ZIMSEC O Level · Biology · Form 3, Form 4
Respiration
Respiration releases energy from glucose in all living cells. Energy is used for movement, active transport, growth, keeping warm and building molecules.
- Aerobic: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (in mitochondria).
- Anaerobic in muscles: glucose → lactic acid + little energy. This causes an oxygen debt that is repaid by deep breathing after exercise.
- Anaerobic in yeast: glucose → ethanol + carbon dioxide (fermentation, used in brewing and baking).
Gas exchange in humans
Air passes through the nose, trachea, bronchi and bronchioles to the alveoli.
Alveoli are adapted by: a huge surface area, walls one cell thick, a moist lining, and a rich capillary supply that keeps the concentration gradient steep.
Breathing
- Inhaling: intercostal muscles contract, ribs move up and out, diaphragm contracts and flattens, chest volume increases, pressure falls, air rushes in.
- Exhaling: the reverse.
Effects of smoking
Tar causes cancer; nicotine is addictive; carbon monoxide reduces oxygen carried by blood; smoke damages cilia, leading to bronchitis and emphysema.
Exam tip: Never write that respiration “produces” energy – energy is released.
Check yourself
- Write the balanced equation for aerobic respiration.
- Give two ways alveoli are adapted for gas exchange.
- Explain why a runner keeps breathing heavily after a race.
Show answers
- C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy)
- Any two: large surface area, thin walls, moist, good blood supply.
- To repay the oxygen debt by breaking down the lactic acid built up during anaerobic respiration.
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