A-Level Biology AQA Notes

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Anaerobic Respiration

Respiration is the process, which occurs in living cells, that releases energy stored in organic molecules such as glucose.
The energy released during respiration is used to synthesise molecules of ATP, which can be used as an immediate source of energy
The first stage of respiration is glycolysis which occurs in the cytoplasm of cells.
- There is a net yield of 2 pyruvate, 2 reduced NAD and 2 ATP molecules

If oxygen is not available as the final electron acceptor, glycolysis can continue in anaerobic respiration
Glycolysis can continue if reduce NAD is reoxidised so that NAD is available to accept a hydrogen atom again
In mammals, the lactate fermentation pathway is used:

Lactate can be converted to glycogen in the liver or oxidised further to release energy, when oxygen is available.

In plants and fungi, the ethanol fermentation pathway is used:
- pyruvate + reduced NAD → ethanol + carbon dioxide + oxidised NAD

Aerobic Respiration

If respiration is aerobic, pyruvate enters the mitochondrial matrix by active transport
Next, the link reaction occurs

The final stage of aerobic respiration is oxidative phosphorylation,
Reduced NAD and FAD donate electrons to the electron transfer chain in the inner mitochondrial membrane. The release of energy as the electrons pass down the electron transfer chain is used to create a proton gradient across the inner mitochondrial membrane into the inter-membranal space. The proton gradient is used to synthesis ATP by oxidative phosphorylation, catalysed by ATP synthase (chemiosmotic theory).
Oxygen combines with the protons that have diffused through the ATP synthase channel and the electrons that have been passed along the electron transfer chain, acting as the final electron acceptor. It helps maintain the proton gradient for the electron transfer chain to continue

½ O2 + 2e- + 2H+ → H2O

Aerobic respiration produces 32 ATP. 30 more than anaerobic respiration.
Sugars such as glucose are not the only substances that can be used as a respiratory substrate.
Lipids release more energy than carbohydrates, due to more carbon-hydrogen bonds

Substrate	Process in respiration
Lipid	Hydrolysed to fatty acids and glycerol. Glycerol is phosphorylated and converted to triose phosphate, which enters the glycolysis pathway The fatty acid part is broken down into 2-carbon fragments which are subsequently converted into acetyl CoA, also generating reduce NAD & FAD
Protein	Protein is hydrolysed to amino acids. In the liver, the amino group is removed (deamination), and the amino group is converted to urea and removed in the urine. The remaining amino acid can then be converted to an intermediate

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