhythm of stomatal opening (at night) and closing (during the day) is a key water-saving strategy.

In simple words: The CAM cycle helps desert plants save water by opening their pores at night to take in \( CO_2 \) and closing them in the day. This way, they can still make food without losing precious water.

🎯 Exam Tip: Highlight water conservation (closed stomata during the day) and the temporal separation of \( CO_2 \) fixation (night) and the Calvin cycle (day) as the main significances of the CAM cycle.

 

Question 13. Compare the C3 and C4 on the basis of ATP production.
Answer:

C4 plants require more ATP per glucose molecule compared to C3 plants due to the additional carbon fixation steps, but this makes them more efficient in hot, dry conditions.In simple words: C3 plants need 8 ATPs and 12 NADPH to make one glucose. C4 plants need 30 ATPs and 12 NADPH to make one glucose, which is more energy but helps them in hot places.

🎯 Exam Tip: The critical distinction is the higher ATP requirement for C4 plants (30 ATPs per glucose) compared to C3 plants (18 ATPs per glucose), along with the NADPH requirements for both.

 

Question 14. What will be the quanta requirement for the complete light reaction which releases 6 oxygen molecules?
Answer: For a complete light reaction, if the evolution of one oxygen molecule requires 8 quanta of light, then for 6 oxygen molecules, the total quanta of light required would be \( 6 \times 8 = 48 \) quanta. This amount of light energy is necessary to drive the electron transport and water splitting processes that generate 6 oxygen molecules.In simple words: If making one oxygen molecule needs 8 units of light, then making 6 oxygen molecules will need 48 units of light in total for the whole process.

🎯 Exam Tip: Remember the quantum requirement for a single oxygen molecule (8 quanta) and then multiply it by the number of oxygen molecules needed to find the total for a complete process.

 

Question 15. Draw the Graphical representation of any 3 factors affecting photosynthesis.
Answer:

Here are graphical representations of three factors affecting photosynthesis:

This graph shows that as light intensity increases, the rate of photosynthesis also increases up to a point, after which it plateaus, indicating saturation.

This graph illustrates that an increase in \( CO_2 \) concentration boosts the rate of photosynthesis until it reaches a maximum, after which other factors become limiting.

This graph shows that photosynthesis has an optimal temperature range, increasing up to a peak and then declining as temperatures become too high, affecting enzyme activity.
In simple words: These graphs show that photosynthesis goes faster with more light and carbon dioxide up to a point. For temperature, it works best in a middle range, slowing down if it's too hot or too cold.

🎯 Exam Tip: When drawing graphs for factors affecting photosynthesis, always label both axes clearly. Remember the typical curve shapes: increasing then plateauing for light and CO2, and a bell-shaped curve for temperature due to enzyme activity.

 

V. 5 Mark Questions

 

Question 1. Distinguish between Photosystem – I and photosystem – II.
Answer:
Photosystem – I:

1. The reaction center of Photosystem I (PSI) is P700, which means it absorbs light best at a wavelength of 700 nm.
2. PSI is involved in both cyclic and non-cyclic photophosphorylation.
3. It is not directly involved in the photolysis (splitting) of water or the release of oxygen.
4. PSI receives electrons from Photosystem II (PSII) during non-cyclic photophosphorylation.
5. It is located in the unstacked regions of the granum, facing the chloroplast stroma.
6. The ratio of chlorophyll to carotenoids in PSI is 20:30:1.

Photosystem – II:

1. The reaction center of Photosystem II (PSII) is P680, which means it absorbs light best at a wavelength of 680 nm.
2. PSII participates only in the non-cyclic pathway of electron flow.
3. Photolysis of water (splitting of water molecules) and the subsequent evolution of oxygen take place directly within PSII.
4. It receives electrons from the splitting of water.
5. PSII is found in the stacked regions of the thylakoid membrane, facing the lumen of the thylakoid.
6. The ratio of chlorophyll to carotenoids in PSII is 3:7:1.

The interplay between PSI and PSII ensures the efficient conversion of light energy into chemical energy during photosynthesis.In simple words: Photosystem I (PSI) uses P700 and works in both cyclic and non-cyclic processes, but doesn't split water. Photosystem II (PSII) uses P680 and only works in non-cyclic processes, and it's where water is split to release oxygen.

