3.3 The Chemistry of Food

A free GCSE Biology lesson for AQA (8461) from OBA School: carbohydrates, lipids and proteins: their building blocks, uses in the body and a balanced diet. 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 the structure of carbohydrates, lipids and proteins in terms of their building blocks
• Name the chemical elements in each type of food molecule
• Describe what each food molecule is used for in the body
• Explain why protein shape 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 made of one or two sugar units, e.g. glucose, sucrose.
• Complex carbohydrate: A long chain of simple sugar units, e.g. starch, glycogen, cellulose.
• 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.
• Balanced diet: Eating the right amounts of each nutrient for your needs.

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)

Carbohydrates

Simple sugars (e.g. glucose, fructose, sucrose, lactose) 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 body’s main source of energy, released by respiration. Foods rich in starch include bread, pasta, rice and potatoes.

Lipids

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

Key fact: Fats and oils are both lipids. Fats are solid at room temperature; oils are liquid.

Proteins and why shape matters

Proteins are long chains of amino acids. There are about 20 different amino acids, and their order decides how the chain folds. Each protein therefore has its own 3-D shape, and its shape lets it do its job.

• Enzymes: biological catalysts with an active site (see 3.4).
• Hormones: some are proteins, e.g. insulin.
• Antibodies: bind to specific antigens on pathogens.
• Structural proteins: e.g. collagen in skin and muscle proteins.

Proteins are needed for growth and repair. High temperatures or extreme pH change the shape of a protein, so it may stop working (it is denatured).

A balanced diet and food labels

A balanced diet contains carbohydrates, lipids and proteins plus vitamins, minerals, fibre and water, in the right amounts. Too much energy-rich food leads to weight gain; too little protein slows growth and repair.

Worked example: reading a food label

A 40 g serving of granola contains 25.6 g carbohydrate, 6.0 g fat and 4.4 g protein.
Percentage of the serving that is carbohydrate = 25.6 ÷ 40 × 100 = 64%.
Mass of fat in a 250 g pack = 6.0 ÷ 40 × 250 = 37.5 g.

Answer: 64%; 37.5 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 proteins but not in carbohydrates or lipids? (Choose from: Carbon, Hydrogen, Oxygen, Nitrogen.)

Answer: Nitrogen

Working: Proteins contain nitrogen (N) as well as C, H and O.

Question 2. True or false? Animals store glucose as starch in their liver and muscles.

Answer: False

Working: False. Animals store glucose as glycogen; plants store starch.

Question 3. How many fatty acid molecules are joined to one glycerol molecule in a lipid? (Choose from: 1, 2, 3, 4.)

Answer: 3

Working: A lipid is one glycerol joined to three fatty acids.

Related lessons

• Previous lesson: 3.2 The Human Digestive System
• Next lesson: 3.4 Catalysts and Enzymes
• In this chapter: 3.1 Tissues and Organs
• In this chapter: 3.5 Factors Affecting Enzyme Action
• In this chapter: 3.6 How the Digestive System Works

Continue in the full course

This lesson is section 3.3 (AGCBH0303) of our free GCSE Biology 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 Biology course.