A level 生物之 Carbohydrates (1)--满满的干货

Carbohydrates contain only the elements of carbon, hydrogen and oxygen. The group includes monomers, dimers and polymers, which as below:

Monosaccharides

All have the formula (CH2O)n, where n is between 3 and 7. The most common and important monosaccharide is glucose, which is a six-carbon sugar. It's formula is C6H12O6 and its structure is below:

Glucose forms a six-sided ring. The six carbon atoms are numbered as shown, so we can refer to individual carbon atoms in the structure. In animals glucose is the main transport sugar in the blood, and its concentration in the blood is carefully controlled.

There are many monosaccharides, with the same chemical formula (C6H12O6), but different structural formulae. These include fructose and galactose. Common five-carbon sugars (where n = 5, C5H10O5) include ribose and deoxyribose (found in nucleic acids and ATP).

Disaccharides

Disaccharides are formed when two monosaccharides are joined together by a glycosidic bond. The reaction involves the formation of a molecule of water (H2O):

 

This shows two glucose molecules joining together to form the disaccharide maltose. Because this bond is between carbon 1 of one molecule and carbon 4 of the other molecule it is called a 1-4 glycosidic bond.

This kind of reaction, where water is formed, is called a condensation reaction. The reverse process, when bonds are broken by the addition of water (e.g. in digestion), is called a hydrolysis reaction.

• polymerisation reactions are condensation reactions

• breakdown reactions are hydrolysis reactions

There are three common disaccharides:

• Maltose   (or malt sugar) is glucose & glucose. It is formed on digestion of starch by amylase, because this enzyme breaks starch down into two-glucose units. Brewing beer starts with malt, which is a maltose solution made from germinated barley. Maltose is the structure shown above.

• Sucrose   (or cane sugar) is glucose & fructose. It is common in plants because it is less reactive than glucose, and it is their main transport sugar. It's the common table sugar that you put in tea.

• Lactose   (or milk sugar) is galactose & glucose. It is found only in mammalian milk, and is the main source of energy for infant mammals.