Aerobic and anaerobic respiration
What is respiration?
is a chemical reaction that happens continuously in all living cells, releasing energy from glucose. It is an reaction — it transfers energy to the environment.
Organisms need this energy for:
- chemical reactions that build larger molecules
- movement (muscle contraction)
- keeping warm
Respiration can be (using oxygen) or (without oxygen). Respiration is not the same as breathing.
Key terms
Aerobic respiration
uses oxygen. It mostly takes place in the , and releases a lot of energy because the glucose is completely oxidised.
Word equation:
glucose + oxygen → carbon dioxide + water
Symbol equation:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Notice that this is the reverse of the photosynthesis equation.
Key terms
Anaerobic respiration in muscles
When there isn't enough oxygen — for example, during hard exercise — muscle cells respire .
Word equation:
glucose → lactic acid
Symbol equation:
C₆H₁₂O₆ → 2C₃H₆O₃
The glucose is only partly (incompletely) oxidised, so much less energy is released than in aerobic respiration.
Key terms
Anaerobic respiration in plants and yeast
In plant cells and yeast cells, anaerobic respiration produces ethanol and carbon dioxide instead of lactic acid.
Word equation:
glucose → ethanol + carbon dioxide
Symbol equation:
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
In yeast, this process is called .
Key terms
Uses of fermentation
Fermentation by yeast is economically important:
- Bread-making — the carbon dioxide produced makes the dough rise. The ethanol evaporates during baking.
- Alcoholic drinks — the produced is the alcohol in beer and wine.
Comparing aerobic and anaerobic respiration
- Oxygen: aerobic needs oxygen; anaerobic doesn't.
- Products: aerobic makes carbon dioxide and water; anaerobic makes lactic acid (animals) or ethanol and carbon dioxide (plants and yeast).
- Energy released: aerobic releases much more, because glucose is completely oxidised; anaerobic releases less, because oxidation is incomplete.