CarbohydratesEdexcel A-Level Biology B: Revision notes
Section 1
Monosaccharides: glucose and ribose
Carbohydrates contain carbon, hydrogen and oxygen. Monosaccharides are single sugar units (monomers); disaccharides are two monosaccharides joined; polysaccharides are many monosaccharides joined into a polymer.
Glucose is a hexose (six carbon atoms, C₆H₁₂O₆) that exists as a ring. It has two isomers. In α-glucose the hydroxyl (OH) on carbon 1 is below the ring; in β-glucose it is above the ring. Ribose is a pentose (five carbons, C₅H₁₀O₅) and is a component of RNA. Fructose and galactose are other common monosaccharides.
Do not say glucose has 'different atoms' in α and β forms. They contain the same atoms; only the position of the OH on carbon 1 differs.
Section 2
Condensation and hydrolysis: disaccharides
Monosaccharides join by a condensation reaction: a molecule of water is removed and a glycosidic bond forms between them. The reverse is hydrolysis: water is added and the glycosidic bond is broken.
Three disaccharides:
- maltose = glucose + glucose
- sucrose = glucose + fructose
- lactose = glucose + galactose
Condensation: glucose + glucose → maltose + H₂O. Hydrolysis uses one water molecule for every glycosidic bond broken.
Learn the three disaccharides as pairs: maltose is glucose + glucose, sucrose is glucose + fructose, lactose is glucose + galactose.
Section 3
Starch and glycogen: energy stores
Starch is the storage polysaccharide of plants and is a mixture of two polymers of α-glucose:
- amylose: unbranched chains coiled into a helix, so very compact
- amylopectin: branched chains, with many free ends for rapid hydrolysis
Glycogen is the storage polysaccharide of animals (and fungi). It is also α-glucose but more highly branched than amylopectin, with shorter chains, so glucose can be released very quickly in active tissues such as muscle and liver.
All are large, insoluble and compact: they hold lots of glucose in a small space, do not diffuse out of cells and do not change the water potential of the cell.
Section 4
Cellulose: structural strength
Cellulose is a polymer of β-glucose. For glycosidic bonds to form between β-glucose units, every other molecule is inverted (rotated 180°). This gives long, straight, unbranched chains.
Chains lie parallel and hydrogen bonds form between them, creating bundles called microfibrils. Each hydrogen bond is weak, but there are very many, so cellulose has high tensile strength. It is found in plant cell walls, where it supports the cell and prevents it bursting when it takes in water by osmosis.
Section 5
Structure and function: summary
Relate structure to function in exam answers:
- Glucose: small and soluble, so it is transported easily; its bonds release energy in respiration.
- Starch: compact, insoluble store of glucose that does not affect water potential.
- Glycogen: highly branched, so many ends allow rapid hydrolysis to supply respiration.
- Cellulose: straight chains with hydrogen bonds between them give strength.
Worked example: a polysaccharide is made of 500 glucose units. Number of water molecules removed = 500 − 1 = 499, because each glycosidic bond formed releases one water.
Do not write that cellulose is 'strong because of covalent bonds'. The strength comes from the very many hydrogen bonds between parallel chains.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Carbohydrates
- A brewing technician is studying how barley grains are converted into fermentable sugars. Barley grains store starch. When the grains germinate, enzymes break the starch down into maltose, which is then broken down further so that yeast can ferment the sugars.Describe what happens when a molecule of maltose is hydrolysed.2 marks
- A biochemistry student is drawing the structures of three monosaccharides. She draws α-glucose, β-glucose and ribose, the sugar found in RNA.Ribose, maltose and glycogen are all carbohydrates. Classify each as a monosaccharide, a disaccharide or a polysaccharide, and state how these three groups differ in structure.2 marks
- A student is investigating milk. Lactose is the main carbohydrate in milk. A dairy company treats milk with an enzyme that hydrolyses lactose, producing 'lactose-free' milk for customers who cannot digest lactose.Describe how a molecule of lactose is formed from its monomers.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).