Questions: Mitochondria: Powerhouses of Energy Conversion

Question 1

A metabolic poison makes the inner mitochondrial membrane freely permeable to protons. What is the immediate consequence for ATP production?

• A. The Krebs cycle stops because it requires a sealed inner membrane to function
• B. ATP synthase can no longer use the proton gradient to drive ATP synthesis, so oxidative phosphorylation collapses
• C. NADH cannot be generated because electrons back up in the electron transport chain
• D. Glucose can no longer enter the mitochondrial matrix because the transporter is inactivated

Answer: (B)

ATP synthase is a molecular turbine that harvests the energy of protons flowing down their electrochemical gradient from the intermembrane space into the matrix. If the inner membrane becomes permeable to protons, they flow through the membrane directly rather than through ATP synthase — the gradient is dissipated without doing useful work. This is called 'uncoupling': electron transport can continue (and does, generating heat), but ATP synthesis collapses. Some uncoupling agents are actually used therapeutically and as weight-loss targets because they increase metabolic rate while reducing ATP output.

Question 2

The majority of ATP produced from a single glucose molecule is generated by:

• A. Glycolysis in the cytoplasm, which directly converts glucose into ATP via substrate-level phosphorylation
• B. The Krebs cycle in the mitochondrial matrix through substrate-level phosphorylation
• C. Chemiosmotic coupling at ATP synthase, powered by the proton gradient built by the electron transport chain
• D. Direct phosphorylation of ADP by NADH in the intermembrane space

Answer: (C)

Glycolysis yields 2 net ATP per glucose; the Krebs cycle yields 2 ATP via substrate-level phosphorylation. Both together account for roughly 4 ATP. The electron transport chain uses the NADH and FADH₂ generated by both pathways to pump protons, building a gradient that drives ATP synthase to produce approximately 30–32 additional ATP — roughly 90% of the total. This is why mitochondria are essential for aerobic organisms: anaerobic glycolysis alone produces only ~4 ATP per glucose versus ~36 with oxidative phosphorylation.

Question 3

Most of a cell's ATP from glucose oxidation is generated directly in the Krebs cycle through the same substrate-level phosphorylation mechanism that operates in glycolysis.

• T. True
• F. False

Answer: False

This is a common misconception about where ATP comes from. The Krebs cycle does generate ATP via substrate-level phosphorylation (about 2 per glucose), but its primary output is electron carriers: NADH and FADH₂. These carriers deliver high-energy electrons to the electron transport chain, which uses them to pump protons and drive ATP synthase — the process that generates ~90% of the cell's ATP from glucose. The Krebs cycle is best understood as a 'loading dock' that reduces NAD⁺ and FAD to carry electrons to the ETC, not as the primary ATP-producing step.

Question 4

The extensive folding of the inner mitochondrial membrane into cristae is functionally important: more folds mean more surface area for electron transport chain complexes, enabling greater proton pumping and higher ATP output.

• T. True
• F. False

Answer: True

Cristae are not merely structural features — they directly determine the cell's energy capacity. A mitochondrion in a metabolically active cell (e.g., a cardiac muscle cell or liver hepatocyte) has densely packed cristae, maximizing the surface area available for ETC complexes and ATP synthase. A mitochondrion with sparse cristae produces less ATP. This is why the relationship between mitochondrial architecture and function is a direct one: the physical structure of the membrane is inseparable from the cell's energy-producing capacity.

Question 5

Why is the impermeability of the inner mitochondrial membrane essential to ATP synthesis? What would happen to energy production if the inner membrane freely allowed protons to pass through?

Model answer: The inner membrane's impermeability is what allows the electron transport chain to build a proton gradient — a store of potential energy. When ETC complexes pump protons from the matrix into the intermembrane space, the protons can only re-enter the matrix through ATP synthase (the only proton channel in an intact inner membrane). The energy released as protons flow down their gradient is captured by ATP synthase's rotating mechanism to synthesize ATP. If the membrane were freely permeable to protons, they would equilibrate instantly, eliminating the gradient. ATP synthase would have no driving force, and oxidative phosphorylation would cease — the cell would have to rely on the much less efficient substrate-level phosphorylation of glycolysis and the Krebs cycle alone.

This is why compartmentalization is described as 'inseparable' from function. The mitochondrion's double-membrane architecture isn't incidental — it creates the spatial separation required for chemiosmotic coupling. Some metabolic poisons (like DNP, dinitrophenol) work precisely by making the inner membrane permeable to protons, uncoupling electron transport from ATP synthesis. The electrons keep flowing (generating heat) but ATP synthesis collapses — which is why such compounds have been misused as weight-loss agents despite their lethality.