RowQ
The Vault
RowQ
The Vault
CBSE Class 11 Biology · 10 questions · 24 marks
Every calorie an animal eats can be traced back to a chloroplast trapping a photon, and this chapter takes apart that conversion step by step. The light reaction captures solar energy as ATP and NADPH using chlorophyll and an electron transport chain across the thylakoid membrane, while the light-independent Calvin cycle uses that ATP and NADPH to fix carbon dioxide into sugar in the stroma. A third pathway, the C4 cycle, shows how some plants have re-engineered this process to thrive in hot, dry conditions.
Splitting of the water molecule to release oxygen during the light reaction is associated with:
Answer
Photosystem II is correct — the oxygen-evolving complex linked to PS II splits water into protons, electrons and oxygen, and the electrons replace those lost by PS II's reaction centre chlorophyll (P680) when it absorbs light.
Which enzyme catalyses the initial fixation of atmospheric CO2 in the mesophyll cells of a C4 plant?
Answer
PEP carboxylase is correct — this enzyme has a far greater affinity for CO2 than RuBisCO and no affinity for oxygen, letting it fix CO2 efficiently in the mesophyll even at the low CO2 concentrations that occur when stomata are partly closed.
In cyclic photophosphorylation, the product formed is:
Answer
ATP only, with no NADPH and no oxygen release is correct — because only PS I operates in a closed loop, no water is split and no NADP+ is reduced, so this route contributes extra ATP without adding to the NADPH or oxygen output.
The first stable product of CO2 fixation in the Calvin cycle is:
Answer
3-phosphoglyceric acid is correct — RuBisCO adds CO2 to the 5-carbon RuBP, and the unstable 6-carbon intermediate immediately splits into two molecules of the 3-carbon acid PGA, which gives the C3 pathway its name.
Assertion (A): C4 plants such as maize show little or no photorespiration compared with C3 plants. Reason (R): In C4 plants, CO2 is concentrated around RuBisCO in the bundle sheath cells, which suppresses the oxygenase activity of the enzyme.
Answer
Both A and R are true and R is the correct explanation of A — because malic acid delivers a steady, concentrated supply of CO2 directly to RuBisCO inside the bundle sheath, the enzyme's carboxylase activity is strongly favoured over its oxygenase activity, so little RuBP is lost to photorespiration.
Explain what is meant by chemiosmosis in the context of ATP synthesis during the light reaction.
Answer
Chemiosmosis is the mechanism by which ATP is synthesised using energy stored in a proton gradient across a membrane. During the light reaction, protons released from water splitting inside the thylakoid lumen, together with protons pumped in during electron transport, build up a high proton concentration inside the lumen relative to the stroma. Because the thylakoid membrane is otherwise impermeable to protons, they can only flow back down this gradient through the CF0-CF1 particles of ATP synthase, and this flow provides the energy that drives the enzyme to combine ADP and inorganic phosphate into ATP.
What is Kranz anatomy, and how does it support the C4 pathway of photosynthesis?
Answer
Kranz anatomy is the distinctive leaf structure of C4 plants in which a ring of large, chloroplast-rich bundle sheath cells surrounds each vascular bundle, and these are in turn surrounded by a layer of mesophyll cells, giving a wreath-like arrangement around the vein. This structure supports the C4 pathway by physically separating the two carbon-fixing steps: CO2 is first captured in the mesophyll cells by PEP carboxylase and converted to a 4-carbon acid, which is then transported into the tightly packed bundle sheath cells, where it releases CO2 at a high local concentration right around RuBisCO before the Calvin cycle proceeds in bundle sheath chloroplasts. The close, compact packing of the bundle sheath cells minimises the loss of this concentrated CO2 back to the intercellular spaces.
Describe the light reaction of photosynthesis, including the roles of both photosystems and the fate of the electrons removed from water.
