RBSE Solutions Class 12 Biology Chapter 10 Photosynthesis

RBSE Class 12 Biology Chapter 10 Multiple Choice Questions

 

Question 1. Which metal element is found in the centre of the chlorophyll molecule?
(a) Fe
(b) Mg
(c) Ne
(d) Cu
Answer: (b) Mg
In simple words: The main metal in the middle of a chlorophyll molecule is magnesium. This atom is key for chlorophyll to capture sunlight during photosynthesis.

🎯 Exam Tip: Remember that magnesium is crucial for chlorophyll's function in absorbing light energy.

 

Question 2. PS-II is related to
(a) Photolysis of water
(b) Reduction of \( \text{CO}_2 \)
(c) Processing math: 100%

 

Question 3. In PS-I and PS-II the reaction centres respectively are -
(a) \( \text{P}_{700} \) and \( \text{P}_{680} \)
(b) \( \text{P}_{600} \) and \( \text{P}_{700} \)
(c) \( \text{P}_{580} \) and \( \text{P}_{700} \)
(d) \( \text{P}_{700} \) and \( \text{P}_{580} \)
Answer: (a) \( \text{P}_{700} \) and \( \text{P}_{680} \)
In simple words: PS-I works best at 700 nm light, and PS-II works best at 680 nm light. These numbers show the wavelengths of light they absorb most effectively.

🎯 Exam Tip: Remember the specific light absorption wavelengths for PS-I and PS-II, as they are fundamental to understanding the light-dependent reactions of photosynthesis.

 

Question 4. \( \text{O}_2 \) evolution is related with which of the following?
(a) PS - I
(b) PS - II
(c) Phytochrome
(d) All the above
Answer: (b) PS - II
In simple words: Oxygen comes out during photosynthesis because PS-II helps split water molecules. This process, called photolysis, is where oxygen gas is produced.

🎯 Exam Tip: Connect oxygen evolution to the photolysis of water, which occurs specifically in Photosystem II.

 

Question 5. Which of the following process occurs in \( \text{C}_4 \) plants and not in \( \text{C}_3 \) plants.
(a) Glycolysis
(b) Photorespiration
(c) Transpiration
(d) Photosynthesis
Answer: (b) Photorespiration
In simple words: \( \text{C}_4 \) plants are better at avoiding photorespiration, which wastes energy, unlike \( \text{C}_3 \) plants. \( \text{C}_4 \) plants have special adaptations to minimize this inefficient process.

🎯 Exam Tip: Understand the anatomical and biochemical differences (like Kranz anatomy and PEP carboxylase) that allow \( \text{C}_4 \) plants to suppress photorespiration.

 

Question 6. Unit of photosynthesis is -
(a) Quantasome
(b) Microsome
(c) Peroxisome
(d) Processing math: 100%

 

Question 8. Select correct statement for photosynthesis -
(a) CO and \( \text{H}_2\text{O} \) are oxidised
(b) \( \text{CO}_2 \) and \( \text{H}_2\text{O} \) are reduced
(c) \( \text{H}_2\text{O} \) is reduced and \( \text{CO}_2 \) oxidized
(d) \( \text{H}_2\text{O} \) is oxidized and \( \text{CO}_2 \) reduced
Answer: (d) \( \text{H}_2\text{O} \) is oxidized and \( \text{CO}_2 \) reduced
In simple words: Water gives away electrons (gets oxidized), and carbon dioxide takes electrons (gets reduced) to make food. This process converts light energy into chemical energy.

🎯 Exam Tip: Clearly distinguish between oxidation (loss of electrons, in water) and reduction (gain of electrons, in carbon dioxide) in photosynthesis.

 

Question 9. What is the source of oxygen, liberated in photosynthesis?
(a) \( \text{H}_2\text{O} \)
(b) \( \text{CO}_2 \)
(c) \( \text{H}_2\text{O} \) and \( \text{CO}_2 \) both
(d) Neither \( \text{O}_2 \) nor \( \text{CO}_2 \)
Answer: (a) \( \text{H}_2\text{O} \)
In simple words: The oxygen we breathe, made by plants, comes from water. This is a result of water molecules being split during the light reactions.

🎯 Exam Tip: It's a common misconception that oxygen comes from carbon dioxide; emphasize that water is the source through photolysis.

 

Question 10. What is the site of the dark reaction of photosynthesis?
(a) Grana of chloroplast
(b) Stroma of chloroplast
(c) The outer membrane of the chloroplast
(d) The inner membrane of the chloroplast

 

Question 12. The first stable product of the \( \text{C}_4 \) cycle is -
(a) Pyruvic acid
(b) Oxaloacetic acid
(c) Malic acid
(d) None of the options
Answer: (b) Oxaloacetic acid
In simple words: In the \( \text{C}_4 \) plant cycle, the very first product made is oxaloacetic acid. This molecule has four carbon atoms.

