1. Breakdown of Glucose

• Food is used by cells to release energy.
• First step of respiration:
  • Glucose (6-carbon compound) → Pyruvate (3-carbon compound)
  • Occurs in the cytoplasm.

2. Fate of Pyruvate

(A) Aerobic Respiration (Presence of Oxygen)

• Occurs in mitochondria.
• Pyruvate → Carbon dioxide + Water + Large amount of energy.
• More energy is released than in anaerobic respiration.

(B) Anaerobic Respiration in Yeast

• Occurs in absence of oxygen.
• Pyruvate → Ethanol + Carbon dioxide + Energy.
• Process is called fermentation.

(C) Anaerobic Respiration in Muscles

• Occurs during oxygen deficiency.
• Pyruvate → Lactic acid + Energy.
• Accumulation of lactic acid causes muscle cramps.

3. Gas Exchange in Plants

• Takes place through stomata.
• Large intercellular spaces help diffusion of gases.
• Exchange occurs by diffusion.

At Night

• Photosynthesis stops.
• CO₂ release is the major activity.

During Day

• CO₂ produced in respiration is used in photosynthesis.
• Oxygen release becomes the major event.

4. Respiration in Animals

• Animals have specialized organs for gas exchange.
• Oxygen is taken in and carbon dioxide is removed.

5. Aquatic Animals

• Use oxygen dissolved in water.
• Dissolved oxygen is much less than atmospheric oxygen.
• Therefore, breathing rate is faster.
• Fishes take water through mouth and pass it over gills.

6. Terrestrial Animals

• Use atmospheric oxygen.
• Respiratory surfaces:
  • Have large surface area.
  • Are thin and delicate.
  • Are kept inside the body for protection.
• Air passages carry air to respiratory surfaces.