Carbohydrates

AS and A Level Science

Carbohydrates

Carbohydrates contain 3 elements:

1. Carbon (C)

2. Hydrogen (H)

3. Oxygen (O)

Carbohydrates are found in one of three forms:

1. Monosaccharides

2. Disaccharides (both sugars)

3. Polysaccharides

Monosaccharides

General formula:

(CH2O)n where n is a number between 3 and 9. They are classified according to the number of carbon atoms. The monosaccharides you will have to know fall into these categories:

C = 3 = triose

C = 4 = tetrose

C = 5 = pentose

C = 6 = hexose

Trioses: (e.g. glyceraldehydes), intermediates in respiration and photosynthesis.

Tetroses: rare.

Pentoses: (e.g. ribose, ribulose), used in the synthesis of nucleic acids (RNA and DNA), co-enzymes (NAD, NADP, FAD) and ATP.

Hexoses: (e.g. glucose, fructose), used as a source of energy in respiration and as building blocks for larger molecules.

All but one carbon atom have an alcohol (OH) group attached. The remaining carbon atom has an aldehyde or ketone group attached.

Ring form:

Due to the bond angles between the carbon atoms, it is possible for pentoses and hexoses to form stable ring structures. The carbon atoms are numbered 1 to 5 in pentoses and 1 to 6 in hexoses.

Depending on the orientation of the OH group on carbon 1, the monosaccharide can have either a or b configurations.

Disaccharides and glycosidic bonds

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Ring form:

Due to the bond angles between the carbon atoms, it is possible for pentoses and hexoses to form stable ring structures. The carbon atoms are numbered 1 to 5 in pentoses and 1 to 6 in hexoses.

Depending on the orientation of the OH group on carbon 1, the monosaccharide can have either a or b configurations.

Disaccharides and glycosidic bonds

These are formed when two monosaccharides are condensed together. One monosaccharide loses an H atom from carbon atom number 1 and the other loses an OH group from carbon 4 to form the bond.

The reaction, which is called a condensation reaction, involves the loss of water (H2O) and the formation of a 1,4-glycosidic bond. Depending on the monosaccharides used, this can be an a-1,4-glycosidic bond or a b-1,4-glycosidic bond.

The reverse of this reaction, the formation of two monosaccharides from one disaccharide, is called a hydrolysis reaction and requires one water molecule to supply the H and OH to the sugars formed.

Examples of Disaccharides

Sucrose: glucose + fructose,

Lactose: glucose + galactose,

Maltose: glucose + glucose.

Maltose: glucose + glucose.

Sucrose is used in many plants for transporting food reserves, often from the leaves to other parts of the plant. Lactose is the sugar found in the milk of mammals and maltose is the first product of starch digestion and is further broken down to glucose before absorption in the human gut.

Biochemical tests

All monosaccharides and some disaccharides including maltose and lactose are reducing sugars. These can be tested for, by adding Benedict's reagent to the sugar and heating in a water bath. If a reducing sugar is present, the solution turns green, then yellow and finally produces a brick red precipitate. Non-reducing sugars can also be tested for using Benedict's reagent but first require addition of an acid and heating to hydrolyse (break apart) the sugar. The acid must then be neutralised using an alkali like sodium hydroxide before carrying out the test as described above.

Polysaccharides

Functions of carbohydrates

Biochemical test

Iodine solution or potassium iodide solution can be used to test for the presence of starch. A positive result changes the solution from an orange-brown to a blue-black colour. - refer to gcse and title biochemical test for carbohydrate.