🎯 Exam Tip: Remember that oxaloacetic acid is a 4-carbon compound, which gives the \( \text{C}_4 \) pathway its name.

 

Question 13. For reduction of \( \text{CO}_2 \), reducing the power generated during non-cyclic photophosphorylation is depicted as -
(a) \( \text{24H}^+ \)
(b) \( \text{36H}^+ \)
(c) \( \text{18H}^+ \)
(d) \( \text{12H}^+ \)
Answer: (a) \( \text{24H}^+ \)
In simple words: The energy units needed to change carbon dioxide into food in non-cyclic photosynthesis are shown as \( \text{24H}^+ \). This indicates the protons and electrons used in the reduction process.

🎯 Exam Tip: Understand the stoichiometry of ATP and NADPH produced in non-cyclic photophosphorylation and how they are consumed in the Calvin cycle.

 

Question 14. The wavelength of photosynthetically active radiation is -
(a) 340 - 450 nm
(b) 400 - 700 nm
(c) 500 - 600 nm
(d) Processing math: 100%

 

Question 1. Define photosynthesis.
Answer: Photosynthesis is the vital process green plants use to create their own food. They take in carbon dioxide from the air and water from the soil, then use sunlight as energy to transform these into carbohydrates (sugars). This magical process powers most life on Earth.
In simple words: Photosynthesis is how plants make food. They use sunlight, water, and air (carbon dioxide) to make sugar.

🎯 Exam Tip: A complete definition must mention carbon dioxide, water, sunlight, and the production of carbohydrates (food).

 

Question 2. What is the first stable product of the reaction of photosynthesis?
Answer: The first stable product formed during carbon dioxide fixation in photosynthesis differs based on the plant type. In \( \text{C}_3 \) plants, it is phosphoglyceric acid (PGA), a compound with three carbon atoms. However, in \( \text{C}_4 \) plants, the first stable product is oxaloacetic acid (OAA), which contains four carbon atoms.
In simple words: \( \text{C}_3 \) plants first make PGA as food, while \( \text{C}_4 \) plants first make OAA.

🎯 Exam Tip: Remember to specify the different initial stable products for \( \text{C}_3 \) and \( \text{C}_4 \) plants (PGA and OAA respectively), as this highlights key differences in their photosynthetic pathways.

 

Question 3. Write the formula of Chl. 'a' and Chl 'b'. What is the main difference between chlorophyll 'a' and chlorophyll 'b'.
Answer: The chemical formula for chlorophyll 'a' is \( \text{C}_{55}\text{H}_{72}\text{O}_5\text{N}_4\text{Mg} \). The chemical formula for chlorophyll 'b' is \( \text{C}_{55}\text{H}_{70}\text{O}_6\text{N}_4\text{Mg} \). The main difference between chlorophyll 'a' and chlorophyll 'b' lies in a specific chemical group attached to the third carbon of the second pyrrole ring. Chlorophyll 'a' has a methyl group (\( \text{-CH}_3 \)) at this position, while chlorophyll 'b' has an aldehyde group (\( \text{-CHO} \)) instead. This small difference causes them to absorb light at slightly different wavelengths.
In simple words: Chlorophyll 'a' and 'b' have similar formulas, but chlorophyll 'a' has a \( \text{CH}_3 \) group, and chlorophyll 'b' has a \( \text{CHO} \) group. This makes them absorb different colors of light.

🎯 Exam Tip: Focus on the structural difference (methyl vs. aldehyde group) and its impact on light absorption, which allows plants to capture a wider range of the light spectrum.

 

Question 4. Write a full form of NADP.
Answer: The full form of NADP is Nicotinamide Adenine Dinucleotide Phosphate. This molecule is important in photosynthesis as an electron carrier, helping to transfer energy.
In simple words: NADP stands for Nicotinamide Adenine Dinucleotide Phosphate. It helps carry energy in plants.

🎯 Exam Tip: Remember that NADP+ acts as an electron acceptor, getting reduced to NADPH during the light reactions of photosynthesis.

 

Question 5. Name the cell organelles which participate in photorespiration.
Answer: Photorespiration involves three specific cell organelles: chloroplasts, peroxisomes, and mitochondria. This complex process occurs in sequence across these different compartments within the cell.
In simple words: Photorespiration takes place in three parts of the cell: chloroplasts, peroxisomes, and mitochondria.

🎯 Exam Tip: Remember the three organelles involved in photorespiration (chloroplasts, peroxisomes, and mitochondria) and their sequential roles in the pathway.

 

Question 7. What are the site of light reaction and the dark reaction of the process of photosynthesis?
Answer: In photosynthesis, the light-dependent reactions occur within the grana, which are stacks of thylakoids inside the chloroplasts. The light-independent (dark) reactions, also known as the Calvin cycle, take place in the stroma, the fluid-filled space surrounding the grana within the chloroplast. These two stages are linked but happen in different locations.
In simple words: Light reactions happen in the grana inside chloroplasts. Dark reactions happen in the stroma, which is the space around the grana.

