BY1 Carbohydrates

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  • Created by: mickey243
  • Created on: 09-02-14 09:39


Carbohydrates are organic compounds containing the elements carbon,hydorgen and oxygen.

In carbohydrates, the basic monomer unit is called the monosaccharide

Two mosaccharides combine together to form a disaccharide.

Many monosaccharide molecules combine to form a polysaccharide

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Monosaccharides provide the building blocks for the larger carbohydrates.

They have the general formula CH2O and their name is determined by the number of carbon atoms.

Monosaccharides have two functions:

1.they are used as a source of energy in respiration. Carbon-hydrogen bods are broken to release energy, which is transferred to make adenosine triphosphate

2.they act as building blocks for larger carbohydrates

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Glucose has a general formula of C6H12O6 and is a hexose sugar.

Glucose exists as two isomers- alpha and beta glucose. They both have the same molecular formula but they differ in the arragment of atoms and this has a significant effect on the properties of their polymers.

Beta glucose differs from alpha glucose in the position of -OH and -H on carbon 1.

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Disaccharides consist of two monosaccharide units linked together by a formation of glycosidic bond and the elimination of water. This is called a condensation reaction.

alpha glucose+alpha glucose=maltose

glucose and galactose=lactose

glucose and fructose=sucrose

Disaccharides are used for storage and transport. For example, carbohydrate is transported in the sieve tube of phloem in the form of sucrose.

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Testing for the presence of sugars

A Benedict's reagent is used to test for the presence of glucose in a solution.

you add an equal volume to the solution that's being tested and heat in the boiling water bath.

All monosaccharides are reducing sugars because they contain an aldehyde group,meaning that they donate an electron to the raegent.

If reducing sugars such as glucose is present, the solution will gradually turn though green,yellow and orane to red.

Some saccharides, such as sucrose are non-reducing sugars and so a negative test will be achieved. Sucrose needs to be broken down into its constituent monosaccharides by heating with hydrochloric acid.Benedict's solution needs an alkaline conditions to work and so the solution is neutralised by adding alkali until fizzing stops. Afterwards, the Benedict's reagent is added and heated as before.

The Benedict's test is qualitative ot at best semi-quantitative

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Polysaccharides + starch

Polysaccharides are large, complex molecules called polymers. They are formed when many monosaccharides join together by glycosidic bonds.


Glucose is the main source of energy in cells and so it needs to be stored adequately.However, glucose dissolves in water and this would lower the water potential, and so water woud enter the cell by osmosis. To avoid this problem, glucose is converted into starch which is a long polysaccharide and therefore insoluble.

Starch is made up of many alpha glucose monomers and consists of two polymers, amylose and amylopectin. Amylose is linear and straight whereas amylopectin is branched. This makes the molecule more complex and compact, and also means that glucose can be obtained by hydrolysis from many different endings at the same time.


Glycogen is the main source of energy in animals and is very similar to amylopectin but is more branched and so provides energy faster

To test for the presence of starch, an orange-brown iodine solution is added and a blue-black colour is produced.

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Cellulose is a structural polysaccharide. It is present in the cell walls.

Cellulose consists of many long parallel chains of beta glucose molecules cross linked to each other by hydrogen bonding. Being made up og beta glucose, the chain has adjacent glucose molecules rotated by 180 degrees. This allows hydrogen bonds to be formed between hydroxyl groups of adjacent parallel chains and helps to give cellulose its structural stability.

many cellulose molecules become tightly cross-linked to form bundles called microfibris. These microgibrils are in turn held together in budles called fibres. A cell wall has several layers of fibres running in different directions to increase the strength. Despite their strength, cellulose fibres are freely permeable, allowing water and other solutes to penetrate through to the cell membrane.

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Chitin is a polysaccharide found in insects. It is similar to cellulose but has amino acids added to form a mucopolysaccharide.

It is strong, waterproof and lightweight and froms the exoskeleton of insects.

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