🎯 Exam Tip: For distinguishing PSI and PSII, remember their reaction center chlorophylls (P700 vs. P680), their involvement in cyclic/non-cyclic pathways, and whether they perform water photolysis.

 

Question 2. Structure of Chloroplast.
Answer: The chloroplast is an organelle where photosynthesis occurs. It has a complex internal structure designed for this process.
A chloroplast contains an outer and inner membrane, enclosing a fluid-filled space called the stroma. Within the stroma are stacks of disc-shaped structures called thylakoids, which form grana. The thylakoids are interconnected by frets or stroma lamellae.In simple words: A chloroplast is like a small factory in a plant cell where food is made. It has two layers outside, and inside it's filled with liquid (stroma) and stacks of flat sacs (grana) connected by tubes.

🎯 Exam Tip: When describing the chloroplast structure, mention the double membrane, stroma, thylakoids (forming grana), and stroma lamellae (frets) as key components.

 

Question 3. Notes on photosystem and Reaction centre.
Answer: Photosystems are functional units located within the thylakoid membranes of chloroplasts, responsible for capturing light energy during photosynthesis.

• Thylakoid membranes contain two main photosystems: Photosystem I (PSI) and Photosystem II (PSII).
• PSI is found in the unstacked regions of the granum, facing the chloroplast stroma.
• PSII is located in the stacked regions of the thylakoid membrane, facing the lumen (inner space) of the thylakoid.
• Each photosystem consists of a central core complex (CC) and light-harvesting complex (LHC), also known as antenna molecules. The LHCs gather light energy and pass it to the core complex.
• The core complex contains the specific reaction center chlorophylls, along with associated proteins, electron donors, and acceptors.
• PS II - CC II consists of reaction center P680 and LHC - II.
• Light harvesting complex consists of several chlorophylls, carotenoids and xanthophyll molecules.
• The main function of LHC is to harvesting light energy and transfer it to their respective reaction centre.

These photosystems work together to convert light energy into chemical energy, driving the photosynthetic process.In simple words: Photosystems are parts of the plant cell that catch sunlight. They have a main part called the reaction center and other parts that collect light. There are two types, Photosystem I and Photosystem II, which work together in different places.

🎯 Exam Tip: Define photosystems as functional units for light capture. Mention their two main types (PSI, PSII), their location (stacked vs. unstacked thylakoids), and their components (core complex, light-harvesting complex).

 

Question 4. Difference between photosystem I and Photosystem II.
Answer:

Both photosystems are essential for light-dependent reactions, working in tandem to transfer energy and electrons.In simple words: Photosystem I (P700) works in both cyclic and non-cyclic processes and gets electrons from PSII. Photosystem II (P680) splits water to make oxygen and only works in non-cyclic processes.

🎯 Exam Tip: Distinguish PSI and PSII by their reaction center pigments (P700 vs. P680), their role in water photolysis (PSI none, PSII performs it), and their location in the thylakoid (unstacked vs. stacked regions).

 

Question 5. Differences between Cyclic Photophosphorylation and Non – cyclic photophosphorylation.
Answer:

Non-cyclic photophosphorylation is the more common and complete pathway, producing both ATP and NADPH needed for the dark reactions.In simple words: Cyclic photophosphorylation only uses Photosystem I, makes only ATP, and recycles electrons. Non-cyclic photophosphorylation uses both Photosystem I and II, makes both ATP and NADPH, and doesn't recycle electrons, splitting water instead.

🎯 Exam Tip: Focus on the involvement of photosystems (PSI only vs. PSI & PSII), products formed (ATP only vs. ATP & NADPH), and electron flow (cyclic vs. non-cyclic with water splitting) for differentiation.

