Carbohydrates, proteins and lipids are the three main food molecules. Each has different building blocks and a different main job.
This lesson opens the SPM Biology chemical composition of cells chapter. The next lesson links structure to function.
What does the comparison look like?
| Carbohydrates | Proteins | Lipids | |
|---|---|---|---|
| Elements | Carbon, hydrogen, oxygen | Carbon, hydrogen, oxygen, nitrogen | Carbon, hydrogen, oxygen |
| Building blocks | Simple sugars such as glucose | Amino acids | Glycerol and fatty acids |
| Main functions | Energy source | Growth, repair, enzymes | Energy store, insulation |
| Examples | Glucose, starch | Egg white, enzymes | Cooking oil, butter |
Learn each column as a set of facts, then test yourself by covering the table.
Worked example: matching molecules to a breakfast
A student eats toast with peanut butter and a boiled egg.
- The bread provides starch, a carbohydrate. It is broken down to glucose, which cells use for energy.
- The egg provides protein. Its amino acids are used for growth and repair.
- The peanut butter provides lipids. They can be stored as an energy reserve and act as insulation under the skin.
Each food is linked to one molecule group, its building block and its function in one line.
The mistake that costs marks
The common slip is to swap the building blocks, for example writing “proteins are made of glucose”.
| Wrong | Right | |
|---|---|---|
| Protein building block | Glucose | Amino acids |
| Carbohydrate building block | Amino acids | Simple sugars |
| Lipid building blocks | Sugars | Glycerol and fatty acids |
| Nitrogen | In carbohydrates | In proteins |
A quick anchor: proteins contain nitrogen, and amino acids contain an amino group, so the two go together.
Check yourself
State the building blocks and one function of each group: (a) carbohydrates, (b) proteins, (c) lipids.
Answer
(a) Carbohydrates: built from simple sugars, used as an energy source.
(b) Proteins: built from amino acids, used for growth and repair or as enzymes.
(c) Lipids: built from glycerol and fatty acids, used as an energy store or for insulation.
What to study next
Continue with relating molecular structure to function. The biological data interpretation trainer gives practice with questions that use these molecules, and the mistake log and paper-error review records slips.
If you would like a teacher to quiz you on the table, see online one-to-one Biology tuition.