Breathing moves air, gas exchange moves gases across a surface by diffusion, and respiration releases energy from glucose inside cells. Three different processes, in three different places. Questions often test whether you can tell which one a statement describes.
This lesson follows alveolar structure and diffusion and sets up the equations in comparing aerobic and anaerobic models.
What does each term mean?
| Term | What it is | Where it happens |
|---|---|---|
| Breathing (ventilation) | Air moves in and out by muscle action | Thorax and airways |
| Gas exchange | Oxygen and carbon dioxide diffuse across a surface | Alveoli and capillaries, and also at body cells |
| Respiration | Chemical reactions that release energy from glucose | Inside cells |
Gas exchange is a physical process, since nothing new is made. Respiration is chemical, since glucose and oxygen are changed into other substances.
Energy is released in respiration, not made or used up. The energy is used for muscle contraction, growth and keeping warm, among other things.
How do you keep them apart in an answer?
- Ask “is something moving, or is something being broken down?”
- If air is moving in or out, write breathing or ventilation.
- If gases are crossing a surface down a gradient, write gas exchange by diffusion.
- If glucose is being broken down to release energy, write respiration.
- Name the place: lungs, alveoli, blood or cells.
Worked example
Follow one oxygen molecule from a breath in a student’s lungs to a muscle cell, and name the process at each step. All of this is a simplified model.
Step 1: the molecule is in the air drawn into the lungs when the student inhales. This is ventilation.
Step 2: it diffuses from the air in the alveolus, across the thin wall, into the blood in a capillary. This is gas exchange, and it is diffusion down a concentration gradient.
Step 3: the blood carries it through the circulation to the muscle. This is transport, covered in circulation and blood.
Step 4: it diffuses from the blood into a muscle cell. This is another example of gas exchange, at the body cell.
Step 5: inside the cell, it reacts with glucose to release energy, forming carbon dioxide and water. This is aerobic respiration.
The word equation is: glucose + oxygen → carbon dioxide + water (+ energy released). Carbon dioxide then travels back the other way and is breathed out.
The mistake to watch for
The classic mix-up is to treat respiration as breathing, or to place respiration in the lungs.
Mistaken answer: “Respiration happens in the lungs, where we breathe in oxygen and breathe out carbon dioxide.”
This describes breathing and gas exchange, and it puts respiration in the wrong place.
The correction: breathing and gas exchange happen at the lungs, and respiration happens in every living cell, including cells of the muscle, the brain and the lungs themselves. Another slip is to say respiration “produces energy”. Energy is released from glucose, not created.
Check yourself
1. A student says, “Oxygen diffuses into the blood, so respiration happens in the blood.” Identify the error.
Show answer
Diffusion of oxygen into the blood is gas exchange, not respiration. Respiration is the chemical breakdown of glucose to release energy and it happens inside cells.
2. Name the process described: “Carbon dioxide diffuses from the blood into the alveolus.”
Show answer
Gas exchange, by diffusion down a concentration gradient.
3. True or false, with a reason: “Plants only respire at night.”
Show answer
False. Plants respire all the time in all living cells. At night only respiration runs, and in the light photosynthesis also runs, so the two happen together during the day.
Where this leads next
Next, compare aerobic and anaerobic models to see what respiration releases with and without oxygen. Then try the mixed practice set.
Students who keep losing marks to this mix-up can practise naming the process in every sentence with a teacher in online one-to-one Biology tuition.