Respiration
Respiration is the exothermic process that transfers energy from glucose in every living cell. Learn the difference between aerobic and anaerobic respiration.
What is respiration?
Respiration is an exothermic reaction that takes place continuously in every living cell. It transfers energy from glucose to power life processes such as movement, growth, and maintaining body temperature.
Respiration is not the same as breathing. Breathing is the mechanical process of moving air in and out of the lungs. Respiration is the chemical reaction that happens inside cells.
Respiration is an exothermic reaction that transfers energy from glucose, which is used for all living processes. It occurs continuously in every living cell.
Aerobic respiration
Aerobic respiration uses oxygen and is the most efficient way of transferring energy from glucose. It takes place in the mitochondria of cells.
Word equation
glucose + oxygen → carbon dioxide + water
Balanced symbol equation
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Aerobic respiration transfers much more energy per glucose molecule than anaerobic respiration, which is why organisms preferentially use aerobic respiration when oxygen is available.
Anaerobic respiration
Anaerobic respiration occurs when oxygen is not available — for example, during intense exercise when muscles cannot get oxygen fast enough.
In animals
glucose → lactic acid
Lactic acid builds up in muscles and causes fatigue and cramp. After exercise, the body needs extra oxygen to break down the lactic acid — this is called the oxygen debt.
In plants and yeast (fermentation)
glucose → ethanol + carbon dioxide
This process is used in brewing (making alcohol) and baking (the CO₂ makes bread rise).
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen needed? | Yes | No |
| Where it happens | Mitochondria | Cytoplasm |
| Products (animals) | CO₂ + water | Lactic acid |
| Products (yeast) | CO₂ + water | Ethanol + CO₂ |
| Energy transferred | More (complete breakdown) | Less (incomplete breakdown) |
Exercise and respiration
During exercise, your body needs to transfer more energy to your muscles. Several changes happen:
- Heart rate increases — pumps blood faster to deliver more oxygen and glucose
- Breathing rate and depth increase — takes in more oxygen and removes more CO₂
- Glycogen is converted to glucose — stored glycogen in muscles is broken down
If the muscles cannot get oxygen fast enough, they switch to anaerobic respiration, producing lactic acid. After exercise, you continue to breathe heavily to repay the oxygen debt — the extra oxygen needed to break down accumulated lactic acid in the liver.
The equations are essentially reversed, but they are not truly 'opposite' processes. Photosynthesis stores energy in glucose (endothermic), while respiration releases energy from glucose (exothermic). They also occur in different organelles — chloroplasts vs mitochondria.
Every cell in your body needs a continuous supply of energy for life processes — maintaining body temperature, muscle contraction, nerve impulses, protein synthesis, cell division, and active transport. If respiration stopped, cells would die very quickly.
Test yourself on respiration
Use our free quiz to check your understanding of aerobic and anaerobic respiration.
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