Aerobic respiration: glycolysis and the Krebs cycleEdexcel A-Level Biology A: Revision notes

Aerobic respiration splits the respiratory substrate (glucose) to release carbon dioxide as a waste product, and reunites the hydrogen with atmospheric oxygen to form water, releasing a large amount of energy used to make ATP:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$

It is a many-stepped process. Each step is catalysed by a specific intracellular enzyme, so energy is released in small, controlled amounts rather than as heat all at once.

Glycolysis occurs in the cytoplasm and needs no oxygen:

1. Phosphorylation: two ATP are used to add phosphate groups to glucose (a hexose), making it more reactive.
2. The hexose bisphosphate splits into two triose phosphate molecules.
3. Each triose phosphate is oxidised (loses hydrogen, accepted by NAD to give reduced NAD) and converted to pyruvate, making four ATP.

Net gain per glucose: 2 ATP, 2 reduced NAD, 2 pyruvate.

Without oxygen, reduced NAD cannot be reoxidised at the electron transport chain. In animals pyruvate therefore accepts hydrogen from reduced NAD and becomes lactate, which regenerates NAD so glycolysis can continue making a little ATP.

Lactate diffuses into the blood and is carried to the liver. There it is converted back to pyruvate, which is oxidised in aerobic respiration, or to glycogen. This needs oxygen, which is why breathing stays deep after exercise (repaying the oxygen debt).

Pyruvate enters the mitochondrial matrix (by active transport) where it is decarboxylated (CO₂ removed) and dehydrogenated (hydrogen passed to NAD) to form acetate. The acetate combines with coenzyme A to form acetyl CoA.

Per glucose, the link reaction gives 2 CO₂ and 2 reduced NAD.

Acetyl CoA (2C) combines with a 4C compound to form a 6C compound. Two decarboxylations release 2 CO₂ and dehydrogenations produce 3 reduced NAD and 1 reduced FAD. One ATP is made by substrate-level phosphorylation and the 4C compound is regenerated.

Per glucose (two turns): 4 CO₂, 6 reduced NAD, 2 reduced FAD, 2 ATP.

Why in mitochondria: the enzymes are in the matrix, next to the cristae where reduced coenzymes are reoxidised, and the double membrane keeps the conditions and substrates concentrated. Glycolysis needs no oxygen or mitochondria, so it occurs in the cytoplasm.

A respirometer measures oxygen uptake. Living organisms such as germinating seeds, small invertebrates or yeast are placed in a tube with soda lime (or potassium hydroxide) to absorb the CO₂ produced. As oxygen is used, the pressure falls and coloured liquid in a capillary tube moves towards the organisms. Rate = distance moved (or volume) ÷ time.

Control temperature with a water bath, keep the mass of organism constant, and use a second tube containing glass beads as a control. Repeat to find a mean. An alternative for yeast is to time the decolourisation of methylene blue or DCPIP, which are decolourised as the cells give up hydrogen.