Describe the role of the stomata in gas exchange during photosynthesis.

6.1 Plant Nutrition – Photosynthesis

Learning Objective

Describe the role of the stomata in gas exchange during photosynthesis and relate this to the overall process of photosynthesis, leaf structure and the plant’s nutritional needs.

1. Overview of Photosynthesis

• Definition: Photosynthesis is the process by which green plants convert light energy into chemical energy, producing carbohydrate (glucose) and oxygen from carbon dioxide and water.
• Key pigment – chlorophyll a: absorbs blue (≈ 430 nm) and red (≈ 660 nm) light and transfers the captured energy to the reaction centre of the thylakoid membranes.
• Word equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• Balanced (Cambridge‑approved) equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• Fate of the carbohydrate produced (Cambridge‑specified uses & storage):
  • Glucose – immediate use in cellular respiration.
  • Starch – stored in chloroplasts, roots, tubers and seeds.
  • Cellulose – structural component of cell walls.
  • Sucrose – transported in the phloem to growing parts.
  • Nectar – produced in specialised glands to attract pollinators.
• Essential nutrients for photosynthesis
  • Nitrate (NO₃⁻) – required for amino‑acid and protein synthesis, essential for growth of new leaf tissue.
  • Magnesium (Mg²⁺) – sits at the centre of the chlorophyll molecule; without Mg²⁺ chlorophyll cannot capture light.

2. Leaf Structure – Parts Relevant to Gas Exchange

3. Structure of a Stoma

• Guard cells – kidney‑shaped, contain chloroplasts; turgor changes open or close the pore.
• Pore (aperture) – the actual opening through which gases move.
• Epidermal cells – provide mechanical support around the guard cells.

4. Role of Stomata in Gas Exchange

1. Blue light activates proton pumps in the guard‑cell plasma membrane.
2. Potassium ions (K⁺) enter the guard cells; water follows osmotically, increasing turgor.
3. Swelling of the guard cells pulls them apart, opening the pore.
4. CO₂ diffuses: atmosphere → stomatal pore → spongy mesophyll → chloroplasts → Calvin cycle.
5. O₂ produced in the chloroplasts diffuses outward along the same pathway and leaves the leaf.
6. Water vapour also escapes (transpiration), creating a negative water potential that assists the upward pull of water from the roots.

5. Gases Exchanged via Stomata

6. Factors Influencing Stomatal Opening (Cambridge Core)

• Light intensity – blue light triggers guard‑cell proton pumps → opening.
• Internal CO₂ concentration – low internal CO₂ → stomata open; high CO₂ → closure.
• Water availability – drought → abscisic acid (ABA) accumulates → K⁺ channels close → guard cells lose turgor → stomata close.
• Temperature – high temperature raises transpiration; if water is limiting, stomata tend to close.
• Hormonal signals – ABA is the principal hormone promoting closure during stress.

7. Suggested Practical Investigations (Cambridge Syllabus)

1. Effect of light intensity on rate of photosynthesis – measure O₂ evolution or CO₂ uptake with a gas‑evolution apparatus while varying light levels.
2. Effect of CO₂ concentration – use sealed chambers with different CO₂ concentrations and record the change in O₂ production.
3. Effect of temperature – repeat the light‑intensity experiment at several temperatures (e.g., 15 °C, 25 °C, 35 °C) and compare rates.
4. Light/dark gas‑exchange in an aquatic plant (e.g., Elodea) – observe bubble formation (O₂) under light and its cessation in darkness.

8. Summary

The stomata are specialised gateways on the leaf epidermis that balance two vital needs: (1) allowing CO₂ to enter for the Calvin cycle and O₂ to leave as a waste product, and (2) limiting water loss through transpiration. Their opening is tightly regulated by light, internal CO₂, water status, temperature and hormonal signals, ensuring optimal photosynthetic efficiency while protecting the plant from dehydration.