Digesting food and carrying it around the body does not, by itself, give a living thing any energy. To release energy, the nutrients from that food have to be broken down by a chemical process, and that work happens inside the cell in the presence of oxygen. The oxygen needed for it comes from breathing. Respiration is the process in which the nutrients in food are broken down inside the cell and energy is produced.
The whole process can be written in one short line:
| Nutrient (glucose) + oxygen → water + carbon dioxide + energy |
It is the energy released by this reaction that lets an organism do every kind of work and run every one of its life processes. Look at the right hand side of that line and notice something. Two of the three things produced are waste. The water and the carbon dioxide have to be excreted, which is exactly what the last topic was about. Respiration is where a body gets its energy and where most of its waste is made, at the same moment, in the same place.
You can hold your breath. You cannot hold your respiration.
This is the most confused pair of words in the whole unit, and the textbook asks about it directly. Are breathing and respiration the same process? They are not, and the difference is not a small one. It is a difference of place.
| Breathing | Respiration | |
|---|---|---|
| What it is | Only an exchange of gases. Air moves in, air moves out | A chemical process that breaks food down and releases energy |
| Where it happens | In one place: the chest, in the lungs | Inside every single cell, everywhere in the body, all at once |
| Can it be stopped on purpose? | Yes. You can hold your breath right now | No. Every cell goes on respiring whatever you decide |
| Does a plant do it? | It has no lungs and no chest, so not in this sense | Yes, in every cell, day and night, exactly as you do |
Now try something. Hold your breath and count. After forty or fifty seconds it becomes very uncomfortable and you have to let go. Most people think that is because the body has run out of air. It is not. There is still oxygen in your blood at that moment. What has actually happened is that your cells never stopped respiring, and every one of them has gone on producing carbon dioxide with nowhere for it to go. It is the build-up of that carbon dioxide, not the shortage of oxygen, that forces you to breathe.
That is the clearest proof of the difference. Breathing is something you do with your chest and can pause. Respiration is something billions of cells are doing right now and cannot pause, and holding your breath does not slow it down by even a little.
Plants respire too, and the day hides it
Both animals and plants respire. Both take in oxygen and give out carbon dioxide. Students often doubt this, because everybody is taught that plants give out oxygen, and both statements seem impossible at once. They are not, and the explanation is worth learning properly because examiners ask it every year.
A plant is running two processes, not one. Photosynthesis makes food and gives out oxygen, and it only happens in the light. Respiration breaks food down and gives out carbon dioxide, and it never stops at all. During the day both are running, and photosynthesis is by far the faster of the two, so the gas you can measure leaving the plant is oxygen and the respiration is hidden underneath it. At night photosynthesis stops completely, respiration carries on exactly as before, and now the gas leaving the plant is carbon dioxide.
So the plant has not changed its behaviour between day and night. It is respiring at the same steady rate the whole time. What changed is that the louder process switched off and let you hear the quieter one.
Activity 5.5 asks for germinating gram or peas, and that choice is not accidental. Germinating seeds respire very fast, because a sprouting seed is doing a great deal of building work. And they have no green leaves yet, so they are not photosynthesising at all. That means the only gas they are producing is carbon dioxide, with no oxygen production to hide it. If you tried the same experiment with a leafy branch in daylight you would get a confusing result, because photosynthesis would be masking the very thing you were trying to detect. A well designed experiment removes the thing that would confuse the answer, and this one does it by choosing its material carefully.
Why you are hungry two hours after eating
The textbook asks two questions here that seem simple and are worth answering properly. Why does the body become weak when you go a long time without food and water? And why do you feel hungry a couple of hours after a meal?
Both have the same answer, and it follows straight from the equation. Respiration is happening in every cell continuously, and every second of it uses up nutrients. Nothing about that stops when you stop eating. So the supply of nutrients falls while the demand carries on exactly as before, less energy is released, and the body has less to work with. Hunger is not the stomach being empty. It is the body noticing that the raw material for respiration is running low, and it arrives a couple of hours after a meal because that is roughly how long the last delivery lasts.
The organs animals use, from simplest to most developed
Animals use different methods and different organs for respiration. From unicellular animals up to developed ones, the organs used include the cell membrane, pores, spiracles, skin, gills, the nose, the mouth and lungs. It is the same ladder you met in transportation and excretion, and it is driven by the same thing: how far the gas has to travel and how much of it is needed.
This comes up as an examination question, so it is worth being clear. A fish takes oxygen that is dissolved in water. Water flows over its gills, the oxygen passes into the blood in the gills, and the blood then carries that oxygen to every cell in the fish. An insect does the opposite. It takes air in through small holes along the sides of its body called spiracles, and those holes lead into a network of fine tubes that go directly to its tissues. The air is delivered to the cells as air, and an insect body does not use blood to carry oxygen at all. So a fish uses gills plus blood, and an insect uses tubes and skips the blood, and this is one reason insects cannot grow large: those air tubes only work over short distances.
The human respiratory system
We breathe through the lungs. The system is made of the nose, the air passages and the lungs. The nose connects to the trachea, and the trachea connects to the two lungs inside the chest. A lung is a bag-like structure, and inside it are countless tiny air sacs called alveoli. When we breathe in, the air that enters through the nose travels down the trachea and reaches the alveoli.
The blood in the capillaries wrapped around each alveolus absorbs oxygen from that air. At the same moment the blood releases the carbon dioxide it has been carrying into the air in the alveolus, and so becomes pure. That purified blood then carries the oxygen to every cell. Inside the cell, with the help of that oxygen, food is broken down and energy is produced, and living things use that energy to run their life processes.
Notice why the lung is full of countless small sacs instead of being one big empty bag. It is the surface trick again, and this is the fifth time it has appeared in this course. A cell stays small so that its surface can serve its inside. A root grows hairs. A leaf spreads out flat. A flatworm flattens. And a lung divides itself into millions of tiny sacs, so that a small space in your chest can hold an enormous surface for gases to cross. Whenever a body needs a lot of something to pass in or out, the answer in biology is nearly always the same: make more surface.
This is an examination question and it is a good one, because answering it properly needs all three topics of this unit at once. When you run, your muscle cells respire much faster. That means two things happen together. They need much more oxygen and they produce much more carbon dioxide. Breathing faster moves gas in and out of the lungs more quickly, which handles the exchange with the air. But gas at the lungs is no use to a muscle in your leg, so the heart beats faster to move blood between the lungs and the muscles more quickly. Two pumps, working on two halves of one delivery problem. Panting alone would fill your lungs with oxygen that never arrived anywhere, and a racing heart alone would carry blood to and from lungs that had nothing fresh to give it.
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