School Assignments for Class 11 Biology: Chapter 11 Photosynthesis In Higher Plants
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Practice Class 11 Biology Assignments: Chapter 11 Photosynthesis In Higher Plants
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Ques. Which one of the following organisms is correctly matched with its three characteristics?
(a) Pea: C3 pathway, endospermic seed, vexillary aestivation
(b) Tomato: twisted aestivation, axile placentation, berry
(c) Onion: bulb, imbricate aestivation, axile placentation
(d) Maize: C3 pathway, closed vascular bundles, scutellum
Answer: C
Question. Thylakoids possess photosynthetic complexes called
(a) photosystem I and II
(b) cyt b6 f complex
(c) ATP synthetase
(d) all of these.
Answer. D
Question. Which of these is a type of phycobilin pigments?
(a) Phycocyanin
(b) Allophycocyanin
(c) Phycoerythrin
(d) All of these
Answer. D
Question. CAM plants do not show photorespiration due to
(a) keeping stomata open during day time
(b) using PEP carboxylase
(c) fixing CO2 into organic acid in night and releasing CO2 during day
(d) performing Calvin cycle at night.
Answer. C
Question. Which of the following is correct about chlorophyll a and b in the leaves of higher plants?
(a) Both are present in equal proportion.
(b) Chlorophyll a is more than chlorophyll b.
(c) Chlorophyll a is less than chlorophyll b.
(d) Chlorophyll b is ten times more than chlorophyll a.
Answer. B
Question. Which of the following best represents Hill reaction?
(a) Photolysis of water
(b) Photolysis of water releasing oxygen
(c) Photolysis of water in light resulting in reduction of NADP and release of oxygen
(d) Photolysis of water and release of hydrogen
Answer. C
Question. The reactions of Calvin cycle are not directly dependent on light, but they usually do not occur at night. Why?
(a) Night is often too cold for these reactions to occur.
(b) CO2 concentration in night is too high for these reactions to occurs.
(c) Plants usually open their stomata at night.
(d) Calvin cycle is dependent on the products of light reaction.
Answer. D
Question. Identify the parts marked as A , B and C in the given figure showing ATP synthesis through chemiosmosis. (Img 36)
(a) Thylakoid lumen F0 F1
(b) Thylakoid lumen F1 F0
(c) Chloroplast lumen F0 F1
(d) Chloroplast lumen F1 F0
Answer. A
Question. Read the given statements and select the correct option.
Statement 1 : 6 molecules of CO2, 12 molecules of NADPH+ + H+ and 18 ATP are used to form one hexose molecule. (Img 36)
Statement 2 : In light reaction formation of ATP and NADPH take place.
(a) Both the statements are true and statement 2 is a correct explanation for statement 1.
(b) Both the statements are true but statement 2 is not a correct explanation for statement 1.
(c) Statement 1 is true but statement 2 is false.
(d) Both the statements are false.
Answer. A
Question. The Z scheme of photophosphorylation follows the following sequence:
Which of the following options is correct for A, B, C, D transfer of electrons?
A B C D
(a) Uphill Downhill Uphill Downhill
(b) Downhill Uphill Downhill Uphill
(c) Downhill Uphill Uphill Downhill
(d) Uphill Downhill Downhill Uphill
Answer. A
Question. In a condensed schematic representation of dark reaction of photosynthesis given below, steps are indicated by alphabets. Select the option where the alphabets are correctly identified.
(a) A = CO2 fixation, B = Reduction,
C = Phosphorylation, D = Regeneration
(b) A = Regeneration, B = CO2 fixation,
C = Reduction, D = Phosphorylation
(c) A = CO2 fixation, B = Phosphorylation,
C = Reduction, D = Regeneration
(d) A = CO2 fixation, B = Phosphorylation,
C = Regeneration, D = Reduction
Answer. C
Question. A typical light response curveof photosynthesis is shown. The limiting factor/s for photosynthesis at M and N is/are
(a) temperature and CO2 respectively
(b) CO2 and light respectively
(c) only CO2
(d) light and CO2 respectively.
Answer. B
Question. Warburg effect refers to
(a) decreased photosynthetic rate at very high O2 concentration
(b) increased photosynthetic rate at very high O2 concentration
(c) decreased photosynthetic rate at very low O2 concentration
(d) increased photosynthetic rate at very low O2 concentration.