 

Question 6. Compare and contrast the photosynthetic processes in C3 and C4 plants.
Answer:
Contrast the photosynthetic processes in C3 and C4 plants:
C3 Plants:

• Carbon dioxide fixation happens only in mesophyll cells.
• RUBP (Ribulose-1,5-bisphosphate) is the only \( CO_2 \) acceptor.
• The first stable product is a 3-carbon compound, PGA (Phosphoglyceric acid).
• Kranz anatomy, a specialized leaf structure, is not present.
• Granum is present in mesophyll cells.
• They have normal chloroplasts.
• The optimum temperature for photosynthesis is usually between 20°C to 25°C.
• \( CO_2 \) fixation occurs if the atmospheric concentration of \( CO_2 \) is about 50 ppm or higher.
• C3 plants are less efficient and experience higher photorespiration rates, especially in hot conditions.
• RUBP carboxylase (Rubisco) is the enzyme used for carbon fixation, but it can also act as an oxygenase.
• 18 ATPs are generally used to synthesize one glucose molecule.
• C3 plants are efficient at low \( CO_2 \) concentrations.
• Examples include Paddy, Wheat, and Potato.

In simple words: C3 plants fix carbon dioxide only in mesophyll cells, use RUBP as the \( CO_2 \) acceptor, and form a 3-carbon product. They don't have Kranz anatomy, work best at cooler temperatures, and are less efficient in hot, dry places.

🎯 Exam Tip: For C3 plants, key points include \( CO_2 \) fixation only in mesophyll cells, RUBP as the acceptor, a 3-carbon first product, lack of Kranz anatomy, and lower temperature optimum.

 

Question 4. Difference between photosystem I and Photosystem II.
Answer:

In simple words: Photosystem I and II are like two teams working in photosynthesis. PS I uses P700 as its center and can work in both cyclic and non-cyclic ways. PS II uses P680 and mainly handles splitting water to release oxygen and electrons.

🎯 Exam Tip: Remember the specific reaction center pigments (P700 for PSI, P680 for PSII) and their involvement in water photolysis to easily differentiate them.

 

Question 5. Differences between Cyclic Photophosphorylation and Non - cyclic photophosphorylation.
Answer:

In simple words: Cyclic photophosphorylation involves only Photosystem I and produces ATP by recycling electrons. Non-cyclic photophosphorylation uses both Photosystem I and II, splits water, releases electrons, and produces both ATP and NADPH, with electrons moving in one direction.

🎯 Exam Tip: Focus on whether electrons are recycled (cyclic) or flow in one direction (non-cyclic) and the products formed (ATP only vs. ATP & NADPH) as key distinguishing points.

 

Question 6. Compare and contrast the photosynthetic processes in C3 and C4 plants.
Answer: The photosynthetic processes in C3 and C4 plants have several key differences:

C3 Plants:

• \( \text{CO}_2 \) fixation takes place in mesophyll cells only.
• The only \( \text{CO}_2 \) acceptor is RUBP.
• The first stable product formed is 3C - PGA.
• Kranz anatomy is not present.
• Granum is present in mesophyll cells.
• They have normal chloroplasts.
• Optimum temperature ranges from 20°C to 25°C.
• \( \text{CO}_2 \) fixation occurs at 50 ppm.
• They are less efficient due to higher photorespiration.
• RUBP carboxylase enzyme is used for fixation.
• 18 ATPs are used to synthesize one glucose molecule.
• They are efficient at low \( \text{CO}_2 \) levels.
• Examples include Paddy, Wheat, and Potato.

C4 Plants:

• \( \text{CO}_2 \) fixation occurs in both mesophyll and bundle sheath cells.
• PEP (phosphoenolpyruvate) in mesophyll and RUBP in bundle sheath cells are the \( \text{CO}_2 \) acceptors.
• The first stable product is 4C - OAA (oxaloacetic acid).
• Kranz anatomy is present, which is a specialized leaf structure.
• Granum is present in mesophyll cells but absent in bundle sheath.
• They have dimorphic chloroplasts, meaning two different types.
• Optimum temperature ranges from 30°C to 45°C.
• \( \text{CO}_2 \) fixation can happen even at less than 10 ppm.
• They are more efficient due to less photorespiration.
• PEP carboxylase and RUBP carboxylase enzymes are both used.
• 30 ATPs are needed to produce one glucose molecule.
• They are efficient at higher \( \text{CO}_2 \) levels.
• Examples include Sugarcane, Maize, and Sorghum. These plants are often found in warmer climates.

