Photosynthesis & Respiration

In the light-dependent reactions of photosynthesis, what is the immediate source of the electrons that replace those lost from the chlorophyll of photosystem II?

• AReduced NADP
• BThe photolysis of water✓
• CThe breakdown of ATP
• DCarbon dioxide fixation

Answer: Photosystem II loses electrons when light energy excites them; these are replaced by electrons released from the photolysis (light-driven splitting) of water, which also yields protons and oxygen.

During the Calvin cycle (light-independent reactions), which molecule combines with carbon dioxide in the carbon-fixation step catalysed by rubisco?

• AGlycerate 3-phosphate (GP)
• BGlyceraldehyde 3-phosphate (TP)
• CRibulose bisphosphate (RuBP)✓
• DPhosphoglycerate kinase (PGK)

Answer: Rubisco catalyses the combination of CO₂ with the 5-carbon acceptor ribulose bisphosphate (RuBP), producing an unstable 6-carbon compound that splits into two molecules of glycerate 3-phosphate (GP).

In the Calvin cycle, the reduction of glycerate 3-phosphate (GP) to triose phosphate (TP) directly requires which products of the light-dependent reactions?

• AATP and reduced NADP✓
• BATP and oxygen gas
• CReduced NAD and FAD
• DCO₂, water and heat

Answer: GP is reduced to TP using both ATP (as a phosphate and energy source) and reduced NADP (as the source of hydrogen/electrons); both are supplied by the light-dependent reactions.

Where in a eukaryotic cell does the link reaction (conversion of pyruvate to acetyl coenzyme A) take place?

• AThe intermembrane space
• BThe inner mitochondrial membrane
• CThe mitochondrial matrix✓
• DThe cytoplasm

Answer: Pyruvate produced by glycolysis is actively transported into the mitochondrial matrix, where it is decarboxylated and oxidised and the resulting 2-carbon acetyl group is bound to coenzyme A.

In glycolysis, what is the NET yield of ATP and reduced NAD per molecule of glucose?

• A4 ATP and 2 reduced NAD
• B1 ATP and 1 reduced NAD
• C2 ATP and 1 reduced NAD
• D2 ATP and 2 reduced NAD✓

Answer: Glycolysis uses 2 ATP in phosphorylation and generates 4 ATP, giving a net gain of 2 ATP, plus 2 molecules of reduced NAD per glucose.

During oxidative phosphorylation, what is the final acceptor of electrons at the end of the electron transport chain?

• AReduced NAD
• BCarbon dioxide
• CPyruvate
• DOxygen✓

Answer: Oxygen is the terminal electron acceptor; it combines with electrons from the chain and protons to form water. Without oxygen the chain backs up and oxidative phosphorylation halts.

According to the chemiosmotic theory, how does the proton gradient across the inner mitochondrial membrane directly drive ATP synthesis?

• AProtons are pumped across the membrane by ATP synthase using energy from ATP
• BProtons flow back through ATP synthase, and this flow drives the phosphorylation of ADP✓
• CProtons reduce oxygen at the end of the chain, and the energy released is captured directly as ATP
• DProtons combine with electrons in the matrix to form reduced NAD, which phosphorylates ADP directly

Answer: Electron transport pumps protons into the intermembrane space, creating an electrochemical gradient. Protons diffuse back into the matrix through the channel/enzyme ATP synthase, and this proton flow drives the synthesis of ATP from ADP and inorganic phosphate.

In mammalian anaerobic respiration, what is the metabolic role of reducing pyruvate to lactate?

• AIt generates additional ATP directly from pyruvate
• BIt produces ethanol and carbon dioxide, which are excreted
• CIt regenerates oxidised NAD so that glycolysis can continue✓
• DIt restores the oxygen supply to the muscle tissue

Answer: Reducing pyruvate to lactate oxidises reduced NAD back to NAD. This regenerated NAD allows glycolysis to keep producing a small yield of ATP in the absence of oxygen; no extra ATP is made in the reduction step itself.

Where does the light-dependent reaction take place?

