C1.2 Cell respirationIB Biology HL: Revision notes

ATP (adenosine triphosphate) is a nucleotide that distributes energy within cells. It is soluble, cannot cross membranes freely and releases a useful amount of energy on hydrolysis. ATP → ADP + phosphate releases energy; making ATP requires energy from cell respiration.

ATP powers active transport, synthesis of macromolecules and movement (whole cells or chromosomes). Cell respiration uses carbon compounds, mainly glucose and fatty acids. It is not the same as gas exchange, the diffusion of O₂ and CO₂.

Aerobic: glucose + oxygen → carbon dioxide + water; needs mitochondria; high ATP yield. Anaerobic (humans): glucose → lactate; cytoplasm only; low ATP yield; carbohydrate only.

Rate of respiration can be measured as O₂ uptake or CO₂ output per unit time per unit mass. Variables include temperature, substrate and oxygen availability. Use controls such as boiled seeds or glass beads.

Oxidation is loss of electrons; removing hydrogen (with its electron) from a substrate, dehydrogenation, oxidises it. NAD accepts the hydrogen and is reduced. Redox reactions always pair oxidation with reduction.

Glycolysis (cytoplasm) converts glucose to two pyruvate in steps, each catalysed by a different enzyme:

1. phosphorylation of glucose using 2 ATP;
2. lysis into two 3C sugars;
3. oxidation, reducing NAD;
4. ATP formation, 4 ATP.

Net yield: 2 ATP and 2 reduced NAD per glucose.

Without oxygen, NAD must be regenerated from reduced NAD, or glycolysis stops. Pyruvate accepts the hydrogen:

• Humans: pyruvate → lactate.
• Yeast: pyruvate → ethanol + carbon dioxide.

The pathways are the same up to pyruvate; only NAD regeneration and the final products differ. Net yield is 2 ATP per glucose. In baking, CO₂ makes dough rise and ethanol evaporates during baking; in brewing, ethanol is the product wanted.

In the link reaction (matrix), pyruvate is oxidised (reducing NAD) and decarboxylated (CO₂ released), forming a 2C acetyl group, which coenzyme A carries into the Krebs cycle as acetyl-CoA.

In the Krebs cycle, the acetyl group joins oxaloacetate (4C) to form citrate (6C). Two decarboxylations and four oxidations (dehydrogenations) regenerate oxaloacetate, producing CO₂, reduced NAD and ATP.

Reduced NAD from glycolysis, the link reaction and the Krebs cycle passes a pair of electrons to the first carrier of the electron transport chain on the inner mitochondrial membrane, becoming NAD again.

Energy released as electrons flow along the chain pumps protons from the matrix into the intermembrane space, creating a proton gradient. In chemiosmosis, protons flow back through ATP synthase, which couples this energy to the phosphorylation of ADP.

Oxygen is the terminal electron acceptor: it takes electrons from the chain and protons from the matrix, forming metabolic water, so electrons keep flowing.

Lipids release more energy per gram than carbohydrates because they contain less oxygen and more oxidisable hydrogen and carbon.

Glycolysis and anaerobic respiration occur only with carbohydrate. Fatty acids are broken into 2C acetyl groups that enter the Krebs cycle via acetyl-CoA, so lipids can only be respired aerobically.