B3.3 The Chemistry of Food

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: carbohydrates, lipids and proteins: their building blocks, how they are built up and broken down, and their uses. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe carbohydrates, lipids and proteins in terms of their building blocks and elements
• Explain that enzymes build large molecules up and break them down
• Describe what each food molecule is used for in the body
• Explain why the shape of a protein matters for its function
• Use food label data to calculate masses and percentages

Key words

• Carbohydrate: Sugars and starches, made of carbon, hydrogen and oxygen.
• Simple sugar: A small carbohydrate with one or two sugar units, e.g. glucose, sucrose.
• Starch: A long chain of glucose units; the energy store in plants.
• Lipid: Fats (solid at room temperature) and oils (liquid); made of glycerol and fatty acids.
• Protein: A long chain of amino acids folded into a particular shape.
• Amino acid: The small building-block molecule of a protein.

Building blocks of food molecules

The three main food molecules are all large molecules built from smaller units. Digestion breaks them back into these units.

• Carbohydrates; Building blocks Simple sugars such as glucose (C₆H₁₂O₆); Elements C, H, O
• Lipids; Building blocks Glycerol + 3 fatty acids; Elements C, H, O
• Proteins; Building blocks Amino acids; Elements C, H, O, N (some also contain S)

Building up and breaking down

Enzymes in the gut break down large food molecules into small soluble ones. After absorption, enzymes inside cells (synthesis enzymes) build the small molecules back up into the large molecules the body needs.

Key fact: Enzymes do not only “cut” molecules. Many enzymes join small molecules together.

Carbohydrates and lipids

Simple sugars (e.g. glucose, fructose, sucrose) taste sweet and dissolve easily. Complex carbohydrates are long chains of sugar units: plants store glucose as starch and build cell walls from cellulose; animals store glucose as glycogen in the liver and muscles. Carbohydrates are the main source of energy, released by respiration.

Each lipid molecule is one glycerol joined to three fatty acids. Lipids store about twice as much energy per gram as carbohydrate, insulate the body, protect organs and form part of cell membranes.

Proteins and why shape matters

Proteins are long chains of amino acids. The order of the amino acids decides how the chain folds, so each protein has its own 3-D shape, and its shape lets it do its job.

• Enzymes: biological catalysts with an active site (see B3.4).
• Hormones: some are proteins, e.g. insulin.
• Antibodies: bind to specific antigens on pathogens.
• Structural proteins: e.g. collagen in skin, and muscle proteins for growth and repair.

High temperatures or extreme pH change the shape of a protein, so it may stop working (it is denatured).

Food labels

Worked example: reading a food label

A 55 g serving of crackers contains 37.4 g carbohydrate, 5.5 g fat and 6.6 g protein.
Percentage of the serving that is carbohydrate = 37.4 ÷ 55 × 100 = 68%.
Mass of protein in a 200 g pack = 6.6 ÷ 55 × 200 = 24 g.

Answer: 68%; 24 g

Exam tip: Always show the division and the × 100 in percentage questions; a correct method can earn marks even if you slip later.

Common mistakes

• Saying lipids are made of “fatty acids” only. Each lipid is glycerol + three fatty acids.
• Forgetting that proteins contain nitrogen, unlike carbohydrates and lipids.
• Calling glycogen a plant store. Plants store starch; animals store glycogen.
• Thinking a denatured protein has lost its amino acids. The chain is still there, but its shape has changed.

Revision questions

Try these, then check your answers below each one.

Question 1. Which element is found in all proteins but not in carbohydrates or lipids? (Choose from: Nitrogen, Carbon, Oxygen, Hydrogen.)

Answer: Nitrogen

Working: Carbohydrates and lipids contain C, H and O. Proteins also contain nitrogen (and some contain sulfur).

Question 2. The diagram shows simple models of three food molecules, P, Q and R. Which model represents a lipid? (Choose from: P, Q, R.)

Answer: Q

Working: Q is one glycerol joined to three fatty acids, so it is a lipid. P is a chain of identical glucose units (starch) and R is a chain of different amino acids (protein).

Question 3. True or false? Plants store glucose as glycogen.

Answer: False

Working: False. Plants store glucose as starch. Animals store it as glycogen in the liver and muscles.

Related lessons

• Previous lesson: B3.2 The Human Digestive System
• Next lesson: B3.4 Catalysts and Enzymes
• In this chapter: B3.1 Tissues and Organs
• In this chapter: B3.5 Factors Affecting Enzyme Action
• In this chapter: B3.6 How the Digestive System Works

Continue in the full course

This lesson is section B3.3 (AGCSC0303) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.