• AOn the thylakoid membranes of the chloroplast✓
• BIn the stroma of the chloroplast
• COn the inner membrane of the mitochondrion
• DIn the matrix of the mitochondrion

Answer: The thylakoid membranes hold the photosystems and electron carriers needed to capture light. The light-independent reaction runs in the stroma; the mitochondrial locations belong to respiration.

What is produced by the light-dependent reaction for use in the Calvin cycle?

• AATP and reduced NAD
• BADP and oxidised NADP
• CATP and reduced NADP✓
• DGlucose and oxygen gas

Answer: Photophosphorylation makes ATP and photolysis of water provides electrons and protons to reduce NADP. Both are consumed in the stroma reducing glycerate 3-phosphate. Reduced NAD belongs to respiration.

What is the source of the oxygen released in photosynthesis?

• APhotolysis of carbon dioxide
• BPhotolysis of water✓
• CBreakdown of glucose in the stroma
• DReduction of NADP in the thylakoid

Answer: Light splits water into protons, electrons and oxygen; the oxygen is a by-product and leaves the plant. Isotope labelling of the oxygen in water confirmed this directly.

What does RuBisCO catalyse in the Calvin cycle?

• AThe fixing of carbon dioxide onto ribulose bisphosphate✓
• BThe fixing of carbon dioxide onto glycerate 3-phosphate
• CThe reduction of glycerate 3-phosphate to triose phosphate
• DThe regeneration of ribulose bisphosphate from triose phosphate

Answer: RuBisCO joins CO₂ to the 5-carbon RuBP, giving an unstable 6-carbon compound that splits into two molecules of GP. Reduction of GP and regeneration of RuBP are separate later steps requiring ATP and reduced NADP.

Where does glycolysis occur?

• AIn the matrix of the mitochondrion
• BOn the cristae of the mitochondrion
• CIn the stroma of the chloroplast
• DIn the cytoplasm of the cell✓

Answer: Glycolysis is anaerobic and happens in the cytoplasm, splitting glucose into two pyruvate molecules with a small net ATP yield. Only if oxygen is present does pyruvate enter the mitochondrion.

What is the role of oxygen in aerobic respiration?

• AActing as the initial electron donor to the electron transport chain
• BCombining directly with glucose during the process of glycolysis
• CProviding the phosphate group needed to convert ADP into ATP
• DActing as the final electron acceptor in the electron transport chain✓

Answer: Oxygen accepts electrons at the end of the chain and combines with protons to form water. Without it the carriers stay reduced, the chain halts and ATP synthesis by oxidative phosphorylation stops.

What happens in the link reaction?

• APyruvate is carboxylated and reduced to acetylcoenzyme A
• BGlucose is phosphorylated and split into two triose phosphates
• CAcetylcoenzyme A combines with a four-carbon compound
• DPyruvate is decarboxylated and oxidised to acetylcoenzyme A✓

Answer: Pyruvate entering the mitochondrial matrix loses a carbon as CO₂ and is oxidised, reducing NAD, before joining coenzyme A. The combination with the 4-carbon compound is the first step of the Krebs cycle.

How does chemiosmosis generate ATP?

• AProtons are pumped through ATP synthase against their gradient
• BProtons flow back through ATP synthase down their gradient✓
• CElectrons flow directly through ATP synthase to make ATP
• DATP synthase transfers a phosphate group from glucose to ADP

Answer: Electron carriers pump protons into the intermembrane space, and the protons return through ATP synthase, driving its rotation and the phosphorylation of ADP. Transferring phosphate from a substrate is substrate-level phosphorylation, a different mechanism.

Why does anaerobic respiration yield far less ATP than aerobic respiration?

• AOnly glycolysis operates, and its ATP yield is small✓
• BOnly the Krebs cycle operates, and its ATP yield is small
• CThe electron transport chain runs but produces no ATP
• DGlucose is only partly broken down into carbon dioxide

Answer: Without oxygen the chain stops, so the link reaction and Krebs cycle cannot run and only glycolysis's small net yield of 2 ATP remains. Fermentation exists to regenerate NAD so glycolysis can continue.

