Maharashtra Board Class 11 Biology Chapter 12 Photosynthesis Solutions

Photosynthesis Class 11 Exercise Question Answers Solutions Maharashtra Board

Class 11 Biology Chapter 12 Exercise Solutions Maharashtra Board

Biology Class 11 Chapter 12 Exercise Solutions

1. Choose Correct Option

Question (A) A cell that lacks chloroplast does not
(a) evolve carbon dioxide
(b) liberate oxygen
(c) require water
(d) utilize carbohydrates
Answer: (b) liberate oxygen
In simple words: Cells without chloroplasts cannot perform photosynthesis, which is the process that produces oxygen. Therefore, they cannot release oxygen.

🎯 Exam Tip: Understanding the function of chloroplasts is key to identifying what a cell lacking them cannot do, particularly in the context of photosynthesis products.

Question (B) Energy is transferred from the light reaction step to the dark reaction step by
(a) chlorophyll
(b) ADP
(c) ATP
(d) RuBP
Answer: (c) ATP
In simple words: ATP (adenosine triphosphate) is the primary energy currency of the cell, produced during the light reactions and then used to power the dark reactions (Calvin cycle) of photosynthesis.

🎯 Exam Tip: Remember ATP and NADPH are the direct products of light reactions that carry energy to the dark reactions for carbon fixation.

Question (C) Which one is wrong in photorespiration?
(a) It occurs in chloroplasts
(b) It occurs in day time only
(c) It is characteristic of C4-plants
(d) It is characteristic of C3-plants
Answer: (c) It is characteristic of C4-plants
In simple words: Photorespiration is a process that typically occurs in C3 plants under specific conditions, not C4 plants, which have adaptations to minimize it.

🎯 Exam Tip: C4 plants have evolved a mechanism (Kranz anatomy, PEP carboxylase) to concentrate CO2, thereby suppressing photorespiration, which is a key distinguishing feature from C3 plants.

Question (D) Non-cyclic phosphorylation differs from cyclic photophosphorylation in that former
(a) involves only PS
(b) Include evolution of 02
(c) involves formation of assimilatory power
(d) both (b) and (c)
Answer: (d) both (b) and (c)
In simple words: Non-cyclic photophosphorylation involves both PS-I and PS-II, leading to water photolysis (oxygen evolution) and the production of ATP and NADPH (assimilatory power).

🎯 Exam Tip: Distinguishing between cyclic and non-cyclic photophosphorylation is crucial. Focus on whether water is split (O2 released) and if NADPH is produced in addition to ATP.

Question (E) For fixation of 6 molecules of CO2 and formation of one molecule of glucose in Calvin cycle, requires
(a) 3 ATP and 2 MADPPE
(b) 18 ATP and 12 NADPH2
(c) 30 ATP and 18 NADPH2
(d) 6 ATP and 6 NADPIT2
Answer: (b) 18 ATP and 12 NADPH2
In simple words: The Calvin cycle requires 18 ATP and 12 NADPH2 molecules to convert six molecules of carbon dioxide into one molecule of glucose.

🎯 Exam Tip: Memorize the energy requirements for glucose synthesis in the Calvin cycle (18 ATP and 12 NADPH2) as this is a frequently tested value.

Question (F) In maize and wheat, the first stable products formed in bundle sheath cells respectively are
(a) OAA and PEPA
(b) OAA and OAA
(c) OAA and 3PGA
(d) 3PGA and OAA
Answer: (c) OAA and 3PGA
In simple words: In C4 plants like maize, OAA is the first stable product in bundle sheath cells, while in C3 plants like wheat, 3PGA is the first stable product.

🎯 Exam Tip: Identify maize as a C4 plant and wheat as a C3 plant. The first stable product of carbon fixation differs between these two photosynthetic pathways.

Question (G) The head and tail of chlorophyll are made up of
(a) porphyrin and phytin respectively
(b) pyrrole and tetrapyrrole respectively
(c) porphyrin and phytol respectively
(d) tetrapyrole and pyrrole respectively
Answer: (c) porphyrin and phytol respectively
In simple words: Chlorophyll molecules consist of a porphyrin ring (head) that contains magnesium and absorbs light, and a long phytol chain (tail) that anchors it to the thylakoid membrane.

🎯 Exam Tip: Knowing the chemical structure of chlorophyll, specifically the porphyrin head and phytol tail, helps understand its function and location within the chloroplast.

Question (H) The net result of photo-oxidation of water is release of ................
(a) electron and proton
(b) proton and oxygen
(c) proton, electron and oxygen
(d) electron and oxygen
Answer: (c) proton, electron and oxygen
In simple words: Photolysis of water, also known as photo-oxidation, splits water molecules into protons (H+), electrons (e-), and oxygen gas (O2), which are crucial for light reactions.

🎯 Exam Tip: The splitting of water (photolysis) is a fundamental process in non-cyclic photophosphorylation, providing electrons to PSII, protons for ATP synthesis, and releasing oxygen as a byproduct.

Question (I) For fixing one molecule of CO2 in Calvin cycle are required.
(a) 3ATP + 1NADPFE
(b) 3ATP + 2NADPH2
(c) 2ATP + 3NADPH2
(d) 3ATP + 3NADPFE
Answer: (b) 3ATP + 2NADPH2
In simple words: To fix one molecule of CO2 in the Calvin cycle, the plant uses 3 ATP molecules and 2 NADPH2 molecules as energy and reducing power.

🎯 Exam Tip: Remember the ATP and NADPH2 ratio for *one* CO2 fixation (3 ATP, 2 NADPH2) versus *six* CO2 fixations for one glucose molecule (18 ATP, 12 NADPH2).

Question (J) In presence of high concentration of oxygen, RuBP carboxylase converts RuBP to .........................
(a) Malic acid and PEP
(b) PGA and PEP
(c) PGA and malic acid
(d) PGA and phosphoglycolate
Answer: (d) PGA and phosphoglycolate
In simple words: When oxygen levels are high, RuBisCO acts as an oxygenase, converting RuBP into 3-PGA and 2-phosphoglycolate, initiating photorespiration.

🎯 Exam Tip: The dual nature of RuBisCO (carboxylation vs. oxygenation) is a critical concept. High O2 leads to oxygenation, producing phosphoglycolate, which is a hallmark of photorespiration.

Question (K) The sequential order in electron transport from PSII to PSI of photosynthesis is
(a) FeS, PQ, PC and Cytochrome
(b) FeS, PQ, Cytochrome and PC
(c) PQ, Cytochrome, PC and FeS
(d) PC, Cytochrome, FeS, PQ
Answer: (c) PQ, Cytochrome, PC and FeS
In simple words: Electrons flow from Photosystem II to Photosystem I through an electron transport chain involving plastoquinone (PQ), cytochrome complex, and plastocyanin (PC), and then to ferredoxin (FeS) via PSI.

🎯 Exam Tip: Tracing the path of electrons through the Z-scheme of non-cyclic photophosphorylation, remembering the sequence of electron carriers (PQ, Cytochrome, PC, FeS) is vital.

2. Answer The Following Questions

Question (A) Describe the light-dependent steps of photosynthesis. How are they linked to dark reactions?
Answer:
The light dependent steps of photosynthesis include cyclic and non-cyclic photophosphorylation,
1. Cyclic photophosphorylation:
a. Illumination of photosystem-l causes electrons to move continuously out of the reaction center of photosystem-l and back to it.
b. The cyclic electron-flow is accompanied by the photophosphorylation of ADP to yield ATP. This is termed as Cyclic photophosphorylation.
c. Since this process involves only pigment system I, photolysis of water and consequent evolution of oxygen does not take place.

ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र चक्रीय फोटोफॉस्फोराइलेशन को दर्शाता है। इसमें प्रकाश ऊर्जा के प्रभाव में PS I से इलेक्ट्रॉन निकलते हैं, जो विभिन्न इलेक्ट्रॉन वाहकों (FRS, Ferredoxin, Cytochrome b6, Cytochrome f, Plastocyanin) से होकर वापस PS I में लौट आते हैं। इस प्रक्रिया के दौरान ADP और Pi मिलकर ATP का निर्माण करते हैं, लेकिन पानी का विभाजन नहीं होता और ऑक्सीजन नहीं निकलती।
2. Non-cyclic photophosphorylation:
a. It involves both photosystems- PS-I and PS-II.
b. In this case, electron transport chain starts with the release of electrons from PS-II.
c. In this chain high energy electrons released from PS-II do not return to PS-II but, after passing through an electron transport chain, reach PS-I, which in turn donates it to reduce NADP to NADPH.
d. The reduced NADP+ (NADPH) is utilized for the reduction of CO2 in the dark reaction.
e. Electron-deficient PS-II brings about oxidation of water-molecule. Due to this, protons, electrons and oxygen atom are released.
f. Electrons are taken up by PS-II itself to return to reduced state, protons are accepted by NADP+ whereas oxygen is released.
g. As in this process, high energy electrons released from PS-II do not return to PS-II and it is accompanied with ATP formation, this is called Non-cyclic photophosphorylation.

ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र गैर-चक्रीय फोटोफॉस्फोराइलेशन को दर्शाता है, जिसे Z-स्कीम भी कहते हैं। इसमें PS II से निकलने वाले इलेक्ट्रॉन विभिन्न वाहकों (PQ, Cyt b6, Cyt f, PC) से होते हुए PS I तक पहुँचते हैं, जहाँ से वे NADP+ को NADPH2 में अपचयित करते हैं। इस प्रक्रिया में पानी का विभाजन (photolysis) होता है, जिससे ऑक्सीजन और प्रोटॉन निकलते हैं, और ATP का भी निर्माण होता है।
3. Link between light-dependent and dark reactions:
1. The light reaction gives rise to two important products, a reducing agent NADPH2 and an energy rich compound ATP. Both these are utilized in the dark phase of photosynthesis.
2. ATP and NADPH2 molecules function as vehicles for transfer of energy of sunlight into dark reaction leaving to carbon fixation. In this reaction CO2 is reduced to carbohydrate.
3. During dark reaction, ATP and NADPH2 are transformed into ADP, iP and NADP which are transferred to the grana in which light reaction takes place.
In simple words: Light-dependent reactions capture sunlight energy to produce ATP and NADPH2. These energy-rich molecules then fuel the dark reactions, where CO2 is converted into sugars, establishing a direct link between the two phases of photosynthesis.

🎯 Exam Tip: Clearly differentiate between cyclic and non-cyclic photophosphorylation regarding ATP/NADPH production and O2 evolution. Emphasize that ATP and NADPH2 are the vital links transferring energy from light to dark reactions.

(B)

Question (a) Distinguish between Respiration and Photorespiration
Answer:

🎯 Exam Tip: When distinguishing, focus on the energy outcome (production vs. wastage), light dependency, and the types of organisms/conditions under which each process occurs.

Question (b) Distinguish between Cyclic photophosphorylation and Non-cyclic photophosphorylation
Answer:

🎯 Exam Tip: Pay close attention to which photosystems are involved, the final electron acceptor, the products formed (ATP, NADPH2), and whether photolysis of water and oxygen evolution occur.

Question (C) Answer the following questions.
1. What are the steps that are common to C3 and C4 photosynthesis?
2. Differentiate between C3 and C4 plants.
Answer:
Steps that are common to C3 and C4 photosynthesis are Carboxylation, Reduction, Glucose synthesis, Regeneration.
[Note: Students are expected to refer the given Q.R code for detail understanding the common steps between C₁ and C4 plants.]
In simple words: Both C3 and C4 photosynthesis pathways share fundamental steps like initial carbon fixation, reduction of carbon compounds, glucose formation, and regeneration of the CO2 acceptor.

🎯 Exam Tip: While C3 and C4 pathways have distinct initial carbon fixation mechanisms, the subsequent stages of the Calvin cycle (reduction, sugar synthesis, regeneration) are common to both.

Question (D) Are the enzymes that catalyze the dark reactions of carbon fixation located inside the thylakoids or outside the thylakoids?
Answer:
Carbon fixation occurs in the stroma by series of enzyme catalyzed steps. The enzymes that catalyze the dark reactions of carbon fixation are located outside the thylakoids.
In simple words: The enzymes for the dark reactions (carbon fixation) are found in the stroma, which is the fluid-filled space outside the thylakoid membranes within the chloroplast.

🎯 Exam Tip: Remember that light reactions occur on the thylakoid membranes, while dark reactions (Calvin cycle) occur in the stroma, highlighting the compartmentalization of photosynthesis.

Question (E) Calvin cycle consists of three phases, what are they? Explain the significance of each of them.
Answer:
The entire process of dark reaction was traced by Dr. Melvin Calvin along with his co-worker, Dr. Benson. Hence, the process is called as Calvin cycle or Calvin-Benson cycle. Since the first stable product formed is a 3-carbon compound, it is also called as C3 pathway and the plants are called C14 plants.
Calvin carried out experiments on unicellular green algae (Chlorella), using radioactive isotope of carbon, C14 as a tracer.
It is also called synthesis phase or second phase of photosynthesis.
The cycle is divided into the following phases:
1. Carboxylation phase:
a. Carbon dioxide reduction starts with a five-carbon sugar ribulose-1,5-bisphosphate (RuBP). It is a 5- carbon sugar with two phosphate groups attached to it.
b. RuBP reacts with CO2 to produce an unstable 6 carbon intermediate in the presence of Rubisco.
c. It immediately splits into 3 carbon compounds called 3-phosphoglyceric acid.
d. RuBisCO is a large protein molecule and comprises 16% of the chloroplast proteins.
2. Glycolytic reversal:
a. 3-phosphoglyceric acid form 1,3-diphosphoglyceric acid by utilizing ATP molecule.
b. These are then reduced to glyceraldehyde-3-phosphate (3-PGA) by NADPH supplied by the light reactions of photosynthesis.
c. In order to keep Calvin cycle continuously running there must be sufficient number of RuBP and regular supply of ATP and NADPH.
d. Out of 12 molecules of 3-phosphoglyceraldehyde, two molecules are used for synthesis of one glucose molecule.
3. Regeneration of RuBP:
a. 10 molecules of 3-phosphoglyceraldehyde are used for the regeneration of 6 molecules of RuBP at the cost of 6 ATP.
b. Therefore, six turns of Calvin cycle are needed to get one molecule of glucose.
Significance:
1. Carboxylation: RuBisCO is the most abundant enzyme in the world. It is responsible for fixing carbon in the form of CO2 into sugar. As a result of Carboxylation, the first stable product of carbon fixation i.e. 3- PGA is synthesized.
2. Reduction/Glycolytic reversal: NADPH2 donates electrons to 1, 3-Bisphoshoglycerate to form 3- phosphoglyceraldehyde molecules. During this process ADP and NADP are generated which are used in light reaction.
3. Regeneration of RuBP: Some 3-phosphoglyceraldehyde molecules are involved in production of glucose while others are recycled to regenerate the 5-carbon compound RuBP which used to accept new carbon molecules. Thus, regeneration of RuBP is required for Calvin cycle to run continuously.
In simple words: The Calvin cycle involves carboxylation (CO2 fixation by RuBP), reduction (using ATP and NADPH2 to form sugars), and regeneration (rebuilding RuBP for continuous CO2 uptake), ensuring the continuous production of carbohydrates and recycling of key molecules.