🎯 Exam Tip: Accurately identifying the location of light and dark reactions within the chloroplast (grana and stroma respectively) is key to understanding photosynthesis.

 

Question 8. What is the law of limiting factor?
Answer: Blackman's Law of Limiting Factors states that when a process, such as photosynthesis, is affected by multiple factors, its rate is determined by the factor that is in the shortest supply or at its minimum level. For instance, if light is too low, increasing \( \text{CO}_2 \) won't speed up photosynthesis.
In simple words: If many things affect how fast something works, the slowest thing controls the whole speed.

🎯 Exam Tip: Clearly state that the rate is limited by the factor that is *least available* or *most deficient* relative to optimal conditions.

 

Question 9. Which protein is found in the greatest amount in the biosphere?
Answer: The protein found in the greatest amount in the entire biosphere is RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). This enzyme plays a crucial role in the Calvin cycle by fixing atmospheric carbon dioxide.
In simple words: The most common protein on Earth is RuBisCO, which helps plants take carbon dioxide from the air.

🎯 Exam Tip: Highlight both the acronym (RuBisCO) and its full name (Ribulose-1,5-bisphosphate carboxylase/oxygenase) as it is a major enzyme in carbon fixation.

 

Question 10. In which part of visible spectrum "red drop" occurs.
Answer: The "red drop" phenomenon, where the efficiency of photosynthesis decreases sharply, happens when plants are exposed to light with wavelengths longer than 680 nm within the visible spectrum. This shows that both Photosystem I and Photosystem II need to work together for maximum efficiency.
In simple words: Red drop happens when plants get only deep red light (over 680 nm). Photosynthesis becomes less effective at these long wavelengths.

🎯 Exam Tip: Link the red drop effect to the separate functions of Photosystem I and II and the requirement for both to be active for optimal photosynthesis.

 

Question 11. In photosynthesis which pigments serve as accessory pigments.
Answer: Carotenoids and phycobilins serve as important accessory pigments. These pigments absorb light energy at different wavelengths than chlorophyll 'a' and then pass that energy on to the reaction centers, broadening the range of light the plant can use.
In simple words: Carotenoids and phycobilins are like helpers that catch extra light for photosynthesis.

🎯 Exam Tip: Remember that accessory pigments expand the spectrum of light usable for photosynthesis and protect chlorophyll from damage.

 

Question 12. Why photorespiration is considered as a harmful or destructive activity?
Answer: Photorespiration is considered a harmful or destructive activity because, during this process, already produced food (carbohydrates) is oxidized using oxygen, but no useful energy in the form of ATP is generated. Instead, it consumes energy and releases carbon dioxide, making it an inefficient and wasteful process for the plant.
In simple words: Photorespiration is bad because it wastes food and energy, instead of making more. It uses oxygen but doesn't make ATP.

🎯 Exam Tip: Emphasize that photorespiration consumes previously fixed carbon and ATP without producing more ATP or NADPH, thus reducing photosynthetic efficiency.

 

Question 2. Write a brief account of the structure of chloroplast.
Answer: Chloroplasts are essential organelles found mainly in the mesophyll cells of plant leaves and other green parts.

• Each chloroplast is shaped like a disc or a lens and is enclosed by a double membrane.
• Its internal space is divided into two main areas.
• The fluid-filled matrix inside is called the stroma, which contains ribosomes, circular DNA, and various enzymes. This liquid helps many reactions happen smoothly.
• Within the stroma, there are flattened sac-like structures called thylakoids. These thylakoids are often stacked up like coins, forming structures known as grana.
• A single chloroplast can have many grana, which are sometimes connected by stroma lamellae.
• The grana are where photosynthetic pigments like chlorophyll and carotenoids are located, making them the site of the light-dependent reactions of photosynthesis.
• The dark reactions (Calvin cycle) of photosynthesis are completed in the stroma region of the chloroplast.

In simple words: Chloroplasts are small, disc-shaped parts in plant cells that have two coverings. Inside, there's a liquid called stroma and stacks of coin-like parts called grana. Grana hold the color pigments and do the light part of food-making, while stroma does the dark part.

🎯 Exam Tip: When describing chloroplast structure, always include the double membrane, stroma, thylakoids, and grana, and mention where light and dark reactions occur.

 

Question 3. What is the contribution of Blackman in plant physiology?
Answer: F.F. Blackman made two significant contributions to plant physiology. In 1905, he proposed that photosynthesis involves two distinct phases: the light-dependent reactions and the light-independent (dark) reactions. He also introduced the "Law of Limiting Factors," which states that when multiple factors influence a process, the rate of that process is determined by the factor that is in the least supply. For instance, if light is too low, increasing \( \text{CO}_2 \) won't speed up photosynthesis.
In simple words: Blackman said photosynthesis happens in two steps: light and dark reactions. He also said that if many things affect a plant process, the thing that is shortest in supply will limit how fast it can go.

