Eduqas Biology for A Level Book 1: 2nd Edition

Phospholipids Phospholipids are a special type of lipid. Each molecule has the unusual property of having one end that is soluble in water and one that is not. The diagram shows that one end of the molecule has a lot of oxygen atoms, in the glycerol group and the phosphate, and so this end of the molecule interacts with water and is hydrophilic . It is described as the polar head of the molecule. As in triglycerides, the fatty acid tails do not have any oxygen atoms and do not interact with water so they are hydrophobic and are non-polar. Structure of a phospholipid Waxes Waxes are lipids and melt above about 45ºC. They have a waterproofing role in both animals, such as in the insect exoskeleton, and plants, in the leaf’s cuticle. Properties of lipids The differences in the properties of fats and oils come from variations in the fatty acids. If the hydrocarbon chain has only single carbon–carbon bonds, then the fatty acid is saturated , because all the carbon atoms are linked to the maximum possible number of hydrogen atoms. That is, they are saturated with hydrogen atoms. The fatty acid chain is a straight zigzag, as the photograph of the model on p23 shows, and the molecules can align readily, so fats are solid. They remain semi-solid at body temperature and are useful for storage in mammals. Animal lipids often contain saturated fatty acids. A saturated fatty acid, CH 3 (CH 2 ) 14 COOH, palmitic acid If any carbon–carbon bond is not a single bond, the molecule is unsaturated and the chain gets a kink. The molecules cannot align uniformly and the lipid does not solidify readily. This is why unsaturated lipids are oils, which remain liquid at room temperature. Plant lipids are often unsaturated and occur as oils, such as olive oil and sunflower oil. If one carbon–carbon double bond is present, the lipids are mono-unsaturated, whereas if there are many carbon–carbon double bonds, the lipids are described as polyunsaturated. Fig 07 H C H H 3 C H C H H C H H C H H C H H C H H C H H C H H C H H C H H C H H C H H C H H C C H O O H saturated fatty acid H C H H 3 C H C H H C H H C H H C H H C H H C H H C H C H H C H H C H C H H C H H C H H C H H C C H O O H unsaturated fatty acid fatty acid tail fatty acid tail CH 2 O – C polar head is hydrophilic (interacts with water) non-polar tails are hydrophobic (do not interact with water) O CHO – C CH 2 O O phosphate Stretch & challenge A molecule with one non-single C–C bond is mono-unsaturated. The triglycerides in olive oil are rich in such fatty acids. A molecule with several non-single carbon– carbon bonds is polyunsaturated. The triglycerides in sunflower oil and oily fish, such as salmon, are rich in these fatty acids. Study point Note how to spell hydrophilic. It has one ‘l’ and only one ‘y’. Link Cell membranes are described on p54. 1.4 Knowledge check Match the terms 1–4 with their meanings. 1. Hydrophilic 2. Hydrophobic 3. Saturated 4. Unsaturated A. Non-polar; not able to interact with water molecules. B. Contains at least one carbon–carbon bond that is not single. C. All carbon–carbon bonds are single bonds. D. Polar; able to interact with water molecules. Eduqas Year 1/AS Biology 22 Key terms Hydrophilic: Polar; a molecule or ion that can interact with water molecules because of its charge. Hydrophobic: Non-polar; a molecule that cannot interact with water molecules because it has no charge. Saturated fatty acid: all carbon- carbon bonds are single. Unsaturated fatty acid: at least one carbon-carbon bond is not single.