Energy and Respiration: Question 4

Worked solution

Part (a): Building and using the proton gradient

Reduced NAD and reduced FAD, formed earlier in glycolysis, the link reaction and the Krebs cycle, deliver hydrogen atoms to the start of the electron transport chain. Each hydrogen atom splits into a proton (H⁺) and an energetic electron.

• The electrons pass along a series of carrier proteins embedded in the inner mitochondrial membrane, forming the electron transport chain. As electrons move from one carrier to the next, they release energy in a series of small steps (details of the individual carriers are not needed).
• This released energy is used to actively transfer protons from the mitochondrial matrix across the inner membrane into the intermembrane space, so that protons build up to a much higher concentration there than in the matrix. This creates a proton (electrochemical) gradient across the inner membrane.
• Protons cannot easily cross the inner membrane on their own, but can move back down their concentration gradient, from the intermembrane space into the matrix, by facilitated diffusion through ATP synthase, an enzyme (and channel protein) spanning the inner membrane.
• As protons flow through it, ATP synthase uses the energy released to synthesise ATP from ADP and inorganic phosphate. This overall coupling of the proton gradient to ATP synthesis is called chemiosmosis.

Part (b): The role of oxygen

At the very end of the electron transport chain, oxygen acts as the final electron acceptor: it combines with the electrons that have passed along the chain and with protons from the matrix to form water.

If no oxygen were available:

• Electrons reaching the end of the chain would have nowhere to go, so the final carrier (and, in turn, each carrier before it) would remain reduced and unable to accept further electrons. The whole electron transport chain would back up and stop.
• With the chain stalled, no further energy would be released to pump protons into the intermembrane space, so the proton gradient could not be maintained.
• Without a proton gradient, protons would no longer flow through ATP synthase, so ATP could no longer be synthesised by oxidative phosphorylation. Even though the link reaction and Krebs cycle could briefly continue, they too would soon stall as reduced NAD and FAD would no longer be reoxidised.

Part (c): Mitochondrion structure and function

A mitochondrion’s structure closely matches its role in aerobic respiration:

• The inner membrane is folded into cristae, greatly increasing its surface area. This provides much more space to hold electron transport chain carriers and ATP synthase molecules, allowing far more oxidative phosphorylation to occur than a flat, unfolded membrane would permit.
• The matrix contains the enzymes required for the link reaction and the Krebs cycle, as well as the mitochondrion’s own DNA and ribosomes, allowing these reactions to take place close to the inner membrane, so that the reduced NAD and FAD they produce are readily available to the electron transport chain.
• The inner membrane is largely impermeable to protons, except through ATP synthase. This allows the proton gradient generated by the electron transport chain to build up and be maintained across the membrane, rather than protons leaking straight back into the matrix, which would prevent chemiosmosis from working.

Final answers

• (a) Reduced NAD/FAD release H atoms that split into protons and electrons; electrons pass along the electron transport chain, releasing energy used to pump protons into the intermembrane space; protons diffuse back through ATP synthase (chemiosmosis), driving ATP synthesis.
• (b) Oxygen is the final electron acceptor, forming water; without oxygen, the chain backs up, no proton gradient is maintained, and oxidative phosphorylation stops.
• (c) Cristae increase inner membrane surface area for the electron transport chain and ATP synthase; the matrix holds the link reaction/Krebs cycle enzymes; the inner membrane is impermeable to protons except via ATP synthase, maintaining the gradient for chemiosmosis.