Biological molecules


Monomers and polymers

·       Monomers are small, basic molecular units e.g. monosaccarides, amino acids and nucleotides

·       Polymers are large, complex molecules composed of long chains of monomers joined together

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Carbohydrates (1)

Carbohydrates are made from monosaccarides, they all contain the elements, carbon, oxygen and hydrogen. The monomers that they are made from are monosaccarides e.g. glucose, fructose and galactose.


Glucose is a hexose sugar – a monosaccaride with six carbon atoms in each molecule. There are two types of glucose, Alpha and Beta – they are isomers (molecules with the same molecular formula as each other but atoms are connected in a different way). The way in which they are different is the change in h-oh in alpha and the change of oh-h in beta glucose


Condensation reactions join monnosaccarides together by forming a glycosidic bond as the molecule of water is released and therefore forming a dissaccaride. Hydrolysis reactions break polymers apart by breaking the chemicl bond between the monomers using a water molecule and therefore the polymer is broken down into its constituent monosaccarides

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Carbohydrates (2)

Using the benedicts test for sugars: Benedicts reagent is added to the sample which is then heated in a water bath. If the test is positive (a reducing sugar) it will form a brick red precipitate. The colour will remain a constant blue if the sample is a non reducing sugar. A further test can be performed if the result is negative, the sample will be added to dilute hydrochloric acid, carefully heated and neutrilised by sodium hydrogencarbonate, then the benedicts test can be tried again. If the test is positive (non-reducing sugar) a coloured precipitate will form and if negative (no contained sugar) will remain blue.


Polysaccarides are formed when more than two monosaccharides are joined together by condensation reactions, each of the monosaccarides joined by a glycosidic bond.


STARCH: Cells get their energy from glucose and plants store the excess glucose as starch, if the plants require more energy, the starch is broken down into glucose. Starch is a mixture of amylose and amylopectin.

AMYLOSE: A long, unbranched chain of a-glucose. Coiled structure making it compact and good for storage

AMYLOPECTIN: A long, branched chain of a-glucose. Side branches allow the enzymes to break down the molecule to get to the glycosidic bonds easily and release glucose quickly.

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Carbohydrates (3)

Starch is insoluble in water and doesn’t affect water potential, so it doesn’t cause water to enter cells by osmosis which would make them swell. This makes starch good for storage.

Iodine test: add iodine solution to sample, P= blue-black colour, N= brown-orange colour from solution


GLYCOGEN: Glycogen is the main energy storage material in Animals. Animals get energy from glucose but store excess glucose as glycogen- a polysaccharide of a-glucose. Contains loads of side branches, glucose can be released quickly, important for movement in animals. Compact molecule making good for storage.

CELLULOSE: The major component of cell walls in plants. Cellulose is made of long, unbranched chains of b-glucose. When the b-glucose molecules bond, they form straight cellulose chains. The cellulose chains are linked together by hydrogen bonds to form strong fibres called microfibrils. The strong fibres provide structural support.  

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Triglycerides: Triglycerides have one molecule of glycerol with three fatty acids attached to it.

Fatty acid molecules have long tails made of hydrocarbons which are hydrophobic. These tails make lipids insoluble in water. Triglycerides are formed by condensation reactions. The fatty acid joins to a glycerol molecule, when the ester bond is formed, a molecule of water is released, this is repeated twice more to form a triglyceride.

The long hydrocarbon tails contain lots of energy and this is released when they are broken down. Lipids can therefore relesse about twice as much energy per gram as carbohydrates. They are also insoluble which means that they dont affect the water potential and cause water to enter the cell by osmosis.

Saturated and unsaturated fatty acids:

Saturated fatty acids do not contain double bonds between the carbon atoms whereas unsaturated fatty acids contain at least one double bond between the carbon atoms and therefore are unsaturated by hydrogen.


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Lipids (2)


These lipids are found in cell membranes. They have a structure of a glycerol joined to two fatty acids and a phosphate group. The phosphate group is hydrophillic and the tails are hydrophobic. This means that a bilayer  can be formed and the cell membranes can control what enters and leaves the cell because the centre of the bilayer is hydrophobic eaning that water soluble substances cannot easily pass through and the membrane acts as a barrier to those substances.

Emulsion test for Lipids:

Shake the test substance with ethanol for a minute so that it dissolves and then pour the solution into water. Any Lipid that is present will show up as a milky emulsion. The more lipid present, the more noticeable the milky colour will be.  

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Proteins are made from long chains of Amino acids, a dipetide is formed when two amino acids join together and a polypeptide is formed when more than two AA join. Proteins made from 1 or more polypeptides.

Amino acids have the same general structure – a carboxyl group (-COOH), an amine or amino group     (NH2) and an R group (variable side group). All living things share a bank of only 20 amino acids. They only differ between their R group.

Amino acids are linked together by condensation reactions to form polypeptides. A molecule of water is released during the reaction. The bonds formed between amino acids are called peptide bonds. The reverse reaction happens during digestion.

Proteins have four structural levels:

·       Primary structure – sequence of amino acids in chain

·       Secondary structure- alpha helix or beta pleated sheet, the way in which the protein is shaped with bonds

·       Tertiary structure- where the bonds form at the different parts of the polypeptide chain and forms final 3D shape of the protein for single chain proteins

·       Quaternary Structure – the way the polypeptide chains are assembled together, final 3D shape for whole protein


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proteins (2)

Proteins have a variety of functions: 

·       Enzymes

·       Antibodies- involved in the immune response 

·       Transport proteins – e.g. channel proteins in cell membranes, transport molecules and ions 

·       Structural proteins – chains lying parrellel with cross links between in keratin and collagen 

The buriet test for proteins: 

The test solution needs to be alkaline by adding a few drops of sodium hydroxide solution. Then add some copper sulfate solution, positive=purple solution and negative=blue solution

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