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.
Oxidative phosphorylation
The coenzymes NAD and FAD became reduced in the earlier stages of respiration by accepting hydrogen and oxidising another molecule. Now, 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 carrier protein complexes which are found in the inner mitochondrial membrane. This is called the electron transport chain. Each time an electron passes to the next carrier, it loses some energy. That energy is used to pump H+ ions from the mitochondrial matrix into the intermembrane space against a concentration gradient. This maintains a high 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). The H+ ions diffuse through a membrane-bound enzyme called ATP synthase. This enzyme synthesises ATP from ADP and inorganic phosphate (Pi). It is the diffusion of H+ ions through the 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. Without oxygen, the electron transport chain would grind to a halt and oxidative phosphorylation would not be able to happen. This would mean that oxidised NAD and oxidised FAD would not be not regenerated, and 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 which A-Level biology exam board you are studying!
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.
In glycolysis, the link reaction, and the Krebs cycle we ended up with 10 reduced NAD and 2 reduced FAD being produced from one molecule of glucose. These all go forward into oxidative phosphorylation to provide H+ ions and electrons.
- Edexcel A teaches that each reduced NAD results in 3ATP and each reduced FAD results in 2ATP.
- OCR A and AQA teach that each reduced NAD results in 2.5ATP and each reduced FAD results in 1.5ATP. This is closer to what latest research suggests.
So if you are on Edexcel A, one molecule of glucose results in 38ATP overall. For OCR A or AQA, one molecule of glucose results in 32ATP.
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 move down the electron transport chain which provides energy for transport of H+ ions into the intermembrane space.
- Diffusion of H+ ions through ATP synthase drives synthesis of ATP.
- Oxygen accepts electrons and combines with H+ ions to form water.
Next time we will look at the process of anaerobic respiration.
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