In simple words: C3 plants fix carbon dioxide only in one cell type and are less efficient in hot, dry conditions. C4 plants use two cell types for carbon fixation, have a special leaf structure (Kranz anatomy), and are more efficient in warmer, brighter conditions because they avoid photorespiration.

🎯 Exam Tip: When comparing C3 and C4 plants, focus on anatomical differences (Kranz anatomy), initial \( \text{CO}_2 \) acceptors, primary products, and efficiency under different environmental conditions.

 

Question 7. Explain Non cyclic photophosphorylation.
Answer: Non-cyclic photophosphorylation, also known as the Z-scheme, is a process where light energy is used to make ATP and NADPH. It starts when Photosystem II (PSII), with its reaction center P680, absorbs light and gets excited. Electrons from PSII are then passed through a chain of electron carriers, including pheophytin, plastoquinone, cytochrome b6-f complex, and plastocyanin, before reaching Photosystem I (PSI), with its reaction center P700. During this electron flow, ATP is generated. The electrons lost by PSII are replaced by splitting water molecules in a process called photolysis, which also releases oxygen, protons (\( \text{H}^{+} \)), and electrons. Once the electrons reach PSI, it also absorbs light and gets excited. The excited electrons from PSI are then passed to ferredoxin, which ultimately reduces \( \text{NADP}^{+} \) to \( \text{NADPH} + \text{H}^{+} \). In this pathway, electrons are not recycled back to their original photosystem but instead move in one direction from water to \( \text{NADP}^{+} \). This entire electron movement creates a 'Z' shape when drawn, giving it the name Z-scheme. The entire process consists of three main stages, ensuring both ATP and NADPH are produced for the Calvin cycle.
In simple words: Non-cyclic photophosphorylation is a process in plants that uses light to create energy (ATP) and reducing power (NADPH). It works like a Z-shaped path where electrons move from water, through two different photosystems, and end up making NADPH, while also creating ATP along the way. Water is split to replace the lost electrons and release oxygen.

🎯 Exam Tip: When drawing the Z-scheme, ensure clear labeling of Photosystem I (P700) and Photosystem II (P680), and correctly indicate the flow of electrons, as well as the points of ATP and NADPH synthesis.

 

Question 8. Explain Calvin cycle or C3 cycle?
Answer: The Calvin cycle, also known as the C3 cycle or dark reactions, is a crucial process where plants use the ATP and NADPH generated during the light-dependent reactions to convert carbon dioxide into sugars. This cycle takes place in the stroma of the chloroplast. It is temperature-dependent and is sometimes called a thermochemical reaction. The cycle was discovered by Melvin Calvin and his colleagues. The Calvin cycle has three main phases:

Phase I: Carboxylation (Carbon Fixation):

• The cycle begins with a 5-carbon sugar called Ribulose-1,5-Bisphosphate (RuBP).
• The enzyme RUBISCO helps RuBP combine with one molecule of carbon dioxide.
• This combination forms an unstable 6-carbon compound.
• The unstable 6-carbon compound quickly breaks down into two molecules of a 3-carbon compound called 3-Phosphoglyceric Acid (3-PGA). This is why it's called the C3 cycle, as the first stable product has 3 carbons.

Phase II: Glycolytic Reversal/Reduction:

• In this phase, the 3-PGA molecules are converted into Glyceraldehyde-3-Phosphate (G3P).
• This conversion requires energy from ATP and reducing power from NADPH, which were produced during the light reactions.
• Specifically, 3-PGA is first phosphorylated by ATP to form 1,3-Bisphosphoglyceric Acid, then reduced by NADPH to form G3P.
• For every six molecules of \( \text{CO}_2 \) fixed, two molecules of G3P are produced. One of these G3P molecules is used to make glucose or other sugars, while the rest continue in the cycle.

Phase III: Regeneration:

• The remaining G3P molecules are used to regenerate the initial 5-carbon RuBP, so the cycle can continue.
• This regeneration involves several intermediate compounds with 6C, 5C, 4C, and 7C atoms.
• This process also requires ATP. For each \( \text{CO}_2 \) molecule fixed, 3 ATPs and 2 NADPH + \( \text{H}^{+} \) are consumed.
• To fix six \( \text{CO}_2 \) molecules and produce one glucose molecule, 18 ATPs and 12 NADPH + \( \text{H}^{+} \) are needed in total. The net gain for the cycle is the formation of hexose sugar.

