Biological MoleculesEdexcel IGCSE Biology: Revision notes
Section 1
What elements make up carbohydrates, proteins and lipids?
The three main groups of biological molecules are built from different chemical elements:
| Molecule | Elements present |
|---|---|
| Carbohydrates | Carbon, hydrogen, oxygen |
| Proteins | Carbon, hydrogen, oxygen, nitrogen (some also contain sulfur) |
| Lipids (fats and oils) | Carbon, hydrogen, oxygen (in a different ratio to carbohydrates) |
Section 2
How are carbohydrates, proteins and lipids built from smaller units?
Large biological molecules are built by joining together many smaller repeating units:
- Carbohydrates: simple sugars (such as glucose) join together to form larger carbohydrates — starch (in plants) and glycogen (in animals) are both made of many glucose units joined in a chain
- Proteins: built from amino acids joined together in a chain, which folds into a specific shape
- Lipids: each lipid molecule is built from one glycerol molecule joined to three fatty acid molecules
Section 3
How do we test food samples for glucose, starch, protein and fat?
| Test | Reagent/method | Positive result |
|---|---|---|
| Glucose (reducing sugar) | Benedict's solution, heated in a water bath | Colour changes from blue to brick-red/orange precipitate |
| Starch | Iodine solution, added directly | Colour changes from orange-brown to blue-black |
| Protein | Biuret test (sodium hydroxide + copper sulfate solution) | Colour changes from blue to purple/lilac |
| Fat | Ethanol (emulsion test), shaken with the sample then mixed with water | A cloudy white emulsion forms |
For the Benedict's test, students often forget it must be heated — at room temperature, no colour change occurs even with glucose present.
Section 4
What are enzymes, and how do they work?
Enzymes are biological catalysts — proteins that speed up the rate of metabolic reactions in cells without being used up themselves. Each enzyme has an active site, a specifically shaped region that fits its target molecule (the substrate), similar to a lock and key. Because the active site shape is specific, each enzyme usually only catalyses one particular reaction.
The lock-and-key model: the enzyme's active site is the lock, and only the substrate with the matching shape (the key) fits and reacts.
Section 5
How do temperature and pH affect enzyme activity?
Temperature: as temperature increases, enzyme activity increases up to an optimum, because particles move faster and collide more often. Above the optimum, the enzyme's active site changes shape (denatures) as bonds holding its structure break, so the substrate no longer fits and activity rapidly decreases to zero.
pH: each enzyme has an optimum pH. Away from this optimum, the shape of the active site is altered, reducing how well the substrate binds and lowering the rate of reaction; at extreme pH the enzyme denatures completely.
Both effects can be investigated experimentally — for example, measuring how quickly amylase breaks down starch (tested with iodine) at a range of temperatures or pH values, keeping all other variables (enzyme/substrate concentration, volume) constant.
Say 'denatured', not 'killed' or 'destroyed' — enzymes are proteins, not living things, so examiners want precise language about the active site changing shape.
Must Know
- Carbohydrates and lipids contain carbon, hydrogen and oxygen; proteins also contain nitrogen
- Starch/glycogen are chains of glucose; proteins are chains of amino acids; lipids = glycerol + 3 fatty acids
- Benedict's test (glucose, heated, blue to brick-red), iodine test (starch, blue-black), biuret test (protein, blue to purple), emulsion test (fat, cloudy white)
- Enzymes are protein catalysts with a specifically shaped active site that fits the substrate (lock and key)
- Activity increases with temperature up to an optimum, then denatures and activity falls sharply
- Activity is highest at the optimum pH; extreme pH changes active site shape and denatures the enzyme
That's the notes covered.
Carry on to the next subtopic.