Nitrogenous reserve material stored in the plants for their use as food is various kinds of proteins and amino acids.
(1) PROTEINS
Proteins are very complex, organic, nitrogenous compounds, essentially containing carbon (C), hydrogen (H), oxygen (O) and nitrogen (N). All plant proteins contain sulphur (S) and some also phosphorus (P). Proteins are very important as food being the source of nitrogen and they also form an integral part of protoplasm and nucleus, particularly as nucleoprotein.
There are various kinds of them found in plant body, particularly in their storage organs. They are mostly insoluble in water but all are soluble in strong acids and alkaloids. A common form of insoluble or sparingly soluble protein abundantly found in the endosperm of the castor seed is the aburone grain. Each aleurone grain is a solid, ovate or rounded body known as the crystalloid and a small rounded mineral body called the globoid. The crystalloid is protein in nature while the globoid is a double phosphate of calcium and magnesium. The occurrence of crystalloid and globoid is not always constant in the aleurone grain. Aleurone grains vary in size. When they occur with starch they are very small as in pea but in oily seeds they are very much larger as in castor.
Fatty seeds contain higher percentage of proteins than starchy seeds e.g. rice contains only 7% of proteins, wheat 12% while sunflower seeds contain proteins as high as 30%. Starchy seeds of leguminous plants contain high percentage of protein as fatty seeds e.g. in the pulses there is an average of about 25% of proteins, in Soya bean protein contents are 35% or more.
Average percentage composition of proteins may be given thus carbon 50-54%, Hydrogen about 7%, oxygen 20-25%, nitrogen 16-18%, sulphur .4% and phosphorus .4%.
Tests of Proteins: (i) Proteins are coloured yellowish brown with strong iodine solution.
(ii) Xanthoproteic reaction: Add some strong nitric acid a white precipitate is formed, on boiling it turns yellowish. After cooling add a little strong ammonia and yellow colour changes to orange.
(2) AMINO COMPOUNDS
Amino acids and amines are the simplest forms of all nitrogenous food material and occur in solution in the cell sap, they are found abundantly in the growing regions of plants, less frequently in storage tissues. When translocation is necessary, proteins become converted into amines and amino acids. They travel to the growing regions where the protoplasm is very active and are directly assimiled by it. They are also the initial stages in the formation of proteins. They contain carbon, hydrogen, oxygen and nitrogen and sometimes also sulphur (as in cystine). There are 20 different amino acids known to be constituents of proteins.
When a protein is heated in strong acid at 100°ZC for several years it is broken or hydrolyzed into its constituent building blocks called amino acids. These are 20 different amino acids in the proteins of all organisms, although certain proteins may contain fewer of them and certain organisms may contain special amino acids that represent slight modifications of 20 principle ones. All amino acids derived from proteins are L-amino acids (except for glycine which is not optically active), a fact that strongly suggests the common origin of all living matter on earth.
.jpg)