CHEMICAL COMPOSITION OF SEEDS Mr. John L.Terlabie

CHEMICAL COMPOSITION...contd. Most seeds have high LIPID content. Except for certain fruits, the occurrence of high lipids concentrations differentiates seeds from other plant organs. Lipids are concentrated in the endosperm or cotyledon storage tissues in Angiosperms. Endosperm is generally solid. Lipids provide the highest amount of potential energy per unit weight

CHEMICAL COMPOSITION...contd. The seed lipid reserve is generally hydrolyzed in situ to glycerol and fatty acid by lipases. The fatty acids are utilized for phospholipids and glycolipids. These are required as constituents of organelles. However, most are converted to sugars and transported to the seedling (body) for growth.

CHEMICAL COMPOSITION...contd. They dissolve in organic solvents e.g. benzene, acetone, alcohol, ether but do not dissolve in H2O. The value of seed-borne lipids for food and industrial uses is enormous. In contrast to animal fats, their highly unsaturated chemical nature has caused increased interest in them for health purposes. High lipid content is usually associated with decreased protein content e.g. soybean, rapeseed.

CHEMICAL COMPOSITION...contd. All the basic protoplasmic components are present in seeds. This include amino acid, vitamins, nucleic acid, co-enzymes and minerals. Phosphorus content in most seeds is often very high. WHY? This element occurs most commonly as Phytin-Salts of inositol hexaphosphate.

CHEMICAL COMPOSITION...contd. All seed contain PROTEIN as the protoplasmic component. Protein are huge N2 containing molecules with huge structures, greater part of which yield amino acid upon hydrolysis of peptide bond. They are so important to plant and animal life that all physiological reactions of living cells revolve around their physical and chemical properties and those of related compounds.

CHEMICAL COMPOSITION...contd. Aside H2O, they are the principal components of all protoplasm of both plant and animal cells. Seeds often contain unusual amino acids which are not constituents of proteins. This may act as N2 store and may be present to the extent of several percent of the seed’s dry weight. Chlorophyll is absent in seeds.

CHEMICAL COMPOSITION...contd. In some cases, e.g. legumes, large quantities of special reserved proteins with a distinctive chemical composition are also stored. The storage proteins occur as membrane enveloped protein bodies (Aleurone bodies) They may occasionally occur as crystals. The protein reserve in all cases is sufficient for growth before the plant becomes self-supporting.

CHEMICAL COMPOSITION...contd. Seed storage proteins are less complex than protoplasmic protein and less likely to be tied up with lipids. The great majority of seed protein are metabolically inactive and serve merely as food reserve for use by the growing embryo during germination. Active ones are few but they are extremely important to the developing and germinating seed. As enzyme, they catalyze the metabolic processes in digestion, translocation and utilization of stored food reserves; no growth can occur without them.

CHEMICAL COMPOSITION...contd. CARBOHYDRATES are the major storage substances in seeds. Carbohydrates occur mainly as starch or hemicellulose. Other carbohydrates that occur in non storage forms are peptin and mucilage.

CHEMICAL COMPOSITION...contd. Starch is the principal and most wide spread storage carbohydrate. They are metabolically inactive reserves stored until needed during germination. Starch is stored in two related forms; amylose and amylopeptin which are two polymers of D-glucose. Amylose stains blue when exposed to iodine and its 100% digestible by alpha amylase.

CHEMICAL COMPOSITION...contd. Amylopeptin on the other hand is only about 50% digestible by alpha amylase and stains purplish-red when exposed to iodine. However, they are both hydrolyzed by alpha and beta amylase during normal metabolism and germination. Starch is stored as grains (up to 5µm in diameter) as formed in the amyloplast. Hemicellulose are normal cell wall components in all tissues.

CHEMICAL COMPOSITION...contd. Hemicellulose are usually found in the thickened tertially layers of cell walls of the endosperm or cotyledon instead of the interior region of the endosperm. They are composed principally of mannans with small amount of glucose, galactose, arabinose as side chain on the main linear polymers of mannose residues. Where they form a major seed reserve, they are laid down as heavy cell wall thickening, which almost fill the lumen, as in seeds of the date palm-Phoenix dactylifera.

Small amounts of soluble sugars are usually present. These sugars are concentrated mainly in the growing parts of the embryo i.e. shoot and root apices where meristematic activities is highest.

CHEMICAL COMPOSITION...contd. Water content is often very low ranging between 5-20% on a fresh weight basis compared to 80-90% of most plants in an active state. This largely accounts for the very low metabolic rate in ungerminated seed. Most of the water in dry seeds are firmly bound to colloides and inaccessible for hydrolytic reactions (not unfreezable). This is only removable by temperature approaching 1000C or by storage in high vacuum. Due to this dehydration, the cells and the subcellular organelles in dry seeds are often shrunken.

CHEMICAL COMPOSITION...contd. Seeds also contain growth promoters, such as cytokinin, and auxin. There is also the presence of growth inhibitors and most of them are phenolic compounds. OTHER COMPOUNDS: Tannins: Used for tanning process as found in cocoa. Tannins contain phenolic and hydroxyl compounds. Alkaloids: Complex cyclic compounds containing N2.

CHEMICAL COMPOSITION...contd. Most are white solids, however, nicotine is a liquid at room temperature. Glucoside Hormones: Vitamin