7. THE USE OF CARBON BY PLANT

by Professor H. K. Porter and Dr H. Jones

Green plants live and grow by taking in from their surroundings a few simple substances, each composed of a small number of atoms, and by building up from them complex substances consisting of very large numbers of atoms linked together. These form the plant's tissues. The building up, or synthesis, of such compounds requires energy, which is supplied by sunlight through the medium of the green pigment chlorophyll. The absorption of this energy and its use are distinct processes. First, the radiant energy of the light waves is converted into chemical energy which is trapped in a specific form ready for use. Then the trapped energy is used to bring about the primary incorporation of carbon from carbon dioxide in the air into the material of the plant. The actual fixation of carbon does not require light. The principal product in terms of weight is carbohydrate, and the whole series of chemical reactions leading to carbohydrate formation, known at photosynthesis, is the basis of all chains of food production. This is because in the living world, apart from a few bacteria, only the green plant is capable of carrying it out.

The carbon dioxide (CO) absorbed from the air supplies both the carbon and the oxygen for carbohydrate molecules. The hydrogen of carbon-hydrate is supplied by water, which is thus a raw material in photosynthesis. It is absorbed by the roots and carried up into the leaves, as are other materials that the plant needs simple nitrogenous substances and a number of mineral salts - to maintain the continuous functioning of the leaves. Any nutrient necessary for plant growth must play some part in the machinery of photosynthesis. Nitrogen and phosphorus are perhaps of particular importance: nitrogen is a constituent of the special proteins, known as enzymes, which control all cellular reactions, and phosphorus is the atom which, combined in a complex organic molecule, enables the radiant energy of the sun to be trapped. Other elements necessary for reactions that we know about are magnesium, a constituent of chlorophyll; iron and copper which are combined into some of the enzyme proteins; and potassium which is necessary for a number of cell processes. The precise site in the plant where photosynthesis takes place is within small bodies containing chlorophyll, known as chloroplasts. These are found in several layers of specialized cells within the leaf, the cells being interspersed with large air spaces which facilitate access to the carbon dioxide. The chloroplasts contain all the enzymes necessary to promote the reactions of photosynthesis and are equipped to start vital syntheses when exposed to light and air. Leaves thus act as collecting agents for carbon by continually repeating a relatively small number of steps, in which minute amounts of carbon dioxide are transformed in innumerable chloroplasts. The concentration of carbon dioxide in the air being only 0.03 per cent, carbon is amassed into the compass of the plant from a large volume of air. To supply the requirements of even a single annual plant, such as the sunflower or tomato, the air volume depleted during growth to maturity is of the order of 50,000 cubic feet.

(from Science Survey, Volume 2, Vista Books, London)