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  Classification of lipids, Biology tutorial

Introduction:

Lipids have been categorized and sub classified into different kinds. Indices on which these classifications are made are essentially because of their composition and properties. Classification allows one to easily classify a lipid that could be isolated from plant or animal tissue.

Classification of lipids:

Lipids have been generally categorized in saponifiable and non saponifiable lipids. Saponifiable lipids are those lipids which yield salt of fatty acids upon alkaline hydrolysis while non saponifiable lipids are not generally subjected to hydrolysis. Example of saponifiable lipids comprises: Phosphoacylglycerols, Acylglycerols, Sphingolipids and Waxes. While example of non-saponifiable lipids are Terpenes, Steroids, Prostaglandins and related compounds.

Acylglycerols:

These are the most plentiful and extensive of all lipids, they are also known as neutral lipids. Acylglycerols are compounds in which one or more of three hydroxyl groups (OH) is esterified to fatty acids. Acylglycerols can either be mono when only one -OH group is esterified, diacylglycerol when two of the three hydroxyl groups are esterified to fatty acid or triacylglycerol when all three -OH groups of glycerol are esterified to fatty acids. Triacyglycerols also known as triglycerides are form in which lipids is good storage form of energy and form in which lipid is stored in adipose tissues. They are generated from reaction.

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The R'and R" may be the similar or different fatty acids.

Triacyglycerols are hydrophobic and don't form stable micelles. They may be hydrolyzed to glycerol and 3 fatty acids by enzymes (lipases) or strong alkali. Properties of triacylglycerol are determined to the great extent by those fatty acids contained in it.

Phosphoacylglycerols:

Phosphoacyglycerols are derivatives of (L - Glycerol - 3 - phosphate).

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Parent compound of phosphoacylglycerols, (phosphatidic acid) is derived from L-glycerol - 3- phosphate by esterification of its two -OH groups.

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All phosphoacylglycerols (phosphoglycerides) or phospholipids contain negative charge around pH 7. They are ampiphatic (having polar head group and non polar hydrophobic tails). Examples of phosphoacylycerols comprise phosphatidylcholine (lecithine), phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine

Sphingolipids:

Sphingolipids are second largest class of membrane lipids. They are complex lipids made up of long chain fatty acids one molecule of long chain amino alcohol or its derivative known as sphingosine and polar head alcohol. Sphingolipids have no glycerol backbone and are of three kinds. Structure of Sphingosine (Parent compound of Sphingolipids). In ceramides, long chain fatty acid in amide linkage to sphingosine.

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Sphingomyelins:

These are the most common class of sphingolipids. In sphingomeyelins-OH group at C-1 of sphingosine is esterfied to phosphocholine or phosphoethanolamine as one of the H atoms of the -NH2 group attached to C - 2 of sphingosine is linked to the fatty acid. Sphingomyelins look like phosphatidylcholines in general properties and three dimensional structures.

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Glycosphingolipids:

These are kind of sphingolipids which occur largely on outer surface of plasma membranes. They contain head groups with one or more sugars joined directly to- OH at C - 1 of sphingosine moiety. They don't have phosphate. Cerebrosides are example of Glycosphingolipids that have a single sugar joined to sphingosine.

Globosides are other example of sphingolipids which are neutral (uncharged) with two or more sugars generally D - glucose, D - gatalactose or N-acetyl neuraminicacid. Cerebrosides and globosides are generally known as neutral glycolipids as they do not have charge at pH 7.0

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Gangliosides:

These are maybe the most complex of all phospholipids. They contain oligosaccharides as their polar groups and one or more residues of N -acetylneuraminic acid also known as sialic acid.

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Waxes:

These classes of saponifiable lipids are esters of long chain fatty acids (14 - 36 Carbon atoms) and long chain monohydric alcohols (16-22 atoms). They are very insoluble in water and are chemically inert.

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Steroids:

These are non-saponifiable lipids which are derivatives of cyclopentanoperhydroxyphenanthrene (steroid nucleus). Steroid nucleus is basically planar, rigid comprising of four faced rings. Most steroids in humans have methyl groups at position 10 and 13 and often a side chain at position 17.

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Sterols have steroid nucleus with one or more (-OH) groups free or esterified to fatty acids. Most abundant sterols in animal tissue is cholesterol that has structure.

Terpenes:

Terpenes are other examples of non-sapofiable lipids which are made up of two or more isoprene units. Isoprene is 5 five carbon compound with structure. Terpenes are essentially hydrocarbons found in plants. Isoprene units are combined in head - to - tail fashion to create variety of compounds that include: β- carotene, rubber, carotenoids, limonene etc.

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