Answer. A
Question. Read the following statements and find out the incorrect one.
(a) Second step of Calvin cycle (i.e., reduction) involve utilisation of 2 molecules of ATP for reduction and 2 of NADPH for phosphorylation per CO2 molecule fixed.
(b) The regeneration step requires one ATP for phosphorylation to form RuBP.
(c) It is probably to meet the differences in number of ATP and NADPH used in dark reaction that the cyclic phosphorylation takes place.
(d) Plants that are adapted to dry tropical regions have the C4 pathway.
Answer. A
Question. During light reaction, as electrons move through photosystems, protons are transported across the membrane. This happens because of
(a) the primary acceptor of e– (located towards the outer surface of the membrane) transfers its electron not to an e– carrier but to H+ carrier
(b) the primary acceptor of e– transfers only its e– to e– carrier
(c) the primary acceptor of e– transfers only H+ to the next carrier
(d) NADP-reductase is present in grana.
Answer. A
Ques. PGA as the first CO2 fixation product was discovered in photosynthesis of
(a) bryophyte
(b) gymnosperm
(c) angiosperm
(d) alga.
Answer: D
Ques. The chemiosmotic coupling hypothesis of oxidative phosphorylation proposes that adenosine triphosphate (ATP) is formed because
(a) a proton gradient forms across the inner membrane
(b) there is a change in the permeability of the inner mitochondrial membrane toward adenosine diphosphate (ADP)
(c) high energy bonds are formed in mitochondrial proteins
(d) ADP is pumped out of the matrix into the intermembrane space.
Answer: A
Ques. Chemiosmotic theory of ATP synthesis in the chloroplasts and mitochondria is based on
(a) membrane potential
(b) accumulation of Na+ ions
(c) accumulation of K+ ions
(d) proton gradient.
Answer: D
Ques. In C3 plants, the first stable product of photosynthesis during the dark reaction is
(a) malic acid
(b) oxaloacetic acid
(c) 3-phosphoglyceric acid
(d) phosphoglyceraldehyde.
Answer: C
Ques. For assimilation of one CO2 molecule, the energy required in form of ATP and NADPH2 are
(a) 2 ATP and 2 NADPH2
(b) 5 ATP and 3 NADPH2
(c) 3 ATP and 2NADPH2
(d) 18 ATP and 12 NADPH2.
Answer: C
Ques. For the synthesis of one glucose molecule the Calvin cycle operates for
(a) 2 times
(b) 4 times
(c) 6 times
(d) 8 times.
Answer: C
Ques. Carbon dioxide acceptor in C3-plants is
(a) PGA
(b) PEP
(c) RuDP
(d) none of these.
Answer: C
Ques. The mechanism of ATP formation both in chloroplast and mitochondria is explained by
(a) chemiosmotic theory
(b) Munch’s hypothesis (mass flow model)
(c) relay pump theory of Godlewski
(d) Cholodny-Wont’s model.
Answer: A
Ques. What will be the number of Calvin cycles to generate one molecule of hexose?
(a) 8
(b) 9
(c) 4
(d) 6
Answer: D
Ques. The primary acceptor, during CO2 fixation in C3 plants, is
(a) phosphoenolpyruvate (PEP)
(b) ribulose 1, 5-diphosphate (RuDP)
(c) phosphoglyceric acid (PGA)
(d) ribulose monophosphate (RMP).
Answer: B
Ques. The carbon dioxide acceptor in Calvin cycle/C3-plants is
(a) phosphoenol pyruvate (PEP)
(b) ribulose 1, 5-diphosphate (RuDP)
(c) phosphoglyceric acid (PGA)
(d) ribulose monophosphate (RMP).
Answer: B
Ques. Which technique has helped in investigation of Calvin cycle?
(a) X-ray crystallography
(b) X-ray technique
(c) Radioactive isotope technique
(d) Intermittent light
Answer: C
Ques. Dark reactions of photosynthesis occur in
(a) granal thylakoid membranes
(b) stromal lamella membranes
(c) stroma outside photosynthetic lamellae
(d) periplastidial space.
Answer: C
Ques. Carbon dioxide joins the photosynthetic pathway in
(a) PS I
(b) PS II
(c) light reaction
(d) dark reaction.
Answer: D
Ques. Phosphoenol pyruvate (PEP) is the primary CO2 acceptor in
(a) C4 plants
(b) C2 plants
(c) C3 and C4 plants
(d) C3 plants.
