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Lipids consist of a polar or hydrophilic (attracted to water) head and one to three nonpolar or hydrophobic (repelled by water) tails (Fig. 1). Since lipids have both functions, they are called amphiphilic. The hydrophobic tail consists of one or two (in triglycerides, three) fatty acids. These are unbranched chains of carbon atoms (with the correct number of H atoms), which are connected by single bonds alone ( saturated fatty acids) or by both single and double bonds ( unsaturatedAn unsaturated fat is a fat or fatty acid in which there is one or more double bonds between carbon atoms of the fatty acid chain. Such fat molecules are monounsaturated if each contains one double bond, and polyunsaturated if each contain more than one. fatty acids). The chains are usually 14-24 carbon groups long.

For lipids present in biological membranes, the hydrophilic head is from one of three groups:

1. GlycolipidGlycolipids are carbohydrate-attached lipids. Their role is to provide energy and also serve as markers for cellular recognition. They occur where a carbohydrate chain is associated with phospholipids in the cell surface membrane. The carbohydrates are fos, whose heads contain an oligosaccharideAn oligosaccharide is a saccharide polymer containing a small number (typically three to six) of component sugars, also known as simple sugars. They are generally found either O or N linked to compatible amino acid side chains in proteins or to lipid moie with 1-15 saccharide (sugar) residues.
2. Phospholipids, whose heads contain a positively charged group that is linked to the tail by a negatively charged phosphate group.
3. SterolSterols are a subgroup of steroids with a hydroxyl group in the 3-position of the A-ring. They are lipids synthetised from Acetyl coenzyme A. Important sterols include cholesterol, phytosterols, and some steroid hormones.s, whose heads contain a planar steroid ring, for example, cholesterolCholesterol is a steroid lipid, found in the cell membranes of all body tissues, and transported in the blood plasma, of all animals. Most cholesterol is produced internally, not dietary in origin. It is present in higher concentrations in tissues which e (only in animals).

In an aqueous environment, the heads of lipids are turned towards the environment, and the tails are turned towards a hydrophobic region of another molecule. With lots of lipids present, the tails "prefer" to turn toward each other, forming a hydrophobic region. This can be a bilayer or a micelle (Fig. 2). Micelles are spheres and can only reach a certain size, whereas bilayers have no limit to their extension. They can also form tubules.

Driven by hydrophilic and hydrophobic forces, the nonpolar tails of lipids (U) tend to cluster together, forming a lipid bilayerIn biology and chemistry, a lipid bilayer is a membrane or zone of membrane composed only of lipid. Lipid bilayer is the foundation of all biological membranes, and is a prerequisite of cell-based life. Within a critical range of concentrations, certain k (1) or a micelleIn chemistry, a micelle (also micella plural micellae is a particular grouping of molecules. Surfactants form micelles when they reach the critical micelle concentration (CMC). Surfactants are chemical amphipathic compounds. It means that they contain bot (2). The polar heads (P) face the aqueous environment.

Lipid bilayers form the foundation of all biological membranes.