Carbohydrates, proteins and lipids

Structure and functions in living organisms · Biological molecules and enzymes · note 1 of 5

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In short

Carbohydrates, proteins and lipids are the three main groups of biological molecules. All contain carbon, hydrogen and oxygen, and proteins also contain nitrogen. They are large molecules built from smaller units: starch and glycogen from simple sugars (glucose), proteins from amino acids, and lipids from one glycerol and three fatty acids.

Carbohydrates, proteins and lipids (fats and oils) are the three main groups of biological molecules. They are all made from a small number of chemical elements.

Elements in biological molecules
Biological molecule	Elements present
Carbohydrates	Carbon, hydrogen and oxygen (C, H, O)
Proteins	Carbon, hydrogen, oxygen and nitrogen (C, H, O, N), and sometimes sulfur (S)
Lipids (fats and oils)	Carbon, hydrogen and oxygen (C, H, O)

Carbohydrates, proteins and lipids are large molecules made up from smaller basic units. Many small units are joined together to make one large molecule.

Large molecules and their basic units
Large molecule	Made from
Starch	Many simple sugars (glucose) joined together
Glycogen	Many simple sugars (glucose) joined together
Protein	Amino acids
Lipid	Fatty acids and glycerol (one glycerol and three fatty acids)

Starch is the storage carbohydrate in plants. Glycogen is the storage carbohydrate in animals. Both are made from glucose, which is a simple sugar.

Three panels: a chain of glucose units making starch or glycogen (C, H, O); a chain of amino acids making a protein (C, H, O, N, sometimes S); one glycerol joined to three fatty acids making a lipid (C, H, O). — Large molecules are built from smaller units: glucose → starch or glycogen, amino acids → protein, glycerol + three fatty acids → lipid.

Exam tip:

A lipid is made from one glycerol and three fatty acids. Learn the numbers 1 and 3, and remember that, of the three groups, only proteins contain nitrogen.

Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026

Frequently asked questions

How do enzymes speed up reactions?

Enzymes speed up reactions by acting as biological catalysts. The substrate fits into the enzyme's active site because their shapes are complementary, forming an enzyme-substrate complex. The reaction takes place at the active site, then the products leave and the enzyme is free to be used again, because it is not used up.

Why do enzymes denature at high temperatures?

Enzymes denature at high temperatures because the heat damages the enzyme's structure, so the shape of the active site changes. The substrate is no longer complementary to the active site and cannot fit, so the rate of reaction falls quickly towards zero. Denaturing is permanent, and enzymes are not alive, so they are not 'killed'.

What happens to enzymes at low temperatures?

At low temperatures enzymes work slowly but are not denatured. The enzyme and substrate particles have less kinetic energy and move more slowly, so they collide less often and fewer enzyme-substrate complexes form. As the temperature rises towards the optimum, around 37 °C for many human enzymes, collisions increase and the rate rises.

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