What is respiration?
Respiration is the process in which food (mainly glucose) is broken down inside cells to release energy. It is not the same as breathing: breathing only moves air in and out; respiration happens inside every living cell.
The energy is stored in a molecule called ATP (adenosine triphosphate), the cell's energy currency. When ATP breaks into ADP, energy is released to contract muscles, make proteins or carry nerve signals.
Breakdown of glucose: aerobic and anaerobic respiration
Step 1 (same for all): in the cytoplasm, glucose (6 carbon) → 2 pyruvate (3 carbon each) + a little energy.
- Aerobic (with oxygen), in mitochondria: pyruvate → carbon dioxide + water + a lot of energy. Most plants and animals.
- Anaerobic in yeast (no oxygen): pyruvate → ethanol + carbon dioxide + little energy. This is fermentation.
- Lack of oxygen in our muscles: pyruvate → lactic acid + little energy. Lactic acid build-up causes cramps.
| Aerobic | Anaerobic |
|---|---|
| Needs oxygen | No oxygen |
| Cytoplasm + mitochondria | Cytoplasm only |
| Full breakdown → CO₂ + H₂O | Partial → ethanol + CO₂, or lactic acid |
| Much energy (≈ 38 ATP per glucose) | Little energy (2 ATP) |
The human respiratory system
- Nostrils and nasal passage: fine hairs and mucus filter dust and germs.
- Pharynx and larynx (voice box).
- Trachea (windpipe): rings of cartilage stop it from collapsing when air pressure is low.
- Bronchi → bronchioles → alveoli: tubes branch smaller and smaller and end in balloon-like sacs.
- Diaphragm and rib muscles: breathing in, ribs lift and the diaphragm flattens, the chest cavity gets bigger and air rushes in; breathing out, they relax and air is pushed out.
The lungs always hold some air (residual volume), so there is time for oxygen and CO₂ to be exchanged.
Alveoli and transport of gases
The walls of alveoli are one cell thin and covered with capillaries. If all the alveoli of an adult were spread out, they would cover roughly 80 m², about the size of a badminton court. This huge area makes gas exchange fast.
Haemoglobin, the red pigment in red blood cells, has a strong liking for oxygen and carries it to the tissues. Carbon dioxide dissolves in water, so it is carried mostly in dissolved form in blood plasma.
Why do land animals breathe slower than fish? Water holds far less dissolved oxygen than air, so fish must pass water over their gills quickly; land animals get plenty of oxygen from air.
Respiration in plants and exam focus
Plants exchange gases by diffusion through stomata in leaves and lenticels in stems and roots. At night they only respire, so they give out CO₂. In daylight photosynthesis is faster, so the net gas they give out is oxygen.
Exam tip: the flow chart of glucose breakdown (three pathways), aerobic vs anaerobic table, role of alveoli and cartilage rings, and 'why aquatic animals breathe faster' come up almost every year.
Key formulas and definitions
- Glucose → (cytoplasm) → pyruvate + little energy
- Pyruvate + O₂ → (mitochondria) → CO₂ + H₂O + much energy
- Pyruvate → (yeast, no O₂) → ethanol + CO₂ + energy
- Pyruvate → (muscle, low O₂) → lactic acid + energy
- ATP: energy currency of the cell (ATP → ADP + energy)
- Air path: nostrils → pharynx → larynx → trachea → bronchi → bronchioles → alveoli
Worked examples
1. Why does idli batter rise when left overnight?
Yeast and bacteria in the batter respire anaerobically. Yeast turns sugar into ethanol and carbon dioxide; the CO₂ gas forms bubbles that make the batter rise and become soft.
2. Why does the trachea not collapse when we breathe in?
Its wall has C-shaped rings of cartilage that keep it open even when the air pressure inside drops.
3. A runner gets leg cramps after a sprint. Explain.
During a fast sprint, oxygen supply to the leg muscles falls short. The muscle cells switch to anaerobic respiration and pyruvate is turned into lactic acid. The build-up of lactic acid causes cramps; it goes away with rest as oxygen returns.
4. How is the lung designed to increase the area for gas exchange?
The air tubes branch again and again and end in millions of alveoli. Their total surface is about 80 m²; the walls are very thin and covered by capillaries, so gases diffuse quickly.
5. Why do aquatic animals breathe much faster than land animals?
The amount of oxygen dissolved in water is very low compared with the oxygen in air. To get enough, fish must move water over their gills rapidly.
Common mistakes
- Treating breathing and respiration as the same thing. Breathing is air movement; respiration is chemical breakdown of food in cells.
- Saying anaerobic respiration happens in mitochondria. It happens only in the cytoplasm.
- Writing that CO₂ is carried mainly by haemoglobin. It is carried mostly dissolved in plasma.
- Thinking plants breathe in CO₂ for respiration. For respiration plants also take in O₂ and give out CO₂.