Answer
The light reaction takes place on the thylakoid membrane and begins when light-harvesting pigment complexes in Photosystem II absorb photons and funnel the energy to a special pair of chlorophyll a molecules called P680 at the reaction centre. This absorbed energy excites an electron in P680 to a higher energy level, and the excited electron is picked up by an electron acceptor and passed along a chain of carriers embedded in the thylakoid membrane. To replace the electron lost from P680, an enzyme complex associated with PS II splits a water molecule, a process called photolysis, releasing two protons, two electrons (which replace those donated to the transport chain) and one atom of oxygen, which combines with another to form the O2 that is released as a by-product of photosynthesis. As the electrons pass down the transport chain toward Photosystem I, they release energy that is used to actively pump additional protons from the stroma into the thylakoid lumen, building up a strong proton gradient across the membrane. Meanwhile, PS I absorbs light independently at its own reaction centre, P700, and re-energises the electrons arriving from the transport chain to an even higher energy level. These re-energised electrons are passed via a short second chain of carriers to the enzyme NADP+ reductase, which uses them, together with a proton from the stroma, to reduce NADP+ to NADPH. The proton gradient built up in the lumen is then used by ATP synthase (built of a CF0 channel embedded in the membrane and a CF1 head projecting into the stroma) to synthesise ATP as protons flow back into the stroma, a process called chemiosmosis. The overall light reaction thus converts light energy into the chemical energy of ATP and the reducing power of NADPH, while releasing oxygen as a by-product, and both products are then delivered to the stroma to power the Calvin cycle.
Explain the three phases of the Calvin cycle and state how many turns of the cycle are needed to produce one molecule of glucose, with reasoning.
Answer
The Calvin cycle, which occurs in the stroma of the chloroplast and operates in all photosynthetic plants including C4 plants (in their bundle sheath cells), proceeds through three phases. In the carboxylation phase, the enzyme RuBisCO catalyses the fixation of one molecule of CO2 onto the five-carbon acceptor RuBP (ribulose bisphosphate), forming an unstable six-carbon intermediate that immediately splits into two molecules of the three-carbon compound 3-phosphoglyceric acid, or PGA; this is the rate-limiting, defining step of the cycle. In the reduction phase, each molecule of PGA is first phosphorylated using ATP and then reduced using the hydrogen and electrons carried by NADPH, converting it into glyceraldehyde-3-phosphate, or G3P; this step consumes both of the products of the light reaction. Of every six molecules of G3P produced from a full cycle, one leaves the cycle to be used in the synthesis of glucose and other carbohydrates, while the remaining five continue into the third phase, regeneration, in which they are rearranged and phosphorylated using further ATP to reconstitute three molecules of the CO2 acceptor RuBP, allowing the cycle to continue. Because only one carbon atom is fixed and only one out of every six G3P molecules is diverted to sugar synthesis per turn of the cycle, six turns of the cycle, fixing six molecules of CO2, are required to accumulate the twelve G3P molecules needed to withdraw two G3P (equivalent to six carbons) and combine them into one molecule of the six-carbon sugar glucose, while the remaining ten G3P molecules are used to regenerate the six RuBP molecules consumed. This makes the overall stoichiometry of the Calvin cycle six CO2, eighteen ATP and twelve NADPH consumed for every glucose molecule synthesised.
Two crop plants are grown side by side under the same hot, sunny, water-limited field conditions. Plant P has leaves showing a distinct wreath-like ring of large chloroplast-containing cells around each vein; its measured rate of photosynthesis stays high even as midday temperature rises. Plant Q lacks any such ring; its rate of photosynthesis drops sharply around midday even though light is abundant. (a) Identify the photosynthetic pathway most likely operating in plant P and the anatomical feature responsible for its name. (b) Name the enzyme that performs the initial CO2 fixation in plant P and state one biochemical reason it is more efficient than RuBisCO under these conditions. (c) Explain why plant Q's photosynthetic rate falls at midday in terms of stomatal behaviour and RuBisCO's dual activity. (d) Suggest one reason plant P would be a better choice for cultivation in a hot, water-scarce region.
Answer
(a) Plant P is most likely a C4 plant, and the anatomical feature responsible for the name given to its leaf structure is Kranz anatomy — the wreath-like ring of large, chloroplast-rich bundle sheath cells surrounding each vascular bundle. (b) The initial CO2-fixing enzyme in plant P is PEP carboxylase. It is more efficient under hot, water-limited conditions because it has a very high affinity for CO2 and, unlike RuBisCO, has no affinity for oxygen at all, so it keeps fixing carbon efficiently even when CO2 is scarce inside the leaf. (c) In plant Q, the heat and water scarcity around midday cause the stomata to close partially to reduce water loss, which lowers the internal CO2 concentration while oxygen remains abundant. Under this low CO2, high O2 condition, RuBisCO increasingly acts as an oxygenase rather than a carboxylase, a wasteful process called photorespiration that consumes RuBP and fixed carbon without producing ATP or sugar, so the net photosynthetic rate falls. (d) Plant P would perform better in such a region because its two-step CO2 pump concentrates CO2 around RuBisCO in the bundle sheath cells regardless of how far the stomata are closed, largely avoiding photorespiration and sustaining a higher rate of sugar production even at high temperature and under water stress.
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