Interaction and interdependence practice questions.

In aerobic respiration, what is the final electron acceptor of the electron transport chain?

• Carbon dioxide
• Oxygen
• NAD+
• Pyruvate

Answer: Oxygen

Oxygen accepts electrons at the end of the chain and combines with protons to form water. Without oxygen, the chain backs up and NADH cannot be reoxidised, halting oxidative phosphorylation.

How many molecules of carbon dioxide are released per glucose from the link reaction and Krebs cycle combined?

• 2
• 4
• 6
• 8

Answer: 6

The link reaction releases 2 CO2 per glucose (one per pyruvate) and the Krebs cycle releases 4 CO2 (two per turn, two turns), giving 6 CO2 in total.

In a chloroplast, where is ATP synthase located and where is ATP produced during the light-dependent reactions?

• Outer membrane; produced in the stroma
• Thylakoid membrane; produced on the stromal side
• Inner membrane; produced in the intermembrane space
• Stroma; produced in the thylakoid lumen

Answer: Thylakoid membrane; produced on the stromal side

ATP synthase spans the thylakoid membrane. Protons accumulate in the thylakoid lumen and flow out through ATP synthase, so ATP is synthesised on the stromal side where it is later used in the Calvin cycle.

What is the main product of non-cyclic photophosphorylation, besides ATP, that is used in the Calvin cycle?

• FADH2
• NADPH
• Oxygen
• Glucose

Answer: NADPH

Non-cyclic electron flow reduces NADP+ to NADPH at photosystem I. NADPH, together with ATP, provides the reducing power and energy needed to convert glycerate-3-phosphate to triose phosphate in the Calvin cycle.

The 'induced fit' model of enzyme action states that:

• The active site is a rigid shape exactly matching the substrate
• The substrate changes to fit the active site permanently
• The active site changes shape slightly as the substrate binds
• Enzymes are used up during the reaction

Answer: The active site changes shape slightly as the substrate binds

In the induced fit model the active site is flexible and moulds around the substrate as it binds, straining bonds and lowering activation energy, unlike the older rigid lock-and-key idea.

In end-product inhibition of a metabolic pathway, the final product usually acts as a:

• Competitive inhibitor of the last enzyme
• Allosteric inhibitor of an early enzyme
• Coenzyme for the first enzyme
• Substrate for the first enzyme

Answer: Allosteric inhibitor of an early enzyme

The end product binds allosterically to an enzyme early in the pathway, changing its shape and slowing production. This negative feedback prevents wasteful overproduction of the product.