🎯 Exam Tip: Understand the purpose and key reactions of each phase: Carboxylation for CO2 uptake, Reduction for sugar synthesis, and Regeneration for maintaining the cycle's continuity.

Question (F) Why are plants that consume more than the usual 18 ATP to produce 1 molecule of glucose favoured in tropical regions?
Answer:
1. C4 plants are favoured in tropical regions as they require 30 ATP to produce 1 molecule of glucose.
2. High temperature in tropical regions leads to closure of stomata to reduce rate of transpiration. Due to this availability of CO2 decreases.
3. PEP carboxylase present in mesophyll cells can fix CO2 even at low concentration. This helps the plant in efficient assimilation of atmospheric carbon dioxide.
4. C4 plants contain a special leaf anatomy called Kranz anatomy which minimizes the losses due to photorespiration.
5. It helps C4 plants to survive in conditions of high daytime temperatures, intense sunlight and low moisture.
In simple words: C4 plants are favored in tropical regions because their specialized Kranz anatomy and PEP carboxylase enzyme allow them to efficiently fix CO2 and minimize wasteful photorespiration, even under high temperatures, intense light, and low CO2 availability.

🎯 Exam Tip: Highlight the adaptations of C4 plants (Kranz anatomy, PEP carboxylase) that enable them to thrive in hot, dry, and high-light environments by circumventing photorespiration, despite their higher ATP cost.

Question (G) What is the advantage of having more than one pigment molecule in a photocenter?
Answer:
Advantages of having more pigment molecules in a photocenter are as follows:
1. Having more than one pigment molecule in photocenter means more sunlight being captured and thus facilitating more effective light reaction.
2. It will provide protection to chlorophyll molecule against photo-oxidation.
3. More pigments will capture more energy to start the initial reactions, which is not possible by single pigment.
In simple words: Multiple pigment molecules in a photocenter enhance light absorption, protect chlorophyll from damage by excessive light, and efficiently funnel energy to the reaction center, thus increasing photosynthetic efficiency.

🎯 Exam Tip: Focus on how accessory pigments broaden the light absorption spectrum, transfer energy to chlorophyll a, and act as photoprotectants, increasing the overall efficiency and resilience of photosynthesis.

Question (H) Why does chlorophyll appear green in reflected light and red transmitted light? Explain the significance of these phenomena in terms of photosynthesis.
Answer:
1. Chlorophyll is a light absorbing pigment. It absorbs light in red and blue regions of the visible light spectrum. Absorption spectrum of chlorophyll for red light is maximum so chlorophyll appears red in transmitted light. Green light is not absorbed but reflected so chlorophyll appear green in reflected light.
2. Chlorophyll predominantly absorbs red and violet-blue light and it allows plants to use this light as a form of energy for photosynthesis process.
3. It is most effective wavelength of light in photosynthesis as it has exactly right amount of energy to excite electrons of chlorophyll and boost them out of their orbits to higher energy level.
In simple words: Chlorophyll appears green because it reflects green light, but in transmitted light, it can appear red because it absorbs red light most strongly for photosynthesis. This strong absorption of red and blue light is vital for photosynthesis as it provides the necessary energy to excite electrons and drive light-dependent reactions.

🎯 Exam Tip: Explain both the reflection/absorption phenomenon for chlorophyll's color and its direct link to the efficiency of light energy capture for photosynthesis, particularly in the red and blue regions.

Question (I) Explain why photosynthesis is considered the most important process in the biosphere.
Answer:
Photosynthesis is considered to be the most important process in the biosphere due to following reasons:
1. Photosynthesis is the biochemical process through which all plants (primary producers) produce food.
2. It is responsible for release of oxygen in the atmosphere.
3. Heterotrophs are directly or indirectly dependent on autotrophs for energy and other related resources. Therefore, photosynthesis is considered the most important process in the biosphere.
In simple words: Photosynthesis is crucial because it produces almost all the food and oxygen essential for life on Earth, directly supporting autotrophs and indirectly sustaining all heterotrophs.

🎯 Exam Tip: Emphasize the twin roles of photosynthesis: producing organic food molecules (the base of all food chains) and releasing oxygen (essential for aerobic respiration), making it indispensable for life.

Question (J) Why is photolysis of water accompanied with non-cyclic photophosphorylation?
Answer:
1. Photolysis of water provides new electrons to Photosystem – II.
2. The water molecule is lysed into three components:
a. Protons (H+) which are used as part of reactions that makes NADPH.
b. Second component formed is electrons which replaces the electrons lost by PS-II.
c. The third component is oxygen (02) which is released into the atmosphere.
3. Photosystem I sends electrons to reduce NADP+.
4. Then, Photosystem II sends replacement electrons to Photosystem I.
5. Finally, photolysis of water replaces the electrons lost by Photosystem II.
6. Water is the ultimate source of electrons for photosynthesis.
In simple words: Photolysis of water is integral to non-cyclic photophosphorylation because it replenishes the electrons lost by Photosystem II, provides protons for ATP and NADPH2 synthesis, and releases oxygen as a byproduct.

🎯 Exam Tip: Stress that water photolysis in non-cyclic photophosphorylation is essential for maintaining the electron flow, preventing PSII from becoming electron-deficient, and generating key products like oxygen and protons.

Question (K) In C-4 plants, why is C-3 pathway operated in bundle sheath cells only?
Answer:
1. Decarboxylation of malic acid occurs in bundle sheath cells of C4 plants. Due to which concentration of CO2 increases in bundle sheath cells.
2. The enzymes required for Calvin cycle i.e. RuBisCO is present in bundle sheath cells.
3. In presence of high concentration of CO2, RuBisCO acts as carboxylase and bring about carboxylation of RuBP.
4. Hence, in C-4 plants, C-3 pathway is operated in bundle sheath cells only.
In simple words: In C4 plants, the C3 pathway (Calvin cycle) takes place exclusively in bundle sheath cells because malic acid is decarboxylated there, creating a high CO2 concentration that allows RuBisCO to function efficiently as a carboxylase.

🎯 Exam Tip: The spatial separation of initial CO2 fixation (mesophyll) and the Calvin cycle (bundle sheath) in C4 plants, driven by CO2 concentration, is a key feature to explain.

Question (L) What would have happened if C-4 plants did not have Kranz anatomy?
Answer:
Photorespiration would occur if C4 plants did not have Kranz anatomy.
In simple words: Without Kranz anatomy, C4 plants would lose their ability to concentrate CO2 around RuBisCO, leading to increased photorespiration and reduced photosynthetic efficiency, similar to C3 plants.

🎯 Exam Tip: Emphasize that Kranz anatomy is a crucial adaptation in C4 plants that creates a high CO2 environment in bundle sheath cells, thus preventing photorespiration and enabling efficient photosynthesis.

Question (M) Why does RuBisCO carry out preferentially carboxylation than oxygenation in C4 plants?
Answer:
1. In C4 plants, CO2 taken from the atmosphere is accepted by a 3-carbon compound, phosphoenolpyruvic acid in the chloroplasts of mesophyll cells.
2. This leads to the formation of 4-carbon compound oxaloacetic acid with the help of enzyme phosphoenolpyruvate carboxylase.
3. It is converted to another 4-carbon compound called malate.
4. Malate is transported to chloroplasts of bundle sheath cells where malate is converted to pyruvate and releases C02 in the cytoplasm thus increasing the concentration of CO2 in the bundle sheath cells.
5. Chloroplasts of bundle sheath cells contains enzymes of Calvin cycle.
6. Thus, due to high concentration of CO2, RuBisCO participates in carboxylation and not in oxygenation.
In simple words: RuBisCO in C4 plants primarily performs carboxylation because a mechanism called Kranz anatomy actively concentrates CO2 in the bundle sheath cells, effectively outcompeting oxygen for RuBisCO's active site.

