AS Biology - Carbohydrates


Introduction to carbohydrates

The genral formula of carbohydrates is Cx(H2O)x. Sugars are usually monosaccharides, single sugar units, disaccharides, in which 2 sugar units are joined together throught the process of condensation. Long straight or branched chains of sugar untis are polysaccharides.

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Monosaccharides are single sugar units with the general formula of (CH2O)n, where n represents the number of carbon atoms in the molecule. Monosaccharides have between 3 and 7 carbon atoms, but the most common number is 6. For example, the monosaccharides glucose, fructose and galactose all have 6 carbon atoms and are known as hexose sugars.

A hexose sugar has a ring structure formed by 5 carbon atoms and 1 oxygen atom; the sixth carbon sticks out at the top or bottom of the ring. The carbon atoms in the molecule are numbered. Number 1 is on the right side of the molecule. The side branches stick out of the top or bottom of the ring, and their posistion determines the type of sugar molecule and it's properties.

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Disaccharides are two single sugar unis joined together in a condensation reaction. A condensation reaction releases a water molecule from the two single sugar units after they have combined. The bond that forms between the two glucose molecules are glycosidic bonds. Common disaccharides are found in food are sucrose, maltose and lactose.

The bond in Maltose is known as a 1,4 glycosidic bond because it forms between carbon 1 on one molecule and carbon 4 on the other molecule. 

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Disaccharides; part 2.

The glycosidic bonds between 2 sugar units can be split by hydrolysis. This is the reverse of condensation; water is added to the bond and the molecule splits into two. Hydrolysis of carbohydrates takes place in the gut, and when carbohydrates stores in a cell are broken down to release sugars. 

Condensation and hydrolsis reactions. In a condensation reaction, sub units are joined together in a reaction which involves the removal of a molecule of water. For example, two glucose molecules can be combined to form one maltose molecule + one molecule of water. Many glucose molecules can be joined to create a starch molecule by the same method. Hydrolysis is the same reaction in reverse - a maltose molecule can be broken down to two glucose molecules by the insertion of a water molecule

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Polysaccharides are polymers made up from simple sugar monomers joined by glycosidic links into long chains.There are 3 main types found in food; starch and glucose in plants, and glycogen in animals.They are hard to dissolve and do not taste sweet. 

Starch and glycogen act as engery storage molecules within cells. These polysaccharides are suitable for storage because they are compact molecues with low solubilty in water. This means that they do not affect the concentration of water in the cytoplasm and so do not affect the movement of water into or out of the cell by osmosis

Starch, the storage carbohydrate found in plants, is made up of a mixture of 2 molecules. Amylose and amylopectin

Amylose: between 200 and 5000 glucose molecules with 1,4 glycosidic links. The posistion of the bonds causes the cain to coil into a spiral shape.

Amylopectin: has side branches. A 1,6 glycosidic links holds each side branch onto the main chain. 

The compact spiral structure of starch and it's insoluble nature make it an excellent storage molecule. It doesn't diffuse across cell membrane and has very little osmotic effect within the cell.

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Polysaccharides; part 2

Bacteria, fungi and animals store glycogen instead of starch. Glycogen is another polymer made out of glucose molecules. Its numerous side branches means that it can be rapidly hydrolysed, giving easy access to stored engergy. In humans glycogen is stored in the muscles and liver. 

Cellulose has up to 10000 glucose molecules and they are joined to form a chain with no branches. Cellulose is indigestible in the human gut, cellulose has an important function in the movement of material through the digestive tract. Dietary fibre is thought to be important in the prevention of diseases such as coronoary heart disease, diabetes and bowl ancer.

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