GCSE Biology Respiration revision

First, kill the common confusion: respiration is not breathing. Breathing moves air; respiration is the chemical reaction inside cells that transfers energy from glucose. It runs in every living cell, all the time, and it is exothermic — it transfers energy to the surroundings, warming you up.

Aerobic respiration uses oxygen: glucose + oxygen → carbon dioxide + water. It happens inside the mitochondria, the organelles often called the cell’s power stations. The energy transferred is not a vague “effort” — it powers the cell’s chemistry: building larger molecules, contracting muscles, keeping warm, and driving active transport.

Working muscles need more glucose and more oxygen, and they need it delivered. The body answers with three coordinated increases: heart rate rises so blood is pumped faster, breathing rate rises so oxygen enters faster, and breath volume rises so each breath carries more air. Together they boost the supply of oxygenated blood to the muscles.

Muscles also keep a private fuel store: glycogen, a glucose polymer packed into muscle cells. When exercise starts, glycogen is converted back to glucose to feed respiration without waiting for the blood supply.

Vigorous exercise can outstrip even this. When oxygen delivery cannot match demand, muscle cells switch to anaerobic respiration — and that switch has a cost.

When oxygen runs short, muscle cells respire anaerobically: glucose is broken down without oxygen into lactic acid — glucose → lactic acid. Because the glucose is only partially broken down, much less energy is transferred per glucose molecule than in aerobic respiration. It is emergency power: fast, but inefficient.

Lactic acid accumulates in the muscles and causes fatigue — the burning ache at the end of a sprint. It also creates oxygen debt: the amount of extra oxygen the body needs after exercise to break down the lactic acid that built up. That is why you keep breathing deeply for minutes after stopping.

(Higher) The repayment has a route: blood carries lactic acid from the muscles to the liver, where it is oxidised to carbon dioxide and water using the extra oxygen. Breathing and heart rate stay elevated until the debt is cleared — the recovery period is the repayment schedule.

Plant cells and yeast can also respire without oxygen: glucose → ethanol + carbon dioxide. In yeast this is fermentation, and it is economically huge. In bread-making, the carbon dioxide bubbles expand in the dough and raise it; the ethanol evaporates in the oven. In brewing, the ethanol is the point — the drink and its strength.

In waterlogged root cells, the same pathway runs involuntarily and damages the plant — ethanol is toxic when it accumulates. The lesson for answers: name the products, then attach the application (bread rises, beer ferments).

Metabolism is the sum of all the reactions happening in a cell or in the body. The energy transferred by respiration is the payroll that funds it. Metabolic reactions come in two kinds:

Note the last row: protein is not stored. Eat more than you need and the surplus amino acids are stripped of their nitrogen in the liver and the nitrogen leaves as urea. Metabolism is a network — respiration simply pays the bills.