If we classify the foods commonly stored by organisms we find that they fall into three: insoluble compounds—cellulose, starch, oils and fats—all of which because of their insolubility cannot affect protoplasm or the concentration of materials in protoplasm ; soluble but non-diffusible compounds—inulin, complex sugars like sucrose, and proteins— which, although soluble, because of their non-diffusibility also do not affect cell metabolism ; and soluble diffusible substances—the simple sugars. These last, according to their concentration, can affect the movement of water into or out of cells. Two conditions, however, can prevent this happening :—
In the onion bulb glucose is stored—the whole structure has its fluid at the same concentration, enclosed not only by the surface membranes of each fleshybut by the fibrous brown making a waterproof covering. It is possible for this storage of glucose to occur in tiny vacuoles within the cells, containing the glucose in solution enclosed by a similar non-permeable protoplasmic membrane, since one of the most outstanding properties of protoplasm is its ability to adapt itself to changing conditions.
The glucose, however, may not be present as such, although we may detect it by ordinary chemical tests. This substance is very commonly found combined with other substances— generally to give a soluble compound. Such are the glucosides —tannins and anthocyanins.
Before any of these substances can be utilized on the spot or elsewhere they must be mobilized, I.e. made suitable to use or rendered transportable respectively. The process of mobilization is in every case carried out by the enzyme particular to each substance for the chemical change involved.
Uses of Glycogen and Fats to Higher Vertebrates
In vertebrates during muscular activity glucose is used up in the muscles to supply the energy for carrying out the activity. As the glucose present in these tissues is depleted, glycogen is broken down by the agency of adrenalin into glucose for further activity. Should the glycogen reserves in the muscles be insufficient for the purpose, then that which is in the liver is mobilized into glucose, transported in the blood stream to the working tissues and there utilized.
Amongst those animals which hibernate, the lipides play an important part. The animal during hibernation has all its activities slowed down almost to a standstill, so that food is required only for the tissue respiration necessary to maintain life. Such animals invariably lay down a large quantity of fat for the purpose before becoming dormant, to emerge later in an emaciated condition. The lipide here stored under the skin also serves the purely passive role of acting as a heat insulator, which function it has in all warm-blooded animals in cold weather or in cold water, e.g. in the Eskimo, the bear, dolphin and whale. Another passive function it possesses is that of holding organs in place, e.g. the kidneys in mammals.
Reserve Food in Eggs
The eggs which are laid by the great majority of animals differ from those which are produced by mammals in possessing a relatively enormous amount of reserve protein, known as yolk, rendering the egg inert as compared with the active sperm. This reserve is to enable the organism to develop to such a stage that it can fend for itself when it hatches, whereas the young mammal is supplied with nourishment in blood by the mother before birth, and by milk afterwards.