Answer: A
Ques. A plant in your garden avoids photorespiratory losses, has improved water use efficiency, shows high rates of photosynthesis at high temperatures and has improved efficiency of nitrogen utilisation.
In which of the following physiological groups would you assign this plant?
(a) CAM
(b) Nitrogen fixer
(c) C3
(d) C4
Answer: D
Ques. Bundle sheath cells
(a) are rich in PEP carboxylase
(b) lack RuBisCO
(c) lack both RuBisCO and PEP carboxylase
(d) are rich in RuBisCO.
Answer: D
Very Short Answer Type Questions
Question. Why is the development of proton gradient across the thylakoid membrane essential for photosynthesis?
Answer. Development of proton gradient across the thylakoid membrane is essential for ATP synthesis during photosynthesis.
Question. Succulents are known to keep their stomata closed during the day to check transpiration. How do they meet their photosynthetic CO2 requirements?
Answer. Succulents (water storing) plants such as cacti, fix CO2 into organic compound using PEP carboxylase at night, when the stomata are open.
Question. (i) NADP reductase enzyme is located on_______ . (ii) Breakdown of proton gradient leads to release of_______ .
Answer. (i) Grana-lamellae (ii) Energy in form of ATP.
Short Answer Type Questions
Question. A cyclic process is occurring in C3 plant, which is light dependent and needs O2. This process doesn’t produce energy rather it consumes energy. (a) Name the given process. (b) Is it essential for survival? (c) What are the end products of this process? Where does it occur?
Answer. (a) : Photorespiration is the light dependent process of oxygenation of ribulose biphosphate (RuBP) and release of carbon dioxide by the photosynthetic organs of plant. (b) Photorespiration is either a necessary evil of plant metabolism or it may have some adaptive functions that are not apparent. Photorespiration allows plant leaves to use up excess light energy and reduce photooxidative damage when the plant is water-stressed and stomata are closed. (c) The end product of this process is phosphoglycerate. The site of photorespiration is chloroplast whereas peroxisome and mitochondria are required for completing the process.
Question. How temperature affects the photosynthesis in C3 and C4 plants?
Answer. The light reaction are temperature sensitive but not much affected by temperature change however, dark reaction is temperature controlled because it involves enzymatic reactions. The optimum temperature is 10°–25°C for C3 plants and 30–40°C for C4 plants. When temperature is increased from minimum to optimum, the rate of photosynthesis doubles for every 10°C rise in temperature. At low temperature, enzymes become inactive. C4 plants show little photosynthesis even at not so low temperature (2–10°C) because their enzyme pyruvate phosphate dikinase is particularly sensitive to it. C3 plants show different responses to lower temperatures depending upon their adaptability. At high temperature, C3 plants are more affected because of increased affinity of RuBisCO to oxygen.
Question. How cyclic photophosphorylation is different from non-cyclic photophosphorylation?
Answer. Differences between Cylic and non-cyclic photophosphorylation are as follows: (Table 44)
Question. There is a clear division of labour within the chloroplast. Identify P, Q and R in the given structure of chloroplast and state their specific functions in photosynthesis.
Answer. P is stroma lamellae that is the site of cyclic photophosphorylation and synthesis of ATP. Q is granum where both cyclic and non-cyclic photophosphorylation takes place that results in synthesis of NADPH and ATP. R is stroma which plays role in CO2 fixation and synthesis of sugar.
Question. Why photorespiration is absent in C4 plants?
Answer. Ribulose biphosphate carboxylase oxygenase (RuBisCO), the main enzyme of Calvin cycle which fixes CO2, acts both as oxygenase and carboxylase. In presence of high concentration of O2, the enzyme RuBisCO acts as oxygenase and splits a molecule of ribulose–1, 5-biphosphate into one molecule each of 3-phosphoglyceric acid and 2-phosphoglycolic acid. This process is called photorespiration. It causes the loss of fixed CO2 and wastes the work already done. In C4 plants RuBisCo is located only in bundle sheath cells where photosynthetic release of oxygen does not occur. Bundle sheath cells have a high intracellular concentration of CO2 due to flow of C4 acids and their decarboxylation to release CO2. Therefore, RuBisCO functions purely as carboxylase in C4 plants and no photorespiration occurs.
