The Oxygen Cycle

Another popular project in primary schools is the one about how animals breathe in air, how their lungs then take the oxygen out of the air, how the oxygen is then used to power the animals' muscles, converting it into carbon dioxide in the process, and then how the unwanted carbon dioxide is released into their lungs to be breathed out into the air. (In short, animals turn oxygen into carbon dioxide.) The second half of the story is that plants take in carbon dioxide from the air through pores in their leaves, they use the energy from sunlight to split it into carbon and oxygen, they use the carbon to make new plant material, and then they release the unwanted oxygen back into the air. (in short, plants turn carbon dioxide into oxygen.) This is a good balance - plants make oxygen for animals to breathe, and animals make carbon dioxide for plants to breathe, and the whole thing is powered by the sun. Now, I don't think you need to spend a month of your life doing a project on this or to cover a wall in a giant picture of it. Let's just save ourselves a lot of time and move straight on so that we can learn more.

There is a third part of the story which is very rarely told in primary schools: animals and plants are both made of carbon, plus a lot of other things such as water (which is in turn made of oxygen and hydrogen), but it's mainly the carbon that holds everything together. Animals get all their carbon by eating plants (or by eating other animals which eat plants; or by eating other animals which eat animals which eat plants; or by eating animals which eat animals which eat animals which eat plants, but in every case the chain ends with animals that eat plants). Carbon is a very good building material - it's what gives trees their enormous strength - but it's also an extremely powerful fuel, which is why materials rich in carbon burn so well: wood, coal, oil and petrol/gasoline are all rich in carbon, and we burn masses of the stuff to power cars and planes, and even more of it to make electricity. Animals use carbon as a fuel too, and they "burn" it in their muscles to power themselves. Carbon can only be burned if there is enough oxygen around it, because the whole burning process depends on combining carbon with oxygen to make carbon dioxide. This is why animals have to breathe in oxygen, and it also explains why they breathe out carbon dioxide as a waste product. Car engines are much the same as animals: they burn petrol (mixed with air so that there is plenty of oxygen in with the carbon), and then they put our carbon dioxide as a waste product. The only difference is that animals have found a way to burn carbon in their muscles without producing any flames or smoke, though they do get hot in the process. So, plants give us carbon to burn and oxygen to burn it with - we depend on them completely. Plants get their energy from the sun, using sunlight to turn carbon dioxide into oxygen which they release into the air and carbon which they use to build themselves, thereby making wood and coal which we can burn, and all sorts of leaves, fruits and vegetables that we can eat.

There's another part of the story which is rarely told in primary schools, and that is about how plants existed long before there were any animals on the Earth. Volcanos played the part of animals in those days: they put out lots of carbon dioxide from deep inside the Earth and the plants then took it in and used the sun's energy to split it into oxygen (which they released as a waste product) and carbon (which they used to build themselves). There was no oxygen cycle in those days because the vocanos didn't take in any oxygen, and the result of this was that the amount of oxygen in the air went up from virtually none to the point where a fifth of all the gas in the air was oxygen, and it was this high level of oxygen that made it possible for animals to exist. The amount of oxygen was actually much higher at one point, but it reached the stage where fires kept breaking out all over the place and the trees were repeatedly burned down. Things only became stable when animals appeared on the scene and started to use up oxygen, preventing the oxygen level from getting too high. You should also be aware that plants removed carbon dioxide from the air to such a degree that it only makes up half a percent of the air around us, though that's still plenty to allow plants to grow. We are now burning a lot of oil and coal and putting massive amounts of carbon dioxide back into the air, so the amount of this gas in the air is double what it was a couple of hundred years ago, and the result of this is that the planet is gradually warming up. While it's true that the world used to be a great deal hotter in the past, humans didn't exist until long after it had cooled down: we are designed to live on a cold planet and it is very much in our interest to keep it cool. We need to find better ways of powering our cars and planes and of heating our houses.

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