Organic matter is that which is living or has been alive, e.g. wood or coal. Inorganic matter is that which is not living and never has been alive, e.g. water or iron. Inorganic substances can be built up into organic compounds by living organisms. All living matter is made up of small units called cells. Some organisms, such as bacteria, consist of one single cell; others, like human beings, consist of many hundreds of millions of cells, all functioning together to make a complete whole. All living cells, however simple, have certain characteristics which are always present. These characteristics are: * Activity
* Digestion and absorption of food
* Growth and repair
Activity This is the most striking characteristic of living matter. It is more apparent in the animal world than in the vegetable world, as the animal must move about in search of food, but even in plants the buds can be seen to break out in spring, and with a microscope, activity becomes as obvious in the plant as in the animal. There can never be activity without energy. Cars are driven by energy derived from petrol, trains are generally moved by electrical energy and in living matter energy is also obtained by burning fuel. The fuel for the human body is the food eaten, particularly Carbohydrates and fats. Oxygen is also necessary for the combustion of fuel, and living things obtain their oxygen from the air or water in which they live. Combustion of fuel also produces waste products, such as carbon dioxide and water, which must be disposed of. The combustion of fuel in living matter produces some energy for work and some energy in the form of heat. The body is economical in that it produces more work energy and less heat for each unit of food consumed, but the heat produced by combustion is not altogether wasted for some heat is needed. The body can remain healthy only...