Explain how the internal structure of a leaf is adapted for photosynthesis.

6.1 Plant Nutrition – Photosynthesis

Learning Objective

Explain how the internal structure of a leaf is adapted for photosynthesis and describe the basic processes, products and factors involved in photosynthesis as required by the Cambridge IGCSE (0610) syllabus.

1. Definition & Equations

• Definition: Photosynthesis is the process by which green plants use light energy to convert carbon dioxide (CO₂) and water (H₂O) into carbohydrate (glucose) and oxygen (O₂).
• Word equation: CO₂ + H₂O → glucose + O₂
• Balanced chemical equation: 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

2. Role of Chlorophyll & Other Pigments

• Chlorophyll a & b: Located in thylakoid membranes; absorb mainly blue (≈ 430 nm) and red (≈ 660 nm) light, reflect green.
• Carotenoids (β‑carotene, lutein, etc.): Extend the range of usable light (blue‑green) and protect chlorophyll from photodamage.
• Absorbed photons raise electrons to a higher energy level, initiating the light‑dependent reactions that produce ATP and NADPH.

3. Main Products of Photosynthesis & Their Uses

4. Supporting Nutrients (Core Requirements)

5. Leaf Structure – Adaptations for Efficient Photosynthesis

5.1 Epidermis & Cuticle

• Upper (adaxial) and lower (abaxial) epidermal layers protect the leaf.
• Cells are thin and transparent, allowing maximum light penetration.
• The cuticle is thin enough not to impede light yet waxy enough to minimise water loss.

5.2 Stomata

• Pores surrounded by guard cells that open/close to regulate CO₂ uptake and water‑vapour loss.
• Most abundant on the lower epidermis to avoid direct sunlight and reduce transpiration.
• Guard‑cell turgor is controlled by K⁺ ions, linking water status to gas exchange.

5.3 Mesophyll

• Palisade mesophyll: Columnar cells tightly packed beneath the upper epidermis; high chloroplast density and elongated shape maximise light capture – the main site of the light‑dependent reactions.
• Spongy mesophyll: Loosely arranged cells with large intercellular air spaces; facilitate rapid diffusion of CO₂ to, and O₂ from, photosynthetic cells – important for the Calvin cycle (light‑independent reactions).

5.4 Chloroplasts

• Abundant in both mesophyll layers, especially palisade cells.
• Thylakoid membranes are organised into stacks (grana) that increase surface area for light‑absorbing pigments and the electron‑transport chain.
• The stroma contains the enzymes of the Calvin cycle.

5.5 Vascular Bundles (Veins)

• Xylem: Transports water and mineral nutrients from the roots to the mesophyll – essential for the light‑dependent reactions.
• Phloem: Carries the sugars (mainly sucrose) produced in the mesophyll to other parts of the plant.
• Veins run close to the mesophyll to minimise diffusion distances for both water and photosynthates.

5.6 Leaf Shape, Thickness & Surface Area

• A broad, thin lamina maximises light interception while keeping the diffusion path for gases short.
• In most dicots the leaf is dorsiventral (different upper and lower sides) to optimise light capture and gas exchange.

6. Light‑Dependent Reactions (Supplementary)

• Location: Thylakoid membranes of the chloroplasts.
• Key steps:
  1. Photolysis of water (H₂O) – light energy splits water, releasing O₂, electrons and H⁺ ions.
  2. Photosystem II (PSII): Absorbs photons, excites electrons which pass to the plastoquinone pool.
  3. Electron transport chain: Energy from electrons pumps H⁺ into the thylakoid lumen, creating a proton gradient.
  4. ATP synthesis: H⁺ flow back through ATP synthase drives formation of ATP (photophosphorylation).
  5. Photosystem I (PSI): Re‑excites electrons; final electron acceptor NADP⁺ is reduced to NADPH.
• Outputs: O₂ (released to the atmosphere), ATP and NADPH (used in the Calvin cycle).
• Pigment roles: Chlorophyll a is the primary electron donor; chlorophyll b and carotenoids broaden the range of usable wavelengths.

7. Calvin Cycle – Light‑Independent Reactions (Supplementary)

• Location: Stroma of the chloroplast.
• Three phases:
  1. Carbon fixation: CO₂ combines with ribulose‑1,5‑bisphosphate (RuBP) – catalysed by Rubisco – to form two molecules of 3‑phosphoglycerate (3‑PGA).
  2. Reduction: ATP and NADPH from the light‑dependent reactions convert 3‑PGA into glyceraldehyde‑3‑phosphate (G3P). One G3P leaves the cycle to form glucose and other carbohydrates.
  3. Regeneration: The remaining G3P molecules are rearranged, using ATP, to regenerate RuBP, allowing the cycle to continue.
• Overall outcome: For every 3 CO₂ fixed, one G3P is exported; six G3P molecules are needed to synthesise one molecule of glucose (C₆H₁₂O₆).

8. Limiting Factors (Core)

9. Photoinhibition & Wavelength Effects (Supplementary)

• Photoinhibition: Excessive light (especially high‑energy blue light) damages the D1 protein of PSII, decreasing the efficiency of the light‑dependent reactions.
• Wavelength effectiveness:
  • Blue (≈ 430 nm) and red (≈ 660 nm) photons are most efficiently absorbed by chlorophyll a and b.
  • Green light (≈ 550 nm) is largely reflected, contributing little to photosynthesis.
  • Carotenoids absorb in the blue‑green region, extending the usable spectrum and providing photoprotection.

10. Suggested Practical Investigations (IGCSE Level)

• Starch test (iodine colour change): Dark‑adapt leaves for 24 h, expose one leaf to strong light for 2 h while keeping another in darkness. Boil, de‑colourise with alcohol, add iodine. The light‑exposed leaf turns blue‑black (starch present), confirming the need for light.
• Floating leaf‑disc assay (CO₂ effect): Punch uniform leaf discs, place them in sodium bicarbonate solution, and expose to different light intensities. Measure the time for discs to rise (oxygen production). Vary CO₂ by adding more NaHCO₃ to see the effect.
• Temperature experiment: Use a water bath to keep leaf samples at 10 °C, 20 °C, 30 °C and 40 °C while measuring oxygen evolution with a gas‑collection apparatus.
• Wavelength experiment (supplementary): Shine monochromatic light (blue, green, red) on identical leaf sections and compare the rate of oxygen evolution to illustrate wavelength effectiveness.

11. Quick Reference – Key Equations

Overall photosynthetic reaction:

$$6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + \text{light energy} \;\longrightarrow\; \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2$$

Light‑dependent reactions (simplified):

$$2\,\text{H}_2\text{O} \xrightarrow{\text{light}} 4\,\text{H}^+ + 4\,e^- + \text{O}_2$$

$$\text{ADP} + P_i + \text{light} \;\rightarrow\; \text{ATP}$$

$$\text{NADP}^+ + 2e^- + \text{H}^+ \;\rightarrow\; \text{NADPH}$$

12. Suggested Diagram