Question. (a) Why does the rate of photosynthesis in plants decrease at higher temperatures? (b) A plant is provided with all the neccessities required for photosynthesis, i.e., water, CO2. To provide the light, the plant is kept in moonlight. Will this plant perform photosynthesis? Explain.
Answer. (a) Temperature does not influence light reactions of photosynthesis but affects the enzyme controlled dark reactions. The optimum temperature for photosynthesis is 18 to 35°C. When temperature is increased from minimum to optimum, the rate of photosynthesis doubles for every 10°C rise in temperature. Above the optimum temperature, the rate of photosynthesis shows an initial increase for short duration but later declines. (b) The plant will not perform photosynthesis as moonlight does not carry enough energy to excite the chlorophyll molecules-reaction centres PS I and PS II. Hence lightdependent reactions are not initiated.
Question. Represent systematically the process of ATP synthesis through chemiosmosis in chloroplast.
Answer. According to chemiosmotic theory, ATP synthesis is linked to the development of a proton gradient across thylakoid membrane. Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase. The systematic process of ATP synthesis is as follows : (i) The protons or hydrogen ions that are produced by the splitting of water accumulate within the lumen of the thylakoids. (ii) As electrons move through the photosystems, protons are released into the inner side or the lumen side of the membrane. (iii) Along with electrons that come from the acceptor of electrons of PS I, protons are necessary for the reduction of NADP+ to NADPH + H+. These protons are also removed from the stroma. (iv) Hence, within the chloroplast, protons in the stroma decrease in number, while in the lumen there is accumulation of protons. This creates a proton gradient across the thylakoid membrane. (v) The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the F0 of the ATPase. (vi) The break down of the gradient provides enough energy to cause a conformational change in the F1 particle of the ATPase, which makes the enzyme synthesise several molecules of energy-packed ATP.
Question. (a) Which pathway prevents photorespiration in plants? Explain it. (b) What is the role of RuBisCO?
Answer. (a) : C4 pathway prevents photorespiration in plants. In C4 plants, RuBisCO is located only in bundle sheath cells where photosynthetic release of oxygen does not occur. Bundle sheath cells have a high intracellular concentration of CO2 due to flow of C4 acids and their decarboxylation to release CO2. Therefore RuBisCO functions purely as carboxylase in C4 plants such as sugarcane and hence photorespiration is prevented. (b) RubisCO or RuBP carboxylase – oxygenese has dual nature. It has affinity for both CO2 and O2 but has more affinity for CO2 than O2. Thus, the concentrations of two determines which of the two will bind to the enzyme. In a normal condition of C3 plants, when CO2 and O2 concentrations are normal, it acts as carboxylase and fix CO2 by combining with ribulose biphosphate and C3 cycle operates normally, producing glucose molecule as by product of photosynthesis. In C3 plants when O2 concentration goes up and CO2 goes down, it starts acting as an oxygenase enzyme and C2 cycle (photorespiration) starts where RuBP binds with O2 to from phosphoglycolate and phosphoglyceric acid. In C4 plants, RuBP acts as carboxylase and accepts liberated CO2 to form phosphoglyceric acid.
Long Answer Type Questions
Question. Where does Calvin cycle take place in chloroplast? Describe the three phases of Calvin cycle.