A respirometer contains germinating peas and potassium hydroxide, which absorbs carbon dioxide. A second tube is identical but contains glass beads of the same volume as the peas instead of the peas.

What is the purpose of the tube containing glass beads?

• ATo show volume changes caused by temperature or pressure, so they can be corrected for✓
• BTo absorb the carbon dioxide the peas release, so that only oxygen uptake is measured
• CTo supply the peas with oxygen, so that their respiration does not become anaerobic
• DTo act as a heat store, so that the peas are kept at a steady temperature throughout

Answer: The glass beads do not respire, so any movement of the liquid in their tube is caused by changes in temperature or atmospheric pressure. Subtracting this from the movement in the pea tube leaves the change due to oxygen uptake by respiration. Absorbing carbon dioxide is the job of the potassium hydroxide, and the beads neither supply oxygen nor control temperature.

In an investigation into the effect of light intensity on the rate of photosynthesis in pondweed, a lamp is moved from 10 cm to 25 cm from the plant. Light intensity is proportional to 1 ÷ d², where d is the distance from the lamp.

What is the light intensity at 25 cm as a fraction of the intensity at 10 cm?

• A0.40
• B0.064
• C0.16✓
• D6.25

Answer: Relative intensity = (1 ÷ 25²) ÷ (1 ÷ 10²) = 10² ÷ 25² = 100 ÷ 625 = 0.16. The tempting 0.40 is 10 ÷ 25, which treats intensity as inversely proportional to distance rather than its square; 6.25 is the inverse ratio, and 0.064 cubes the ratio of distances.

Rate of oxygen production by a plant (arbitrary units) at 25 °C: Light intensity (arbitrary units): 1, 2, 4, 8 At 0.04% CO₂: 10, 19, 26, 27 At 0.10% CO₂: 10, 20, 37, 52

Which factor limits the rate at light intensity 8 and 0.04% CO₂?

• ALight intensity, as the rate at 0.04% CO₂ is still rising slightly between 4 and 8 units
• BCarbon dioxide concentration, as raising it at that light intensity almost doubles the rate✓
• CTemperature, as the rate at 0.04% CO₂ has levelled off while light intensity increases
• DChlorophyll content, as at low light intensity the rate is the same at both concentrations

Answer: At light intensity 8, increasing CO₂ from 0.04% to 0.10% raises the rate from 27 to 52, so CO₂ is the factor holding the rate back. The tiny rise from 26 to 27 shows light is no longer the main limit. A levelling-off alone does not show temperature is limiting, because raising CO₂ at the same temperature still increases the rate; the equal rates at low light show that light, not chlorophyll, limits there.

The herbicide DCMU blocks the transfer of electrons out of photosystem II in chloroplasts.

What happens to the concentrations of glycerate 3-phosphate (GP) and ribulose bisphosphate (RuBP) in the leaves of a treated plant kept in the light?

• AGP rises and RuBP falls, as GP cannot be reduced without ATP and reduced NADP✓
• BGP falls and RuBP rises, as rubisco can no longer fix carbon dioxide onto RuBP
• CGP and RuBP stay the same, as the Calvin cycle does not use light energy directly
• DGP and RuBP both rise, as photolysis stops and less oxygen competes for rubisco

Answer: With electron flow blocked, the light-dependent reaction makes little ATP or reduced NADP. RuBP still combines with CO₂ to form GP, but GP cannot be reduced to triose phosphate, so GP accumulates; without triose phosphate and ATP, RuBP cannot be regenerated, so it falls. Rubisco still works, so a fall in GP would follow lack of CO₂, not lack of light-dependent products.

If aerobic respiration of one glucose molecule yields 32 molecules of ATP and anaerobic respiration yields 2, how many times more ATP per glucose does aerobic respiration produce?

• A30
• B64
• C16✓
• D34

Answer: 32 ÷ 2 = 16 times as much. The tempting 30 is the difference (32 − 2) rather than the ratio; 64 multiplies the two yields, and 34 adds them. Anaerobic respiration yields so little because only glycolysis produces ATP.