C1.2 Cell respiration

Every active process in a cell — building molecules, pumping ions, contracting muscle — is paid for in the same currency: ATP. Cell respiration is the controlled release of energy from organic compounds to make that ATP, and it goes on continuously in every living cell. For C1.2 the essential story is how glucose is broken down step by step, why oxygen makes such a dramatic difference to the energy yield, and how the structure of the mitochondrion is suited to its role. Keep ATP at the centre of your thinking and the topic stays coherent.

ATP: the energy currency of the cell

Cell respiration is the controlled release of energy from organic compounds (chiefly glucose) to produce ATP (adenosine triphosphate). ATP is described as the cell’s energy currency because it is the immediate, usable source of energy for cellular work and can be quickly made and broken down again and again.

Energy is stored in the bond to ATP’s third phosphate group. When ATP is hydrolysed to ADP + P_i, a useful amount of energy is released to drive processes such as active transport, synthesis and movement. The ADP is then re-phosphorylated back to ATP using energy from respiration, so the molecule is constantly recycled. ATP is suited to this role because it releases a manageable quantity of energy in a single step and is soluble, so it moves easily within the cell.

Anaerobic respiration

Respiration can occur with or without oxygen. Anaerobic respiration releases energy from glucose without using oxygen. It yields only a small amount of ATP, but it is rapid and allows ATP production to continue when oxygen is scarce. The pathway differs between organisms:

• In humans (and other animals), glucose is converted to lactate (lactic acid). This supplies extra ATP during intense exercise when the lungs and circulation cannot deliver oxygen fast enough; the lactate is later broken down when oxygen is again available.
• In yeast and plants, glucose is converted to ethanol and carbon dioxide. This alcoholic fermentation is the basis of brewing and of the rising of bread dough.

Both pathways take place in the cytoplasm and regenerate the molecules needed to keep the first stage of respiration running, even though the energy yield is low.

Aerobic respiration and the role of oxygen

Aerobic respiration uses oxygen to break glucose down completely to carbon dioxide and water, releasing far more energy and so yielding much more ATP per glucose molecule than anaerobic respiration. The overall word equation is:

glucose + oxygen → carbon dioxide + water (+ energy as ATP)

The large difference in yield is a favourite exam point: aerobic respiration produces a great deal more ATP per glucose than anaerobic respiration, because the glucose is oxidised fully rather than only partly. Oxygen is essential as the final acceptor that allows this complete breakdown to proceed; without it, the later stages cannot continue and the cell must fall back on the low-yield anaerobic pathways.

Glycolysis and the mitochondrion

All respiration begins with glycolysis (literally sugar splitting), which takes place in the cytoplasm and does not require oxygen. In glycolysis a glucose molecule is partially broken down into smaller molecules (pyruvate), producing a small net gain of ATP. Because it needs no oxygen, glycolysis is common to both aerobic and anaerobic respiration — a useful unifying point.

If oxygen is present, the products of glycolysis pass into the mitochondrion, where the oxygen-requiring stages occur and most of the ATP is generated. The mitochondrion’s structure suits this function: it has a double membrane, and the highly folded inner membrane forms cristae that greatly increase the surface area available for the reactions of aerobic respiration, while the fluid matrix inside holds the enzymes for earlier aerobic steps. More active cells, such as muscle and liver cells, contain many mitochondria to meet their high ATP demand.