• Carbohydrates are organic compounds containing carbon, hydrogen and oxygen
  • They have two main functions; To act as a source of energy in both plants and animals ans to play a structural role in plant cell walls

There are three types;

  • Monosaccharides - Simple sugars
  • Disaccharides - Double sugars formed from 2 monosaccharides
  • Polysaccharides - Large molecules formed from many monosaccharides

Every carbohydrate shares the basic formula CnH2nOn but the number and arrangement canchange

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  • Relatively small organic molecules and provide the building blocks for the larger carbohydrates 
  • The number and arrangement of atoms of each element, including the way they are joined together produce different carbohydrates, however they all share the same formula CnH2nOn
  • This means there are the same number of carbon atoms as oxygen atoms and twice as many hydrogen atoms as carbon and oxygen

The name of the monosaccharide is determined by the number of carbon atoms in the molecule

Triose: 3 carbon atoms, e.g. pyruvate, use in metabolism

Pentose: 5 carbon atoms, e.g. ribose, component of nucleic acids e.g. DNA

Hexose: 6 carbon atoms, e.g. glucose, the main energy source


  • The most common and importent monosaccharide
  • It is a six carbon sugar 
  • It's structure may be straight but is may also have a ring structure
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Glucose is an example of a structural isomer, where molecules have the same formula but can have different structures e.g. alpha and beta glucose

The only difference between alpha and beta glucose is the position of the OH group and the hydrogen atom attached to carbon 1

This is a small structural difference but has a major effect on the biological roles of alpha and beta glucose

Fuctose and Galactose

  • Although fructose is a pentagon shape it is a hexose sugar
  • Fructose is the main sugar in fruits and nectar. It is very soluble and sweeter than glucose
  • Galactose is important in the production of glycolipids and glycoproteins. It is less soluble than frucose and not as sweet

Ribose and Deoxyribose

  • They have carbons - they are pentose sugars
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Test for monosaccharides

Test for monosaccharides - The reducing sugars test

  • All monosaccharides are referred to as reducing sugars because they can reduce the blue copper sulphate in benedicts solution

1. Mix the test solution with an equal volume of benedict's solution

2. Heat (70 degrees dentigrade)

3. A red precipitate shows a reducing sugar is present. If it stays blue a reducing sugar is not present


  • Double sugars formed when 2 monosaccharides join together
  • The link between the monosaccharide rings is called a glycosidic bond
  • The reaction realeases water, so is called a condensation reaction
  • Disaccharides can be formed by joining together;
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  • Two similar monosaccharides, like maltose which is made from two alpha glucose molecules
  • Two different monosaccharides as with sucrose and lactose

Maltose = glucose + gluose. Found in malt sugar

Sucrose = fructose + fructose. Found in cane sugar

Lactose = galactose + galactose. Found in milk sugar


  • Disaccharides can be broken back down into monosaccharides e.g. during digestion
  • Hydrolysis is the splitting of the glycosidic bond by the chemical addition of a water molecule
  • Hydrolysis can be increased by adding acid and heat or adding enzymes
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Test for Disaccharides

Non-reducing sugars

Disaccharides are referred to as non-reducing sugars as they cannot reduce the blue copper sulphate in benedicts solution

1. Add 2cm3 of test solution to a test tube

2. Add 10 drops of hydrochloric acid

3. Mix and heat

4. Neutralise with an alkali 

5. Add 2cm3 benedicts solution 

6. Mix and Heat (70 degrees centigrade)

7. A red precipitate shows a reducing sugar is present. If it stays blue there is not a reducing sugar present

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  • Large, complex molecules called polymers
  • They are molecules made up of many monosaccharides joined together by glycosidic bonds formed by condensation reactions
  • There are two main groups, categorised by function, which in turn is dependant on whether they are made up of either alpha or beta glucose molecules

Storage - alpha glucose

  • Starch - main storage material in plants
  • Glycogen - main storage material in animals

Structural - beta glucose

  • Cellulose - plants
  • Chitin - insects
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  • Starch is a polysaccharide made up of many alpha glucose molecules arranged into two different structural units

1. Amylase

  • Long, unbranched chain of alpha glucose with 1-4 glycosidic bonds 
  • It makes up about 20% of starch 
  • The angles of the glycosidic bonds cause it to curl into a helix 
  • It's coiled, compact structure makes it very good for storage

2. Amylopectin

  • Long, branched chain of alpha glucose with some 1-4 glycosidic bonds, but many more 1-6 glycosidic bonds that give it it's branched structure
  • It's side branches make it particularly good for storage of glucose because the enzymes that break down the molecule can get at the glycosdic bonds easily to break them and release the glucose
  • It makes up about 80% of starch
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Implications of the properties of starch. Starch is;

  • Compact, so it doesn't take up much space in cells
  • Insoluble - It is a large molecule so it cannot leave the cell via the cell membrane and has no osmotic effect on cells
  • Easily broken donw to disaccharides and monosaccharides to provide energy for respiration

Testing for starch

  • Mix the test solution with 3 drops of iodine solution (orange/brown)
  • If the colour changes to blue/black colour, it dhows that starch is present
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  • Glycogen is a polymer of glucose 
  • It has a similar structure to amylopectin - 1-4 and 1-6 glycosidic bonds, but many more 1-6 glycosidic bonds creating more side branches
  • It is very compact and found in animal liver and muscle cells
  • Lots of branches means stored glycogen can be released quickly. This is important for energy release in animals

Implications of properties of glucose

  • Compact shape - Takes up very little space inside cells
  • Insoluble - It is a large molecule so it cannot leave the cell via the cell membrane, therefore it has no osmotic effect on cells
  • It is easily hydrolysed by the enzyme glycogen phosphorylase to provide glucose for respiration
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  • Cellulose is made up of many beta glucose molecules, linked to each other by 1-4 glycosidic bonds forming long, unbranched chains
  • However, to form the bonds, the OH group of each beta glucose molecule must be adjacent to each other
  • To make this possible, every other beta glucose molecule is rotated 180 degrees so the OH groups line up
  • The bonds between beta glucose units alternate upwards and downwards so overall the chain of molecules is straight
  • These chains of molecules are then cross linked by hydrogen bonds
  • The chains group together to form strong fibres called microfibrils
  • Many microfibrils form a cellulose fibre. It is the arrangement of the microfibrils that give plant cell walls their strength and rigidity and prevent the cell from bursting when they take in excess water
  • Enzymes that break down glycosidic bonds in starch cannot reach the bonds in cellulose so those enzymes canno break it down and the structural support is maintained
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  • Chitin is similar in structure to cellulose but has amino acids added to form a mucopolysaccharide 
  • It is strong, waterproof and lightweight
  • It forms the exoskeleton of insects. It is also found in the cell walls of fungi
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