The overall structure of all cereal grains is basically similar differing from one cereal to another in detail.
The percentage of endosperm, germ and bran of cereal are 83, 1 AVi and 2Vi, respectively.
Bran or pericarp:
The outer layer, epidermis of the cereal consists of thin-walled long rectangular cells. Next to the epidermis is the hypoderm of varying thickness.
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The innermost layer of pericarp tears during the ripening of the seed and in the mature grain they are represented by a layer of branching hypha-like cells called tube cells.
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The seed coat or testa is a thin single or double layer. The inner layer of testa of wheat is often deeply pigmented which gives the grain its characteristic colour.
Next to testa is a hyaline layer (nucellar tissue) which is colourless and devoid of any obvious cellular structure.
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Aleurone cell layer:
The endosperm is surrounded by one or more layers of cells known as aleurone. In wheat, the aleurone is a single layer of thick-walled cubicle cells and constitutes 7 per cent of grain weight.
The cells contain about 20 per cent each of protein, oil and mineral matter. The cells are also rich in nicotinic acid. The aleurone cells also contain tiny grains of phytic acid with some protein.
Endosperm:
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The endosperm itself consists of cells of various sizes, shapes and different composition.
The endosperm cell consists mainly of starch and protein, the starch being in the form of spherical granules which are single granules or tightly packed together and embedded in a matrix of protein.
The size and shape of the starch granule in the endosperm cells vary from one cereal to another.
Embryo:
The germ or embryo consists of many parts. It is separated from endosperm by scutellum which has the function of mobilizing the stored food in the endosperm and transmitting them to the embryo when the grain germinates.
The germ and scutellum are rich in protein and fat. Most of the B vitamins in the grain are present in the scutellum.