🎯 Exam Tip: Explain the CO2-concentrating mechanism (the C4 pathway, including PEP carboxylase and malate transport) as the reason for RuBisCO's preferential carboxylation activity in C4 plants.

Question (N) What would have happened if plants did not have accessory pigments?
Answer:
1. Accessory pigments are light absorbing molecules which are found in photosynthetic organisms.
2. They transfer the absorbed light to chlorophyll-a and thus increasing the photosynthetic rate.
3. In absence of accessory pigments less amount of light will be absorbed and also there would be no protection provided to chlorophyll molecule from photo-oxidation.
In simple words: Without accessory pigments, plants would absorb less light, have a reduced photosynthetic rate, and lack protection for chlorophyll from photo-oxidative damage, severely hindering their survival and growth.

🎯 Exam Tip: Focus on the dual role of accessory pigments: broadening the light absorption spectrum for photosynthesis and protecting chlorophyll from harmful excess light energy.

Question (O) How can you identify whether the plant is C3 or C4? Explain / Justify.
Answer:
1. By observing the cross section of a leaf we can identify whether the plant is a C3 plant or a C3 plant.
2. C4 plants possess a special anatomy of leaves called Kranz anatomy. In Kranz anatomy two types of chloroplasts are present, agranal in bundle sheath cells and granal in mesophyll cells.
3. In C3 plants Kranz anatomy is absent.
In simple words: C3 and C4 plants can be identified by examining their leaf cross-section for Kranz anatomy: C4 plants exhibit this distinctive ring-like arrangement of bundle sheath cells with specialized chloroplasts, whereas C3 plants do not.

🎯 Exam Tip: The presence or absence of Kranz anatomy (distinctive bundle sheath cells with agranal chloroplasts and mesophyll cells with granal chloroplasts) is the primary anatomical feature for differentiating C3 and C4 plants.

Question (P) In C4 plants, bundle sheath cells carrying out Calvin cycle are very few in number. Then also, C4 plants are highly productive. Explain.
Answer:
1. C4 plants have special type of leaf anatomy called Kranz anatomy.
2. In C4 plants, CO2 fixation occurs twice.
3. In these plants, chloroplasts of mesophyll cells contain enzyme PEP carboxylase which fixes atmospheric CO2.
4. Thus, first CO2 fixation occurs in mesophyll cells.
5. Decarboxylation of malic acid in bundle sheath cells results in increase in CO2 concentration.
6. Thus, RuBisCO acts as carboxylase and brings about carboxylation of RuBP.
7. Due to this oxygenation of RuBP and photorespiration is prevented.
8. Thus, despite of having less number of bundle sheath cells carrying out Calvin cycle, C4 plants are highly productive.
In simple words: C4 plants are highly productive despite fewer bundle sheath cells because their Kranz anatomy and C4 pathway concentrate CO2 in these cells, making RuBisCO highly efficient and preventing photorespiration, especially in hot, dry environments.

🎯 Exam Tip: Explain that the productivity of C4 plants stems from their ability to suppress photorespiration through the CO2-concentrating mechanism facilitated by Kranz anatomy, leading to efficient carbon fixation even with limited bundle sheath cells.

Question (Q) What is functional significance of Kranz anatomy?
Answer:
1. Leaves of C4 plants show some structural peculiarities called Kranz anatomy.
2. The chloroplast of mesophyll cells contain enzyme PEP Carboxylase, which can fix CO2 at low concentration.
3. Thus, light reaction and evolution of 02 occurs in mesophyll cells.
4. Decarboxylation of malate occurs in bundle sheath cells, which results in release of CO2, due to which concentration of CO2 in bundle sheath cells increases.
5. Enzyme RuBisCO present in bundle sheath cells acts as carboxylase in presence of high CO2 concentraion and catalyses carboxylation of RuBP.
6. Thus, possibility of oxygenation of RuBP is avoided and photorespiration does not take place.
In simple words: Kranz anatomy functionally separates initial CO2 fixation from the Calvin cycle, creating a high CO2 environment in bundle sheath cells for RuBisCO. This minimizes photorespiration and maximizes photosynthetic efficiency, particularly in hot and arid conditions.

🎯 Exam Tip: Focus on how Kranz anatomy's structural design allows for a CO2-concentrating mechanism, ensuring RuBisCO functions as a carboxylase and prevents photorespiration, thereby enhancing photosynthetic output.

3. Correct The Pathway And Name It.

Question 1. Correct the pathway and name it.
Answer:

ℹ️ चित्र व्याख्या (Diagram Explanation): यह आरेख C4 पाथवे को दर्शाता है, जिसमें कार्बन डाइऑक्साइड पहले मेसोफिल कोशिका में PEP द्वारा OAA में स्थिर होती है। फिर OAA मैलिक एसिड में बदल जाता है और बंडल शीथ कोशिकाओं में चला जाता है, जहाँ यह CO2 छोड़ता है। यह CO2 तब C3 पाथवे में प्रवेश करती है, जिससे बंडल शीथ कोशिकाओं में CO2 की उच्च सांद्रता सुनिश्चित होती है।
1. The pathway shown is C4 pathway.
2. M. D. Hatch and C. R. Slack while working on sugarcane found four carbon compounds (dicarboxylic acid) as the first stable product of photosynthesis.
3. It occurs in tropical and sub-tropical grasses and some dicotyledons.
4. The first product of this cycle is a 4-carbon compound oxaloacetic acid.
Hence it is also called as C4 pathway and plants are called C4 plants.
In simple words: The diagram illustrates the C4 pathway, where CO2 is initially fixed in mesophyll cells, transported to bundle sheath cells as a 4-carbon compound, and then released for the Calvin cycle, ensuring efficient carbon fixation under high temperatures.

🎯 Exam Tip: Identify the key stages of the C4 pathway—CO2 uptake in mesophyll, conversion to a 4-carbon acid, transport to bundle sheath, and CO2 release for the C3 cycle—and recognize the role of Kranz anatomy in this process.

4. Is there something wrong in following schematic presentation? If yes, correct it so that photosynthesis will be operated.

Question 1. Is there something wrong in following schematic presentation? If yes, correct it so that photosynthesis will be operated.
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह एक गैर-चक्रीय फोटोफॉस्फोराइलेशन का योजनाबद्ध आरेख है। यह आरेख प्रकाश-निर्भर अभिक्रियाओं में इलेक्ट्रॉनों के प्रवाह को दिखाता है, जहाँ इलेक्ट्रॉन पानी से निकलते हैं, PS-II, साइटोक्रोम कॉम्प्लेक्स, PS-I से गुजरते हैं, और NADP+ को NADPH में कम करने के लिए उपयोग किए जाते हैं, साथ ही ATP का उत्पादन भी होता है। ऑक्सीजन पानी के विभाजन से एक उप-उत्पाद के रूप में निकलती है।
Non-cyclic photophosphorylation:
a. It involves both photosystems- PS-I and PS-II.
b. In this case, electron transport chain starts with the release of electrons from PS-II.
c. In this chain high energy electrons released from PS-II do not return to PS-II but, after passing through an electron transport chain, reach PS-I, which in turn donates it to reduce NADP to NADPH.
d. The reduced NADP+ (NADPH) is utilized for the reduction of CO2 in the dark reaction.
e. Electron-deficient PS-II brings about oxidation of water-molecule. Due to this, protons, electrons and oxygen atom are released.
f. Electrons are taken up by PS-II itself to return to reduced state, protons are accepted by NADP+ whereas oxygen is released.
g. As in this process, high energy electrons released from PS-II do not return to PS-II and it is accompanied with ATP formation, this is called Non-cyclic photophosphorylation.
In simple words: The schematic presentation correctly depicts non-cyclic photophosphorylation. Electrons move from water through PS-II and PS-I to reduce NADP+ to NADPH, generating ATP in the process.

