Unlike carbohydrates, which constitute a family of homogeneous compounds, lipids form a very heterogeneous class of compounds of widely differing structures?
may be defined as compounds containing in their molecule an aliphatic chain (chain consisting of -CH2-) of at least 8-carbon atom. Some short-chain fatty acids (like butyric | acid, C4) are the only exceptions to this rule.
The term fats and oils denote mixtures of lipids which are respectively solid (goat fat) I or liquid (mustard oil) at ordinary temperature. Simple lipids yield an alcohol and one or several fatty acids on hydrolysis while complex lipids on hydrolysis liberate alcohol | and fatty acids but also phosphoric acid, carbohydrate etc.
I- Fatty acids
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They are found in small quantities in Free State, but in large quantities as part of lipids. As a general rule, these are mono carboxylic, straight UN branched chain acids containing an even number of carbon atoms (between 4 and 36). They may be saturated or unsaturated and sometimes hydroxylated or branched.
1. Saturated fatty acids
The general formula is: CIT3-(CH2)n-COOH. The most frequent are palmitic acid (C16) and stearic acid (C18). In lower concentration are found the fatty acids with 14 or 20 carbon atoms. Longer fatty acids (up to 36 carbon atoms) are present in numerous cells (bacteria, unicellular eukaryotes, plants, vertebrates). They are generally present in some types of lipids. Milk on the contrary, is rich in short-chain fatty acids, besides the even -carbon fatty acids.
2. Unsaturated fatty acids
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These are fatty acids having one or more double bonds in their hydrocarbon chain.
a. Monounsaturated fatty acids (1 double bond) Cn H2n-1 C00H
Oleic acid (C18) has a double bond between carbon atoms C9 and C1o with a molecular formula-
CH3-(CH2)7-CH= CH-(CH2)7-COOH the common sources of Oleic acid are animal and plant fats.
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b. Polyunsaturated fatty acids (several double bonds)
In the most common of such acids, a methylene group separates the non- conjugated double bonds, e.g. linoleic acid having the molecular formula-
CH3 – (CH2)4 -CH= CH-CH2 -CH=CH-(CH2)7 -COOH
In mammals, polyunsaturated fatty acids can have up to 22 carbon atoms and 6 double bonds, but in plants these acids do not exceed 18 carbon atoms and 4 double bonds. Common sources of linoleic acid are Lin-seed oil and cotton seed oil.
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An important physical property of fatty acids is that their melting points decrease with increasing number of double bonds. For example, the melting point of stearic acid is 70°C whereas that of oleic acid is 13°C and that of linoleic acid is -5.8°C.
3. Hydroxylated fatty acids
Plants can synthesize a series of hydroxylated fatty acids like ricinoleic acid with the molecular formula-
CH3-(CH2)5-CH (OH)-CH2-CH=CH – (CH2)7-COOH
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Some of these hydroxylated fatty acids lead to the formation of cutin that are found in the cuticle of epidermal cells of plants. The common source of ricinoleic acid is castor oil.
4. Branched fatty acids
Example: ismethylhexadecaenoic acid with a molecular formula- (CH3)2-CH-(CH2)i3-COOH
The above type of fatty acid is particularly abundant in Gram+ bacteria.