2. Biological Molecules

Biological Molecules


1. Contain the elements Carbon, Hydrogen and Oxygen.

2. Are split into groups polysaccharides (e.g. starch), disaccharide (e.g. sucrose) and monosaccharides (e.g. glucose).

3. The test for starch involves iodine. If it changes colour from red-orange to blue-black then starch is present.

4. The test for glucose involves Benedict’s Reagent. If, when placed in heat, the solution turns from blue to brick-red then glucose is present.


1. Contain the elements Carbon, Hydrogen, Oxygen and sometimes Sulfur.

2. Made of subunits called amino acids.

3. The test for protein involves the use of a biuret solution (Sodium Hydroxide and Copper Sulphate). If the solution goes from blue to pink-purple then protein is present.


1. Contain the elements Carbon, Hydrogen and Oxygen.

2. Made of a molecule of Glycerol and three fatty acids.

3. The test for lipids involves the use of ethanol. If when mixed the solution goes cloudy then lipid is present.

Enzymes are biological catalysts meaning that they are organic molecules that speed up reactions but remain unchanged. The lock and key model is used to describe the actions of enzymes. Enzymes have an active site which the substrate (reagent) fits into. After the enzymes catalyses the reaction it’s then free to go elsewhere. Because of the varying shapes of the enzymes and substrates, only one type of enzyme can “fit” a substrate, they are, therefore, specific. Enzymes are used to catalyse metabolic reactions.

Enzymes are affected by temperature. The rate of reaction increases as temperature increases as there is more kinetic energy supplied which means more collisions occur between the enzymes and substrates. However, after the optimum temperature the rate of reaction falls as the enzymes are being denatured which means the active site is changed. Enzymes are also affected by pH. If the optimum pH is not reached the the active site is once again changed.

Experiment to show enzyme activity with temperature:

1. Place test tubes of amylase and another with starch in a water bath.

2. After five minutes mix.

3. Take sample and test with iodine.

4. Do this every thirty seconds until the iodine turns blue-black.

5. Repeat the experiment changing the temperature of the water bath to 20, 30, 40, 50, 60 °C


The movement of substances into and out of cells:

Diffusion – the next movement of particles from an area of high concentration to low concentration down a concentration gradient.

Osmosis – the movement of water molecules from an area of high water potential to an area of low water potential through a partially permeable membrane.

Active transport – the movement of particles against a concentration gradient using energy from respiration and carrier proteins.

The movement of substances into and out of cells can be through diffusion, osmosis and/or active transport.

Factors affecting rate of movement:

1. A high concentration gradient will increase the rate of movement.

2. A high temperature will increase the rate of movement as there is more energy meaning more kinetic energy is present. This means the particles move faster.

3. A large surface area to volume ratio will increase the rate of movement as there is more area for the particles to move across.

Experiment for diffusion:

1. Cut potassium permanganate agar jelly cubes into cubes of length 0.5, 1, and 2 cm.

2. Place cubes of jelly at the same time into three beakers containing 75ml hydrochloric.

3. Record the amount of time it takes for the jelly to go from the dyed purple colour to colourless.

Experiment for osmosis:

1. In three test tubes, pour in one 10ml of distilled water, one 10ml 0.85% salt solution and one 10ml 3% salt solution.

2. Add 1ml of fresh blood to each test tube and shake.

3. Look at the sample of each under a microscope.

4. No red blood cells should be seen in test tube one as lysis has occurred. Normal red blood cells should be shown in test tube two and shrunken red bloods cells should be present in test tube three (flaccid and plasmolysed).

Experiment for both diffusion and osmosis:

1. Fill visking tubing with a substance.

2. Place in beaker containing the substance but in different concentrations.

3. The amount of substance in each should change.