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Higher Level Revision Notes
Leaving Cert Higher Level Biology
Chapter 2: Food
Biomolecules, tests for food, and balanced diet.
Subtopics Covered
- Learning Outcomes
- Big Picture: Why Food Matters
- Main Biomolecules in Food
- Carbohydrates
- Lipids and Proteins
- Food Tests - Required Practical Knowledge
What This Pack Includes
- Structured chapter notes formatted for ExamsLogic website reading
- Exam-focused diagrams, definitions, and worked examples
- Interactive practice sections carried over from the source notes
- Independent study guidance based on the official curriculum
- Print-friendly layout for future PDF export when needed
This publication is an independent educational resource developed by ExamsLogic and compiled by experienced educators. It is based on publicly available official curricula, including Cambridge, Pearson Edexcel, IB, and the Irish Leaving Certificate. This product is not endorsed by, affiliated with, or sponsored by any examination board or governing authority. All registered trademarks remain the property of their respective owners.
1. Learning Outcomes
- State the main biomolecules found in food.
- Describe the functions of carbohydrates, lipids, proteins, vitamins, minerals, water and fibre.
- Explain the food tests for starch, reducing sugar, protein and lipid.
- Identify positive and negative results in food tests.
- Explain the meaning of a balanced diet and relate diet to health.
- Apply food-test knowledge to exam-style data and practical investigations.
HL focus: Do not just memorise the colour changes. Link the reagent, procedure, positive result and biological molecule clearly.
2. Big Picture: Why Food Matters
Energy
Carbohydrates and lipids provide energy for respiration and activity.
Growth & Repair
Proteins supply amino acids used to build and repair body tissues.
Health
Vitamins, minerals, water and fibre help maintain normal body functions.
Key idea: Food is not only fuel. It also supplies building materials, protective nutrients and substances needed for regulation.
3. Main Biomolecules in Food
| Food component | Basic units / examples | Main function | Exam detail |
|---|---|---|---|
| Carbohydrates | Glucose, starch, cellulose | Energy source | Starch is a storage carbohydrate in plants. |
| Lipids | Fats and oils; fatty acids + glycerol | Energy storage, insulation, cell membranes | Lipids contain C, H and O but proportionally less oxygen than carbohydrates. |
| Proteins | Amino acids | Growth, repair, enzymes and hormones | Proteins contain nitrogen. |
| Vitamins | Vitamin C, Vitamin D | Needed in small amounts for health | Deficiency causes disease. |
| Minerals | Calcium, iron | Bones, teeth, blood and enzyme function | Iron is needed for haemoglobin. |
| Water | H2O | Solvent and transport medium | Needed for metabolic reactions. |
| Fibre | Cellulose | Helps movement through the gut | Prevents constipation. |
4. Carbohydrates
Types
| Type | Example | Role |
|---|---|---|
| Monosaccharide | Glucose | Immediate energy source |
| Disaccharide | Maltose | Energy source after digestion |
| Polysaccharide | Starch, cellulose | Storage or structural support |
Exam Tip: For starch, iodine changes from brown/orange to blue-black. Write the starting colour and final colour.
5. Lipids and Proteins
Lipids
Lipids include fats and oils. They are used for long-term energy storage, insulation, protection around organs and cell membranes.
Basic units: fatty acids and glycerol.
Proteins
Proteins are used for growth, repair, enzymes, hormones and antibodies.
Basic units: amino acids.
| Nutrient | Element clue | Extra exam note |
|---|---|---|
| Carbohydrate | C, H, O | Usually H:O is 2:1. |
| Lipid | C, H, O | Less oxygen than carbohydrates. |
| Protein | C, H, O, N | Nitrogen is the important clue. |
Examiner Trap: Do not say proteins are mainly used for energy. In exams, the preferred role is growth and repair.
6. Food Tests - Required Practical Knowledge
| Substance tested | Reagent / test | Method summary | Positive result | Negative result |
|---|---|---|---|---|
| Starch | Iodine solution | Add iodine to food sample. | Blue-black | Brown/orange |
| Reducing sugar | Benedict's solution | Add Benedict's and heat in water bath. | Green/yellow/orange/brick-red | Blue |
| Protein | Biuret test | Add sodium hydroxide then copper sulfate / Biuret reagent. | Purple/lilac | Blue |
| Lipid | Brown paper or ethanol emulsion test | Rub on brown paper or mix with ethanol then water. | Translucent grease spot / cloudy white emulsion | No translucent spot / clear |
Common Mistake: Forgetting heat in Benedict's test. Benedict's solution must be heated in a water bath to test for reducing sugars.
