What you'll learn
Respiration is one of the seven characteristics of all living things, and in CSEC Biology it is examined alongside gas exchange because the two are linked: respiration uses oxygen and produces carbon dioxide, and gas exchange is how those gases move between an organism and its environment. This topic covers aerobic and anaerobic respiration and the equations for each, why aerobic respiration yields more energy, the adaptations of gas exchange surfaces, breathing in humans, the composition of inhaled and exhaled air, the standard laboratory investigations, and the relationship between respiration and photosynthesis.
Keep two ideas firmly separate: respiration is a chemical reaction inside cells, while gas exchange and breathing are the physical movement of gases. Confusing them is the commonest reason candidates lose marks in this topic.
Key terms and definitions
Respiration — the release of energy from glucose inside living cells.
Aerobic respiration — respiration using oxygen, releasing a large amount of energy.
Anaerobic respiration — respiration without oxygen, releasing far less energy; gives lactic acid in animals and ethanol plus carbon dioxide in yeast.
Mitochondria — the organelles where most aerobic respiration takes place.
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 accumulated lactic acid.
Fermentation — anaerobic respiration in yeast, producing ethanol and carbon dioxide.
Compensation point — the light intensity at which a plant's photosynthesis and respiration proceed at the same rate.
Exothermic — describing a reaction that releases heat.
Core concepts
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.
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. Cells that are very active — muscle, liver, sperm — contain many of them.
Anaerobic respiration releases energy without oxygen and yields far less 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 remains locked in the lactic acid or ethanol — which is why those products still contain usable energy, and why lactic acid can be broken down later when oxygen becomes available. This is a favourite comparison question: always state both the oxygen point and the complete-versus-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.
Each adaptation exists because of the same underlying rule: diffusion is faster across a larger area, over a shorter distance, and down a steeper gradient. If a question asks you to explain an adaptation rather than list it, give the diffusion reason.
Gas exchange in plants
Plants have no lungs and no transport system for gases, so every part must exchange gases with the air around it. Stomata, mostly on the lower surface of the leaf, open to let carbon dioxide diffuse in and oxygen diffuse out during photosynthesis, and they also allow the gases of respiration to move in the opposite directions. Each stoma is opened and closed by a pair of guard cells, which close it when the plant is short of water to reduce loss by transpiration — a compromise, since closing the stomata also slows gas exchange.
Inside the leaf, the spongy mesophyll has large air spaces between loosely packed cells, giving a very large moist internal surface across which gases diffuse. Woody stems exchange gases through small pores called lenticels.
Breathing in humans
Breathing, or ventilation, moves air in and out of the lungs.
Inhalation: the diaphragm contracts and flattens, and the external intercostal muscles contract to lift the ribs up and out. The volume of the chest increases, the pressure inside falls below atmospheric pressure, and air moves in.
Exhalation: the diaphragm and intercostal muscles 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 then carried by the blood: oxygen mainly as oxyhaemoglobin in the red blood cells, carbon dioxide mostly dissolved in the plasma. Because the blood flowing past the alveoli constantly removes oxygen and delivers carbon dioxide, the concentration gradients are maintained and diffusion continues.
Comparing inhaled and exhaled air
| Gas | Inhaled | Exhaled |
|---|---|---|
| Oxygen | about 21% | about 16% |
| Carbon dioxide | about 0.04% | about 4% |
| Water vapour | variable | saturated |
| Temperature | room temperature | warmer, at body temperature |
Exhaled air contains less oxygen and more carbon dioxide because of respiration in the body's cells. Note that a good deal of oxygen remains in exhaled air — which is why rescue breathing works.
Investigating respiration in the laboratory
Showing that living things release carbon dioxide: breathe out, or pass air from germinating seeds or small animals, through limewater; it turns milky, showing carbon dioxide is produced. A control of fresh air leaves the limewater clear.
Showing that respiration releases heat: place germinating seeds in a vacuum flask with a thermometer, and dead, boiled and sterilised seeds in an identical flask as a control. The temperature rises only in the flask of living seeds, because respiration is exothermic. Sterilising the dead seeds matters, because otherwise microorganisms respiring on them would raise the temperature and spoil the comparison.
Showing that oxygen is used up: germinating seeds sealed in a tube with soda lime, to absorb the carbon dioxide produced, cause a coloured liquid to move towards the seeds, because the oxygen used is not replaced by an equal volume of carbon dioxide.
In each case the control 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 on its own.
