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)
• BTriose phosphate (TP)
• CRibulose bisphosphate (RuBP)✓
• DPhosphoglycerate kinase

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
• CReduced NAD and FAD
• DCO₂ and water

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 by ATP synthase using energy from ATP
• BProtons flow back through ATP synthase, and this flow drives the phosphorylation of ADP✓
• CProtons reduce oxygen to release energy that is captured as ATP
• DProtons combine with electrons in the matrix to form reduced NAD

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 for excretion
• CIt regenerates oxidised NAD so that glycolysis can continue✓
• DIt restores oxygen levels in 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.