Respiration and Excretion
What this domain tests
Respiration and Excretion covers energy release from food, gas exchange, breathing, the kidney and removal of waste. Questions often compare aerobic and anaerobic pathways and trace urine formation.
Core rules
- Aerobic respiration: glucose + oxygen → carbon dioxide + water + energy. It releases much more energy per glucose molecule than anaerobic respiration.
- Anaerobic respiration in muscles: glucose → lactic acid + small energy. In yeast: glucose → ethanol + carbon dioxide + small energy.
- Breathing is mechanical ventilation; respiration is a chemical process in cells. Do not use the terms interchangeably.
- Alveoli have a large surface area, thin walls, moist lining and rich blood supply for gas exchange.
- The kidney filters blood at the glomerulus, reabsorbs useful substances such as glucose and some water and ions in the tubule, and excretes urea and excess salts in urine.
- ADH increases water reabsorption, producing concentrated urine; low ADH produces dilute urine. Dialysis removes urea and excess ions by diffusion across a partially permeable membrane.
Worked example
During sprinting, a runner's muscle oxygen supply is insufficient. Glucose is broken down anaerobically to lactic acid, releasing a small amount of energy. The lactic acid lowers pH and contributes to muscle fatigue. After exercise, extra oxygen is needed to oxidise the lactic acid. In yeast, the same shortage of oxygen leads to ethanol and carbon dioxide, not lactic acid.
Traps
Do not write that anaerobic respiration produces no energy. It produces less energy. Do not say the kidney reabsorbs urea; urea is excreted. In the nephron, filtration is by pressure, while reabsorption uses diffusion and active transport. Plants excrete oxygen and carbon dioxide through stomata and lose water vapour.