🎯 Exam Tip: Mention both Blackman's two-stage concept of photosynthesis and his Law of Limiting Factors, providing a brief explanation for each.

 

Question 5. Explain the difference between \( \text{C}_3 \) and \( \text{C}_4 \) cycle.
Answer: The \( \text{C}_3 \) and \( \text{C}_4 \) cycles are two different pathways plants use for carbon fixation, each with unique characteristics that allow plants to adapt to various environments.

In simple words: \( \text{C}_3 \) and \( \text{C}_4 \) plants have different ways of fixing carbon dioxide. \( \text{C}_4 \) plants are often better in hot, dry places because they avoid photorespiration, which saves energy.

🎯 Exam Tip: When comparing \( \text{C}_3 \) and \( \text{C}_4 \) cycles, focus on key differences like initial carbon fixation product, enzymes involved, Kranz anatomy, and photorespiration.

 

Question 6. \( \text{CO}_2 \) fixation through crassulacean acid metabolism is a physiological adaptation in xerophytic and succulent plants. Explain.
Answer: Crassulacean Acid Metabolism (CAM) is a special adaptation for carbon dioxide fixation seen in succulent plants and those thriving in dry, desert-like (xeric) environments. These plants typically have thick, fleshy leaves. Their stomata, which are pores for gas exchange, open only at night to absorb carbon dioxide and then close during the hot daytime to minimize water loss through transpiration. This strategy helps them avoid excessive water loss during hot daytime conditions when transpiration rates would be very high.
In simple words: CAM is a way dry plants like succulents get carbon dioxide. They open tiny holes in their leaves at night to take in \( \text{CO}_2 \) and close them in the day to save water. This is because they lose less water at night.

🎯 Exam Tip: Explain CAM as an adaptation for water conservation in arid environments, linking it to nocturnal stomatal opening for \( \text{CO}_2 \) uptake.

 

Question 7. What do you understand by photophosphorylation?
Answer: Photophosphorylation is the process by which adenosine triphosphate (ATP) molecules are synthesized from adenosine diphosphate (ADP) and inorganic phosphate (Pi), using light energy. This crucial energy-capturing process occurs in the grana of chloroplasts, where solar energy is converted into the chemical energy stored in ATP. There are two main types: cyclic photophosphorylation and non-cyclic photophosphorylation.
In simple words: Photophosphorylation is how plants make ATP energy using sunlight. It happens in the chloroplasts and has two forms: cyclic and non-cyclic.

🎯 Exam Tip: Define photophosphorylation by its inputs (ADP, Pi, light energy) and output (ATP), and specify its location in the chloroplast (grana).

RBSE Class 12 Biology Chapter 10 Essay Type Questions

 

Question 1. Describe the process of light reaction of photosynthesis.
Answer: The light reaction, also known as the photochemical reaction or Hill's reaction, is the first stage of photosynthesis. It occurs within the grana of chloroplasts and converts sunlight's electromagnetic energy into chemical energy in the form of ATP and NADPH + \( \text{H}^+ \). This process involves several key steps:
1. Excitation of Chlorophyll: Chlorophyll molecules absorb light energy, becoming excited. This causes them to release high-energy electrons, for example, \( \text{Chl} \xrightarrow{\text{Light}} \text{Chl}^* \text{ (Excited state)} \implies \text{Chl}^+ \text{ (oxidised state)} + \text{e}^- \). These energized electrons are then captured by an electron acceptor.
2. Photolysis of Water and Oxygen Release: The energy absorbed by chlorophyll is used to break down water molecules (\( \text{H}_2\text{O} \)). This process, called photolysis, splits water into hydrogen ions (\( \text{H}^+ \)), electrons (\( \text{e}^- \)), and hydroxyl ions (\( \text{OH}^- \)). The oxygen gas (\( \text{O}_2 \)) that plants release comes from this splitting of water, with some also used in the plant's respiration.
\[ \text{2H}_2\text{O} \xrightarrow{\text{Photolysis, Mn}^{+2}} 4\text{H}^+ + 4\text{e}^- + \text{O}_2 \]
3. Formation of NADPH + \( \text{H}^+ \): The hydrogen ions (\( \text{H}^+ \)) liberated from water photolysis reduce NADP+ to form NADPH + \( \text{H}^+ \). This reaction takes place on the surface of the thylakoid membrane, close to the stroma. It requires four hydrogen ions (from two water molecules) and four electrons, which are supplied by Photosystem I (PS-I). This newly formed NADPH + \( \text{H}^+ \) is a strong reducing agent.
\[ \text{2NADP} + 4\text{H}^+ + 4\text{e}^- \implies \text{2NADPH} + 2\text{H}^+ \] These two molecules of NADPH + \( \text{H}^+ \) are crucial for reducing one molecule of carbon dioxide in the subsequent dark reactions.
4. Photophosphorylation (Cyclic and Non-cyclic): This is the process of generating ATP from ADP and inorganic phosphate using light energy.
(a) Cyclic Photophosphorylation: In this process, electrons ejected from Photosystem I (\( \text{P}_{700} \)) pass through a series of electron carriers like ferredoxin (Fd), cytochrome \( \text{b}_6 \), cytochrome f, and plastocyanin. They eventually return to \( \text{P}_{700} \), making it a cycle. During this electron transport, enough energy is released, particularly when electrons move from cytochrome \( \text{b}_6 \) to cytochrome f, to synthesize ATP. This pathway primarily produces ATP.
(b) Non-cyclic Photophosphorylation: Unlike cyclic, electrons ejected from Photosystem II (\( \text{P}_{680} \)) do not return to their original photosystem. Instead, they move through an electron transport chain involving pheophytin, plastoquinone (PQ), cytochrome \( \text{b}_6 \), cytochrome f, and plastocyanin, eventually reaching Photosystem I (\( \text{P}_{700} \)). From \( \text{P}_{700} \), these electrons flow to ferredoxin and are then used to reduce NADP+ into NADPH + \( \text{H}^+ \). Both PS-I and PS-II are involved here, and ATP is also synthesized during this electron flow. This process generates both ATP and NADPH + \( \text{H}^+ \).
Non-cyclic photophosphorylation leads to the production of ATP, the generation of NADPH + \( \text{H}^+ \), and the release of oxygen (\( \text{O}_2 \)). This pathway is also known as the Z-scheme due to its zig-zag electron flow. Here is a comparison of the two types:

Overall, the combined reactions of Photosystem I and Photosystem II result in the release of oxygen through water photolysis, the synthesis of ATP, and the generation of NADPH + \( \text{H}^+ \). Both ATP and NADPH + \( \text{H}^+ \) are then utilized in the dark reactions to reduce carbon dioxide and form carbohydrates. The light reaction transforms light energy into chemical energy, creating ATP and NADPH, which are vital for the plant's food production.
In simple words: The light reaction is the first part of photosynthesis, where sunlight is used to make energy (ATP) and electron carriers (NADPH) in the grana. It involves chlorophyll getting excited by light, water splitting to release oxygen, and the creation of NADPH. This whole process turns light into useful chemical energy.

🎯 Exam Tip: When describing the light reaction, detail the roles of chlorophyll excitation, water photolysis (including the source of oxygen), and the production of ATP and NADPH, clearly linking them to their locations within the chloroplast.

 

Question 2. Explain \( \text{CO}_2 \) fixation by Calvin-Benson cycle.
Answer: The Calvin-Benson cycle, often simply called the Calvin cycle or \( \text{C}_3 \) cycle, describes how plants fix carbon dioxide (\( \text{CO}_2 \)) to produce carbohydrates. This cycle gets its \( \text{C}_3 \) name because the first stable compound formed during \( \text{CO}_2 \) fixation is a three-carbon molecule called phosphoglyceric acid (PGA).
This pathway was extensively studied by Melvin Calvin, Andrew Benson, and their colleagues between 1946 and 1953. They used radioactive carbon-14 labeled carbon dioxide (\( ^{14}\text{CO}_2 \)) in experiments with unicellular green algae like *Chlorella* and *Scenedesmos*. Their work revealed that PGA was the initial stable product of \( \text{CO}_2 \) fixation. For their groundbreaking discoveries, Calvin and Benson were awarded the Nobel Prize in 1961. The formation of carbohydrates from \( \text{CO}_2 \) and \( \text{H}_2\text{O} \) through this cycle involves several detailed steps.
The Calvin-Benson cycle proceeds through three main phases:
1. Carboxylation (or \( \text{CO}_2 \) Fixation): This is the initial step where atmospheric carbon dioxide is incorporated into an organic molecule. The enzyme RuBisCO catalyzes the reaction between carbon dioxide and ribulose-1,5-bisphosphate (RuBP), a five-carbon sugar. This reaction forms an unstable six-carbon intermediate, which immediately splits into two molecules of 3-phosphoglycerate (PGA), a stable three-carbon compound.
\[ \text{6CO}_2 + \text{6 Ribulose-1,5-bisphosphate} + \text{6H}_2\text{O} \xrightarrow{\text{RuBisCO}} \text{12 Phosphoglyceric Acid (PGA)} \] This phase is also known as the carboxylate phase of the dark reaction.
2. Reduction (or Glycolytic Reversal): In this phase, the phosphoglycerate (PGA) molecules are converted into glyceraldehyde-3-phosphate (G3P), which is a sugar. This conversion requires energy from ATP and reducing power from NADPH + \( \text{H}^+ \), both generated during the light-dependent reactions of photosynthesis. Each molecule of PGA is first phosphorylated by ATP to form 1,3-bisphosphoglycerate, which is then reduced by NADPH to glyceraldehyde-3-phosphate (G3P) with the help of triose phosphate dehydrogenase.
\[ \text{12 Diphosphoglyceric acid} + \text{12 NADPH} + \text{12 H}^+ \implies \text{12 Phosphoglyceraldehyde (PGAL)} + \text{12 NADP} + \text{12 H}_3\text{PO}_4 \] For every six molecules of \( \text{CO}_2 \) entering the cycle, 12 molecules of G3P are produced. Two of these G3P molecules are used to synthesize glucose (a six-carbon sugar), which can then be converted into sucrose or starch for storage.
\[ \text{2 G3P} \implies \text{Glucose} \]
3. Regeneration of RuBP: The remaining ten molecules of glyceraldehyde-3-phosphate (G3P) are used to regenerate the starting molecule, ribulose-1,5-bisphosphate (RuBP). This regeneration requires energy from ATP. For six turns of the Calvin cycle, six molecules of RuBP are regenerated from ten molecules of G3P, using six ATP molecules. This process also involves several intermediate sugars.
\[ \text{10 G3P} + \text{6 ATP} \implies \text{6 RuBP} + \text{6 ADP} \] This regeneration ensures that the cycle can continue to fix more carbon dioxide.
The Calvin-Benson cycle efficiently converts inorganic carbon dioxide into organic sugars, forming the foundation of most food chains.
In simple words: The Calvin cycle is how plants use the energy from sunlight to turn carbon dioxide into sugar. It has three main steps: first, carbon dioxide is joined with a sugar; next, this new molecule is turned into actual sugar using energy; finally, the starting sugar is remade so the cycle can keep going.