🎯 Exam Tip: Focus on understanding the direction of electron flow and the products (ATP, NADPH, O2) of non-cyclic photophosphorylation, as it's a key process in photosynthesis.

Practical/ Project:

Question 1. Draw schematic presentation of different processes/ cycles/ reactions related to photosynthesis.
Answer:
Cyclic photophosphorylation:
a. Illumination of photosystem-l causes electrons to move continuously out of the reaction center of photosystem-l and back to it.
b. The cyclic electron-flow is accompanied by the photophosphorylation of ADP to yield ATP. This is termed as Cyclic photophosphorylation.
c. Since this process involves only pigment system I, photolysis of water and consequent evolution of oxygen does not take place.
Non-cyclic photophosphorylation::
a. It involves both photosystems- PS-I and PS-II.
b. In this case, electron transport chain starts with the release of electrons from PS-II.
c. In this chain high energy electrons released from PS-II do not return to PS-II but, after passing through an electron transport chain, reach PS-I, which in turn donates it to reduce NADP to NADPH.
d. The reduced NADP+ (NADPH) is utilized for the reduction of CO2 in the dark reaction.
e. Electron-deficient PS-II brings about oxidation of water-molecule. Due to this, protons, electrons and oxygen atom are released.
f. Electrons are taken up by PS-II itself to return to reduced state, protons are accepted by NADP+ whereas oxygen is released.
g. As in this process, high energy electrons released from PS-II do not return to PS-II and it is accompanied with ATP formation, this is called Non-cyclic photophosphorylation.
Interdependence of light and dark reactions:
1. The light reaction gives rise to two important products, a reducing agent NADPH2 and an energy rich compound ATP. Both these are utilized in the dark phase of photosynthesis.
2. ATP and NADPH2 molecules function as vehicles for transfer of energy of sunlight into dark reaction leaving to carbon fixation. In this reaction CO2 is reduced to carbohydrate.
3. During dark reaction, ATP and NADPH2 are transformed into ADP, iP and NADP which are transferred to the grana in which light reaction takes place.
Calvin cycle: The entire process of dark reaction was traced by Dr. Melvin Calvin along with his co-worker, Dr. Benson. Hence, the process is called as Calvin cycle or Calvin- Benson cycle. Since the first stable product formed is a 3-carbon compound, it is also called as C3 pathway and the plants are called C14 plants. Calvin carried out experiments on unicellular green algae (Chlorella), using radioactive isotope of carbon, C14 as a tracer. It is also called synthesis phase or second phase of photosynthesis.
The cycle is divided into the following phases:
1. Carboxylation phase:
a. Carbon dioxide reduction starts with a five-carbon sugar ribulose-1,5-bisphosphate (RuBP). It is a 5- carbon sugar with two phosphate groups attached to it.
b. RuBP reacts with CO2 to produce an unstable 6 carbon intermediate in the presence of Rubisco.
c. It immediately splits into 3 carbon compounds called 3-phosphoglyceric acid.
d. RuBisCO is a large protein molecule and comprises 16% of the chloroplast proteins.
2. Glycolytic reversal:
a. 3-phosphoglyceric acid form 1,3-diphosphoglyceric acid by utilizing ATP molecule.
b. These are then reduced to glyceraldehyde-3-phosphate (3-PGA) by NADPH supplied by the light reactions of photosynthesis.
c. In order to keep Calvin cycle continuously running there must be sufficient number of RuBP and regular supply of ATP and NADPH.
d. Out of 12 molecules of 3-phosphoglyceraldehyde, two molecules are used for synthesis of one glucose molecule.
3. Regeneration of RuBP:
a. 10 molecules of 3-phosphoglyceraldehyde are used for the regeneration of 6 molecules of RuBP at the cost of 6 ATP.
b. Therefore, six turns of Calvin cycle are needed to get one molecule of glucose.
Photorespiration: Mechanism:
1. Photorespiration involves three organelles chloroplast, peroxisomes and mitochondria and occurs in a series of cyclic reactions which is also called PCO cycle. (Photosynthetic Carbon Cycle)
2. Enzyme Rubisco acts as oxygenase at higher concentration of O2 and photorespiration begins.
3. When RuBP reacts with O2 rather than CO2 to form a 3-carbon compound (PGA) and 2-carbon compound phosphoglycolate.
4. Phosphoglycolate is then converted to glycolate which is shuttled out of the chloroplast into the peroxisomes.
5. In Peroxisomes, glycolate is converted into glyoxylate by enzyme glycolate oxidase.
6. Glyoxylate is further converted into amino acid glycine by transamination.
7. In mitochondria, two molecules of glycine are converted into serine (amino acid) and CO2 is given out.
8. Thus, it loses 25% of photosynthetically fixed carbon.
9. Serine is transported back to peroxisomes and converted into glycerate.
10. It is shuttled back to chloroplast to undergo phosphorylation and utilized in formation of 3-PGA, which get utilized in C3 pathway.
Hatch-Slack pathway: M. D. Hatch and C. R. Slack while working on sugarcane found four carbon compounds (dicarboxylic acid) as the first stable product of photosynthesis.
It occurs in tropical and sub-tropical grasses and some dicotyledons.
The first product of this cycle is a 4-carbon compound oxaloacetic acid. Hence it is also called as C4 pathway and plants are called C4 plants.
Mechanism:
1. CO2 taken from atmosphere is accepted by a 3-carbon compound, phosphoenolpyruvic acid in the chloroplasts of mesophyll cells, leading to the formation of 4-C compound, oxaloacetic acid with the help of enzyme phosphoenolpyruvate carboxylase.
2. It is converted to another 4-C compound, malic acid.
3. It is transported to the chloroplasts of bundle sheath cells.
4. Malic acid (4-C) is converted to pyruvic acid (3-C) with the release of CO2 in the cytoplasm.
5. Thus, concentration of CO2 increases in the bundle sheath cells.
6. Chloroplasts of these cells contain enzymes of Calvin cycle.
7. Because of high concentration of CO2, RuBP carboxylase participates in Calvin cycle and not photorespiration.
8. Sugar formed in Calvin cycle is transported into the phloem.
9. Pyruvic acid generated in the bundle sheath cells re-enter mesophyll cells and regenerates phosphoenolpyruvic acid by consuming one ATP.
10. Since this conversion results in the formation of AMP (not ADP), two ATP are required to regenerate ATP from AMP.
11. xi. Thus, C4 pathway needs 12 additional ATP.
12. The C3 pathway requires 18 ATP for the synthesis of one glucose molecule, whereas C4 pathway requires 30 ATP. Thus, C4 plants are better photosynthesizers as compared to C3 plants as there is no photorespiration in these plants.
13. CAM Pathway: In CAM plants, malic acid accumulates during night, which is formed from Oxaloacetic acid in presence of the enzyme malate dehydrogenase.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह कैल्विन चक्र और RuBP के पुनर्जनन का विस्तृत योजनाबद्ध आरेख है। यह दर्शाता है कि कैसे CO2 RuBP के साथ जुड़कर कार्बोक्सीलेशन चरण में 3-फॉस्फोग्लिसरेट बनाता है, फिर ग्लाइकोलिटिक उत्क्रमण चरण में एटीपी और NADPH का उपयोग करके 3-फॉस्फोग्लिसराल्डिहाइड में कम हो जाता है, और अंततः ग्लूकोज का उत्पादन होता है और RuBP का पुनर्जनन होता है।
In simple words: Photosynthesis involves light-dependent reactions (cyclic and non-cyclic photophosphorylation producing ATP and NADPH) and light-independent reactions (Calvin cycle) where CO2 is fixed into carbohydrates. Photorespiration is a wasteful process, while C4 and CAM pathways are adaptations to optimize CO2 fixation and reduce photorespiration, making C4 plants particularly efficient.

