MCAT Content / Oxidative Phosphorylation / Electron Transport Chain And Oxidative Phosphorylation Substrates And Products General Features Of The Pathway

Electron transport chain and oxidative phosphorylation, substrates and products, general features of the pathway

Topic: Oxidative Phosphorylation

Oxidative phosphorylation, incorporating two interdependent processes – the flow of electrons through electron transport chain down to the oxygen and chemiosmotic coupling -, is the final stage of cellular respiration. 

Highly energetic electrons that are extracted during the decomposition of food molecules by cellular metabolic pathways are stored in electron carriers – NADH and FADH2. Energy stored in these molecules is converted into cellular energy currency (i.e. ATP) only via oxidative phosphorylationActually, most of the ATP production during cellular respiration happens here. Take a look at the diagram below that shows the interconnectedness between cellular energy pathways.

In the mitochondrial matrix, the electron carriers NADH and FADH2 deposit the electrons they gained from glycolysis and the citric acid cycle in the electron transport chain – a series of proteins embedded in the inner mitochondrial membrane. In the electron transport chain, electrons are passed from one molecule to another as a series of redox reactions. The energetically “downhill” movement of electrons through the chain causes pumping of protons into the intermembrane space by the first, third, and fourth complexes. As a result, the electrochemical gradient (i.e. proton gradient) forms across the inner membrane of mitochondria.

This gradient is used by chemiosmotic coupling to make ATP through phosphorylation of ADP. In this process, protons flow down their concentration gradient into the matrix through the membrane protein ATP synthase, causing it to spin (like a water wheel) and catalyze conversion of ADP to ATP. Take a look at the diagram below which summarizes oxidative phosphorylation.

You may wonder where the oxygen takes role in cellular “respiration”? As electrons complete their flow through electron transport chain elements, oxygen accepts them. Also taking up protons from the environment, oxygen forms water in this process. If oxygen isn’t there to accept electrons (for instance, because a person is not breathing in enough oxygen), the electron transport chain will stop running, and ATP will no longer be produced by chemiosmosis. Without enough ATP, cells can’t carry out the reactions they need to function, and, after a long enough period of time, may even die.

Finally, let’s overview the substrates and products of oxidative phosphorylation. NADH, FADH2, oxygen (O2), ADP, Pi (phosphate) are used as substrates. On the other hand ATP, NAD+, FAD and H2O are produced as a result of oxidative phosphorylation reactions.


Practice Questions


Khan Academy   

Regulating electron carrier molecules

Studying metabolism with galvanic cells

Inhibiting the electron transport chain


MCAT Official Prep (AAMC)

Biology Question Pack, Vol. 1 Passage 13 Question 85

Biology Question Pack, Vol 2. Question 18

Section Bank B/B Section Question 71

Section Bank B/B Section Passage 12 Question 95

Section Bank B/B Section Question 97


Key Points

• Reduced electron carriers (NADH and FADH2) from other steps of cellular respiration transfer their electrons to molecules near the beginning of the electron transport chain. In the process, they turn back into NAD+ and FAD, which can be reused in other steps of cellular respiration.

• As electrons are passed down the electron transport chain, they move from a higher to a lower energy level, releasing energy. Some of the energy is used to pump hydrogen ions, moving them out of the mitochondrial matrix and into the intermembrane space. This pumping establishes an electrochemical gradient.

• At the end of the electron transport chain, electrons are transferred to molecular oxygen, which splits in half and takes up hydrogen ions to form water.

• As hydrogen ions flow down their gradient and back into the matrix, they pass through an enzyme called ATP synthase, which harnesses the flow of protons to synthesize ATP.

Key Terms

Oxidative phosphorylation: The final stage of cellular respiration where the combined action of the electron transport chain and chemiosmotic coupling result in ATP production.

Electron transport chain: a series of proteins embedded in the inner mitochondrial membrane that takes up highly energetic electrons from NADH and FADHand uses the energy to form proton gradient across the membrane.

Chemiosmotic coupling: A process in which the movement of protons across the mitochondrial membrane downward their gradient is coupled to ATP production via phosphorylation of ADP. 

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