part 13

120 There are about five million red cells in one cubic millimeter of our blood. One millimeter is a thousandth part of a meter and a tenth of a centimeter. One cubic centimeter—how much space does that take up? Here is a picture of a cube which is one centimeter long, one centimeter wide and one centimeter high. It takes up one cubic centimeter of space. A centimeter is a little less than half an inch.

121 You can put a paper cube this size together for yourself, or cut one out of soap or cheese. To make a paper cube, take a pencil and make six one centimeter squares like this: Now take your scissors and cut round the outer lines. Do not cut the broken lines but make folds there. Put the edges together to make a hollow paper cube, as in the picture. Your cube will be a cubic centimeter in size. A cube has six sides; its sides are equal squares. The surface of a one-centimeter cube is six square centimeters. In one cubic centimeter of our blood there are about five billion red cells.

122 Living things are made up of cells, and cells do not grow to be more than twice the size they were at first. But trees can grow to be many thousands of times the size they were as seeds. The tallest tree known is 364 feet high. One great tree in California is as much as 115 feet round its trunk at the thickest point. How does such growth take place if cells do not grow to be more than twice as large as they were at first? It all takes place through division. The cells which make up the organism are able to divide into two and this division goes on and on. When you were born you had as many as 200 billion cells in your body all coming, in nine months, from the division of one cell 1.175th of an inch across.

123 All organisms which you see in the world about you have grown by cell division. You and I have become what we are through billions of divisions in the cells whose outcome is our cell structure at this minute. In cell division the two halves of the cell (as you see in picture four) become ready to separate. In picture five you see that they have separated. The cell in picture one, by division, has become two separate cells. The daughter cells do in every way the same as their mother cell did. One of the greatest questions scientists are working on is: What keeps the daughter cells doing what they have to do?

124 Even the simplest living organism is far more complex than any machines that people have made. Organisms are built up of parts which are themselves more complex than any machine. And these parts in turn are the most complex things the science of chemistry knows. It is because they are so complex that they can work together in an organism in so many different ways. Chemistry is about the different ways in which different materials are built up. For example: Water is made up of oxygen and hydrogen united in a way which may be represented like this. Carbon dioxide is made up of carbon and oxygen in a way which may be represented like this. Chemists make use of the formula H2O for water in their writing. They use formulas to represent the structure of all material things.

125 The most important of the materials in our bodies are the proteins (from Greek proteios, meaning "having first place"). Among them are materials with the most complex structures known to the chemist. We can get some idea of how complex proteins are by comparing the formula for one of them with the formula for water (H2O) or for carbon dioxide (CO2). Compare this formula for a protein from milk: with the simple formulas for water: or carbon dioxide: The letter N in this formula represents nitrogen and the letter S represents sulphur.