THE PHOTOSYNTHETIC MECHANISM IN LEAVES

The process of making starch from raw materials is only one of what is probably a series of chemical changes. Having given the facts which are capable of simple experimental proof, it is convenient here to summarize the whole process, termed photosynthesis.

The experiments have shown that carbon dioxide, chlorophyll and light are essential, and that oxygen is evolved at the same time. Water is also involved in the conversion of carbon dioxide into carbohydrate. The essential stages in the process are :—

Carbon dioxide enters a plant. In flowering plants and ferns it enters by way of the stomata by gaseous diffusion, dissolves in the water in the cells of the spongy layer and passes by liquid diffusion to the cells of the palisade layer. In aquatic green plants all the raw materials enter by liquid diffusion from the surrounding water.

The chlorophyll in the chloroplasts absorbs energy from the sunlight. By the use of water containing a small proportion of molecules incorporating a radio-active form of ’heavy ’oxygen instead of the usual non-radio-active oxygen, it has been shown that this energy is used in the photolysis of water so that all the oxygen evolved in photosynthesis comes from water and not from carbon dioxide. This may be represented by the following equations :— 2H20^2 +2 2 ^H20 +

By further reactions which can proceed in the dark, the hydrogen in equation is caused to react with carbon dioxide to produce complex organic compounds from which sugar is ultimately produced. During these reactions e.g. Phosphoglyceric acid. the energy absorbed from the sunlight in the photolysis of water is transferred to and stored as potential energy in the sugar. From the sugar this energy may be released as free energy by respiration.

If the sugar is produced so rapidly that it cannot be transported or used immediately, it may be converted into starch which, being insoluble, is readily stored in the leaves.

Such starch, termed transitory, is usually converted into soluble carbohydrate in darkness and transported elsewhere to be used or stored.

Experiment 41—To slioiv the Removal of Starch in Leaves during Darkness

Sycamore leaves are removed from a small potted plant and are tested to prove that starch is present. If found, the main median vein of each of three or four leaves which are left on the same plant is cut at the same time and a thin piece of metal foil inserted. The plant is left in darkness and the following day they are tested for starch in the usual way. The cut veins with the foil prevent the transport of any material towards the leaf stalk.

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