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CBSE Class 11 Biology · 10 questions · 24 marks
Photosynthesis stores energy in glucose; respiration is how every living cell, day and night, cashes that energy in as usable ATP. This chapter follows glucose through glycolysis in the cytoplasm, then either into fermentation when oxygen is absent or into the mitochondrion for the Krebs cycle and the electron transport chain when it is present, and totals up exactly how much ATP each route yields.
Glycolysis, the breakdown of glucose to pyruvic acid, occurs in the:
Answer
Cytoplasm is correct — glycolysis takes place in the cytosol of every living cell and does not require oxygen, which is why it is the one respiratory step common to both aerobic and anaerobic pathways.
In yeast fermenting under anaerobic conditions, pyruvic acid is ultimately converted to:
Answer
Ethanol and carbon dioxide is correct — yeast first decarboxylates pyruvic acid to acetaldehyde, releasing CO2, and then reduces the acetaldehyde to ethanol using NADH, regenerating NAD+ so glycolysis can continue.
The terminal electron acceptor at the end of the electron transport chain in aerobic respiration is:
Answer
Oxygen is correct — molecular oxygen accepts the electrons at the end of the chain and combines with protons to form water, and it is this final step that makes the whole chain, and therefore ATP synthesis by chemiosmosis, dependent on a continuous oxygen supply.
The respiratory quotient (RQ) for the complete aerobic respiration of a carbohydrate such as glucose is approximately:
Answer
Equal to 1 is correct — because glucose respiration releases one molecule of CO2 for every molecule of O2 consumed, the ratio of CO2 evolved to O2 consumed works out to 1, unlike the respiration of fats, which yields an RQ below 1.
Assertion (A): Anaerobic respiration yields far less ATP per glucose molecule than aerobic respiration. Reason (R): In anaerobic respiration, glucose is only partially oxidised to ethanol or lactic acid, leaving most of the chemical energy of glucose still locked within these end products.
Answer
Both A and R are true and R is the correct explanation of A — because fermentation stops at ethanol or lactic acid rather than continuing to CO2 and water, the bulk of the energy originally stored in glucose is never released for ATP synthesis, which is why fermentation nets only about two ATP compared with roughly 36-38 from complete aerobic breakdown.
Distinguish between substrate-level phosphorylation and oxidative phosphorylation, giving one example of each from cellular respiration.
Answer
Substrate-level phosphorylation is the direct transfer of a phosphate group from a high-energy phosphorylated intermediate to ADP, forming ATP without any involvement of a membrane-based electron transport chain; this happens during glycolysis and also during the Krebs cycle, where succinyl CoA is converted to succinic acid with the direct formation of one GTP (equivalent to ATP). Oxidative phosphorylation, by contrast, is the synthesis of ATP driven indirectly by the flow of electrons down the electron transport chain on the inner mitochondrial membrane, which pumps protons to build a gradient that ATP synthase then uses to make ATP; this accounts for the bulk of ATP generated from the NADH and FADH2 produced throughout aerobic respiration.
What is meant by the statement that respiration is an amphibolic pathway? Give two supporting examples.
Answer
Calling respiration an amphibolic pathway means that it is not purely a catabolic, energy-releasing process but also feeds into anabolic, biosynthetic processes, because several intermediates generated during glycolysis and the Krebs cycle are withdrawn from the pathway to serve as starting materials for building other biomolecules rather than being oxidised all the way to CO2 and water. For example, fatty acids being oxidised for energy release acetyl CoA, which normally enters the Krebs cycle, but the same acetyl CoA pool can equally be diverted to synthesise new fatty acids when the cell needs them. Similarly, intermediates of the Krebs cycle such as alpha-ketoglutaric acid and oxaloacetic acid can be drawn off and converted into amino acids through transamination, linking the respiratory pathway directly to protein synthesis.
Trace the fate of pyruvic acid under aerobic conditions from its entry into the mitochondrion through to the regeneration of oxaloacetic acid in the Krebs cycle.
