Lesson overview
How plants make food: What does a leaf need to make starch?
See how a leaf uses energy from light to make glucose from carbon dioxide and water, then test leaves for starch to find out what a leaf needs, and follow where the sugar goes.
Suggested sequence
- Lesson 1 · What photosynthesis makes. Glucose and oxygen from carbon dioxide and water, using energy from light, with chlorophyll in the chloroplasts. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- Lesson 2 · What a leaf needs: the starch investigation. The starch test, destarched plants and controls for light, chlorophyll and carbon dioxide. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · What the plant does with its sugar. Starch, cellulose, respiration, sucrose and nectar; nitrate and magnesium ions. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- End of module · final check. 9 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 9 questions, 15 min, lesson closed.
Learning goals
Part 1: What photosynthesis makes
- Describe photosynthesis as the process by which plants synthesise carbohydrates from raw materials using energy from light (6.1.1)
- State the word equation for photosynthesis: carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll (6.1.2)
- State that chlorophyll is a green pigment that is found in chloroplasts (6.1.3)
- State that chlorophyll transfers energy from light into energy in chemicals, for the synthesis of carbohydrates (6.1.4)
- Supplement: state the balanced chemical equation for photosynthesis, 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (S6.1.10)
Part 2: What a leaf needs: the starch investigation
- Investigate the need for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls (6.1.7)
- Test a leaf for starch, giving the reason for each step and the safety precautions (practical skill for 6.1.7)
- Explain why plants are destarched and why each control differs from its test in one thing only (6.1.7)
- Interpret starch-test results and evaluate how far they show photosynthesis (6.1.7)
Part 3: What the plant does with its sugar
- Outline the use and storage of the carbohydrates made in photosynthesis: starch as an energy store, cellulose for cell walls, glucose in respiration, sucrose for transport in the phloem and nectar to attract insects for pollination (6.1.5)
- Explain the importance of nitrate ions for making amino acids (6.1.6a)
- Explain the importance of magnesium ions for making chlorophyll (6.1.6b)
- Interpret a controlled trial of seedlings grown with and without nitrate or magnesium ions (6.1.6)
Misconceptions to listen for
- “Photosynthesis is how plants release energy from glucose.” — That is respiration. Photosynthesis builds glucose; respiration breaks it down.
- “Plants take in their food from the soil.” — The soil gives water and mineral ions. The plant makes its food itself, in its leaves.
- “Light is a raw material.” — Light supplies energy, but it is not a substance, so none of it becomes part of the glucose.
- “Oxygen is taken in for photosynthesis.” — Oxygen is a product of photosynthesis: the leaf gives it out.
- “The product is starch.” — The product of photosynthesis is glucose. The plant then turns much of the glucose into starch to store it.
- “Carbon dioxide is a product.” — Carbon dioxide is taken in as a raw material; it is respiration that gives it out.
- “The leaf takes in water from the air.” — A leaf loses water vapour to the air. The water for photosynthesis comes up from the roots.
- “Carbon dioxide comes from the soil.” — Carbon dioxide comes from the air, through pores in the leaf.
- “Chlorophyll is used up to make glucose.” — Chlorophyll is not a raw material; the same chlorophyll absorbs light again and again.
- “Leaves are green because chlorophyll absorbs green light.” — A leaf looks green because the green light is reflected, not absorbed.
Scope
Photosynthesis as the making of carbohydrates from raw materials using energy from light (6.1.1); the word equation (6.1.2); chlorophyll in chloroplasts (6.1.3) transferring energy from light into energy in chemicals (6.1.4); the uses and storage of the carbohydrates made (6.1.5); nitrate and magnesium ions (6.1.6); the need for chlorophyll, light and carbon dioxide, with controls, through the starch investigation (6.1.7); the balanced equation (Supplement 6.1.10). Leaf colours in the experiment come from a teaching model.