Respiration (B4.2)
What is respiration?
Respiration is the process by which cells release energy from glucose. It is NOT breathing — that is ventilation. Respiration is a series of chemical reactions occurring in every living cell.
Energy released is used for: muscle contraction, active transport, protein synthesis, maintaining body temperature (mammals/birds), cell division.
Aerobic respiration
Requires oxygen. Occurs in mitochondria. Complete oxidation of glucose.
Word: glucose + oxygen → carbon dioxide + water
Symbol: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Releases much more energy than anaerobic — approximately 36–38 ATP per glucose molecule.
Anaerobic respiration
No oxygen available. Occurs in cytoplasm.
In animal cells / bacteria:
glucose → lactic acid
C₆H₁₂O₆ → 2C₃H₆O₃
In yeast (and plant cells):
glucose → ethanol + carbon dioxide
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Releases much less energy — approximately 2 ATP per glucose.
Oxygen debt
During intense exercise, muscles respire anaerobically producing lactic acid. After exercise stops, lactic acid must be broken down — this requires oxygen: the oxygen debt (excess post-exercise oxygen consumption, EPOC). Lactic acid is transported to the liver where it is converted back to glucose.
Response to exercise
During exercise:
- Heart rate and stroke volume increase → cardiac output increases → more O₂ delivered
- Breathing rate and tidal volume increase → more O₂ absorbed, more CO₂ removed
- Blood flow to muscles increases; blood vessels dilate (vasodilation)
- Glycogen in muscles is broken down to glucose for respiration
Metabolism
Metabolism is the sum of all chemical reactions occurring in a cell or organism. This includes:
- Aerobic and anaerobic respiration
- Photosynthesis (in plants)
- Synthesis of large molecules from smaller ones (e.g. amino acids → proteins; glucose → glycogen)
- Breaking down large molecules
Metabolic rate varies with body size, temperature, activity level.
Common exam errors
- Calling aerobic respiration "breathing" — it is a cellular chemical process.
- Forgetting lactic acid is produced in animals during anaerobic; ethanol + CO₂ in yeast.
- Saying muscles "run out of oxygen" rather than that demand exceeds supply.
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