Page 14 - Curriculum Visions Dynamic Book

The oxygen cycle
There is a continual exchange of oxygen between the atmosphere and the water, the plants and animals and mineral matter. This is called the oxygen cycle.
It begins with the reservoir of carbon dioxide in the air. The process of photosynthesis uses carbon dioxide and water from the soil to produce cellulose, the material from which plants are made. This releases oxygen gas into the air. Photosynthesis ceases when darkness falls and plants then burn off some of the cellulose they have made, returning carbon dioxide to the atmosphere.
Animals use oxygen in the atmosphere for respiration, oxidising the sugars in their food to give energy and releasing carbon dioxide to the atmosphere.
When dead tissue (carbon compounds) decays by a combination of oxidation and microorganism decay, carbon dioxide is released back to the atmosphere.
A slower cycle occurs whenever mineral matter is oxidised, such as in the formation of rocks.
 The decay of leaves is mainly an oxidation process.
Oxygen decomposes organic matter
If you leave vegetables out on a window ledge, they will eventually shrivel and then rot. This process is called decomposing.
Decomposing is a chemical reaction similar to combustion. The organic material of waste food, dead weeds and so on, will react with the oxygen in the air and change chemically. Because all this happens at normal temperatures, the reaction is slow. In many cases microorganisms also play an important role, such as in helping to rot down dead organic material. When butter or milk for example, taste unpleasant,
it is because they are oxidising, often with the help of unseen microorganisms.
A compost heap is made of decomposing vegetable material. The compost is also a good insulator, which means that it keeps the heat inside the compost. If you feel inside a compost heap it is likely that it will be warm, if not hot. This shows that the organic material of the compost is oxidising and forming new materials and at the same time it is releasing heat. This shows that oxidation is an exothermic process.
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