5.7.8 Oxidative Phosphorylation

Question 1

What is oxidative phosphorylation and what is the current model?

Answer 1

Oxidative phosphorylation is the last stage of aerobic respiration
It takes place at the inner mitochondrial membrane
It results in the production of many molecules of ATP and the production of water from oxygen
The current model for oxidative phosphorylation is the chemiosmotic theory
The model states that energy from electrons passed through a chain of proteins in the membrane (the electron transport chain) is used to pump protons (hydrogen ions) up their concentration gradient into the intermembrane space
The hydrogens are then allowed to flow by facilitated diffusion through a channel in ATP synthase into the matrix
The energy of the hydrogens flowing down their concentration gradient is harnessed (a bit like water flowing through a hydroelectric dam) resulting in the phosphorylation of ADP into ATP by ATP synthase

Question 2

Outline the process of oxidative phosphorylation.

Answer 2

Hydrogen atoms are donated by reduced NAD (NADH) and reduced FAD (FADH2) from the Krebs Cycle
Hydrogen atoms split into protons (H+ ions) and electrons
The high energy electrons enter the electron transport chain and release energy as they move through the electron transport chain
The released energy is used to transport protons across the inner mitochondrial membrane from the matrix into the intermembrane space
A concentration gradient of protons is established between the intermembrane space and the matrix
The protons return to the matrix via facilitated diffusion through the channel protein ATP synthase
The movement of protons down their concentration gradient provides energy for ATP synthesis
Oxygen acts as the ‘final electron acceptor’ and combines with protons and electrons at the end of the electron transport chain to form water

Question 3

What is the importance of the electron transport chain?

Answer 3

The electron transport chain is made up of a series of membrane proteins/ electron carriers
They are positioned close together which allows the electrons to pass from carrier to carrier
The inner membrane of the mitochondria is impermeable to hydrogen ions so these electron carriers are required to pump the protons across the membrane to establish the concentration gradient

Question 4

Why is oxygen so important to aerobic respiration?

Answer 4

Oxygen acts as the final electron acceptor. Without oxygen the electron transport chain cannot continue as the electrons have nowhere to go. Without oxygen accepting the electrons (and hydrogens) the reduced coenzymes NADH and FADH2 cannot be oxidised to regenerate NAD and FAD, so they can’t be used in further hydrogen transport.