Carbohydrates and lipids — common mistakes

Pay close attention to detail when drawing or describing structures. For example, ensure you can draw the difference between alpha and beta glucose, or correctly identify an ester bond in a triglyceride. Examiners often award marks for specific keywords like 'glycosidic bond', 'ester bond', 'condensation', 'hydrolysis', 'hydrogen bonds', and 'microfibrils'. Also, be precise about the bonds, e.g., 'α-1,4 glycosidic bond'.

Carbohydrates (e.g., glucose, starch) are for short- and medium-term energy and structural roles (cellulose). They have a general formula Cₓ(H₂O)y and are often polar. Lipids (e.g., triglycerides) are for long-term energy storage, insulation, and membrane structure. They have proportionally less oxygen, are non-polar, and thus insoluble in water, yielding more energy per gram.

Both use these reactions. In carbohydrates, monosaccharides join via condensation to form glycosidic bonds, releasing water. Hydrolysis breaks these bonds by adding water. In lipids, fatty acids and glycerol join via condensation to form ester bonds, releasing water. Hydrolysis breaks these ester bonds. Remember the specific bond type for each molecule class.

The high tensile strength of cellulose, due to hydrogen-bonded microfibrils, provides rigidity to the plant cell wall. This prevents the cell from bursting when it takes in water by osmosis (maintains turgor pressure) and provides overall structural support to the plant.

Saturated fatty acids have no carbon-carbon double bonds in their hydrocarbon tails, making them straight and able to pack closely, resulting in solids at room temperature (fats). Unsaturated fatty acids have one or more C=C double bonds, which create kinks in the tails, preventing close packing and resulting in liquids at room temperature (oils).