🎯 Exam Tip: When asked to draw schematic presentations, ensure clear labeling of all components and indicate the flow of electrons, energy, and key molecules for each process like cyclic, non-cyclic, and Calvin cycle. Emphasize the inputs and outputs.

Question 2. Check the effects of different factors on photosynthesis under the guidance of teacher.
Answer:
External factors which affect photosynthesis are as follows:
1. Light:
a. It is an essential factor as it supplies the energy necessary for photosynthesis.
b. Quality and intensity of light affects the photosynthesis.
c. Highest rate of photosynthesis takes place in red light followed by blue light.
d. The rate of photosynthesis considerably decreases in plants which are growing under a forest canopy.
e. In most of the plants, photosynthesis is maximum in bright diffused sunlight.
f. Uninterrupted and continuous photosynthesis for a very long period of time may be sustained without any visible damage to the plant.
2. Carbon dioxide:
The main source of CO2 in land plants is the atmosphere, which contains only 0.3% of the gas.
b. Under normal conditions of temperature and light, carbon dioxide acts as a limiting factor in photosynthesis.
c. Increase in concentration of CO2 increases the photosynthesis.
d. Increase in CO2 to about 1% is advantageous to most of the plants.
e. Higher concentration of the gas has an inhibitory effect on photosynthesis.
3. Temperature:
a. Like all other physiological processes, photosynthesis also needs a suitable temperature.
b. The optimum temperature at which the photosynthesis is maximum is 25-30 °C. Except in plants like Opuntia, photosynthesis takes place at as high as 55 °C.
c. This is the maximum temperature. Minimum temperature is temperature at which photosynthesis process just starts.
d. In the presence of sufficient light and CO2, photosynthesis increases with the rise of temperature till it becomes maximum. After that there is a decrease or fall in the rate of the process.
4. Water:
a. Water is necessary for photosynthetic process.
b. An increase in water content of the leaf results in the corresponding increase in the rate of photosynthesis.
c. Thus, the limiting effect of water is not direct but indirect.
d. It is mainly due to the fact that it helps in maintaining the turgidity of the assimilatory cells and the proper hydration of their protoplasm.
[Students can refer the given information and perform this activity on their own]
In simple words: Photosynthesis is influenced by external factors like light intensity and quality, carbon dioxide concentration, temperature, and water availability, each playing a critical role in determining the rate of the process.

🎯 Exam Tip: When discussing limiting factors, remember Blackman's Law of Limiting Factors. Explain how each factor individually impacts the rate of photosynthesis and how they interact.

11th Biology Digest Chapter 12 Photosynthesis Intext Questions and Answer

Can you recall? (Textbook Page No. 138)

Question (i) Why energy is essential in different life processes?
Answer:
a. Energy is the basic requirement of life.
b. Without energy no work can be done.
c. All living organisms need energy for reproduction and survival.
d. Sun is the main source of energy, and that energy should be transformed into the usable forms for living organisms to carry out life processes.
Therefore, energy is essential in different life processes.
In simple words: Energy is fundamental for all life functions, enabling organisms to perform work, reproduce, survive, and transform solar energy into usable forms.

🎯 Exam Tip: Understand that energy is a universal requirement for metabolic processes and maintaining cellular organization in all living systems.

Question (ii) How do we get energy?
Answer:
a. Sun is the main source of energy.
b. Plants utilize sunlight, carbon dioxide and water for the process called photosynthesis to produce sugars.
c. Animals make use of these sugars provided by the plants in their own cellular energy factories called mitochondria. Thus, energy is obtained.
In simple words: Energy originates from the sun, is captured by plants through photosynthesis to make sugars, and then consumed by animals to fuel their cellular processes via mitochondria.

🎯 Exam Tip: Explain the flow of energy from the sun through producers (plants) to consumers (animals), highlighting the roles of photosynthesis and cellular respiration.

Use your brainpower (Textbook Page No. 138)

Question. Justify: All life on earth is 'bottled solar energy'.
Answer:
1. Life on earth is dependent on solar energy directly or indirectly.
2. Plants by carrying out photosynthesis converts solar energy into chemical energy by producing carbohydrates.
3. Humans and animals depend on plants for food. Basically, life on earth depends totally on photosynthesis for food and energy.
4. Therefore, all life on earth is bottled solar energy.
In simple words: All life fundamentally relies on solar energy, which is converted into chemical energy (carbohydrates) by plants through photosynthesis, making it the ultimate energy source for most organisms on Earth.

🎯 Exam Tip: Link the concept of solar energy to photosynthesis and food chains, illustrating how energy is captured and transferred throughout ecosystems.

Can you tell? (Textbook Page No. 140)

Question. Draw well labeled diagram of ultrastructure of chloroplast.
Answer:
1. The chloroplasts are discoid and lens shaped in higher plants. Chloroplast is bounded by a double membrane.
System of chlorophyll bearing a double-membrane sac is present inside the stroma.
2. These are stacked one above the other to form grana.
3. Individual sacs in each granum is are known as thylakoid.
4. All the pigments chlorophylls, carotenes and xanthophylls are located in thylakoid membranes.
5. These pigments are fat soluble and are present in lipid part of membrane also they absorb light of specific spectrum in the visible regions.
In simple words: A chloroplast is a double-membraned, lens-shaped organelle in plants containing stacks of thylakoids (grana) within its stroma, where photosynthetic pigments capture light for photosynthesis.

🎯 Exam Tip: When describing cell organelles, always mention their shape, bounding membranes, and key internal structures, relating them to their primary functions.

Use your brainpower (Textbook Page No. 140)

Question. The photosynthetic lamellae taken out from a chloroplast and suspended in a nutrient medium in the presence of CO2 and light. Will they synthesize sugar or not?
Answer:
Photosynthetic ladmellae will not synthesize sugar because sugar synthesis occurs only in stroma, as all the enzymes required for sugar synthesis are present there. In photosynthetic lamellae only light reactions occur. Thus, lamellae cannot synthesize sugar even when CO2, light and other nutrients are provided.
In simple words: Photosynthetic lamellae (thylakoids) perform only light-dependent reactions; sugar synthesis (dark reactions) requires enzymes located in the stroma, so lamellae alone cannot produce sugar.

🎯 Exam Tip: Differentiate clearly between the locations of light-dependent reactions (thylakoids) and light-independent reactions (stroma) within the chloroplast.

Internet my friend (Textbook Page No. 139)

Question. Collect information: Why does chlorophyll appear red in reflected light and green in transmitted light?
Answer:
Chlorophyll is a light absorbing pigment. It absorbs light in red and blue regions of the visible light spectrum. Absorption spectrum of chlorophyll for red light is maximum so chlorophyll appears red in transmitted light. Green light is not absorbed but reflected so chlorophyll appear green in reflected light. [Note: Chlorophyll appear red in transmitted light and green in reflected light.[
In simple words: Chlorophyll primarily absorbs red and blue light for photosynthesis, reflecting green light (making it appear green), but when light is transmitted through it, the less absorbed red light might be more prominent.