🎯 Exam Tip: Outline the three main phases of the Calvin cycle (carboxylation, reduction, regeneration) and the key molecules involved (RuBP, PGA, G3P, ATP, NADPH), and emphasize that it happens in the stroma.

 

Question 3. What do you understand by photophosphorylation? Explain the process in detail.
Answer: Photophosphorylation is the process where ATP molecules are formed from ADP and inorganic phosphate (Pi) in the presence of sunlight. This takes place in the grana region of chloroplasts, storing solar energy as chemical energy in ATP. Arnon et al. (1954) discovered this process, which has two types: cyclic and non-cyclic photophosphorylation.

1. Cyclic Photophosphorylation:
In this type, electrons ejected from the P700 reaction center return to P700 after passing through various electron acceptors like A(FeS), Fd, Cyt \(b_6\), Cyt f, and Plastocyanin. This electron transport releases enough energy to synthesize ATP. Only Photosystem I (PS-I) is involved in this process. This method helps plants make energy when only PS-I is activated by light.

2. Non-Cyclic Photophosphorylation:
In this process, electrons ejected from the P680 reaction center do not return to their original position. Instead, they travel through a series of electron acceptors, leading to ATP synthesis. Both Photosystem I (PS-I) and Photosystem II (PS-II) are involved here. Electrons from P680 flow through pheophytin, plastoquinone (PQ), cytochrome \(b_6\), cytochrome f, and plastocyanin, eventually reaching P700. From P700, electrons flow to ferredoxin, reducing NADP to NADPH + \(H^+\). During this flow, sufficient energy is released when electrons move from plastoquinone to cytochrome \(b_6\), which is used to synthesize ATP. This process is also known as the Z-scheme, and it results in the formation of ATP, generation of NADPH + \(H^+\), and the release of oxygen (\(O_2\)).

In simple words: Photophosphorylation is when plants make ATP energy using sunlight, ADP, and phosphate. This process happens in the chloroplasts and makes chemical energy. It was found by Arnon in 1954 and has two main kinds. Cyclic photophosphorylation involves only Photosystem I, where electrons cycle back to P700, making ATP. Non-cyclic photophosphorylation uses both Photosystem I and II. Electrons do not return but move through a chain, making ATP and NADPH, and releasing oxygen. This is like a Z-shaped path for electrons.

🎯 Exam Tip: Remember to clearly define photophosphorylation and mention both its types for a complete answer, highlighting the key differences in electron flow, participants (PS-I/PS-II), and products.

 

Question 4. Describe the Hatch-Slack cycle and write its significance.
Answer: The Hatch-Slack cycle is the way carbon dioxide fixation happens in \(C_4\) plants. These plants are characterized by forming a four-carbon compound, oxaloacetic acid (OAA), as the first stable product of their dark reaction. M.D. Hatch and C.R. Slack (1966) fully described this carbon pathway, hence its name.

The cycle is common in monocot plants like maize, sugarcane, and pearl millet, as well as some dicots such as Amaranthus and Euphorbia. Since its primary stable carbon compound is a four-carbon acid instead of a three-carbon compound, it's also called the \(C_4\) cycle or dicarboxylic acid cycle.

