Aerobic Respiration

Word equation:

glucose + oxygen → carbon dioxide + water (+ energy)

Balanced symbol equation:

$\text{C}_{6}\text{H}_{12}\text{O}_{6} + 6\text{O}_{2} \to 6\text{CO}_{2} + 6\text{H}_{2}\text{O}$

What's happening? Glucose is OXIDISED — its hydrogens combine with oxygen to make water; its carbons combine with oxygen to make CO₂. The chemical-bond energy in glucose is RELEASED.

Where the energy goes.

• Most is captured as ATP (~30 per glucose).
• Some is released as HEAT (warms the body).

Where it happens.

• Glycolysis (first stage) — in the CYTOPLASM.
• Krebs cycle and electron transport — in MITOCHONDRIA.
• For Cambridge IGCSE: 'mitochondria' is the answer.

Worked qualitative. Why does heavy exercise leave you exhaling lots of CO₂?

• Muscles respire faster → produce CO₂ rapidly.
• Blood carries the CO₂ to lungs.
• Breathing rate rises to expel it.
• Brain detects rising CO₂ in blood → triggers faster breathing.

Cambridge tip. Always BALANCE the equation. The same 6 appears 4 times in the equation: 6 O₂, 6 CO₂, 6 H₂O. Forgetting any one loses a mark.

Aerobic:

• Glucose → CO₂ + H₂O.
• Glucose is COMPLETELY oxidised — no chemical-bond energy left in the products.
• Releases ~30 ATP per glucose.

Anaerobic (animals):

• Glucose → LACTIC ACID.
• Glucose only partly broken down — lactic acid still has chemical energy in its bonds.
• Releases only ~2 ATP per glucose.

Anaerobic (yeast):

• Glucose → ETHANOL + CO₂.
• Same partial breakdown — ethanol still energy-rich.
• ~2 ATP per glucose.

The energy gap: ~15× more from aerobic vs anaerobic. That's why animals fundamentally rely on oxygen.

Worked qualitative. Why can muscles work harder for short bursts than sustained effort?

• Short bursts can use anaerobic respiration → don't need O₂ supply to keep up.
• But anaerobic accumulates lactic acid (toxic) → muscles fatigue + cramp.
• Long sustained effort needs aerobic → demands constant O₂ supply.
• Athletes train to maximise aerobic capacity (more mitochondria, more capillaries, more haemoglobin).

Cambridge tip. Never just say 'aerobic releases more energy'. Cambridge wants the WHY: 'because glucose is completely broken down'.

Structure.

• Sausage-shaped, ~1-10 μm long.
• Surrounded by a DOUBLE MEMBRANE.
• Inner membrane folded into 'CRISTAE' — increases surface area for the enzymes that make ATP.
• Inside the cristae: a fluid called the MATRIX, where Krebs cycle happens.
• Their own DNA and ribosomes (evidence they were once free-living bacteria).

Function.

• Site of AEROBIC RESPIRATION — most stages.
• Convert energy from glucose into ATP.

Numbers vary by cell type:

Cell	Mitochondria	Why
Muscle	Many	Constant contraction needs ATP.
Sperm	Many (in midpiece)	Power swimming.
Liver	Many	Processing chemistry.
Skin (resting)	Few	Low energy demand.
Red blood cell	NONE	Anuclear; only carry O₂; respire anaerobically.

Worked qualitative. Why don't red blood cells have mitochondria?

• They lose their nucleus AND organelles during development.
• More space for haemoglobin.
• They survive on anaerobic respiration (low energy needs — they don't 'do' anything besides carry O₂).
• A nice example of structure-fits-function.

Cambridge tip. When asked WHY a cell has many mitochondria, link it to the cell's high ENERGY DEMAND. 'Has many mitochondria' alone is descriptive, not explanatory.