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Respiration is one of the most fundamental processes in biology. Every living cell carries out respiration to release energy from glucose. This lesson covers aerobic and anaerobic respiration, their equations, and the practical investigation of respiration rates.
Respiration is a series of chemical reactions that take place in every living cell to release energy from glucose.
Respiration is NOT breathing.
Respiration happens continuously in every living cell — in animals, plants, fungi and microorganisms. Even when you are asleep, your cells are respiring.
The energy released by respiration is used for all life processes:
| Use of energy | Example |
|---|---|
| Muscle contraction | Movement; heartbeat; breathing |
| Active transport | Absorbing mineral ions in plant roots; absorbing glucose in the small intestine |
| Maintaining body temperature | Mammals and birds keep a constant internal temperature |
| Building larger molecules | Protein synthesis (joining amino acids); making cellulose in plants; making lipids from fatty acids and glycerol |
| Cell division | Growth and repair |
| Nerve impulses | Transmitting electrical signals |
Exam tip: If asked "why do organisms need to respire?", never just say "to get energy." Always give specific examples of what the energy is used for, such as muscle contraction, active transport, or maintaining body temperature.
Aerobic respiration uses oxygen and is the most efficient way to release energy from glucose. It takes place in the mitochondria of cells.
glucose + oxygen → carbon dioxide + water (+ energy released)
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Let us verify the symbol equation is balanced:
| Element | Left side | Right side |
|---|---|---|
| Carbon (C) | 6 (from C₆H₁₂O₆) | 6 (from 6CO₂) ✓ |
| Hydrogen (H) | 12 (from C₆H₁₂O₆) | 12 (from 6H₂O) ✓ |
| Oxygen (O) | 6 (from C₆H₁₂O₆) + 12 (from 6O₂) = 18 | 12 (from 6CO₂) + 6 (from 6H₂O) = 18 ✓ |
Exam tip: You MUST be able to write both the word equation and the balanced symbol equation for aerobic respiration. This is one of the most commonly examined equations in the whole GCSE.
Anaerobic respiration occurs without oxygen. It is less efficient than aerobic respiration but allows cells to continue releasing some energy when oxygen is in short supply.
Word equation:
glucose → lactic acid (+ small amount of energy)
Key points:
Word equation:
glucose → ethanol + carbon dioxide (+ small amount of energy)
Key points:
| Feature | Aerobic | Anaerobic (animals) | Anaerobic (yeast/plants) |
|---|---|---|---|
| Oxygen needed? | Yes | No | No |
| Where in the cell? | Mitochondria | Cytoplasm | Cytoplasm |
| Products | CO₂ + H₂O | Lactic acid | Ethanol + CO₂ |
| Energy released | Large amount | Small amount | Small amount |
| Glucose breakdown | Complete | Incomplete | Incomplete |
After vigorous exercise, you continue to breathe heavily even though you have stopped moving. This is to repay the oxygen debt.
Exam tip: A common 4-mark question asks you to explain why you continue to breathe heavily after exercise. Your answer should include: lactic acid builds up during anaerobic respiration → lactic acid is transported to the liver → oxygen is needed to break down lactic acid → this extra oxygen is the oxygen debt → so breathing rate stays high to take in more oxygen.
A respirometer measures the rate of oxygen consumption (and therefore the rate of aerobic respiration) in living organisms.
Method (using germinating seeds or small invertebrates like woodlice):
Control: Use a tube with dead organisms (e.g. boiled seeds) or glass beads of equivalent volume to account for changes in temperature and pressure.
Variables:
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