USES OF WATER AND OF FOOD: STORAGE OF FOOD

Uses of Water to Organisms

It is convenient here to summarize the uses of water to organisms.

A. General Uses.

It keeps the colloidal protoplasm fluid.

It forms a natural environment for many organisms.

It is dense enough to buoy up those fragile aquatic organisms which lack a skeleton, e.g. jelly fish and seaweeds.

It is a chemical reagent in all those digestive processes known as hydrolyses.

B. In Plants.

It enables the plant to obtain mineral salts in solution from the soil.

It is used up during photosynthesis.

It is used up during protein synthesis.

It is required for the transport of materials in solution in vascular plants, e.g. the minerals from roots to leaves; and foods down from assimilatory organs, e.g. leaves; and up from underground storage organs, e.g. swollen leaves of bulbs.

Being under pressure due to osmosis, it keeps those parts turgid which are not sufficiently supported by a skeleton of wood or cellulose, e.g. leaves, tulip stems.

During hot weather, as it evaporates into the air spaces inside leaves, it requires heat to do so, and takes it from the leaves, thereby cooling them during transpiration. This prevents delicate leaves from being scorched by the sun.

Seeds will not germinate unless water is present.

The extracellular digestion of food by saprophytic fungi cannot occur in most cases unless the food is damp.

It is required for digestion of food in germinating seeds and in sprouting storage organs.

C. In Animals.

It acts as a solvent for food during digestion and for effecting digestion with the aid of enzymes.

It forms digestive fluids, I.e. water solutions of gases like hydrogen chloride present in gastric juice, and of solids like the salts present in bile. These agents are then readily transported.

It is required to transport other materials internally in solution, e.g. useful food and oxygen in blood for distribution ; nitrogenous wastes, urea, and carbon dioxide in blood for removal ; useless solids in urine for excretion.

The kidneys regulate the amount of water present in blood either by removing water if the liquid is too dilute, or removing salt if it is too concentrated—so that the salinity of the blood is maintained.

In mammals it keeps the skin firm, since blood is under pressure.

In mammals it lowers the temperature of the body, as it evaporates from sweat on the skin during hot weather.

Many mammals and birds use water to wash away ectoparasites.

Uses of Foods

Bearing in mind our definition of a food as a substance containing potential energy, the most important use for food is that of tissue respiration, which can be defined as the setting free of useful energy from food, generally by oxidation, but in certain cases without oxygen gas being involved. All classes of food substances can be utilized in this way, although the carbohydrates, especially the simple sugars, seem to be the most commonly used, and are, in fact, the most suitable for the purpose, since they require least oxygen and when oxidized give only carbon dioxide and water. The complete oxidation of lipides, although yielding similar products, requires much more oxygen per unit weight, whereas the proteins give rise to nitrogenous waste products which are not so easily removed as carbon dioxide., nitrogenous alkaloids and anthocyanins Those substances in the cells which are insoluble are got rid of by the plant when it sheds bark, leaves and petals respectively.

Protoplasm, the living material in every cell of an organism, is such an unstable substance that it is constantly decomposing and, in doing so, producing materials which are of no further use. Food is necessary, therefore, for building up fresh protoplasm to replace that which breaks down. Where the rate at which protoplasm is built up exceeds that at which it breaks down, there will be an increase in the quantity of protoplasm, and the bulk of the organism will therefore become greater. Such a state of affairs is obviously exhibited by all young animals and plants, which require ample food in order that it may be utilized for growth.

An organism may suffer damage as a result of illness or an accident; food will be required to make good the loss. This is perhaps best exemplified by crabs and lobsters which can regenerate a lost limb, the new limb growing larger each time the old shell is cast off.

For growth, repair and regeneration new protoplasm is necessary, but this may not be the only material requirement, for some structures contain non-living material which by giving support, is of value to the organism. Although the actual weight of calcium phosphate in the bones of vertebrates is fairly low, it nevertheless is highly important, since the rigid nature of the bony skeleton depends almost entirely upon the calcium phosphate present. When it is lacking in the diet the animal suffers from rickets, and its legs cannot support the weight of its body. The dead skeletal structures of animals other than vertebrates include the flinty spicules of sponges, rough external parts of corals, the chastas of Annelids like the earthworm, the exoskeleton of all arthropods, the shells of molluscs like the oyster and the snail, and of echinoderms like the starfishes. The skeletons of cartilaginous fishes are somewhat strengthened by chalky deposits. The ’oozes ’forming what is now, or was once, the floor of the sea, are the skeletons of Protozoa.

The dry weight of a plant consists almost entirely of its skeleton, which is dead material when the plant is alive. It consists largely of cellulose in simpler forms of life and in many annuals among flowering plants, whereas the skeleton of higher plants is largely made of wood. In all cases the skeleton is formed by the cell walls which, in woody plants, persist long after the living contents have disappeared. Some plants, notably grasses and the microscopic diatoms, have a skeleton of silica. The selective power of protoplasm in obtaining materials is well exemplified by those organisms, including grasses, which have a flinty skeleton, since silica is almost completely insoluble in water, and, in any case, is present in extremely minute quantities in sea and soil water.

Food within the organism which is not wanted for immediate use is usually stored. The uses of food may therefore be summarized as follows :—

To provide energy for all living processes through respiration.

For growth.

For repair and replacement of broken down protoplasm and tissues.

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