Describe Proteins and Amino compounds present in plant cell
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.
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