Plant Nutrition & Photosynthesis

IGCSE Edexcel Biology
2.18–2.22 Photosynthesis, limiting factors, leaf structure and mineral ions
Key Concepts: Photosynthesis converts light energy into chemical energy stored in glucose. Leaves are adapted for efficient photosynthesis. Mineral ions are absorbed from the soil and are essential for plant growth.

Section A — The Photosynthesis Equation

1. Write the word equation for photosynthesis. [2]
2. Write the balanced symbol equation for photosynthesis. [2]
3. State where in the cell photosynthesis takes place and name the pigment involved. [2]
4. State two uses of glucose produced by photosynthesis in a plant. [2]

Section B — Limiting Factors

5. State three factors that can limit the rate of photosynthesis. [3]
6. A student measures the rate of photosynthesis at different light intensities. Describe how the rate of photosynthesis changes as light intensity increases, including what happens at high light intensities. [3]
7. Explain the effect of increasing CO₂ concentration on the rate of photosynthesis. [2]

Section C — Leaf Structure

8. Label the following features on a cross-section of a leaf and state the function of each. [8]
StructureFunction
Palisade mesophyll layer
Spongy mesophyll layer
Guard cells / stomata
Waxy cuticle
9. Explain two ways in which the palisade cell is adapted for photosynthesis. [4]
10. Describe how stomata open and close, and explain why they close at night. [3]

Section D — Mineral Ions

11. State the role of each mineral ion and the deficiency symptom if it is lacking. [4]
Mineral ion Role in the plant Deficiency symptom
Nitrate (NO₃⁻)
Magnesium (Mg²⁺)
12. Explain why plants need nitrate ions. [2]

Total marks: 37

Mark Scheme

1. Carbon dioxide + water → glucose + oxygen (light energy needed / using light energy) [2]
2. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ [2 — 1 each side correct]
3. Chloroplasts [1]; chlorophyll [1] [2]
4. Any two: used in respiration for energy; converted to starch for storage; used to make cellulose for cell walls; converted to sucrose for transport; used to make amino acids/proteins (with nitrate ions) [2]
5. Light intensity, carbon dioxide concentration, temperature [3]
6. As light intensity increases, rate of photosynthesis increases [1]; at high light intensities, the rate levels off / becomes constant [1]; another factor (CO₂ or temperature) becomes limiting [1] [3]
7. Increasing CO₂ concentration increases the rate of photosynthesis [1] up to a point where another factor (light or temperature) becomes limiting [1] [2]
8. Palisade: packed with chloroplasts, near the top of leaf for maximum light absorption; Spongy: air spaces allow CO₂ and O₂ to diffuse to/from cells; Guard cells/stomata: allow CO₂ in and O₂/water vapour out; Waxy cuticle: reduces water loss by evaporation [8 — 2 per row]
9. Any two: many chloroplasts to absorb more light; positioned at the top of the leaf to receive maximum light; tall, column-shaped cells packed tightly together [4 — 2 per point]
10. Guard cells take in water by osmosis → become turgid → stomata open [1]; at night no photosynthesis occurs so guard cells lose water → become flaccid → stomata close [1]; this reduces water loss at night when photosynthesis is not occurring [1] [3]
11. Nitrate: needed to make amino acids/proteins [1]; deficiency causes stunted growth and yellow/pale leaves [1]; Magnesium: needed to make chlorophyll [1]; deficiency causes yellow leaves (chlorosis) [1] [4]
12. Nitrate ions provide nitrogen [1]; nitrogen is needed to synthesise amino acids and proteins (including enzymes) [1] [2]