Respiration is one of the seven characteristics of all living things. In CSEC Biology it is examined alongside gas exchange, because the two processes are linked: respiration uses oxygen and produces carbon dioxide, and gas exchange is how those gases move between an organism and its environment. It is vital to keep the two ideas separate in your mind — respiration is a chemical reaction inside cells, while gas exchange (and breathing) is the physical movement of gases. Confusing them is one of the commonest reasons candidates lose marks.
What respiration is
Respiration is the controlled release of energy from food (chiefly glucose) inside every living cell. The energy released is used for muscle contraction, active transport, growth, cell division, and keeping warm in mammals and birds. It happens continuously, day and night, in plants and animals alike.
There are two types.
Aerobic respiration uses oxygen and releases a large amount of energy:
glucose + oxygen → carbon dioxide + water (+ energy) C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Most of this takes place in the mitochondria, the organelles often called the "powerhouses" of the cell. Cells that are very active — muscle, liver, sperm — contain many mitochondria.
Anaerobic respiration releases energy without oxygen and yields far less energy from each glucose molecule.
- In human muscle during hard exercise: glucose → lactic acid (+ a little energy). The lactic acid builds up, causing fatigue and an oxygen debt that must be "repaid" by deep breathing afterwards.
- In yeast and plants: glucose → ethanol + carbon dioxide (+ a little energy). This is fermentation, used in baking and brewing.
Why aerobic respiration releases more energy
Because oxygen allows glucose to be broken down completely to carbon dioxide and water. In anaerobic respiration the glucose is only partly broken down, so most of the energy stays locked in the lactic acid or ethanol. This is a favourite comparison question — always state both the oxygen point and the "complete vs incomplete breakdown" point.
Gas exchange surfaces
For respiration to continue, oxygen must reach the cells and carbon dioxide must be removed. This happens across gas exchange surfaces, which share four adaptations you should be able to list:
- Large surface area — more room for diffusion.
- Thin (often one cell thick) — a short diffusion distance.
- Moist — gases dissolve before diffusing.
- Good blood (or air) supply — maintains a steep concentration gradient.
In humans the surface is the alveoli of the lungs; in fish it is the gill filaments and lamellae; in plants it is the spongy mesophyll of the leaf, with gases entering and leaving through the stomata.
Breathing in humans
Breathing (ventilation) moves air in and out of the lungs.
- Inhalation: the diaphragm contracts and flattens, the external intercostal muscles contract and lift the ribs up and out. The volume of the chest increases, pressure inside falls, and air is pushed in.
- Exhalation: the diaphragm and intercostals relax, the ribs move down and in, chest volume decreases, pressure rises, and air is forced out.
At the alveoli, oxygen diffuses from the air into the blood (where its concentration is lower) and carbon dioxide diffuses from the blood into the air. The gases are carried by the blood: oxygen mainly as oxyhaemoglobin in red blood cells, carbon dioxide mostly dissolved in the plasma.
Comparing inhaled and exhaled air
| Gas | Inhaled | Exhaled |
|---|---|---|
| Oxygen | ~21% | ~16% |
| Carbon dioxide | ~0.04% | ~4% |
| Water vapour | variable | saturated |
| Temperature | room | warmer (body temp) |
Exhaled air contains less oxygen and more carbon dioxide because of respiration in the body's cells.
Investigating respiration in the laboratory
CSEC examiners often set practical questions on respiration, so know these classic experiments:
- Showing living things release carbon dioxide: breathe out (or pass air from germinating seeds/small animals) through limewater; it turns milky, showing CO₂ is produced. A control of fresh air leaves the limewater clear.
- Showing respiration releases heat: place germinating seeds in a vacuum flask with a thermometer, and dead (boiled, sterilised) seeds in an identical flask as a control. The temperature rises only in the flask of living seeds, because respiration is exothermic.
- Showing oxygen is used up: germinating seeds sealed in a tube with soda lime (to absorb CO₂) cause a coloured liquid to move towards the seeds, because the oxygen used is not replaced.
In each case the control (boiled seeds or fresh air) is essential — it shows the result is due to living, respiring tissue and not some other factor. Naming and explaining the control is frequently worth a mark.
Respiration and photosynthesis — the gas balance
A common higher-order question asks you to compare the two processes. They are almost opposites:
| Feature | Respiration | Photosynthesis |
|---|---|---|
| When | all the time | only in light |
| Where | all living cells (mitochondria) | green cells (chloroplasts) |
| Gases | uses O₂, releases CO₂ | uses CO₂, releases O₂ |
| Energy | releases energy | stores energy |
In a green plant both happen at once in daylight. At a certain light intensity, called the compensation point, photosynthesis and respiration occur at exactly the same rate, so there is no net exchange of gases with the surroundings. Below it (dim light or darkness) the plant gives out net CO₂; above it the plant takes in net CO₂. Being able to explain the compensation point shows real understanding and is a reliable way to pick up the top marks.
Common exam mistakes
- Writing "breathing" when the question asks about "respiration." Respiration is the chemical reaction; breathing is the muscle-driven movement of air.
- Saying the lungs "make" oxygen — they do not; they are simply the exchange surface.
- Forgetting that plants respire all the time, even when they are photosynthesising. In bright light photosynthesis is faster than respiration, so a plant takes in net CO₂ and releases net O₂; at night only respiration occurs.
- Leaving out "energy" or the word "diffusion" when it would earn a mark.
Key terms to remember
- Respiration — the release of energy from glucose inside living cells.
- Aerobic respiration — respiration using oxygen; releases a lot of energy.
- Anaerobic respiration — respiration without oxygen; releases little energy (gives lactic acid in animals, ethanol + CO₂ in yeast).
- Mitochondria — the cell organelles where most aerobic respiration occurs.
- Gas exchange — the movement of oxygen in and carbon dioxide out across a surface, by diffusion.
- Ventilation (breathing) — the muscle-driven movement of air in and out of the lungs.
- Oxygen debt — the extra oxygen needed after hard exercise to break down lactic acid.
- Compensation point — the light intensity at which a plant's photosynthesis and respiration occur at the same rate.
Quick recap
- Respiration releases energy from glucose in all living cells; aerobic uses oxygen and releases more, anaerobic does not and releases less.
- Aerobic: glucose + oxygen → carbon dioxide + water; anaerobic gives lactic acid (animals) or ethanol + CO₂ (yeast/plants).
- Gas exchange surfaces are large, thin, moist and well supplied with blood or air.
- Breathing changes chest volume and pressure to move air; gases then move by diffusion down their concentration gradients.
- Exhaled air has less O₂ and more CO₂ than inhaled air.