Answer. Calvin cycle takes place in stroma of the chloroplast. Calvin cycle can be described under three stages: carboxylation, reduction and regeneration. (i) Carboxylation: Carboxylation is the fixation of CO2 into a stable organic intermediate. A 5-carbon pentose sugar, ribulose 1,5 bisphosphate (RuBP) is the first acceptor of CO2. Carboxylation of RuBP by atmospheric CO2 in presence of the enzyme RuBP carboxylase or RuBisCO is the first step in Calvin cycle. Six molecules of RuBP combine with six molecules of CO2 to form six molecules of a transient unstable compound 2 carboxy, 3-keto 1, 5 biphosphoribotol. This compound immediately breaks into 2 molecules of phosphoglyceric acid (PGA, 3C) with the help of the enzyme carboxydismutase. 2 carboxy 3-keto 1, 5-biphosphoribotol + H2O → 3-PGA (ii) Reduction: Phosphoglyceric acid or PGA is further phosphorylated by ATP with the help of enzyme triose phosphate kinase. Biphosphoglyceric acid is reduced by NADPH through the agency of enzyme glyceraldehyde 3-phosphate dehydrogenase (triose phosphate dehydrogenase). It produces glyceraldehyde 3-phosphate or 3-phospho-glyceraldehyde (3PGAL). 1,6 biphosphate (F-1, 6BP) is then dephosphorylated first to fructose mono-phosphate (or fructose-6-phosphate, F-6-P). Here, phosphatase enzyme catalyses the reactions. In presence of the enzyme isomerase, some fructose monophosphate molecules are isomerised to glucose monophosphate (G-6-P) and ultimately glucose (6C) is produced. The hexose sugars are further converted to sucrose or to starch. As Calvin cycle takes only one carbon (as CO2) at a time, so it takes six turns of the cycle to produce one molecule of glucose. (iii) Regeneration of the CO2 acceptor – RuBP: In the Calvin cycle, the 5 carbon compound RuBP is constantly required for the fixation of CO2. It is regenerated through a chain of reactions, mediated by different enzymes. Some molecules of fructose-6-phosphate (F-6-P)combine with PGAL to form erythrose-4-phosphate (E-4-P, 4C) and xylulose-5-phosphate (X-5-P, 5C) in presence of transketolase enzyme. Erythrose-4-phosphate combines with DHAP to form sedoheptulose-7- phosphate (Se-7-BP, 7C) in presence of phosphatase enzyme. This 7 carbon compound combines with PGAL to form xylulose-5-phosphate (R-5-P, 5C) and ribose- 5-phosphate (5C). Transketolase is the enzyme in this step. Ribose-5-phosphate is converted to ribulose-5-phosphate (Ru-5-P) by isomerase and xylulose-5-phosphate is converted to ribulose-5-phosphate (5C) by epimerase. Ribulose-5-phosphate is phosphorylated to produce RuBP (5C). ATP is the donor of one phosphate, in this step.
Question. Give a detailed account on factors affecting photosynthesis.
Answer. Various external as well as internal factors affect the rate of photosynthesis. These are discussed as follows : External factors (i) CO2 concentration : Increase in CO2 concentration increases rate of photosynthesis in most C3 plants. When CO2 concentration is reduced, there comes a point at which illuminated plant parts stop absorbing carbon dioxide from their environment. It is known as CO2 compensation point or threshold value. At this value, CO2 fixed in photosynthesis is equal to CO2 evolved in respiration and photorespiration. (ii) Light : At low light intensity the rate of photosynthesis is reduced. As the light intensity increases, the rate of photosynthesis also increases. The light intensity at which a plant can perform maximum amount of photosynthesis is called light saturation point. (iii) Temperature : When temperature is increased from minimum to optimum, the rate of photosynthesis doubles for every 10°C rise in temperature. Above the optimum temperature, the rate of photosynthesis shows an initial increase for short duration but later declines. This decline with time is called time factor. (iv) Oxygen : At a very high oxygen content the rate of photosynthesis begins to decline in all plants. The phenomenon is called Warburg effect. (v) Water : The amount of water used in photosynthesis is very small, the rest is lost in transpiration. Even a slight increase in transpiration reduces the leaf hydration that cuts down photosynthesis by causing stomatal closure. Hence, photosynthesis is very sensitive to dehydration. Internal or plant factors (i) Chlorophyll-Photosynthesis does not occur in the absence of chlorophyll. Therefore, variegated leaves produce less organic food as compared to completely green leaves. (ii) Leaf age : It also affects rate of photosynthesis. As a leaf grows, the rate of photosynthesis rises with the age till it becomes maximum at full maturity. Afterwards, the rate of photosynthesis begins to decline. (iii) Phytohormones : Phytohormones like cytokinins and gibberellins increase the rate of photosynthesis but abscisic acid reduces the same. (iv) Leaf anatomy : Size, structure, position and frequency of stomata, thickness of cuticle and epidermis, vascular strands distribution, etc., influences the CO2 diffusion rate into mesophyll cells, availability of light, rate of translocation of end products, etc.
QUESTIONS
Very Short Answer Questions (1 mark each)
Question 1. Name two photosynthetic pigments belonging to Carotenoids.
Answer: Carotene and Xanthophyll.
In simple words: Carotenoids are accessory pigments that include carotenes and xanthophylls, which help absorb light energy and protect chlorophyll from photo-oxidation.
Exam Tip: Mention both specific chemical classes (carotenes and xanthophylls) to secure full marks.