1. Structural Peculiarity of \(C_4\) Plants:
\(C_4\) plants have a unique leaf anatomy known as 'Kranz anatomy' (meaning 'wreath' in German). In these plants, two types of cells participate in photosynthesis: mesophyll cells and bundle sheath cells. The mesophyll cells contain small chloroplasts with well-developed grana, while the bundle sheath cells have larger chloroplasts without grana. The bundle sheath cells are arranged in one or two layers around the vascular bundles, forming a wreath-like structure. Light reactions occur in the mesophyll cells, and dark reactions are completed in the bundle sheath parenchyma.

2. Mechanism of \(C_4\) Cycle:
Atmospheric \(CO_2\) enters the leaf through stomata and is first absorbed by the mesophyll cells. Here, phosphoenolpyruvate (PEP), a three-carbon compound, acts as the primary \(CO_2\) acceptor. PEP reacts with \(CO_2\) in the presence of carboxylase enzyme to form oxaloacetic acid (OAA), a four-carbon compound.

In the bundle sheath parenchyma, malic acid undergoes decarboxylation, releasing \(CO_2\) and forming pyruvic acid. The released \(CO_2\) then enters the \(C_3\) cycle to form hexose molecules (sugars). Pyruvic acid returns to the mesophyll cells and is converted back into PEP, using ATP, to continue the cycle. This allows efficient carbon fixation.

3. Importance (Significance) of the Hatch-Slack Cycle:
\(C_4\) plants can photosynthesize effectively even at low \(CO_2\) concentrations, which is biologically significant. Photorespiration does not occur in \(C_4\) plants, leading to higher productivity compared to \(C_3\) plants. The PEP carboxylase enzyme is active even when \(CO_2\) levels are low. \(C_4\) plants thrive in dry places with high temperatures (30°C - 45°C), making them more successful in tropical regions.

In simple words: The Hatch-Slack cycle is how certain plants, called \(C_4\) plants, capture carbon dioxide. They first make a four-carbon acid called OAA. These plants have a special leaf structure (Kranz anatomy) with two types of cells working together: mesophyll and bundle sheath cells. This cycle helps them grow well even when there's less \(CO_2\) or in hot, dry weather, because they avoid a wasteful process called photorespiration and fix carbon very efficiently.

🎯 Exam Tip: When describing the Hatch-Slack cycle, always mention the \(C_4\) plants, Kranz anatomy, the first stable product (OAA), and the key enzymes involved to score full marks.

 

Question 5. Write notes on the following:
1. Photosynthetic pigments
2. Photosystem I and Photosystem-II
3. Photolysis of water
4. Importance of the \(C_4\) cycle.
5. Photo-respiration and Photosynthesis
Answer:

1. Photosynthetic Pigments:
These are special colored molecules that absorb light energy for photosynthesis. Chlorophyll 'a' is the main pigment and is universal in all green plants; it acts as the reaction center. Other accessory pigments include chlorophyll 'b', carotenoids (like carotenes and xanthophylls), and phycobilins. These accessory pigments broaden the range of light absorption and transfer energy to chlorophyll 'a'.

2. Photosystem I (PS-I) and Photosystem II (PS-II):
These are protein complexes in the thylakoid membranes that capture light energy.
Photosystem I (PS-I): Its reaction center is P700, meaning it absorbs light best at 700 nm. PS-I participates in both cyclic and non-cyclic photophosphorylation. It is located in the thylakoid membrane, found in both grana and stroma regions of chloroplasts.

Photosystem II (PS-II): Its reaction center is P680, absorbing light best at 680 nm. PS-II participates only in non-cyclic photophosphorylation and is found exclusively in the grana region of the thylakoid membrane. PS-II is primarily responsible for the photolysis of water.

3. Photolysis of Water:
This is the process where water (\(H_2O\)) molecules are split into their components-protons (\(H^+\)), electrons (\(e^-\)), and oxygen (\(O_2\))-in the presence of sunlight and chlorophyll. This reaction occurs on the inner side of the thylakoid membrane. It provides electrons to replace those lost by PS-II and releases oxygen as a byproduct.

\(2H_2O \xrightarrow{\text{Photolysis}} 4H^+ + 4e^- + O_2\)
This light-dependent splitting of water is vital for sustained electron flow in photosynthesis and for releasing the oxygen we breathe.

4. Importance of the \(C_4\) Cycle:
The \(C_4\) cycle is important because \(C_4\) plants can efficiently photosynthesize even at low concentrations of \(CO_2\). This is due to the phosphoenolpyruvate carboxylase enzyme, which is very active in low \(CO_2\) conditions. The \(C_4\) cycle also enables plants to thrive in dry, hot environments with high atmospheric temperatures, making them highly productive and adaptable.

5. Photorespiration and Photosynthesis:
Photorespiration: This is a process where plants use oxygen instead of carbon dioxide, leading to a loss of fixed carbon and no production of ATP or NADPH. It is considered a wasteful process that occurs in photosynthetic parts of the plant in the presence of sunlight.

