Lipids: Essential Biomolecules and Their Roles
Lipids: Structure and Classification
Lipids are a structurally diverse group of biomolecules. The only common characteristic is that a significant part, or all, of their molecule is insoluble in water and soluble in organic solvents (alcohol, acetone, ether, chloroform, etc.). Unlike proteins, nucleic acids, and polysaccharides, lipids do not form large polymers.
Classification of Lipids
Fatty Acids and Derivatives
Fatty acids are long-chain carboxylic acids. The hydrocarbon chain can be saturated or unsaturated, with a number of carbon atoms that typically varies between 14 and 24. Two regions in the fatty acid molecule have very different properties:
- The long hydrocarbon chain is hydrophobic (water-insoluble) and chemically unreactive.
- The carboxyl group is hydrophilic (water-soluble) and highly reactive.
Key Properties of Natural Fatty Acids
- The number of carbon atoms is almost always even. This is due to the mechanism of fatty acid synthesis.
- When a fatty acid is polyunsaturated, the double bonds are never conjugated but are separated by three bonds in the chain.
- Except for rare exceptions, the double bonds are always in the cis configuration; there are only traces of trans isomers.
The physical properties of fatty acids are largely derived from the chain length and degree of saturation. The longer the chain and the fewer the number of double bonds, the less water-soluble the molecule is.
Lipids Containing Fatty Acids
Acylglycerols (Acylglycerides)
These are formed by linking, via ester bonds, one, two, or three molecules of fatty acids to glycerol, producing mono-, di-, or triglycerides. These molecules are major energy reserves, and their calorific value (about 38 kJ/g) is much higher than that of carbohydrates and proteins (about 17 kJ/g). Triglycerides constitute almost 99% of stored lipids.
Phospholipids
Phospholipids are a group of lipids that contain a phosphate group, in addition to fatty acids. These molecules have two regions: a polar (hydrophilic) head and a nonpolar (hydrophobic) tail, giving the molecule an amphipathic character. They are essential components of the lipid bilayers of biological membranes. Structurally, phospholipids are divided into two groups:
- Phosphoacylglycerides
- Phosphosphingolipids
These groups are differentiated based on the alcohol they contain.
Functions of Lipids in the Human Body
- Energy Reserve: Lipids are a long-term energy store that can be used by the body over extended periods. They are stored in anhydrous form, without accompanying water molecules, making them very compact.
- Structural Function: Lipids are the major component of cell membranes, both the plasma membrane and the membranes of intracellular organelles. They also serve as thermal insulation due to their low thermal conductivity, and act as protective lubricants.
- Regulatory Function: Various types of lipids play regulatory roles, acting as hormonal regulators of metabolism and as mediators of information, both inside and outside of cells.