Question 2. How many molecules of ATP are required for synthesis of one molecule of glucose in \(C_3\) and \(C_4\) pathways?
Answer: In the \(C_3\) pathway, 18 ATP molecules are required. In the \(C_4\) pathway, 30 ATP molecules are required.
In simple words: \(C_4\) plants require extra energy (ATP) for the initial fixation of carbon dioxide in mesophyll cells before running the Calvin cycle.
Exam Tip: Remember that \(C_4\) plants use 12 additional ATP molecules for regenerating phosphoenolpyruvate (PEP).
Question 3. What part of sunlight is most suitable for photosynthesis?
Answer: The blue and red regions of the light spectrum are the most suitable and effective for photosynthesis.
In simple words: Chlorophyll pigments absorb light maximally in the blue and red wavelengths of the visible spectrum.
Exam Tip: Always specify both blue and red regions corresponding to the action spectrum peaks.
Question 4. Name the two photosystems.
Answer: Photosystem I (PSI) and Photosystem II (PSII).
In simple words: Photosystems are functional units of light absorption, where PSII absorbs light at \(680\) nm and PSI absorbs light at \(700\) nm.
Exam Tip: Note that they are named in order of their discovery, not in the sequence they function during non-cyclic photophosphorylation.
Question 5. Name two plants that can carry out photosynthesis at night.
Answer: Opuntia and Chenopodium (or Bougainvillea / CAM plants).
In simple words: CAM plants open their stomata at night to take in \(CO_2\) and store it as malic acid, minimizing water loss in arid environments.
Exam Tip: Give examples of Crassulacean Acid Metabolism (CAM) plants when asked about nocturnal carbon fixation.
Question 6. Name the enzyme which is found abundantly in the world.
Answer: RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase).
In simple words: RuBisCO is the most abundant protein on Earth because it is present in all photosynthetic plants that fix carbon.
Exam Tip: Write the full form or standard abbreviation RuBisCO.
Question 7. Name the scientist who proposed the \(C_4\) pathway.
Answer: Hatch and Slack.
In simple words: The \(C_4\) pathway is also known as the Hatch and Slack pathway after the scientists who discovered it.
Exam Tip: Associate Hatch and Slack with \(C_4\) pathway and Calvin with \(C_3\) pathway.
Question 8. Where does carbon fixation occur in chloroplast?
Answer: Stroma of the chloroplast.
In simple words: The light-independent dark reactions (biosynthetic phase) take place in the fluid stroma matrix.
Exam Tip: Distinguish clearly between grana (light reaction) and stroma (dark reaction / carbon fixation).
Question 9. Which compound acts as \(CO_2\) acceptor in Calvin cycle?
Answer: Ribulose 1,5-bisphosphate (RuBP).
In simple words: RuBP is a 5-carbon ketose sugar that accepts \(CO_2\) during the carboxylation stage of the Calvin cycle.
Exam Tip: Mention the carbon number (\(5-C\)) along with the full chemical name for completeness.
Question 10. Name the end products of light reaction.
Answer: ATP, NADPH, and \(O_2\).
In simple words: The photochemical phase produces assimilatory power (ATP and NADPH) and releases oxygen as a by-product from water splitting.
Exam Tip: Do not include glucose here, as glucose is formed during the biosynthetic phase (dark reaction).
Short Answer Questions-II (2 marks each)
Question 11. Why does the rate of photosynthesis decline in the presence of continuous light?
Answer: An increase in incident light beyond a saturation point causes the breakdown or photo-oxidation of chlorophyll pigments, leading to a decline in photosynthetic rate.
In simple words: Too much intense light damages the photosynthetic apparatus, reducing efficiency.
Exam Tip: Mention photo-oxidation of chlorophyll to earn full credit.
Question 12. Why do green plants start evolving carbon dioxide instead of oxygen on a hot sunny day?
Answer: On a hot sunny day, stomata close to conserve water, causing internal \(CO_2\) concentration to fall and \(O_2\) to rise. Under these conditions, the enzyme RuBisCO acts as an oxygenase instead of a carboxylase, initiating photorespiration where \(CO_2\) is released.
In simple words: High temperatures favor photorespiration over photosynthesis in \(C_3\) plants.
Exam Tip: Highlight the dual nature of RuBisCO (carboxylase vs. oxygenase activity).
