aqueous medium lipid bilayer

"I hydrophobic J lipid tails hydrophilic lipid headgroups aqueous medium

Figure 2.19. Lipids.

fatty acids attached to glycerol, but only two of them. In place of the third fatty acid is a polar, often electrically charged head group. The head group is joined to the glycerol through phosphate in a structure called a phosphodiester link. The combination of head group and negatively charged phosphate is able to associate strongly with water—it is said to be hydrophilic. The two fatty acids, on the other hand, form a tail that, like olive oil, is hydrophobic. Phospholipids can therefore neither dissolve in water (because of their hydrophobic tails) nor remain completely separate, like olive oil (because then the head group could not associate with water). Phospholipid molecules therefore spontaneously form lipid bilayers between 5 and 10 nm thick (Fig. 2.19c) in which each part of the molecule is in its preferred environment. Cell membranes are lipid bilayers plus some added protein.

Medical The Kinks Have It: Double Bonds, Membrane Fluidity, Relevance and Evening Primroses 2 1

One of the fatty acids commonly found as a component of animal fat is stearic acid, illustrated as (a) opposite. Its 18 carbon atoms are joined by single bonds, making a long straight molecule. In contrast, oleic acid (b) has a double bond between the ninth and tenth carbons. This introduces a kink in the chain. A fatty acid with kinks is less able to solidify because it is less able to pack in a regular fashion. The more double bonds in a fatty acid, the lower its melting point. Thus stearic acid melts at 69.6oC, while oleic acid melts at 13.4°C and is therefore liquid at room temperature. Fatty acids containing double bonds between the carbon atoms are said to be unsaturated. Polyunsaturated fatty acids have more than one double bond, more than one bend, and therefore even lower melting points. Linoleic acid (c) has two, and melts at —9°C; linolenic acid (d) has three and a melting point of —17°C.

For the triacylglycerols inside our cells, and the phospholipids in our membranes, the same rule applies—the more double bonds in the acyl groups, the lower the melting point. The large masses of triacylglycerols containing stearic acid in animal bodies are liquid at body temperature but form solid lumps of fat at room temperature, while trioleoylglycerol is liquid at room temperature (but should not be kept in the fridge!). Membranes must not solidify—if they did, then they would crack and the cell contents would leak out each time the cell was flexed. Unsaturated fatty acids play an essential role in maintaining membrane liquidity.

Mammals are unable to introduce double bonds beyond carbon 9 in the fatty acid chain. This means that linoleic and linolenic acids must be present in the diet. They are known as essential fatty acids. Fortunately, the biochemical abilities of plants are not so restricted, and plant oils form a valuable source of unsaturated fatty acids.

The normal form of linolenic acid, shown in (d), is a-linolenic acid, which has its double bonds between carbons 9 and 10, 12 and 13, and 15 and 16. Some plant seed oils contain an isomer of linolenic acid with double bonds between carbons 6 and 7, 9 and 10, and 12 and 13. This is called y-linolenic acid (e). The attractive, yellow-flowered garden plant called the evening primrose (Oenethera perennis) has seeds that contain an oil with y-linolenic in its triacyglycerols. The 6 to 7 double bond introduces a kink closer to the glycerol. This is thought to increase fluidity when incorporated into membrane lipids. No one really knows if it really has the marvelous health effects that some claim for it, or if it does, how it works. This ignorance has not stopped people from making lots of money from evening primrose oil. It is included in cosmetics and in alternative medicines for internal and external application. y -Linolenic acid occurs in other plants—the seeds of borage (Borago officinalis) are one of the richest sources. It is even found in some fungi. However, these lack the romance of the evening primrose!

Stearic acid, C18 no double bonds, melts at 69.6° (a)

Oleic acid, C18 one double bond, melts at 13.4° (b)

Linoleic acid, C18 two double bonds, melts at -9°

Linoleic acid, C18 two double bonds, melts at -9°

Linolenic acid, C18 three double bonds, melts at -17° (d)

y-Linolenic acid, C18 three double bonds

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