🎯 Exam Tip: Understand the difference between reflected and transmitted light and how a pigment's absorption spectrum dictates its apparent color.

Activity 1 (Textbook Page No. 139)

Question. Grind the spinach leaves in small quantity of acetone / nail paint remover. Mix the contents properly and filter with filter paper in test tube. Test tube contains green filtrate. Take the test tube in dark-room and put a flash of torch on it. Now, solution appears red. Why does this occur? Which phenomenon is this? Discuss this with your physics, chemistry and biology teachers.
Answer:
Chlorophyll is the green pigment present in chloroplast. It absorbs light in red and blue region of visible spectrum. It does not absorb green light and thus the green light is reflected which is why it appears green. In this experiment, the chlorophyll in test tube appears red when a flash torch is put on it in the dark room.
This is because when the electrons of the chlorophyll molecule are excited in dark in the absence of electron transport chain the electrons release their energy in the form of red light as they return to ground state. This phenomenon observed here is transmission of light.
In simple words: The chlorophyll solution appears red under torchlight in the dark due to fluorescence, where excited chlorophyll electrons release energy as red light when returning to their ground state, a phenomenon distinct from normal reflection.

🎯 Exam Tip: This activity demonstrates fluorescence, a process where absorbed energy is re-emitted as light, highlighting a key property of chlorophyll when isolated from the photosynthetic electron transport chain.

Activity 2 (Textbook Page No. 139)

Question. To separate the chloroplast pigments by paper chromatography. Concentrate the extracted chlorophyll solution by evaporation. Apply a drop of it at one end, 2cm away from edge of a strip of chromatography paper and allow it to dry thoroughly. Take a mixture of petroleum ether and acetone in the ratio of 9:1 at temperature of 40 to 60°C. Hang the strip in the jar with its loaded end dipping in the solvent. Close the jar tightly and keep it for an hour. The pigments separate into distinct green and yellow bands of chlorophyll and carotenoid respectively.
Answer:
Pigments are the molecules which reflects only certain wavelengths of visible light. Chromatography is the technique used to separate the chloroplast pigments. Carotenes form yellow-orange band, chlorophyll forms blue-green band, chlorophyll b forms yellow-green bands.
In simple words: Paper chromatography separates chloroplast pigments based on their differing solubilities and affinities for the stationary and mobile phases, resulting in distinct bands like yellow-orange for carotenes and various greens for chlorophylls.

🎯 Exam Tip: Chromatography is a vital technique for separating mixtures. Focus on how differential solubility and adsorption lead to the separation of pigments into distinct bands.

Can you tell? (Textbook Page No. 139)

Question. Tomatoes, carrots and chillies are red in colour due to the presence of pigments. Name the pigment.
Answer: Red colour pigment present in tomatoes, carrots and chillies is lycopene.
In simple words: The red color in tomatoes, carrots, and chilies is primarily due to the pigment lycopene, a type of carotenoid.

🎯 Exam Tip: Be familiar with common plant pigments (chlorophylls, carotenoids, anthocyanins) and their characteristic colors and functions.

Think about it (Textbook Page No. 140)

Question. Why are a large number of gas bubbles evolved during day time in a pond of water?
Answer:
Photosynthesis occurs in the presence of sunlight. During photosynthesis, plants give out oxygen and take in carbon dioxide. The plants present underwater carryout photosynthesis and release oxygen. Hence, large number of gas bubbles are evolved during day time in a pond.
In simple words: The gas bubbles seen in a pond during daytime are oxygen, a byproduct of photosynthesis performed by submerged aquatic plants in the presence of sunlight.

🎯 Exam Tip: This observation is a direct evidence of photosynthesis, specifically the release of oxygen from the photolysis of water.

Think about it (Textbook Page No. 141)

Question. Does moonlight support photosynthesis?
Answer:
The reactions of photosynthesis take place in the presence of sunlight. The intensity of moonlight is several thousand times less than that of direct sunlight which is insufficient for the light dependent phase of photosynthesis. As the sun sets, rate of photosynthesis also decreases. Therefore, moonlight does not support photosynthesis.
In simple words: Moonlight is too dim to provide sufficient light intensity for the light-dependent reactions of photosynthesis, hence it does not support the process.

🎯 Exam Tip: Light intensity is a critical limiting factor for photosynthesis. Emphasize that below a certain threshold, the process cannot occur efficiently.

Can you tell? (Textbook Page No. 145)

Question. How chlorophyll - a is excited? Show it with a diagram.
Answer:
1. Chlorophyll-a is an essential photosynthetic pigment as it converts light energy into chemical energy and acts as a reaction centre.
2. Initially, it lies at ground state or singlet state but when it absorbs or receives photons (solar energy), it gets activated and goes in excited state or excited second singlet state.
3. In the excited state, chlorophyll-a emits an electron. The emitted electron is energy rich, i.e. has extra amount of energy.
4. Due to the loss of electron (e-), chlorophyll-a becomes positively charged. This is the ionized state.
5. Chlorophyll-a molecule cannot remain in the ionized state for more than 10-9 seconds. Hence the photo-chemical reaction or electron transfer occurs very fast.
6. The energy rich electron is then transferred through various electron acceptors and donors (carriers).
7. During the transfer, the electron emits energy which is utilized for the synthesis of ATP. This shows that light energy is converted into chemical energy in the form of ATP.

ℹ️ चित्र व्याख्या (Diagram Explanation): यह क्लोरोफिल-ए के फोटोएक्साइटेशन को दर्शाने वाला एक योजनाबद्ध आरेख है। यह दिखाता है कि क्लोरोफिल-ए अणु कैसे प्रकाश ऊर्जा को अवशोषित करता है, जिससे उसके इलेक्ट्रॉन उत्साहित अवस्था में चले जाते हैं। इलेक्ट्रॉन के निकलने से अणु आयनीकृत अवस्था में आ जाता है, जो प्रकाश संश्लेषण की शुरुआत का एक महत्वपूर्ण कदम है।
In simple words: Chlorophyll-a gets excited by absorbing solar energy (photons), causing an electron to jump to a higher energy level, making the molecule ionized and initiating the electron transfer chain for ATP synthesis.

🎯 Exam Tip: Explain the energy states of chlorophyll (ground, excited, ionized) and the fate of the excited electron, linking it to the initiation of the light-dependent reactions and ATP production.

Can you tell? (Textbook Page No. 140)

Question. What made Hill to perform his experiment?
Answer:
Robert Hill proved that the source of oxygen evolved during photosynthesis is water and not carbon dioxide. Hence, it is called Hill's Reaction.
1. In this experiment, Hill cultured isolated chloroplasts in a medium containing CO2 free water, haemoglobin and ferric compound.
2. Ferric salts and haemoglobin were added in the medium as hydrogen and oxygen acceptors respectively.
3. When the suspension was illuminated, he observed that haemoglobin turned into oxyhaemoglobin (red colour).
4. This confirmed that water must have oxidized releasing O2, that reacted with haemoglobin. Reduction of ferric compound was also indicated by change in colour.
5. The H2O molecule oxidized to evolve O2 as a by-product. Thus, Hill proved that the source of evolving O2 is H2O and not CO2.
6. This process of splitting up of water molecules under the influence of light in the presence of chlorophyll is called Photolysis of water or Hill Reaction.
7. Hill's reaction can be represented as follows:
\(2H2O + 2A \xrightarrow{\text{Sunlight}} 2AH2 + O2\)

ℹ️ चित्र व्याख्या (Diagram Explanation): यह हिल की अभिक्रिया का योजनाबद्ध प्रतिनिधित्व है। यह दर्शाता है कि कैसे पानी (H2O) क्लोरोफिल की उपस्थिति में और सूर्य के प्रकाश के प्रभाव में एक इलेक्ट्रॉन स्वीकर्ता (A) की उपस्थिति में टूट जाता है, जिससे कम हुआ स्वीकर्ता (AH2) और ऑक्सीजन गैस (O2) एक उप-उत्पाद के रूप में निकलती है।
In simple words: Hill's experiment demonstrated that oxygen released during photosynthesis originates from water, not carbon dioxide, through a light-dependent splitting process called photolysis.