Question 13. Fill in the space, left blank in the given table to bring the difference between \(C_3\) and \(C_4\) plants:
| S. No. | Characteristics | \(C_3\) plants | \(C_4\) plants |
|---|---|---|---|
| 1 | Cell type | Mesophyll | Bundle sheath and mesophyll |
| 2 | \(CO_2\) acceptor | RuBP | Phosphoenol pyruvate (PEP) |
| 3 | First \(CO_2\) fixation product | 3-PGA | OAA (Oxaloacetic acid) |
| 4 | Optimum temperature | \(20^\circ\text{C} - 25^\circ\text{C}\) | \(30^\circ\text{C}\) to \(45^\circ\text{C}\) |
Answer: (a) Bundle sheath
(b) RuBP
(c) OAA (Oxaloacetic acid)
(d) \(20^\circ\text{C} - 25^\circ\text{C}\)
In simple words: The table compares structural and biochemical differences between \(C_3\) and \(C_4\) plant anatomies.
Exam Tip: Memorize Kranz anatomy and temperature optima for quick table completion.
Question 14. State two functions of accessory pigments found in thylakoids.
Answer: (a) Absorption of light of different wavelengths and transfer of energy to chlorophyll \('a'\).
(b) Protection of chlorophyll \('a'\) from photo-oxidation.
In simple words: Accessory pigments widen the absorption spectrum and shield the reaction center from damage by excess light.
Exam Tip: Clearly state energy transfer and photoprotection as the two primary roles.
Question 15. Why do \(C_4\) plants are more expensive than \(C_3\) plants?
Answer: \(C_4\) plants require more energy (\(30\) ATP molecules) in synthesizing one molecule of glucose as compared to \(C_3\) plants (\(18\) ATP molecules).
In simple words: The extra energy expenditure in \(C_4\) plants is the metabolic cost for concentrating \(CO_2\) to eliminate photorespiration.
Exam Tip: Always contrast the ATP count (\(30\) vs \(18\)) when explaining why \(C_4\) pathways are energetically expensive.
Short Answer Questions-I (3 marks each)
Question 16. The figure shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions:
(i) At which point(s) A, B or C in the curve, light is a limiting factor?
(ii) What could be the limiting factor(s) in region A?
(iii) What do region C and D represent on the curve?
Answer: (i) 'B'
(ii) \(CO_2\) concentration and temperature
(iii) 'C' represents constant rate of photosynthesis, and 'D' is the light saturation intensity at which the rate of photosynthesis is maximum.
In simple words: Light intensity limits photosynthesis at lower levels, but other factors become limiting once light saturation is reached.
Exam Tip: Interpret graph axes carefully (X-axis: Light Intensity, Y-axis: Rate of photosynthesis).
Question 17. When and why does photorespiration take place in plants? How does this process result in a loss to the plant?
Answer: Photorespiration takes place in \(C_3\) plants under high temperature, high oxygen concentration, and low carbon dioxide concentration when stomata are partially closed. It results in a loss because it fixes carbon with a release of \(CO_2\) without producing ATP or NADPH, thus wasting previously fixed energy.
In simple words: Photorespiration is a wasteful process where RuBisCO fixes oxygen instead of carbon dioxide.
Exam Tip: Emphasize that no energy-rich molecules (ATP/NADPH) are synthesized during photorespiration.
Question 18. What are the steps that are common to \(C_3\) and \(C_4\) photosynthesis?
Answer: (a) Photolysis of \(H_2O\) and photophosphorylation occur in both \(C_3\) and \(C_4\) plants.
(b) In both types, the dark reaction (Calvin cycle) occurs in the stroma.
(c) Both types undergo the phases of carboxylation and regeneration during sugar synthesis.
In simple words: While initial carbon capture differs, the core energy generation and Calvin cycle sugar production steps are identical.
Exam Tip: Mention light reactions and the Calvin cycle phases as common denominators.
Question 19. Two potted plants were kept in an oxygen free environment in transparent containers, one in total darkness and the other in sunlight. Which one of the two is likely to survive more? Justify your answer by giving the reason.
Answer: The plant kept in sunlight will survive for a longer period because light is essential for photosynthesis, allowing it to manufacture its own food and produce oxygen.
In simple words: Sunlight enables photosynthesis, sustaining plant life and oxygen supply.
Exam Tip: Link survival directly to light-driven food production (photosynthesis).
