International A Level Biology / Chemistry for biologists
Monosaccharides and disaccharides revision notes
Edexcel International A Level Biology (YBI11), chapter 1: Chemistry for biologists, specification point 1.2, 1.4. Free lesson.
Watch the video lesson · International A Level Biology past papers
Carbohydrates are grouped by how many sugar units they contain. Knowing how monosaccharides join and split helps you identify disaccharides and write their reactions.
How to sort carbohydrates by size
Carbohydrates contain carbon, hydrogen and oxygen, and are built from sugar units. The prefixes in their names tell you how many units they have. Monosaccharides have one unit: glucose, fructose and galactose. They are small and soluble. Glucose is a main fuel used in respiration. Disaccharides have two monosaccharides joined together. They must be split into monosaccharides before a cell can use them. Polysaccharides have many monosaccharides joined in a long chain. Starch in plants and glycogen in animals are energy stores. Mono means one, di means two, poly means many, and…
How to recognise and number α-glucose
To number the carbons in α-glucose, use the oxygen in the ring as your landmark. It lies between carbons 1 and 5. The position of the OH group on carbon 1 identifies the alpha form. Start at carbon 1 and count clockwise around the ring. Carbon 4 is on the far side, and carbon 5 is beside the ring oxygen. Carbon 6 is outside the ring in the CH₂OH group, above carbon 5. The OH group on carbon 1 points below the ring in α-glucose. The ring drawing may leave out single hydrogens on ring carbons for clarity, but include them when you draw glucose in an exam. Glucose, fructose and galactose are…
How condensation forms maltose
A condensation reaction joins two molecules and releases water. When two α-glucose molecules join, the OH groups on carbon 1 of one glucose and carbon 4 of the other react. They release H₂O, leaving one oxygen to bridge the two carbons. The oxygen bridge between carbons 1 and 4 is a 1,4 glycosidic bond. The disaccharide made from two glucose molecules is maltose. Hydrolysis is the reverse reaction: water is added, the glycosidic bond breaks, and two glucose molecules are produced.
How to tell maltose, sucrose and lactose apart
Each of these disaccharides contains two monosaccharides joined by a glycosidic bond. All three contain glucose, so learn the other monosaccharide to distinguish them. Maltose is glucose + glucose. It is formed when starch is digested. Sucrose is glucose + fructose. It is table sugar and is also found in phloem sap. Lactose is glucose + galactose. It is the sugar in milk. Sucrose and lactose are easy to swap, so link sucrose with fructose and lactose with galactose.
How to write hydrolysis equations
Hydrolysis uses water to split a glycosidic bond, so include H₂O on the left-hand side of the equation. Check that the numbers of each type of atom balance on both sides. For sucrose, the products are glucose and fructose. Both have the formula C₆H₁₂O₆. The equation balances because the water supplies the extra hydrogen and oxygen atoms needed in the products.