Oxidative Phosphorylation

We’ve reached the final stage of aerobic respiration – oxidative phosphorylation. This step takes place using proteins in the inner mitochondrial membrane. Now we will find out what all the reduced NAD and reduced FAD produced in glycolysis, the link reaction, and the Krebs cycle is used for.

The coenzymes NAD and FAD became reduced in the earlier stages of respiration by accepting hydrogen and oxidising another molecule. Now, at the inner mitochondrial membrane, reduced NAD and reduced FAD release hydrogen atoms. These split into a H⁺ ions (protons) and electrons (e^–), both of which have an important role. The electrons get passed down a series of electron carriers which are found in the inner mitochondrial membrane. This is called the electron transport chain (or electron transfer chain depending on your specification). Each time an electron passes to the next carrier, it releases some energy. That energy is used to pump H⁺ ions from the mitochondrial matrix into the intermembrane space. This maintains a higher concentration of H⁺ ions in the intermembrane space relative to the mitochondrial matrix, so they can diffuse back down that concentration gradient (called an electrochemical gradient because the ions have a charge). The H⁺ ions diffuse through a membrane-bound enzyme called ATP synthase. This enzyme synthesises ATP from ADP and inorganic phosphate (P_i). It is the diffusion of H⁺ ions through the ATP synthase enzyme which drives the synthesis of ATP, and this process is called chemiosmosis.

We are left with an electron that has come off the end of the electron transport chain. It combines with a H⁺ ion and oxygen to form water (a waste product) in the following equation:

½O₂ + H⁺ + e^– → H₂O

So it’s actually not until the very final stage of aerobic respiration that oxygen is used. Oxygen is said to be the final electron acceptor. This is probably one of the biggest key phrases in the whole of A-level biology. Without oxygen, the electron transport chain would grind to a halt because the final electron carrier protein would have nowhere to pass its electron, meaning it cannot accept another electron from the previous protein. Ultimately this means that reduced NAD and reduced FAD would not be able to pass electrons to the electron transport chain, and oxidised NAD and oxidised FAD would not be not regenerated. Consequently the earlier stages of aerobic respiration would also grind to a halt as there would be no NAD or FAD to accept hydrogen and oxidise the carbon-containing compounds. This is how metabolic poisons which target the electron transport chain prevent the cell from producing enough ATP to function. There is a way to allow glycolysis to continue for short amounts of time, and that is to start carrying out anaerobic respiration, which we will look at in the next article.

How much ATP is produced from one molecule of glucose in aerobic respiration?

This is a question which has different answers depending on what you read, but let’s look at an estimation.

The consistent part is that 2ATP are produced in glycolysis and 2ATP are produced in the Krebs cycle (per molecule of glucose). The inconsistent part is how many ATP molecules are produced from each reduced NAD or reduced FAD molecule entering oxidative phosphorylation. However we can say that approximately 28ATP are produced in oxidative phosphorylation, meaning that overall 32ATP are produced from one molecule of glucose in aerobic respiration.

Summary

Oxidative phosphorylation gets its name from using oxygen in a process to phosphorylate ADP to ATP. Here is a summary of the process:

Oxidative phosphorylation happens at the inner mitochondrial membrane.
Reduced NAD and reduced FAD revert to their oxidised form and release hydrogen, which splits into H⁺ ions and electrons.
Electrons are passed down the electron transport chain which provides energy to pump H⁺ ions into the intermembrane space.
Diffusion of H⁺ions through ATP synthase drives synthesis of ATP.
Oxygen accepts electrons and H⁺ ions to form water. Oxygen is the final electron acceptor.