Respiration — releasing energy from glucose
Respiration is a continual chemical reaction in every living cell. It releases energy from glucose to drive other cellular processes. Don't confuse respiration with breathing — breathing is gas exchange; respiration is the chemistry inside cells.
Aerobic respiration
Uses oxygen and gives the most energy:
glucose + oxygen → carbon dioxide + water C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
It happens mainly in mitochondria. The reaction is exothermic — releases energy for things like:
- muscle contraction (movement)
- maintaining body temperature in mammals/birds
- protein synthesis from amino acids
- active transport across membranes
Anaerobic respiration in muscles
When exercising hard, lungs and circulation can't supply enough oxygen. Muscles switch (partially) to anaerobic respiration:
glucose → lactic acid
Less energy released; glucose is only partly oxidised. Lactic acid builds up, causing fatigue (the burning feeling) and an oxygen debt.
Anaerobic respiration in plants and yeast (fermentation)
glucose → ethanol + carbon dioxide
This is fermentation. Yeast use it to make alcohol (brewing, baking — CO₂ raises bread).
Response of body to exercise
During exercise:
- Heart rate rises (more blood flow to muscles)
- Breathing rate / depth rises (more O₂ in, more CO₂ out)
- Glycogen stores in muscles are broken down to glucose
If exercise is hard or long, anaerobic respiration kicks in. The result is the oxygen debt — the extra oxygen needed after exercise to oxidise lactic acid back to CO₂ and water:
lactic acid + O₂ → CO₂ + H₂O
This is why you keep panting after a sprint.
Metabolism
Metabolism is the sum of all the chemical reactions in a cell or the body. It includes:
- Reactions that build up (anabolism): making proteins from amino acids, starch from glucose, lipids from fatty acids and glycerol
- Reactions that break down (catabolism): respiration, digestion, breakdown of excess proteins to urea (in the liver, see B4.3)
The rate of metabolism is set by genes, fitness, age, and crucially body size and activity.
Required practical hints
Investigating respiration rate in seeds: a sealed container with germinating peas has its O₂ removed (taken up by respiration) and CO₂ produced is absorbed by soda lime. The pressure drop is measured with a coloured-fluid manometer or a syringe.
⚠Common mistakes— Common mistakes / exam traps
- "Respiration is breathing" — wrong; respiration is the cellular chemistry; breathing is mechanical.
- "Anaerobic respiration in plants makes lactic acid" — no; in plants and yeast, it makes ethanol + CO₂.
- Saying anaerobic respiration is "more efficient" because it's used in emergencies — actually it's less efficient; it just provides ATP quickly.
- Forgetting that even at rest, cells respire — only stops at death.
Links
Aerobic respiration uses the products of B4.1 (photosynthesis). Glucose handling links to B5.7 (insulin) and the liver to B4.3 (metabolism).
AI-generated · claude-opus-4-7 · v3-deep-biology