Elements and Structure of Carbohydrates, Proteins and Lipids
Carbohydrates, proteins and lipids are all large molecules built from smaller basic units joined together. Learn the elements and the building units — these are straight recall marks.
The chemical elements
| Food group | Elements present |
|---|---|
| Carbohydrates | Carbon, hydrogen, oxygen |
| Lipids (fats and oils) | Carbon, hydrogen, oxygen |
| Proteins | Carbon, hydrogen, oxygen, nitrogen (some also contain sulfur) |
The only thing that separates proteins from the other two is nitrogen. If a question asks how you would tell a protein apart from a carbohydrate by its elements, the answer is nitrogen.
Carbohydrates and lipids contain exactly the same three elements — they differ in the proportions, not the elements. Lipids contain proportionally much less oxygen.
Large molecules from basic units
| Large molecule | Made from |
|---|---|
| Starch | simple sugars (glucose) |
| Glycogen | simple sugars (glucose) |
| Protein | amino acids |
| Lipid | fatty acids and glycerol |
Carbohydrates
Simple sugars such as glucose are the basic unit. Many glucose molecules join in a long chain to make starch (the storage carbohydrate in plants) or glycogen (the storage carbohydrate in animals and fungi).
The chemical formula of glucose is C₆H₁₂O₆ — learn it. It is needed for the respiration and photosynthesis equations elsewhere in the course, and candidates regularly lose marks for not knowing it or for writing an unbalanced equation.
Proteins
A protein is a chain of amino acids joined together. There are about 20 different amino acids, and the order of amino acids in the chain determines the shape and therefore the job of the protein. The chain folds into a specific three-dimensional shape — this matters enormously for enzymes.
Lipids
One molecule of glycerol joined to three fatty acids makes one lipid molecule. Fats are solid at room temperature, oils are liquid — both are lipids.
Common mistake: writing that proteins are used for insulation or as an energy store. That is lipids. Proteins are for growth and repair (and for making enzymes). Examiners report seeing "proteins are used for insulation of energy" — a muddle of two different food groups.
Common mistake: plants need nitrate ions from the soil to make amino acids and proteins; humans get their nitrogen by eating protein. Do not say humans absorb or transport nitrates to make protein — examiners flag this confusion between plant mineral requirements and human nutrition.
Quick self-test
- Which food group contains nitrogen? Protein.
- Glycogen is built from what? Simple sugars / glucose.
- How many fatty acids per lipid molecule? Three, plus one glycerol.
Examiner tip. Do not swap the functions of the food groups. Lipids (not proteins) are the insulator and the energy store; protein is for growth and repair. And remember glucose is C₆H₁₂O₆ — a surprising number of candidates cannot recall this or balance equations that use it.