describe the relationship between the structure of chloroplasts, as shown in diagrams and electron micrographs, and their function

Photosynthesis – Structure & Function of the Chloroplast

Learning Objective

Describe the relationship between the structure of chloroplasts (as shown in diagrams and electron micrographs) and their function in photosynthesis.

Quick‑scan of the Cambridge AS/A‑Level Syllabus (Topic 13 – Photosynthesis)

1. Chloroplast Structure (Syllabus 13.1)

• Double‑membrane envelope

  • Outer membrane – porous, allows passage of small molecules.
  • Inner membrane – less permeable; contains transport proteins for metabolites.

• Inter‑membrane space (thylakoid lumen)

  • Space inside the thylakoid membranes.
  • H⁺ ions pumped here by the electron‑transport chain create a proton‑motive force used by ATP synthase (chemiosmosis).

• Stroma

  • Fluid matrix surrounding the thylakoid system.
  • Contains chloroplast DNA, ribosomes, enzymes of the Calvin cycle and stored starch granules.

• Thylakoid system

  • Flattened sac‑like membranes that house pigments and protein complexes of the light‑dependent reactions.
  • Granum (plural grana) – stacks of thylakoids; provide a large surface area for light‑harvesting complexes, especially Photosystem II.
  • Stroma lamellae (inter‑granal lamellae) – unstacked thylakoid membranes that interconnect grana; the main site of Photosystem I, cyclic electron flow and ATP synthase.

2. Electron‑Micrograph Highlights (Syllabus 13.1)

• Regularly spaced lamellae within each granum give an enormous membrane surface area for pigment‑protein complexes (PSII).
• Clear distinction between stacked (grana) and unstacked (stroma lamellae) regions – explains segregation of PSII (grana) and PSI (stroma lamellae).
• Dense starch granules in the stroma illustrate storage of excess photosynthate.
• The lumen (inter‑membrane space) appears as a dark interior of the thylakoids, indicating where H⁺ accumulates during electron transport.

3. Light‑Dependent Reactions (occur in thylakoid membranes) – Syllabus 13.2

3.1 Photolysis (Water‑splitting) – Photosystem II (mainly in grana)

• Chlorophyll a in PSII absorbs a photon → excites an electron.
• Excited electron is passed to the primary electron acceptor, then into the electron‑transport chain.
• The oxygen‑evolving complex of PSII extracts electrons from H₂O:

  
  2 H₂O → 4 H⁺ (released into the lumen) + O₂ + 4 e⁻.

• Result: O₂ is released, H⁺ contributes to the proton gradient, electrons enter the chain.

3.2 Linear (Non‑cyclic) Electron Transport – PSII → PSI

1. From PSII to plastoquinone (PQ) – electrons reduce PQ to PQH₂.
2. Cytochrome b₆f complex – transfers electrons to plastocyanin (PC) and pumps H⁺ from the stroma into the lumen, enhancing the gradient.
3. Plastocyanin → Photosystem I (mainly in stroma lamellae) – delivers electrons to PSI.
4. Re‑excitation at PSI – a second photon raises the electron to a higher energy level.
5. Ferredoxin (Fd) → NADP⁺ reductase – electrons reduce NADP⁺ + H⁺ → NADPH.

Overall, linear flow produces both NADPH and a proton gradient used for ATP synthesis.

3.3 Cyclic Photophosphorylation – Photosystem I only (stroma lamellae)

• Electrons from PSI are transferred to ferredoxin, then back to the plastoquinone pool and cytochrome b₆f.
• The cycle pumps additional H⁺ into the lumen but does not reduce NADP⁺, so only ATP is generated.
• Important when the ATP demand of the Calvin cycle exceeds that supplied by linear flow.

3.4 Chemiosmosis – ATP Synthase (predominantly in the unstacked stroma lamellae)

• Proton‑motive force (high H⁺ concentration in the lumen, low in the stroma) drives H⁺ back through ATP synthase.
• ATP synthase couples H⁺ flow to phosphorylation of ADP → ATP.
• Location in stroma lamellae ensures proximity to PSI (source of electrons for cyclic flow) and to the Calvin‑cycle enzymes in the stroma.

Overall non‑cyclic equation (simplified)

2 H₂O + 2 NADP⁺ + 3 ADP + 3 Pi + light → O₂ + 2 NADPH + 3 ATP

4. Light‑Independent Reactions – The Calvin Cycle (occurs in the stroma)

Overall Calvin‑cycle equation (per 6 CO₂):

6 CO₂ + 12 NADPH + 18 ATP → C₆H₁₂O₆ + 6 O₂ + 12 NADP⁺ + 18 ADP + 18 Pi

5. Pigments, Absorption & Action Spectra

• Chlorophyll a – primary pigment; absorption peaks ≈ 430 nm (blue) and 660 nm (red).
• Chlorophyll b – accessory pigment; peaks ≈ 453 nm and 642 nm; transfers excitation energy to chlorophyll a.
• Carotenoids (β‑carotene, lutein, etc.) – absorb mainly 400–500 nm; protect against photo‑damage and extend usable wavelength range.

The combined absorption spectrum of all pigments matches the action spectrum for oxygen evolution, confirming that light of these wavelengths drives photosynthesis.

6. Pigment Chromatography (Practical Requirement)

Paper chromatography separates pigments according to polarity. Typical Rf values (solvent = petroleum ether : acetone = 80 : 20) are:

7. Limiting‑Factor Investigations (Syllabus Requirement)

Students should be able to design, carry out and interpret experiments that vary one factor while keeping others constant.

• Light intensity – neutral‑density filters; measure O₂ evolution (gas syringe) or CO₂ uptake.
• Wavelength (colour) of light – coloured filters or LEDs; plot an action spectrum.
• CO₂ concentration – bubble known concentrations of CO₂; record rate of photosynthesis.
• Temperature – run the assay at several temperatures (e.g., 10 °C, 20 °C, 30 °C) to locate the optimum.

Data are presented as graphs (rate vs. variable). The factor that, when increased, no longer raises the rate is identified as the limiting factor.

8. Structure‑Function Summary Table

9. Key Points for Revision

1. Grana increase membrane surface area for PSII and light‑harvesting complexes; stroma lamellae house PSI, cyclic flow and ATP synthase.
2. Photolysis at PSII supplies electrons, releases O₂ and adds H⁺ to the lumen.
3. Linear electron flow (PSII → PSI) produces NADPH and a proton gradient; cyclic flow (PSI only) boosts ATP production.
4. ATP synthase uses the proton gradient across the thylakoid membrane (mainly in stroma lamellae) to synthesise ATP (chemiosmosis).
5. The Calvin cycle operates in the stroma, using ATP and NADPH to fix CO₂ into G3P and ultimately glucose.
6. Chlorophyll a is the only pigment that directly drives photochemistry; chlorophyll b and carotenoids broaden the usable light spectrum and protect the photosystems.
7. Paper chromatography separates pigments; characteristic Rf values confirm their identity.
8. Limiting‑factor experiments demonstrate how light intensity, wavelength, CO₂ concentration and temperature each affect the overall rate of photosynthesis.
9. Electron micrographs provide visual evidence for the specialised structures that enable these functional processes.