Overall equation of C3 or Dark reaction is:
\( 6\text{CO}_2 + 18\text{ATP} + 12\text{NADPH} + \text{H}^{+} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{H}_2\text{O} + 18\text{ADP} + 18 \text{P}_{\text{i}} + 12\text{NADP}^{+} \).
In simple words: The Calvin cycle is how plants turn carbon dioxide into sugar. It happens in three steps: first, carbon dioxide is taken in and combined with a 5-carbon sugar; then, this new compound is changed into a 3-carbon sugar using energy from ATP and NADPH; finally, the 5-carbon sugar is remade so the cycle can keep going. This whole process makes glucose, which the plant uses for food.

🎯 Exam Tip: When explaining the Calvin cycle, remember to clearly state the three phases (carboxylation, reduction, regeneration), the key molecules involved (RuBP, 3-PGA, G3P), and the energy inputs (ATP, NADPH).

 

Question 9. Draw the flow Chart of C4 pathway?
Answer: The C4 pathway is a mechanism used by some plants to efficiently fix carbon dioxide, especially in hot and dry environments. It involves two types of cells: mesophyll cells and bundle sheath cells. The diagram below illustrates this flow. The process helps plants minimize photorespiration and conserve water.

In simple words: The C4 pathway is like a two-step process to capture carbon dioxide. First, in mesophyll cells, CO2 is quickly taken up to form a 4-carbon compound. Then, this 4-carbon compound moves to special bundle sheath cells, where CO2 is released and used in the normal Calvin cycle to make sugar. This helps plants avoid losing carbon dioxide in hot weather.

🎯 Exam Tip: When drawing the C4 pathway, clearly show the roles of both mesophyll and bundle sheath cells, highlighting the initial \( \text{CO}_2 \) fixation by PEP carboxylase and the subsequent Calvin cycle activity.

 

Question 10. Explain CAM Cycle?
Answer: The CAM (Crassulacean Acid Metabolism) cycle is a special way that succulent plants, which live in dry or desert-like conditions, fix carbon dioxide. This pathway helps them survive by conserving water very effectively. The key feature of the CAM cycle is that it separates the carbon fixation process by time, rather than by space (like C4 plants). It works as follows:

• These plants have stomata that close during the day to prevent water loss (scotoactive) and open only at night.
• At night, when the stomata are open, the plants take in \( \text{CO}_2 \). This \( \text{CO}_2 \) is then fixed with the help of PEP (phosphoenolpyruvate) to form a 4-carbon compound called oxaloacetic acid (OAA).
• The OAA is quickly converted into malic acid, which is stored in large vacuoles within the plant cells. This storage of malic acid increases the acidity within the plant.
• During the daytime, when the stomata are closed and light is available for photosynthesis, the stored malic acid is transported out of the vacuole.
• The malic acid is then decarboxylated, meaning \( \text{CO}_2 \) is released from it. This process also decreases the acidity within the plant cells.
• The released \( \text{CO}_2 \) then enters the Calvin cycle (C3 cycle) within the chloroplasts to produce carbohydrates, just like in other plants.
• This "reverse rhythm" of stomata opening at night and closing during the day helps these plants to conserve water through transpiration, while still being able to perform photosynthesis during the day with the internally stored \( \text{CO}_2 \). This makes it advantageous for succulent plants to obtain \( \text{CO}_2 \) from malic acid when stomata are closed.

In simple words: The CAM cycle is a clever trick some desert plants use to save water. They open their leaf pores (stomata) at night to take in carbon dioxide and store it as an acid. Then, during the hot day when their pores are closed to stop water loss, they use that stored carbon dioxide to make food.

🎯 Exam Tip: Emphasize the temporal separation of \( \text{CO}_2 \) uptake (night) and utilization (day) in CAM plants, along with the role of stomata closure in water conservation, as key points in your explanation.

