Aerobic Respiration

Biology: Cambridge International A Level, topic 12.2.3

The Link Reaction

Pyruvate can be used in aerobic respiration by converting it to acetyl coenzyme A. This process is called the link reaction and it takes place in the matrix of the mitochondria.

Decarboxylation

Pyruvate first diffuses from the cytoplasm across the mitochondrial membrane and into the matrix of the mitochondria.
In the mitochondrial matrix, pyruvate is decarboxylated and dehydrogenated to acetate, which is a two-carbon molecule.
CO₂ is produced as a by-product.
NAD is reduced to NADH.

Coenzyme A

Acetate then combines with coenzyme A (CoA) to produce acetyl coenzyme A.
Acetyl coenzyme A is used in the Krebs cycle (the next stage of aerobic respiration).
The link reaction links glycolysis to the Krebs cycle.

Net gain

The net gain from the link reaction is:
1 CO₂ molecule.
1 NADH molecule.

The Krebs Cycle

The Krebs cycle takes place in the matrix of the mitochondria. The products of the cycle are two coenzymes (NADH and FADH₂), ATP and CO₂.

Acetyl coenzyme A (coA)

Acetyl coenzyme A acts as a carrier for the two-carbon acetyl group. It reacts with oxaloacetate (a four-carbon molecule) to produce citrate (a six-carbon molecule).
CoA is now available to be recycled and reused in the link reaction.
The production of citrate allows the Krebs cycle to begin.

6C → 5C

Citrate is converted to a five-carbon molecule (5C) by decarboxylation and dehydrogenation.
CO₂ is produced as a by-product.
NAD is reduced to NADH.

5C → 4C

The five-carbon molecule is decarboxylated and dehydrogenated again to a four-carbon compound.
CO₂ is produced.
NADH are produced.
ATP is also produced by substrate-level phosphorylation.

Regeneration of oxaloacetate

This 4C molecule is then dehydrogenated again to produce another molecule of NADH. FAD is also reduced to FADH₂.
No decarboxylation takes place at this stage.
These intermediate reactions regenerate oxaloacetate. This allows the cycle to continue again.

Net gain

The net gain of the Krebs cycle is:
2 CO₂ molecules.
3 NADH molecules.
1 ATP molecule.
1 FADH₂ molecules.
For each molecule of glucose, there are two cycles (this is because two molecules of pyruvate are produced in glycolysis).

Other respiratory substrates

Fatty acids and amino acids can also be used as respiratory substrates in aerobic respiration.
The substrates are converted to molecules that can easily enter the Krebs cycle.

Oxidative Phosphorylation

Oxidative phosphorylation is the final stage in aerobic respiration.

Inner mitochondrial membrane

Oxidative phosphorylation takes place at the inner mitochondrial membrane.
There are several features of the membrane that allows production of ATP on a large scale:
Three electron carrier proteins (electron transport chain, ETC).
ATP synthase enzyme.
The space between the inner and outer mitochondrial membranes is called the intermembrane space.

Electron transport chain

NADH (from the Krebs cycle) is oxidised by the first electron carrier protein in the inner mitochondrial membrane.
FADH₂ (from the Krebs cycle) is oxidised by the second electron carrier protein in the inner mitochondrial membrane.
This initiates oxidative phosphorylation because NADH and FADH₂ release two protons and two electrons each.
The electrons are then transferred along the ETC.

Proton gradient

As the electrons move down the ETC, they lose energy.
This energy pumps the protons from NADH and FADH₂ into the intermembrane space.
This creates a proton gradient (also known as an electrochemical gradient).

Chemiosmosis

The protons diffuse down the concentration gradient through the ATP synthase enzyme.
As protons flow through the ATP synthase, energy is released.
This energy converts ADP and inorganic phosphate to ATP.
This process is called chemiosmosis.

The final electron acceptor

After the electrons have reached the end of the ETC and protons have flowed through the ATP synthase enzyme, they combine with O₂ to form water (H₂O).
Oxygen is called the final electron acceptor for this reason.

Test yourself on Aerobic Respiration

Where does the link reaction take place?
Which of the following statements about the link reaction are true?
What is the name of the reaction that happens after glycolysis and before the Krebs cycle in aerobic respiration?
Where does the Krebs cycle take place?
Which of the following statements about the Krebs cycle are true?

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