![]() ![]() The two ‘tails’ are based on chains of carbon atoms and are hydrophobic. Figure 2 is a sketch representing an amphiphilic lipid molecule, such as a phospholipid. Such molecules are termed ‘amphiphilic’ and exhibit particularly interesting properties when mixed with water. Many important lipids, including the phospholipids that make up cell membranes, contain both hydrophobic and hydrophilic groups. Insolubility in water is also a useful property when it comes to forming a barrier to keep some substances separate from others. A material like this with a certain amount of fluidity can be extremely useful in certain situations. Think of a lump of lard: if you press on it, it dents. ![]() ![]() Because of the weak bonding between the molecules, lipids tend to be fluid aggregates that can easily change shape. Their hydrophobic nature means that lipids tend to separate from water and from all water-soluble, hydrophilic compounds and to group themselves together as large hydrophobic aggregates, held together by weak hydrophobic interactions. ![]() The interplay between hydrophilic and hydrophobic groups is important in many biological materials. As the name suggests, they have an affinity for water and are more likely to dissolve in it. In contrast, polar substances or groups are termed hydrophilic or ‘water-loving’. Hydrophobic literally means ‘water-hating’ and generally applies to non-polar substances or chemical groups. The reason for their insolubility in water is that lipids contain large hydrophobic groups. Fats and oils are important in cells as energy storage compounds, while the cell wall itself is constructed from a class of lipids known as phospholipids. The terms ‘fat’ and ‘oil’ simply describe whether a lipid is solid (a fat) or liquid (an oil) at room temperature. Lipids make up the fatty components of living organisms and are the major components of margarine, cooking oil, butter and the white fat associated with meat. What they have in common is that they are all insoluble in polar liquids like water, but soluble in organic (carbon-based) solvents: by this I mean the sort of smelly solvents you tend to find in paints, glues and degreasing agents chloroform is one example. Chemically, lipids are a rather varied group of compounds that include all the substances you might already think of as fats or oils. The cell membrane is constructed from lipids. ![]()
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