Biological Molecules

2.5 – Identify the chemical elements present in carbohydrates, proteins and lipids (fats
and oils).
– Carbohydrates: Carbon, Hydrogen, Oxygen
– Lipids: Carbon, Hydrogen, Oxygen
– Proteins: Carbon, Hydrogen, Oxygen, Nitrogen (some may contain Sulphur)
2.6 – Describe the structure of carbohydrates, proteins and lipids as large molecules
made up from smaller basic units: starch and glycogen from simple sugar; protein from
amino acids; lipid from fatty acids and glycerol.
– Glucose and fructose (C6H12O6) are single sugar molecules. Two of those joined
together forms a double sugar molecule – sucrose (C12H22O11) and water.

Starch (found in plants) and glycogen (found in human bodies) are glucose (simple
sugar) polymers. Many sub-units of glucose are joined together to form a chain of
starch or glycogen.
• Single sugars = monosaccharides
• Double sugars = disaccharides
• Polymers of sugars = polysaccharides
– Proteins are made of sub-units called amino acids.
– They all contain different elements (carbon, hydrogen, oxygen, nitrogen).
• Two amino acids also contain sulphur.
– They are linked into long chains and usually twisted into spirals.
– Amino acids can be arranged in any way.
– The arrangement determines the function of a protein.
– There are 2 main types of lipids: fatty acids and glycerol.
– Each glycerol molecule is joined to 3 fatty acid molecules.

2.7 – Describe tests for glucose and starch

Details Iodine Benedict’s Solution
Carbohydrate Starch Glucose
Positive Test Blue/Black colour Brick-red precipitate
Negative Test Yellow/Brown colour Blue colour

• Place some starch powder on a spotting tile.
• Add drops of dilute iodine solution.
• Observe colour change.
• Add a small amount of glucose to a test tube with water.
• Shake test tube to dissolve glucose.
• Add drops of Benedict’s solution.
• Prepare a water bath in a beaker. Heat on tripod and gauze by a Bunsen burner.
• Place test tube in water bath.
• Observe colour change.
2.8 – Understand the role of enzymes as biological catalysts in metabolic reactions.
– Enzymes are biological catalysts.
• Catalyst: A protein that speeds up a metabolic reaction without being chemically
changed itself.
2.9 – Understand how the functioning of enzymes can be affected by changes in
temperature, including changes due to change in active site.
– Every enzyme there is an optimum temperature. Chemical reactions are at the
fastest at the optimum temperature.
• More frequent collisions of the substrate and active site with a higher energy.
• Substrate: The substance in which an enzyme acts on.
• Active site: The area in which the substrate fits in order for reactions to occur.
– Most enzymes have an optimum temperature of 37℃.

– If the temperature goes above optimum, the enzymes denature and its active site is
– When below the optimum temperature, enzymes are inactive and not denatured.
2.10 – Understand how the functioning of enzymes can be affected by changes in
active site caused by changes in pH.
– Most enzymes work at a pH of 7.
– Enzymes in our stomach work best at a pH of 2-3 as our stomach contains
hydrochloric acid.
– Enzymes denature when the pH is both lower or higher than the optimum pH (i.e. not
optimum) and its active site is altered.
2.11 – Describe experiments to investigate how enzyme activity can be affected by
changes in temperature.
– Spots of iodine are placed on the spotting tile.
– 5cm3 of starch suspension is placed in a boiling tube and 5cm3 of amylase
solution in another.
• Use two different syringes to place starch and amylase in the boiling tubes.
– Put boiling tubes in a beaker filled with water at room temperature.
– Place boiling tubes in beaker for 5 minutes and record the temperature.
– Pour amylase solution into the boiling tube containing starch suspension while
keeping it in the beaker.
– Immediately remove a small sample of the mixture and add it to one of the iodine
spots on the spotting tile. Record the colour.

– Keep taking samples every 30 seconds for 10 minutes and continue recording the
– Repeat the experiment for temperatures of the water bath between 20℃ and 60℃.
• At 40℃ the starch was all gone after 1 minute, so 5÷1 = 5cm3/min = fast reaction.