 

Question 11. Give the flow chart of Photo respiration or C2 cycle.
Answer: Photorespiration, also known as the C2 cycle, is a process that occurs in C3 plants under specific conditions (high oxygen concentration, high temperature, and high light intensity) where the enzyme RuBisCO binds with oxygen instead of carbon dioxide. This leads to the production of a 2-carbon compound (phosphoglycolate) instead of two 3-carbon PGA molecules, reducing photosynthetic efficiency. The flowchart below illustrates the movement of compounds through the chloroplast, peroxisome, and mitochondrion during photorespiration. This process consumes energy and releases \( \text{CO}_2 \), rather than fixing it.

In simple words: Photorespiration, or the C2 cycle, is a wasteful process in plants that happens when the RuBisCO enzyme accidentally uses oxygen instead of carbon dioxide. This produces a toxic 2-carbon compound that the plant has to break down, using energy and releasing carbon dioxide, which reduces how much food it can make. It involves steps across the chloroplast, peroxisome, and mitochondrion.

🎯 Exam Tip: Remember that photorespiration is a three-organelle process (chloroplast, peroxisome, mitochondrion) and its primary outcome is a reduction in photosynthetic efficiency due to \( \text{CO}_2 \) release and ATP consumption.

 

Question 12. Differentiate Photorespiration and Dark respiration.
Answer:

In simple words: Photorespiration happens in green cells only when there is light, uses glycolic acid, and is not essential for survival. Dark respiration happens in all living cells at all times, uses carbohydrates or other nutrients, and is vital for life as it produces ATP energy.

🎯 Exam Tip: Focus on the conditions (light vs. dark), locations (specific organelles vs. all cells), substrates, and energy outcomes (ATP yield vs. no ATP yield) to distinguish between photorespiration and dark respiration.

 

Question 13. Give any 5 External factors affecting photosynthesis.
Answer: Photosynthesis, the process by which plants make their own food, is affected by several external factors in the environment. Here are five important ones:

1. Carbon Dioxide:Carbon dioxide (\( \text{CO}_2 \)) is a key raw material for photosynthesis. Normally, the atmosphere contains about 330 ppm (or 0.3%) of \( \text{CO}_2 \). If the \( \text{CO}_2 \) concentration increases, the rate of photosynthesis generally increases. However, if it increases beyond 500 ppm, the rate of photosynthesis can actually slow down or be inhibited. This is because high \( \text{CO}_2 \) levels can also lead to stomatal closure, reducing water loss but also limiting gas exchange.

2. Oxygen:When there is a high concentration of oxygen, it can slow down photosynthesis. This inhibitory effect of oxygen on photosynthesis was first studied by Warburg in 1920 using green algae, Chlorella, and is known as the Warburg effect. High oxygen levels cause the enzyme RuBisCO to bind with oxygen instead of \( \text{CO}_2 \), leading to photorespiration, which wastes energy and carbon.

3. Temperature:Temperature plays a significant role in photosynthesis because it affects the enzymes involved in the process. The optimum temperature for photosynthesis varies greatly among different plants. For most plants, it's typically between 25°C and 35°C. For example, in desert plants like Opuntia, the optimum temperature can be as high as 55°C, while in some lichens, it might be around 20°C. Algae in hot springs can even photosynthesize at 75°C. Both extremely high and low temperatures can cause stomata to close and inactivate the photosynthetic enzymes, thereby reducing the rate of photosynthesis.

4. Water:Water is essential for photosynthesis. The photolysis (splitting) of water provides electrons and protons (\( \text{H}^{+} \)) needed for the reduction of \( \text{NADP}^{+} \) directly. Water also indirectly affects photosynthesis by influencing stomatal movement and the overall hydration of protoplasm (the cell's living content). During water stress, stomata close to conserve water, but this also limits the intake of \( \text{CO}_2 \), significantly reducing photosynthesis. A lack of water also affects the supply of NADPH + \( \text{H}^{+} \).

5. Minerals:Several minerals are vital for the proper functioning of photosynthesis, acting as cofactors for enzymes or components of photosynthetic pigments. The table below shows some important minerals and their effects:

In simple words: Photosynthesis needs a few things from outside: carbon dioxide, oxygen, the right temperature, enough water, and certain minerals. Too much or too little of any of these can make it harder for plants to make food. For example, high oxygen can slow it down, and low water makes plants close their pores.

🎯 Exam Tip: When listing external factors, remember to not only name them but also briefly explain how each factor influences the rate of photosynthesis, referencing relevant mechanisms or effects like the Warburg effect.