Answer
When oxygen is available, the pyruvic acid produced by glycolysis in the cytoplasm is transported across both mitochondrial membranes into the matrix. There it is acted upon by the multienzyme pyruvate dehydrogenase complex in a reaction called oxidative decarboxylation: one carbon atom of pyruvic acid is removed as CO2, the remaining two-carbon fragment is oxidised, and the electrons released are accepted by NAD+ to form NADH, while the two-carbon unit is attached to coenzyme A to form the key intermediate acetyl CoA. Since each glucose molecule yields two pyruvic acid molecules, this step happens twice per glucose. Acetyl CoA then enters the Krebs cycle by condensing with the four-carbon compound oxaloacetic acid (OAA) to form the six-carbon citric acid, releasing coenzyme A to be reused. Citric acid is then processed through a series of enzyme-catalysed steps, being isomerised and oxidised in stages: two of these steps release a molecule of CO2 each while transferring electrons to NAD+, and the cycle also includes one step where FAD is reduced to FADH2 and one step of substrate-level phosphorylation that directly produces one molecule of GTP, equivalent to ATP. Each of these transformations gradually shortens and rearranges the carbon skeleton, and by the final step of the cycle, the four-carbon compound oxaloacetic acid is regenerated exactly as it started, ready to combine with another molecule of acetyl CoA and begin the cycle again. Over one full turn, the two carbons brought in by acetyl CoA are balanced by the two carbons released as CO2, so the cycle can continue indefinitely as long as substrate keeps arriving, and the NADH, FADH2 and GTP generated are the true energy payoff of this stage, to be cashed in as ATP during the electron transport chain.
Explain how the electron transport chain and chemiosmosis together account for the bulk of ATP produced during aerobic respiration, and give the approximate total ATP yield from one glucose molecule.
Answer
The electron transport chain is a series of protein complexes embedded in the inner mitochondrial membrane that receives the reduced coenzymes NADH and FADH2 generated during glycolysis, pyruvate oxidation and the Krebs cycle. As each NADH or FADH2 donates its electrons at the start of the chain, the electrons pass from one complex to the next in a sequence of oxidation-reduction reactions, releasing energy in small manageable steps rather than all at once. Several of these complexes use the released energy to actively pump protons from the mitochondrial matrix across the inner membrane into the intermembrane space, building up a much higher proton concentration there than in the matrix, along with an electrical gradient, together called the proton motive force. At the very end of the chain, the electrons, now at a much lower energy level, combine with protons and molecular oxygen to form water, which is why a continuous oxygen supply is essential to keep the whole chain running. The proton gradient built up across the membrane cannot dissipate directly because the membrane is impermeable to protons except through a specific channel-and-enzyme complex called ATP synthase; as protons flow down their gradient back into the matrix through this complex, the enzyme is driven to catalyse the addition of inorganic phosphate to ADP, forming ATP, in the process called chemiosmosis, essentially identical in principle to ATP synthesis in the chloroplast. Because each NADH that feeds into the chain drives the pumping of enough protons to yield roughly 2.5 to 3 ATP, and each FADH2, entering at a later point in the chain, yields somewhat less, roughly 1.5 to 2 ATP, this stage of respiration is responsible for the great majority of the energy captured from one glucose molecule. Adding together the small direct, substrate-level ATP yields of glycolysis and the Krebs cycle with the much larger indirect yield from oxidative phosphorylation of all the NADH and FADH2 produced along the way gives an approximate net total of 36 to 38 ATP molecules per glucose molecule fully oxidised, compared with the mere two ATP obtained from fermentation.
A sprinter runs an all-out 200-metre race. Her leg muscles rapidly deplete their local oxygen supply well before the race ends, yet the muscles keep contracting, and afterward she experiences muscle soreness and heavy breathing that persists for several minutes ('oxygen debt'). (a) Name the anaerobic pathway operating in her leg muscles during the sprint and its end product. (b) Explain, in terms of NAD+ regeneration, why this pathway allows glycolysis to continue despite the oxygen shortage. (c) Give one reason this pathway yields so much less ATP per glucose than the aerobic pathway would. (d) Suggest a biochemical reason for the heavy, sustained breathing she shows immediately after the sprint ends.
Answer
(a) The pathway operating is lactic acid fermentation (anaerobic respiration in muscle), and its end product is lactic acid, formed from pyruvic acid by the enzyme lactate dehydrogenase. (b) Glycolysis requires a continuous supply of NAD+ to accept electrons at one of its oxidation steps. Under low oxygen the electron transport chain cannot reoxidise NADH back to NAD+, so the reduction of pyruvic acid to lactic acid serves to regenerate NAD+ from the NADH produced in glycolysis, allowing glycolysis to keep running and to keep producing its small, direct ATP yield even without oxygen. (c) This pathway yields far less ATP because glucose is only partially broken down to lactic acid rather than being fully oxidised to CO2 and water; the great majority of the chemical energy originally present in glucose remains locked within the lactic acid molecule and is never captured as ATP, unlike in aerobic respiration where the Krebs cycle and electron transport chain extract that remaining energy. (d) The heavy, sustained breathing after the sprint reflects the need to repay an oxygen debt: extra oxygen is required to support the liver and muscles in oxidising the accumulated lactic acid back to pyruvic acid (and much of it eventually back toward glucose or through the Krebs cycle), a process that itself consumes oxygen and can only proceed once oxygen supply is restored.
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