🎯 Exam Tip: Hill's experiment is crucial for understanding the origin of photosynthetic oxygen. Remember the key reactants (water, electron acceptor) and products (reduced acceptor, oxygen).

Can you tell? (Textbook Page No. 145)

Question. Draw a flowchart of non-cyclic photophosphorylation.
Answer:
Non-cyclic photophosphorylation:
a. It involves both photosystems- PS-I and PS-II.
b. In this case, electron transport chain starts with the release of electrons from PS-II.
c. In this chain high energy electrons released from PS-II do not return to PS-II but, after passing through an electron transport chain, reach PS-I, which in turn donates it to reduce NADP to NADPH.
d. The reduced NADP+ (NADPH) is utilized for the reduction of CO2 in the dark reaction.
e. Electron-deficient PS-II brings about oxidation of water-molecule. Due to this, protons, electrons and oxygen atom are released.
f. Electrons are taken up by PS-II itself to return to reduced state, protons are accepted by NADP+ whereas oxygen is released.
g. As in this process, high energy electrons released from PS-II do not return to PS-II and it is accompanied with ATP formation, this is called Non-cyclic photophosphorylation.
In simple words: Non-cyclic photophosphorylation involves both PS-I and PS-II, where electrons from water move through an electron transport chain to reduce NADP+ to NADPH, simultaneously producing ATP, with oxygen released as a byproduct.

🎯 Exam Tip: Focus on the "Z-scheme" of non-cyclic photophosphorylation, emphasizing the involvement of both photosystems, the continuous flow of electrons, and the production of both ATP and NADPH.

Can you tell? (Textbook Page No. 145)

Question. Describe Calvin's cycle.
Answer:
The entire process of dark reaction was traced by Dr. Melvin Calvin along with his co-worker, Dr. Benson. Hence, the process is called as Calvin cycle or Calvin- Benson cycle. Since the first stable product formed is a 3-carbon compound, it is also called as C3 pathway and the plants are called C14 plants. Calvin carried out experiments on unicellular green algae (Chlorella), using radioactive isotope of carbon, C14 as a tracer. It is also called synthesis phase or second phase of photosynthesis.
The cycle is divided into the following phases:
1. Carboxylation phase:
a. Carbon dioxide reduction starts with a five-carbon sugar ribulose-1,5-bisphosphate (RuBP). It is a 5- carbon sugar with two phosphate groups attached to it.
b. RuBP reacts with CO2 to produce an unstable 6 carbon intermediate in the presence of Rubisco.
c. It immediately splits into 3 carbon compounds called 3-phosphoglyceric acid.
d. RuBisCO is a large protein molecule and comprises 16% of the chloroplast proteins.
2. Glycolytic reversal:
a. 3-phosphoglyceric acid form 1,3-diphosphoglyceric acid by utilizing ATP molecule.
b. These are then reduced to glyceraldehyde-3-phosphate (3-PGA) by NADPH supplied by the light reactions of photosynthesis.
c. In order to keep Calvin cycle continuously running there must be sufficient number of RuBP and regular supply of ATP and NADPH.
d. Out of 12 molecules of 3-phosphoglyceraldehyde, two molecules are used for synthesis of one glucose molecule.
3. Regeneration of RuBP:
a. 10 molecules of 3-phosphoglyceraldehyde are used for the regeneration of 6 molecules of RuBP at the cost of 6 ATP.
b. Therefore, six turns of Calvin cycle are needed to get one molecule of glucose.

ℹ️ चित्र व्याख्या (Diagram Explanation): यह कैल्विन चक्र और RuBP के पुनर्जनन का विस्तृत योजनाबद्ध आरेख है। यह दर्शाता है कि कैसे CO2 RuBP के साथ जुड़कर कार्बोक्सीलेशन चरण में 3-फॉस्फोग्लिसरेट बनाता है, फिर ग्लाइकोलिटिक उत्क्रमण चरण में एटीपी और NADPH का उपयोग करके 3-फॉस्फोग्लिसराल्डिहाइड में कम हो जाता है, और अंततः ग्लूकोज का उत्पादन होता है और RuBP का पुनर्जनन होता है।
In simple words: Calvin cycle, or the C3 pathway, is the dark reaction of photosynthesis where CO2 is fixed and reduced to form glucose in three main phases: carboxylation of RuBP, reduction of 3-PGA to glyceraldehyde-3-phosphate, and regeneration of RuBP.

🎯 Exam Tip: Clearly delineate the three phases of the Calvin cycle (carboxylation, reduction, regeneration) and remember the key enzymes and molecules involved, especially RuBisCO and RuBP.

Can you tell? (Textbook Page No. 147)

Question. Summarise the photosynthetic reaction.
Answer:
\(6CO2 + 12H2O \implies C2H12O6 + 6O2 + 6H2O\)
1. Photosynthesis is a two step process.
The light dependent reactions convert the light energy from the sun into chemical energy.
The light independent reactions convert the chemical energy to synthesize carbohydrates.
2. Light dependent reactions: Light is absorbed by chlorophyll which results in the production of ATP. Photolysis of water produce oxygen and hydrogen. The hydrogen and ATP are used in the light independent reactions and the oxygen is released from stomata.
3. Light independent reactions: ATP and hydrogen are transferred to the site of light independent reactions. The hydrogen is combined with carbon dioxide to form complex organic compounds.
The ATP provides the required energy to power these anabolic reactions and fix the carbon molecules.
In simple words: Photosynthesis is a two-stage process: light-dependent reactions convert light energy into ATP and NADPH (and release oxygen), which then power the light-independent reactions (Calvin cycle) to fix carbon dioxide into carbohydrates.

🎯 Exam Tip: Understand the overall chemical equation for photosynthesis. Emphasize the distinct roles of the light-dependent (energy capture, O2 release) and light-independent (carbon fixation, sugar synthesis) stages and their interdependence.

Can you tell? (Textbook Page No. 147)

Question. Xerophytic plants survive in high temperature. How?
Answer:
1. Xerophytic plants are those that have adapted to dry environments.
2. They have adapted to arid conditions by storing water in stems.
3. Stomata of these plants remain closed during day time to reduce the rate of transpiration to bare minimum.
4. Leaves are modified into spines or are reduced in size to check the loss of water due to transpiration.
5. The waxy surfaces of xerophytic plants prevent the loss of moisture.
6. Thus, they are able to survive in high temperature.
In simple words: Xerophytic plants have evolved specialized adaptations like water storage in stems, closed stomata during the day, modified leaves, and waxy surfaces to efficiently conserve water and survive extreme high temperatures in dry environments.

🎯 Exam Tip: Focus on the structural and physiological adaptations that enable xerophytes to minimize water loss and tolerate high temperatures, such as stomatal regulation and leaf modifications.

Can you tell? (Textbook Page No. 147)

Question. Compare C4 plants and CAM plants.
Answer:

🎯 Exam Tip: When comparing C4 and CAM plants, highlight differences in their geographical distribution, leaf anatomy, stomatal behavior, carbon fixation mechanisms, and the timing of their photosynthetic processes to ensure full marks.