STARCH IN PLANTS

King Country Chronicle, Volume XXVIII, Issue 4527, 5 April 1934, Page 7

STARCH IN PLANTS

RECENT RESEARCH WORK. Starch is such a commonplace article in everyday life that one often ' fails to recognise its importance in ! the economy of the plant itself. Starch plays much the same role in the physiology of plants as it does in animals —that is, it is converted first into sugars, which are then utilised in respiration, and this process supplies energy for the need of the living organism. In animals this energy finds its expression in locomotion or other activities, and in maintaining the body at an even temperature; in plants the energy release 1 in respiration is used largely in building up complex substances, such as the proteins which constitute the bulk of the protoplasm or living matter. In general, therefore, starch forms a "reserve substance," which can be drawn upon to provide simpler carbohydrates (sugars) when necessary. A very important point is, however, that only plants can form carbohydrates from simple inorganic constituents. The animal receives its carbohydrates in the plant food which it ingests. Not all plants, however, store their carbohydrate reserves as starch, although this is true of the great majority of them. Some plants never form starch under ordinary conditions, but, instead, store their reserves as cane sugar; examples are the sugar cane, onion, hyacinth, tulip and many bulbous plants of the* lily type. Two different mechanisms, one leading to sugar accumulation and the other to starch, do not exist in the plant, but only one. The first formed carbohydrate in the leaf is glucose or grape sugar. This is formed in S'reen leaves in sunlight; in most leaves its concentration is from the carbon dioxide of the air by the process termed photosynthesis, and concerning which I shall have more to say later. Glucose, which is readily soluble in water, does not accumulate in any quantity in the leaf, but is condensed almost immediately to form starch, which is insoluble and relatively inactive, and for these reasons is ideal as a storage substance. The extent to which the soluble sugar accumulates in the leaf varies with different species; in about 0.1 per cent, of the fresh weight of the leaf. When this limit is reached, starch formation begins, and the process is an efficient one, for the amount of soluble, sugar remains constant, providing other conditions are constant. In leaves of the sugar type, such as sugar cane, onion and hyacinth, the concentration of sugar at which starch formation begins is veryhigh—about 15 per cent, in the case of hyacinth, and 18 per cent in the case of the sugar cane. High concentrations such as these are rarely reached under natural conditions, but starch formation can be induced in these leaves by floating them on concentrated sugar solutions. In the onion, the critical concentration for sugar formation is so high that starch is never formed. In "sugar leaves" there is still little glucose, but cane sugar is found in quantity. This is also a soluble sugar, less reactive chemically than glucose, but more complex in structure. It is formed of one molecule of glucose and one molecule of fructose (or fruit sugar). Fructose is readily formed from glucose in the leaf and also in the laboratory, so that the formation of cane sugar presents no difficulties. Starch, furthermore, can be formed as readily by floating leaves on cane sugar solution as on glucose solutions. Starch, chemically, is composed of a large number of glucose units condensed together. In the cells of the leaf starch occurs in the form of grains. These are not homogeneous in structure, but show a stratified appearance vtoder the microscope. They can readily be distinguished microscopically, because of the fact that starch forms a deep blue compound when a solution of iodine is poured on it, and this colour is not formed by any other constituent of living cells. The stratified appearance is due to differences in the density of the layers, and, if the starch is heated, water is given off and the rings disappear. The layers are more or less concentric, and are formed about a nucleus known as the hilum. Of special interest is the shape oi the starch grains. Their shape and size vary in every different species of plant. This h?ts been shown in a very exhaustive study of many hundreds of species of plants by an American named Reichert. In potato starch, for example, the grains are egg shaped, with the hilum at one end; in the pea they are spherical, with an irregular hilum; in the poinsetta they are dumb-bell shaped; in the Egyptian lotus the grains are forked or branched.

An interesting parallel case is found in animals, where the red colouring matter of the blood colls— haemoglob-in—.-which carries oxygen to the tissues, has a different crystalline form in each species of animal. This difference in form in both cases can be traced to the same cau'se — namely, to the heterogeneity of protoplasm. Protoplasm is composed laigely of proteins. Thefje are complex sub-

stances formed by the condensation of units known as amino-acids. About eighteen amino-acids are concerned in protein formation and these may combine so variously, and in such different proportions, that the numbers of ways of combination are almost infinite. It is probably that the proteins forming the protoplasm of no two species are exactly identical, and these slight differences in protoplasmic constitution probably confer the differences in structure found in the starch grain and in haemoglobin.

The starch grain is not homogeneous chemically, but consists of a mixture of two different substances, both known as starches. Both of these are formed from >lucose units, but arranged in different ways. These two compounds forming starch are called amylose and amylopectin, and they occur in constant proportions of twothirds of the former and one-third of the latter. The amylopectin forms the envelope of the starch grain, and is probably identical with glycogen, or animal starch, which accumulates in the liver of animals. It is soluble in hot water. Amylose, which forms the bulk of the starch grain, is soluble in cold water. Chemical evidence shows that in amylose the glucose units are united in pairs, while in amylopectin they occur in groups of three. Starch itself cannot be respired, but must first be broken down into its constituent units, which is the soluble sugar, glucose. To transform starch into glucose in the laboratory it is necessary to heat the starch with strong acids, but in the living plant this breakdown is accomplished by means of substances termed enzymes, to which I have made reference before. These enzymes can be extracted from the plan and made to perform the same reactions in the laboratory. One particular group of enzymes converts the starch into glucose at ordinary temperatures, and this is then utilised in respiration.