Respiration

Definition (verbatim memorise):

Respiration is the chemical reactions in cells that BREAK DOWN nutrient molecules and RELEASE ENERGY for metabolism.

Three keywords Cambridge wants:

1. Chemical reactions — not just one step; a series controlled by enzymes.
2. In cells — every living cell, not in lungs or blood.
3. Release energy — energy is stored in glucose; respiration unlocks it. (Energy is NOT made.)

Where it happens.

• AEROBIC respiration mainly in MITOCHONDRIA.
• The first step (glycolysis) happens in the cytoplasm.
• Anaerobic respiration: cytoplasm only.

The substrate.

• Mainly GLUCOSE (C₆H₁₂O₆).
• Also FATS (when glucose runs low — long-distance running).
• Also AMINO ACIDS (in starvation — body breaks down its own muscle).

Worked qualitative. Why do muscles have MANY mitochondria?

• Muscles contract often — high energy demand.
• More mitochondria = more aerobic respiration = more ATP.
• Endurance training increases mitochondrial number → muscles use O₂ more efficiently.

Cambridge tip. Memorise the verbatim definition. Cambridge often asks for it directly.

1. Muscle contraction.

• Every movement (heart, gut, limbs, breathing) needs energy.
• Even at rest, the heart, diaphragm, and gut muscles work continuously.

2. Active transport.

• Pumping ions and molecules across membranes against gradients.
• Examples: nerve impulses (sodium-potassium pump), nutrient absorption in the gut, mineral uptake in roots.

3. Building large molecules (anabolism).

• Protein synthesis (joining amino acids).
• DNA replication.
• Making fats and other macromolecules.
• All require energy to FORM bonds.

4. Maintaining body temperature (in mammals/birds).

• We're warm-blooded — must keep ~37°C even in cold.
• Respiration RELEASES some energy as HEAT (not all is captured as ATP).
• Shivering = rapid muscle respiration → more heat.

5. Nerve impulses.

• Each impulse needs energy to reset the membrane potential.
• Brain alone uses ~20% of total body energy.

6. Cell division and growth.

• Mitosis is energy-intensive.
• Growing organisms (children, healing tissue) respire more.

Worked qualitative. Why do you feel TIRED if you don't eat?

• No glucose intake → less respiration possible.
• Body switches to fat (slow) and then protein (slower) breakdown.
• Energy levels drop → you feel weak and slow.

Cambridge tip. When asked 'what is the energy used for?', always give 4 distinct uses with brief explanations. 4 marks = 4 uses + reasons.

These are commonly confused.

	Breathing (ventilation)	Respiration
What	Movement of AIR in and out of lungs.	Chemical reactions inside cells.
Where	Lungs + diaphragm + intercostals.	Every living cell, mainly mitochondria.
Mechanism	Muscular pump.	Enzymatic breakdown of glucose.
Substance	AIR (O₂ in, CO₂ out).	GLUCOSE → energy + waste.
Process type	Mechanical.	Chemical.

Why we breathe IF respiration is the energy bit?

• Aerobic respiration NEEDS O₂.
• Aerobic respiration MAKES CO₂.
• Breathing brings O₂ to the lungs and removes CO₂.
• Without breathing → no O₂ supply → respiration falters → cells starve → death.

Plants?

• Plants RESPIRE (every cell, all the time, day and night).
• They don't 'breathe' the way mammals do — gases just diffuse in/out via stomata and lenticels.
• During the day they ALSO photosynthesise — taking in CO₂, releasing O₂.
• Net gas exchange depends on which process dominates: in the day, photosynthesis usually wins; at night, only respiration happens.

Worked qualitative. Why do plants put oxygen IN the air during the day but take it OUT at night?

• Day: photosynthesis (uses CO₂, releases O₂) > respiration (uses O₂, releases CO₂). Net release of O₂.
• Night: no photosynthesis (no light). Only respiration. Net consumption of O₂.
• Houseplants in bedrooms: minor effect — humans don't suffocate from a few plants.

Cambridge tip. When the question asks about 'breathing', it's the lungs/mechanism. When it asks about 'respiration', it's the cellular reactions. Read carefully.