Respiration and photosynthesis — the gas balance
| 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, the compensation point, photosynthesis and respiration proceed at exactly the same rate, so there is no net exchange of gases with the surroundings. Below it, in dim light or darkness, the plant gives out net carbon dioxide; above it the plant takes in net carbon dioxide. Being able to explain the compensation point is a reliable way to reach the top marks.
Worked examples
Example 1. A student runs hard for two minutes and afterwards continues to breathe deeply for several minutes. Explain why.
During hard exercise the muscles cannot receive oxygen fast enough, so they respire anaerobically, producing lactic acid. This creates an oxygen debt. Continued deep breathing supplies the extra oxygen needed to break the lactic acid down, which is why heavy breathing persists after the exercise has stopped.
Example 2. Germinating seeds in a vacuum flask cause the temperature to rise, but boiled seeds in an identical flask do not. Explain both observations and state why the boiled seeds are sterilised.
The germinating seeds are alive and respiring, and respiration is exothermic, so the energy released as heat raises the temperature. The boiled seeds are dead and cannot respire, so no heat is released and the temperature does not rise. They are sterilised so that microorganisms growing on them cannot respire and produce heat, which would otherwise make the control behave like the experiment.
Example 3. Explain why exhaled air contains about 16% oxygen rather than none at all.
Only a proportion of the oxygen in each breath diffuses into the blood, because diffusion depends on the concentration gradient and stops being effective once the concentrations approach each other. Air also remains in the airways without reaching the alveoli. The oxygen not absorbed is breathed out again.
Example 4. A plant is kept in dim light. Explain the net movement of carbon dioxide.
Both respiration and photosynthesis occur, but in dim light the rate of photosynthesis is lower than the rate of respiration. More carbon dioxide is therefore produced by respiration than is used in photosynthesis, so there is a net release of carbon dioxide from the plant.
Common mistakes and how to avoid them
Writing "breathing" when the question asks about respiration. Respiration is the chemical reaction releasing energy; breathing is the muscle-driven movement of air. Check which word the question uses.
Saying the lungs "make" oxygen. They do not. They are an exchange surface across which oxygen already in the air diffuses into the blood.
Forgetting that plants respire all the time. Plants respire day and night. In bright light photosynthesis is faster, so there is net uptake of carbon dioxide; at night only respiration occurs.
Leaving out "energy" or "diffusion". Both words carry marks. Respiration releases energy; gases move by diffusion down a concentration gradient.
Saying anaerobic respiration releases no energy. It releases some, just much less, because the glucose is only partly broken down.
Describing inhalation as the lungs pulling air in. The lungs have no muscle of their own. The diaphragm and intercostal muscles change the chest volume, which changes the pressure, and air moves down the pressure gradient.
Exam technique for Respiration and Gas Exchange
Learn both equations, in words and in symbols, and be ready to write either. Marks are given for the correct reactants and products, so an equation missing water or with oxygen on the wrong side loses them.
For breathing questions, answer in the sequence muscles → volume → pressure → air movement. Skipping the pressure step is the commonest omission, and it is usually a marked point.
When comparing aerobic with anaerobic respiration, or respiration with photosynthesis, write comparative sentences rather than two separate lists, and make sure every point covers both sides.
For practical questions, expect to identify the control and say what it shows. Where soda lime or limewater is used, be able to say what each is for.
Finally, read whether a question asks about a human, a fish or a plant. The four adaptations of a gas exchange surface are the same in each, but the named structures are not.
Quick revision summary
- Respiration releases energy from glucose in all living cells, continuously.
- Aerobic: glucose + oxygen → carbon dioxide + water; a large energy yield, mostly in the mitochondria.
- Anaerobic in muscle: glucose → lactic acid, causing fatigue and an oxygen debt. In yeast: glucose → ethanol + carbon dioxide (fermentation).
- Aerobic releases more energy because glucose is broken down completely.
- Gas exchange surfaces are large, thin, moist and well supplied with blood or air — all to speed diffusion.
- Plants exchange gases through stomata and lenticels, across the moist surfaces of the spongy mesophyll.
- Inhalation: diaphragm and intercostals contract, volume up, pressure down, air in. Exhalation: the reverse.
- Oxygen is carried as oxyhaemoglobin; carbon dioxide mostly dissolved in the plasma.
- Exhaled air: less O₂ (about 16%), more CO₂ (about 4%), saturated with water vapour, warmer.
- Limewater detects carbon dioxide; soda lime absorbs it; a control with dead, sterilised seeds proves the effect is due to living tissue.
- At the compensation point photosynthesis and respiration proceed at equal rates, so there is no net gas exchange.