In this article we will discuss about fats and their distribution in plants.
Introduction to Fats:
Fats or glycerides or acylglycerols are esters of high molecular weight fatty acids and trihydroxy alcohol, the glycerol.
Most of the glycerides in nature have all three hydroxyl groups of glycerol esterified with fatty acids and are called as triglycerides (or ‘fats’).
For example, tripalmitin a common fat is an ester of glycerol and palmitic acid:
Monoglycerides and diglycerides having one and two of the three hydroxyl groups of glycerol esterified respectively do not occur in appreciable amounts in living organisms although they are important intermediate metabolites.
A triglyceride is known as ‘simple’ when all the fatty acid residues in it are identical, and ‘mixed’ when two or more of them are different. Natural triglycerides are mostly of mixed type.
In plants the fatty acids usually consists of even number of C-atoms.
Fatty acids constituting the fats may be saturated or unsaturated. Table 14.1 and 14.2 show major saturated and unsaturated fatty acids respectively in plants.
The carbon atoms in fatty acids are usually numbered from the carboxylic carbon side. The carbon atom no. 2 which is adjacent to the carboxylic carbon is also called as a-carbon while the next or no. 3 carbon is β-carbon and so on.
The number written before the colon denotes the no. of C-atoms in the fatty acid while that after the colon is the number of double bonds present. Each of the numbers in parenthesis is the lowest numbered C-atom of the two connected by a double bond, counting being made from the carboxyl carbon side. The letter c which follows immediately denotes that the double bond has cis configuration.
Some plant fatty acids have unusual structure in their molecules. Although these constitute majority of the fatty acids in plants but they are not widespread in plant kingdom and occur only in a few plant species. Table 14.3 shows some of the examples of such fatty acids.
Triglycerides may be solid or liquid at ordinary room temperature and are termed as fats or oils respectively. The fats are rich in saturated fatty acids while the oils are rich in unsaturated fatty acids. In plants, the triglycerides mostly occur as oils. Natural fats and oils also contain minor amounts of other lipids in addition to the glycerides.
The term triglycerides, is however, frequently used synonymously with the term fats and thus the latter represents both fats and oils.
Fats (triglycerides) are poorer in oxygen in comparison to carbon.
Fats are insoluble in water, hence can be stored in the cell without disturbing their osmotic relations.
Fats can be hydrolysed by alkalies into glycerol and salts of fatty acids – a process called as saponification.
Fats together with waxes constitute the ‘neutral lipids’. But while fats are fatty acids esters of trihydroxy alcohol, glycerol, and the waxes are predominantly fatty acid esters of long chain monohydric alcohol. Neutral lipids are readily soluble in non-polar hydrocarbon solvents such as light petroleum and benzene.
Distribution of Fats in Plants:
Fats are widely distributed throughout the plant kingdom. They are especially found in abundance in reproductive tissues (e.g., seeds and fruits) of some higher plants where they form important reserve food material such as in cotyledons of sunflower, rape (Brassica napus) peanuts (Arachis hypogaea), almonds (Prunus amygdalus), endosperm of castor beans, coconut plants etc., and mesocarp of avocado pear (Persea americana).
Some cereal seeds such as wheat and barley (Hordeum distichon) which store starch as chief reserve food in their endosperm have rich fat content in the cells of their aleurone layer. Within the cells of the above mentioned tissues, the fats are stored in cell-organelles called spherosomes (or oleosomes or oil bodies) which are extensively distributed throughout the cytoplasm.