Respiration — releasing energy from glucose

Respiration is NOT the same as breathing. Respiration is a chemical process that occurs in every living cell, releasing energy from glucose. Breathing is the mechanical process of moving air in and out of the lungs.

Aerobic respiration

Aerobic = with oxygen. This is the more efficient form, releasing the maximum energy from glucose.

Word equation: Glucose + Oxygen → Carbon dioxide + Water (+ energy)

Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

This occurs mainly in the mitochondria. The energy released is captured as ATP.

Uses of energy from respiration:

• Muscle contraction (movement)
• Active transport of molecules across membranes
• Protein synthesis (building molecules)
• Cell division
• Maintaining body temperature (in mammals and birds)
• Active ion uptake in plant roots

Anaerobic respiration

Anaerobic = without oxygen. Occurs when oxygen supply is insufficient (e.g., intense exercise, when oxygen cannot be delivered to muscles fast enough).

In animals/humans — produces lactic acid: Word equation: Glucose → Lactic acid (+ small amount of energy)

Lactic acid builds up in muscles, causing fatigue and the "burning" sensation. It is toxic in large amounts. After exercise, extra oxygen is needed to break down lactic acid — this is the oxygen debt (or EPOC — excess post-exercise oxygen consumption).

In plants and yeast — produces ethanol and CO₂: Word equation: Glucose → Ethanol + Carbon dioxide (+ small amount of energy)

This is fermentation — the basis of bread-making (CO₂ makes bread rise) and alcohol production (ethanol).

Comparing aerobic and anaerobic

Feature	Aerobic	Anaerobic
Oxygen needed?	Yes	No
Energy produced	Much (38 ATP per glucose)	Little (2 ATP per glucose)
Products	CO₂ + H₂O	Lactic acid (animals) OR ethanol + CO₂ (plants/yeast)
Location	Mitochondria	Cytoplasm
Duration possible	Sustained	Short bursts only

Gas exchange in humans

Oxygen moves from the lungs (high concentration) into the blood, and CO₂ moves from the blood (high concentration) into the lungs — by diffusion.

The alveoli (air sacs in the lungs) are highly adapted for gas exchange:

• Very large surface area (approximately 70 m² — the size of a tennis court)
• Very thin walls (one cell thick) → short diffusion distance
• Rich blood supply (capillary network) → maintains concentration gradient
• Moist surface → gases dissolve for transport

Gas exchange in plants

Plants exchange gases through stomata (pores mainly on the underside of leaves), controlled by guard cells.

• During daylight: CO₂ in (for photosynthesis), O₂ out.
• At night: O₂ in (for respiration), CO₂ out.