7. Balanced Diet
A balanced diet supplies all food components in the correct amounts for age, sex, activity level and health status.
Too little
Deficiency diseases, poor growth, weak immunity and low energy.
Correct amount
Normal growth, repair, energy release and body regulation.
Too much
Obesity, cardiovascular strain and diet-related health problems.
| Nutrient | Deficiency issue | Useful source |
|---|---|---|
| Vitamin C | Scurvy, poor wound healing | Citrus fruits, vegetables |
| Vitamin D | Weak bones / rickets | Sunlight, oily fish, fortified foods |
| Calcium | Weak bones and teeth | Milk, dairy, leafy greens |
| Iron | Anaemia | Red meat, beans, leafy greens |
8. Interactive Simulator: Food Test Detective
Identify the Biomolecule
Result: Iodine turns blue-black.
Feedback
Ready
Use reagent and colour change to identify the food molecule.
9. Interactive Simulator: Balanced Diet Builder
Choose the Best Missing Nutrient
Scenario: A student has poor wound healing and bleeding gums.
Feedback
Ready
Match the symptom to the likely missing nutrient.
10. High-Value Exam Guidance
Exam Tip 1: In food-test questions, write reagent, procedure and positive result. One missing part can cost marks.
Exam Tip 2: For Benedict's test, mention heating in a water bath.
Exam Tip 3: Use balanced diet language: correct amounts, not simply “healthy food”.
Examiner Trap 1: Iodine tests for starch, not glucose.
Examiner Trap 2: Biuret is for protein; Benedict's is for reducing sugar.
Examiner Trap 3: Fibre is not digested for energy, but it is still important in the diet.
Common Mistake 1: Writing “red” for Benedict's without saying brick-red precipitate.
Common Mistake 2: Calling all carbohydrates sugars. Starch is a carbohydrate but not a reducing sugar.
Examiner Secret 1: Food tests are often assessed through unfamiliar data tables. Focus on colour changes.
Examiner Secret 2: In diet questions, marks often come from linking nutrient to function or deficiency.
11. MCQs with Instant Answers
1. Which reagent tests for starch?
Answer: B. Iodine turns blue-black if starch is present.
2. Which molecule is made of amino acids?
Answer: A. Proteins are built from amino acids.
3. A positive Benedict's test after heating gives:
Answer: C. Reducing sugar changes Benedict's from blue to green/yellow/orange/brick-red.
4. Which nutrient is important for preventing constipation?
Answer: A. Fibre helps movement through the gut.
12. Structured Questions
- State three functions of food in the human body. [3]
- Describe a test for starch in a food sample. [3]
- Describe a test for reducing sugar in a food sample. [4]
- Compare carbohydrates, lipids and proteins in terms of function and basic units. [6]
- Explain what is meant by a balanced diet and give two factors that affect dietary needs. [4]
13. Mark Scheme
Q1. Energy [1]; growth and repair [1]; regulation/protection/health [1].
Q2. Add iodine solution to the food sample [1]; positive result is blue-black [1]; negative result remains brown/orange [1].
Q3. Add Benedict's solution [1]; heat in a water bath [1]; positive result changes from blue to green/yellow/orange/brick-red [1]; indicates reducing sugar [1].
Q4. Carbohydrates provide energy and include glucose/starch [2]; lipids store energy/insulate and are made of fatty acids and glycerol [2]; proteins are for growth/repair and are made of amino acids [2].
Q5. Balanced diet provides all nutrients in correct amounts [2]; factors: age, sex, activity level, pregnancy, health status or occupation [any 2].
Q2. Add iodine solution to the food sample [1]; positive result is blue-black [1]; negative result remains brown/orange [1].
Q3. Add Benedict's solution [1]; heat in a water bath [1]; positive result changes from blue to green/yellow/orange/brick-red [1]; indicates reducing sugar [1].
Q4. Carbohydrates provide energy and include glucose/starch [2]; lipids store energy/insulate and are made of fatty acids and glycerol [2]; proteins are for growth/repair and are made of amino acids [2].
Q5. Balanced diet provides all nutrients in correct amounts [2]; factors: age, sex, activity level, pregnancy, health status or occupation [any 2].
14. Mastery Checklist
- I can list the main food components.
- I can explain the functions of carbohydrates, lipids and proteins.
- I can describe iodine, Benedict's, Biuret and lipid tests.
- I can identify positive and negative food-test results.
- I can explain balanced diet in exam language.
- I can link nutrient deficiency to health problems.