4.1 Biological molecules
The elements in each type of molecule
| Molecule | Elements |
|---|---|
| carbohydrates | carbon, hydrogen, oxygen |
| lipids (fats and oils) | carbon, hydrogen, oxygen |
| proteins | carbon, hydrogen, oxygen, nitrogen (and sometimes sulfur) |
| DNA | carbon, hydrogen, oxygen, nitrogen, phosphorus |
Large molecules from small molecules
Many biological molecules are large molecules made of many small molecules joined together.
| Large molecule | Made from |
|---|---|
| starch (a store of energy in plants) | many glucose molecules |
| glycogen (a store of energy in animals, in liver and muscles) | many glucose molecules |
| cellulose (in plant cell walls) | many glucose molecules |
| proteins | amino acids |
| lipids (fats and oils) | three fatty acids and one glycerol |
| DNA | nucleotides |
The order of the amino acids in a protein decides its shape and its function, and there are about 20 different amino acids.
Functions of the main groups.
- Carbohydrates (sugars, starch): a source of energy.
- Lipids: a store of energy, insulation, and part of cell membranes.
- Proteins: growth and repair, and used to make enzymes, hormones and antibodies.
- DNA: carries the genetic information.
Food tests
| Test for | Reagent | Method | Positive result |
|---|---|---|---|
| starch | iodine solution | add a few drops to the food | blue-black (it stays orange-brown if starch is absent) |
| glucose and maltose (reducing sugars) | Benedict's solution | add Benedict's solution to the food solution and heat in a hot water bath | blue green, yellow, orange, then brick-red precipitate, depending on the amount of sugar |
| protein | biuret reagent (sodium hydroxide and copper(II) sulfate solution) | add to the sample | blue purple (lilac) (stays blue if there is no protein) |
| lipids | ethanol emulsion test | mix with ethanol, shake, then pour into water | a cloudy white emulsion forms (it stays clear if no lipid) |
It contains protein (biuret) and starch (iodine).
Practise this chapter
Every past-paper question on these topics, with the mark scheme.