A level Biology B · Edexcel 9BI0 · Topic 1A
Carbohydrates
Key points
- Monosaccharides include glucose (a hexose), galactose, fructose and the pentoses ribose and deoxyribose. α-glucose and β-glucose differ in the position of the –OH on carbon 1.
- Two monosaccharides join by a glycosidic bond in a condensation reaction to give a disaccharide: maltose (glucose + glucose), sucrose (glucose + fructose), lactose (glucose + galactose).
- Starch is a plant energy store: amylose (1,4 bonds, coiled, compact) and amylopectin (1,4 and 1,6 bonds, branched, so it is broken down quickly).
- Glycogen is the animal store: even more branched than amylopectin, so glucose can be released very quickly.
- Cellulose is made of β-glucose; alternate molecules are flipped, giving straight chains held together by many hydrogen bonds into strong microfibrils for plant cell walls.
- Storage polysaccharides are insoluble, so they do not affect water potential.
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Carbohydrates are made of carbon, hydrogen and oxygen and range from small, soluble sugars to huge polymers. Their structures explain why some are quick energy sources, some are long-term stores and one is the toughest material in a plant.
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