Question 20. (a) In the diagram shown below, label A, B and C. What type of photophosphorylation is possible in this?
(b) Give any two points of difference between cyclic and non-cyclic photophosphorylation.
Answer: (a) A = \(e^-\) acceptor, B = Electron transport system, C = Chlorophyll P700. This represents cyclic photophosphorylation.
(b) Differences between cyclic and non-cyclic photophosphorylation:
1. Cyclic photophosphorylation involves only PSI, whereas non-cyclic involves both PSI and PSII.
2. Cyclic photophosphorylation synthesizes only ATP (no NADPH or \(O_2\)), whereas non-cyclic synthesizes both ATP and NADPH along with the evolution of \(O_2\).
In simple words: Cyclic electron flow loops back to PSI producing only ATP, while non-cyclic flow uses two photosystems to produce both ATP and NADPH.
Exam Tip: Remember: Cyclic = PSI only + ATP only. Non-cyclic = PSI + PSII + ATP + NADPH + \(O_2\).
Long Answer Questions (5 marks each)
Question 21. Describe \(C_4\) pathway in a paddy plant. How is this pathway an adaptive advantage to the plant?
Answer: In \(C_4\) plants, carbon dioxide is first fixed in mesophyll cells by phosphoenolpyruvate (PEP) carboxylase to form oxaloacetic acid (OAA), a 4-carbon compound. OAA is converted into malate or aspartate and transported to bundle sheath cells. In the bundle sheath cells, malate is decarboxylated to release \(CO_2\), which then enters the Calvin cycle.
Adaptive Advantage: The \(C_4\) pathway minimizes photorespiration by concentrating \(CO_2\) around RuBisCO in bundle sheath cells, allowing efficient photosynthesis under high temperatures and low moisture conditions with high water-use efficiency.
In simple words: \(C_4\) plants use a special dual-cell anatomy to concentrate carbon dioxide, avoiding wasteful photorespiration in hot climates.
Exam Tip: Structure your answer with anatomical adaptation (Kranz anatomy) and biochemical steps (PEP fixation followed by Calvin cycle).
Question 22. Explain the process of biosynthetic phase of photosynthesis occuring in plants.
Answer: The biosynthetic phase (dark reaction) uses the ATP and NADPH produced during the light reaction to fix \(CO_2\) into carbohydrates. It takes place in the stroma and proceeds through the Calvin cycle in three main stages:
1. Carboxylation: \(CO_2\) combines with ribulose 1,5-bisphosphate (RuBP) in the presence of RuBisCO to form two molecules of 3-phosphoglyceric acid (3-PGA).
2. Reduction: ATP and NADPH are utilized for the reduction of 3-PGA to form triose phosphate (glucose precursors).
3. Regeneration: The \(CO_2\) acceptor molecule RuBP is regenerated at the expense of ATP to keep the cycle running continuously.
In simple words: The dark reaction converts carbon dioxide into glucose using the energy packets (ATP and NADPH) made during the light reaction.
Exam Tip: Clearly list the three stages: Carboxylation, Reduction, and Regeneration.
Question 23. (a) Give steps of ATP synthesis in chloroplasts through chemiosmosis.
(b) Schematically represent non-cyclic photophosphorylation in plants.
Answer: (a) Steps of ATP synthesis via chemiosmosis:
1. Water splitting on the inner side of the thylakoid membrane releases protons (\(H^+\)) into the lumen.
2. As electrons move through the electron transport system, protons are transported across the membrane from the stroma into the thylakoid lumen, creating a proton gradient.
3. The breakdown of this proton gradient across the membrane through the transmembrane channel of ATP synthase releases energy.
4. ATP synthase uses this released energy to catalyze the formation of ATP from ADP and inorganic phosphate (\(P_i\)).
(b) Schematic representation of non-cyclic photophosphorylation (Z-Scheme):
Light excites PSII (P680), passing electrons down an electron transport chain to PSI (P700), and subsequently reducing \(NADP^+\) to \(NADPH + H^+\), accompanied by photolysis of water releasing \(O_2\).
In simple words: ATP is generated when protons flow down their concentration gradient through ATP synthase, driven by the proton motive force across the thylakoid membrane.
Exam Tip: Draw a clean proton gradient diagram for chemiosmosis and mention the Z-scheme pathway clearly for non-cyclic photophosphorylation.
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