IGCSE Biology / Food and digestion

Carbohydrates, proteins and lipids

Edexcel International GCSE Biology (4BI1), chapter 4: Food and digestion, specification point 2.7, 2.8. Free lesson.

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Carbohydrates, proteins and lipids are built from smaller units and contain different elements. Knowing their building blocks and how they are stored helps you identify them and answer questions about food molecules.

What the three food groups are built from

Large biological molecules are made from smaller units joined together. Learning the units helps you identify the larger molecule. A simple sugar, such as glucose, is a unit of a carbohydrate. Many amino acids join to make a protein, and their order determines which protein is formed. A lipid is built differently: three fatty acids attach to one glycerol molecule. Carbohydrates are made from simple sugars. Examples include starch, glycogen and cellulose. Proteins are chains of amino acids. There are 20 different amino acids, and their order matters. Lipids are made from fatty acids…

Which elements are in carbohydrates and proteins

The elements in a molecule can help you identify its food group. Carbohydrates contain carbon, hydrogen and oxygen. Proteins also contain nitrogen, which is the key difference to look for. Some proteins contain sulfur as well. Carbohydrates contain carbon, hydrogen and oxygen only. Proteins contain carbon, hydrogen, oxygen and nitrogen. Some also contain sulfur. Of these three food groups, only proteins contain nitrogen.

Where carbohydrates and proteins are stored

Plants and animals store carbohydrates in different forms. Plants store carbohydrate as starch, while animals store it as glycogen. Protein is used to build body structures and is not a storage molecule in either plants or animals. Plants store carbohydrate as starch. Animals store glycogen in the liver and muscles. Protein is not stored as a reserve in plants or animals.

How lipids differ

Lipids contain carbon, hydrogen and oxygen, like carbohydrates, but they have proportionally less oxygen. Their building blocks are fatty acids and glycerol, rather than a chain of one repeating unit. Lipids release about twice as much energy per gram as carbohydrates. Plants store lipids as oil; animals store them as fat. A lipid contains fatty acids joined to glycerol. Lipids have proportionally less oxygen than carbohydrates.

How to identify a protein from its elements

If a long-chain molecule contains carbon, hydrogen, oxygen and nitrogen, the nitrogen identifies it as a protein. The repeating units in the chain are amino acids.

More lessons in Food and digestion

• Testing for starch, sugars, protein and fat
• The seven parts of a balanced diet
• What changes how much energy you need
• The parts of the digestive system
• Teeth, chewing and peristalsis
• Amylase, protease and lipase
• Bile, the liver and emulsifying fat
• The villus and absorption
• Practical: measuring the energy in a food sample