Photosynthesis: This is the main process where plants synthesize carbohydrates (sugars) using \(CO_2\) and \(H_2O\) as raw materials, with sunlight as the energy source. This vital process takes place in the chlorophyll-containing parts of plants and is fundamental for life on Earth.

In simple words: Photosynthetic pigments like chlorophyll catch sunlight. Photosystems I and II are tiny factories that use this light to make energy. Photolysis of water is when water splits into hydrogen, electrons, and oxygen using light. The \(C_4\) cycle helps certain plants grow well in hot, dry places by being very good at using carbon dioxide. Photorespiration is a less efficient process that wastes energy, while photosynthesis is the main process plants use to make food from sunlight and carbon dioxide.

🎯 Exam Tip: For "write notes" questions, ensure each point is clearly defined and its role or significance is briefly explained. Use distinct sub-headings for clarity.

 

Question 6. Discuss in detail the factors affecting the process of photosynthesis.
Answer: The rate of photosynthesis is influenced by several factors, which can be categorized into two main groups: external (atmospheric) factors and internal (plant-related) factors.

(1) External Factors or Atmospheric Factors:
1. Light: Photosynthesis uses the visible spectrum of sunlight (400-700 nm). The rate is highest when blue and red light are supplied together. Chlorophyll absorbs more energy from these specific light colors.
    (a) Light Intensity: The amount of light absorbed depends on leaf structure (phyllotaxy). Typically, only a small percentage of light is absorbed by chlorophyll. Increasing light intensity generally boosts photosynthesis until another factor, like temperature or \(CO_2\), becomes limiting. Very high light intensity can damage chloroplasts and other organelles through photooxidation, a process called solarization. The "compensation point" is when \(CO_2\) used in photosynthesis equals \(CO_2\) produced in respiration.

    (b) Duration of Light: Plants can perform photosynthesis for long periods under continuous light. Normally, 10-12 hours of light are sufficient for adequate photosynthesis.

2. Temperature: Photosynthesis occurs across a wide temperature range. Some plants, like Juniperus, can photosynthesize at -35°C, while xerophytes can do so at 55°C. The optimal temperature for photosynthesis varies among plant species, often around 25-35°C for \(C_3\) plants and higher for \(C_4\) plants.

3. Carbon Dioxide (\(CO_2\)): Atmospheric \(CO_2\) is about 0.03% (300 ppm). As \(CO_2\) increases, photosynthesis initially rises until other factors become limiting. An increase up to 1.0% boosts the rate, but beyond this, it can become toxic, causing stomata to close and stopping gas exchange. \(CO_2\) acts as a primary raw material for sugar synthesis.

4. Water: Water is a crucial reactant, acting as a hydrogen donor. Only about 1.0% of absorbed water is used in photosynthesis; therefore, it usually isn't a direct limiting factor. However, severe water shortage indirectly affects photosynthesis by causing stomata to close, which reduces \(CO_2\) uptake and lowers leaf water potential. This causes stress on the plant, ultimately slowing photosynthesis.

5. Oxygen (\(O_2\)): Higher \(O_2\) concentrations can inhibit photosynthesis. Oxygen competes with \(CO_2\) at the active site of the RuBISCO enzyme. In \(C_3\) plants, increased \(O_2\) makes RuBISCO act as an oxygenase, leading to photorespiration and reducing \(CO_2\) fixation.

(2) Internal Factors:
1. Chlorophyll: Chlorophyll is the primary pigment converting light energy into chemical energy. The rate of photosynthesis generally increases with more chlorophyll, provided other factors are optimal. Plants with sufficient chlorophyll can absorb maximum sunlight for efficient food production.

2. Amount of Stored Food: The accumulation of end products of photosynthesis (like sugars or starch) in plant cells can slow down the process. If these products are not quickly transported away, they can inhibit further photosynthesis. This feedback mechanism regulates the plant's energy production. Also, the number of stomata and their opening duration on leaves affect \(CO_2\) absorption, influencing the rate of photosynthesis.

In simple words: Many things change how fast plants make food (photosynthesis). Outside factors include light (how bright, what color, and how long), temperature (too hot or too cold slows it down), carbon dioxide (plants need it, but too much can be bad), water (needed for the process, and too little closes tiny leaf holes), and oxygen (too much can make plants wasteful). Inside factors include how much chlorophyll a plant has (more color means more food) and if too much food is already stored in the leaves, or how many tiny holes are on the leaves.

🎯 Exam Tip: When discussing factors, clearly separate external and internal categories. For each factor, explain how it affects the rate and mention any specific conditions (e.g., optimal range, limiting effects).

There is no educational content (questions, answers, or relevant headings) found within page 29 of the provided PDF. Therefore, no HTML output can be generated for the specified page range.