 

Question 14. Explain the test tube funnel experiment.
Answer: The test tube funnel experiment is a simple demonstration to prove that oxygen gas is released during photosynthesis. Here’s how it works:

Aim: To show that oxygen is evolved during photosynthesis.

Procedure:

1. Take a beaker filled with water. Place some aquatic plant, like Hydrilla, at the bottom of the beaker.
2. Add a small amount of sodium bicarbonate (\( \text{NaHCO}_3 \)) to the water. This provides carbon dioxide for the plant to use.
3. Cover the Hydrilla plant with an inverted funnel. Ensure the funnel's stem is submerged in the water.
4. Carefully invert a test tube full of water over the stem of the funnel, making sure no air bubbles are trapped inside the test tube at the beginning.
5. Place this entire setup in sunlight.

Observation: After some time in sunlight, you will observe small air bubbles being released from the Hydrilla plant. These bubbles rise through the funnel's stem and collect at the top of the inverted test tube, displacing the water. To confirm that the collected gas is oxygen, carefully remove the test tube by closing its mouth with your thumb to prevent gas from escaping. Then, introduce a burning matchstick or glowing splint into the test tube. The matchstick will burn more brightly, or the glowing splint will re-ignite.

Inference: The observation that the matchstick burns more brightly confirms that the gas collected is oxygen. This experiment clearly shows that Hydrilla plants perform photosynthesis in the presence of sunlight and release oxygen as a byproduct.
In simple words: In this experiment, a water plant is put in a beaker with a funnel and a test tube over it, then placed in sunlight. Bubbles form, collect in the test tube, and make a burning matchstick glow brighter, proving the plant makes oxygen during photosynthesis.

🎯 Exam Tip: When performing or describing this experiment, always remember to add sodium bicarbonate to ensure enough \( \text{CO}_2 \) is available, and use a burning splint to confirm the presence of oxygen.

 

Question 15. Explain the experiment to determine rate of photosynthesis by Witmott's bubbler.
Answer: Witmott's bubbler experiment is used to measure the rate of photosynthesis by counting the number of oxygen bubbles released by an aquatic plant over time. It provides a simple way to observe how different factors affect photosynthesis. Here's a breakdown:

Procedure:

• Wilmott's bubbler consists of a wide-mouthed bottle, fitted with a single-holed cork.
• A glass tube is inserted through the cork, having a wider opening at the lower end to allow easy insertion of a Hydrilla plant.
• The upper end of the glass tube is narrower and extends into a smaller bottle filled with water, which acts as a water reservoir.
• The main bottle is filled with water, and a Hydrilla plant is placed inside its wider part.
• It's crucial to cut the Hydrilla plant inside the water to prevent any air bubbles from entering the plant stem, which could interfere with the experiment.
• The entire apparatus is placed in sunlight, or near a light source.
• The rate of photosynthesis is determined by counting the number of gas bubbles released by the Hydrilla plant per minute. This count can be observed when the bubbles are of a consistent size.

In simple words: Wilmott's bubbler is an experiment to see how fast a water plant does photosynthesis. It uses a special setup where the plant releases oxygen bubbles, and by counting these bubbles, we can tell how quickly photosynthesis is happening. The setup needs to be placed in light, and the plant must be cut underwater to get accurate results.

🎯 Exam Tip: Remember to mention the continuous flow of bubbles as an indicator of photosynthesis rate and the importance of cutting the Hydrilla underwater to avoid air blockages.

 

Question 16. Differentiate photosynthesis in plants and Bacterial photosynthesis.
Answer:

In simple words: Plant photosynthesis uses both Photosystem I and II, splits water to release oxygen, and uses chlorophyll in chloroplasts. Bacterial photosynthesis only uses Photosystem I, does not release oxygen because it uses other electron donors like \( \text{H}_2\text{S} \), and uses bacteriochlorophyll in simpler structures called chromatophores.

🎯 Exam Tip: Focus on the electron donor (water vs. \( \text{H}_2\text{S} \)), oxygen evolution (present vs. absent), types of photosystems involved, and the pigments/structures used as key differences between plant and bacterial photosynthesis.