Box 25 Saturated or unsaturated

You may well have heard of saturated and unsaturated fats in the context of the sorts of foods we should and shouldn'tbe eating. This terminology derives from the type of fatty acids which make up the different types of fat.

Each carbon atom in the hydrocarbon chain of a saturated fatty acid such as stearic acid is bonded to the maximum possible number of hydrogen atoms (i.e. it is saturated with them).

Fatty acids containing one or more double bonds have fewer hydrogen atoms and are said to be unsaturated.

Compare the structures of stearic acid and oleic acid below. Both have identical structures except that oleic acid has two fewer hydrogen atoms and in their place a C==C double bond. A kink or bend is introduced into the chain at the point of the double bond; this means that adjacent fatty acids do not pack together so neatly, leading to a drop in the melting point. The presence of unsaturated fatty acids in membrane phospholipids makes the membrane more fluid.

Stearic acid (18.0) (saturated)

Oleic acid (18.1) (monounsturated)

Linoleic acid (18.2) (polyunsaturated)

Oleic acid (18.1) (monounsturated)

Linoleic acid (18.2) (polyunsaturated)

The second main group of lipids to be found in living cells are phospholipids. These have a similar structure to triacylglycerols, except that instead of a third fatty acid chain, they have a phosphate group joined to the glycerol (Figure 2.26), introducing a hydrophilic element to an otherwise hydrophobic molecule. Thus, phospholipids are an example of an amphipathic molecule, with a polar region at one end of the

Carboxyl Hydrocarbon group chain

H OHHHHHHHHHHHHHHH

I CCCCCCCCCCCCHCC

Fatty acid (palmitic acid), C15H31COOH

Glycerol Ester linkage

H OHHHHHHHHHHHHHHH

i II i i i i i i i i i i i i i i i H-C-O-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-H Palmitic acid (C15H31COOH) + H2O lllllllllllllll v 15 31 ' 2 HHHHHHHHHHHHHHH (Saturated)

OHHHHHHHHHHHHHHHHH

H-C-O-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-^C-H Stearic acid (C17H35COOH) + H2O

HHHHHHHHHHHHHHHHH (Saturated) OHHHHHHHH

^ fy ^ Oleic acid (C17H33COOH) + H2O (Unsaturated)

Figure 2.25 Fatty acids are linked to glycerol to form an acylglycerol. When all three -OH groups on the glycerol are esterified, the result is a triacylglycerol or triglyceride. The three fatty acids may or may not be the same. In the example shown, one of the fatty acids is unsaturated (see Box 2.5)

Choline

Phosphate

Glycerol

OHHHHHHHH

HHHHHHHH

OHHH

Fatty acids

Figure 2.26 Phospholipids introduce a polar element to acylglycerols by substituting a phosphate at one of the glycerol -OH groups. A second charged group may attach to the phosphate group; the phospholipid shown is phosphatidylcholine

Hydrophobic tail groups

Hydrophobic tail groups

Hydrophilic head groups

Figure 2.27 Phospholipids can form a bilayer in aqueous surroundings. A 'sandwich' arrangement is achieved by the polar phosphate groups facing outwards and burying the fatty acid chains within. Water is thus excluded from the hydrophobic region, a key property of biological membranes (see Figure 3.5)

Hydrophilic head groups

Figure 2.27 Phospholipids can form a bilayer in aqueous surroundings. A 'sandwich' arrangement is achieved by the polar phosphate groups facing outwards and burying the fatty acid chains within. Water is thus excluded from the hydrophobic region, a key property of biological membranes (see Figure 3.5)

molecule and a non-polar region at the other. This fact is essential for the formation of a bilayer when the phospholipid is introduced into an aqueous environment; the hydrophilic phosphate groups point outwards towards the water, while the hydrophobic hydrocarbon chains 'hide' inside (Figure 2.27, and c.f. Figure 2.8, micelle formation).

This bilayer structure forms the basis of all biological membranes (see Chapter 3), forming a barrier around cells and certain organelles. Phospholipids generally have another polar group attached to the phosphate; Figure 2.25 shows the effect of substituting serine.

The structural diversity of lipids can be illustrated by comparing fats and phospho-lipids with the final group of lipids we need to consider, the steroids. As can be seen from Figure 2.28, these have a completely different form, but still share in common the property of hydrophobicity. The four ring planar structure is common to all steroids, with the substitution of different side groups producing great differences in function. Cholesterol is an important component of many membranes.

It would be wrong to gain the impression that living cells contain only molecules of the four groups outlined above. Smaller organic molecules play important roles as precursors or intermediates in metabolic pathways (see Chapter 6), and several inorganic ions such as potassium, sodium and chloride play essential roles in maintaining the living cell. Finally, some macromolecules comprise elements of more than one group, for example, lipopolysaccharides (carbohydrate and lipid) and glycoproteins (protein and carbohydrate).

Figure 2.28 All steroids are based on a four-ringed structure. The presence of an -OH group on the lower left ring makes the molecule a sterol. Cholesterol plays an important role in the fluidity of animal membranes by interposing itself among the fatty acid tails of phospholipids. The only bacterial group to contain sterols are the mycoplasma; however some other groups contain hopanoids, which have a similar structure and are thought to play a comparable role in membrane stability

Figure 2.28 All steroids are based on a four-ringed structure. The presence of an -OH group on the lower left ring makes the molecule a sterol. Cholesterol plays an important role in the fluidity of animal membranes by interposing itself among the fatty acid tails of phospholipids. The only bacterial group to contain sterols are the mycoplasma; however some other groups contain hopanoids, which have a similar structure and are thought to play a comparable role in membrane stability

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