Water, carbohydrates and lipidsEdexcel A-Level Biology A: Revision notes

Water is a polar molecule. The electrons in each O–H covalent bond are shared unequally, so oxygen is slightly negative (δ−) and each hydrogen slightly positive (δ+). A molecule with two oppositely charged ends is a dipole.

This makes water an excellent solvent for charged and polar substances. Water dipoles are attracted to ions and surround them, separating them from a lattice (hydration), and they also dissolve polar molecules such as glucose and amino acids. Because of this, blood plasma, xylem sap and tissue fluid can transport dissolved ions, sugars, amino acids and waste products around organisms. Non-polar molecules such as lipids do not dissolve, which is why they travel in blood as lipoproteins.

Monosaccharides are single sugar units, the monomers of carbohydrates. Glucose (C₆H₁₂O₆), fructose and galactose are hexoses. They are small and soluble, and glucose is the main respiratory substrate.

Disaccharides are two monosaccharides joined together:

• maltose = glucose + glucose
• sucrose = glucose + fructose
• lactose = glucose + galactose

Disaccharides are soluble, so they are suitable for transport (sucrose in plants) and as a food source (lactose in milk), but they must be hydrolysed to monosaccharides before cells can use them.

Monosaccharides join by a condensation reaction: a hydroxyl group from each sugar reacts, a molecule of water is removed and a glycosidic bond (a C–O–C link) forms between them.

$\text{glucose} + \text{glucose} \rightarrow \text{maltose} + \text{H}_2\text{O}$

The reverse is hydrolysis: a molecule of water is added across the glycosidic bond, which breaks it. Digestive enzymes such as amylase and maltase catalyse hydrolysis, releasing monosaccharides small enough to be absorbed. Repeated condensation makes polysaccharides, and repeated hydrolysis breaks them down.

Polysaccharides are polymers of many monosaccharides joined by glycosidic bonds. Starch and glycogen are made of α-glucose and are the energy stores of plants and animals.

• Starch is a mixture of amylose (unbranched, 1,4 links, coiled into a helix so it is compact) and amylopectin (1,4 links with 1,6 branch points).
• Glycogen is similar to amylopectin but more highly branched with shorter chains.

They suit storage because they are large and insoluble (no osmotic effect, cannot diffuse out of cells), compact, and made of many glucose units. Branching gives many free ends, so enzymes hydrolyse the stored polymer quickly. Glycogen's greater branching suits animals' high energy demand. Beta-glucose and cellulose are not needed in this topic.

A triglyceride is one glycerol molecule joined to three fatty acids. Each fatty acid's carboxyl (–COOH) group reacts with a hydroxyl (–OH) group on the glycerol in a condensation reaction, releasing three molecules of water and forming three ester bonds.

$\text{glycerol} + 3\ \text{fatty acids} \rightarrow \text{triglyceride} + 3\,\text{H}_2\text{O}$

Triglycerides are non-polar and insoluble in water. They are an energy store because the long hydrocarbon tails contain many C–H bonds, releasing more energy per gram than carbohydrate.

Saturated fatty acids have only single C–C bonds in the hydrocarbon tail, so each carbon holds the maximum number of hydrogen atoms. The tails are straight and pack closely, so saturated fats (butter, lard) are usually solid at room temperature.

Unsaturated fatty acids contain one or more C=C double bonds (monounsaturated has one, polyunsaturated several). Each double bond puts a kink in the tail, so molecules cannot pack closely and unsaturated fats (olive oil) are usually liquid at room temperature.

Diets high in saturated fat are linked to raised blood cholesterol and a greater risk of cardiovascular disease.