explain that during photophosphorylation: energetic electrons release energy as they pass through the electron transport chain (details of carriers are not expected), the released energy is used to transfer protons across the thylakoid membrane, prot

Photosynthesis – Energy Transfer and Carbon Assimilation (Cambridge IGCSE/A‑Level)

Learning Objectives

• Describe the structure of a chloroplast and relate each part to its function.
• Identify the main photosynthetic pigments, their absorption maxima, and how they are separated by thin‑layer chromatography (TLC).
• Explain the light‑dependent reactions (non‑cyclic and cyclic photophosphorylation), including water‑splitting, the Z‑scheme, the flow of electrons, proton pumping and the formation of ATP and NADPH.
• Outline the light‑independent (Calvin) cycle, the role of ATP and NADPH, and the key enzymes involved.
• Analyse how light intensity, CO₂ concentration, temperature and water availability limit the overall rate of photosynthesis.
• Integrate the above processes to answer typical Cambridge AO1, AO2 and AO3 exam questions.

1. Chloroplast Structure (AO1)

Diagram suggestion: a labelled cross‑section of a chloroplast showing outer/inner membranes, stroma, grana, lamellae, thylakoid lumen, and the relative positions of PS II (stacked regions) and PS I (lamellae).

2. Photosynthetic Pigments (AO1 + AO2)

Interpreting absorption and action spectra (AO2)

• Absorption spectrum: plots the amount of light absorbed by a pigment at each wavelength. Peaks correspond to the λ_max values shown above.
• Action spectrum: plots the rate of photosynthesis (or O₂ evolution) against wavelength of incident light. The shape mirrors the combined absorption spectra of chlorophyll a and b, confirming that these pigments drive the light reactions.
• In exam questions, students may be asked to compare an action spectrum with an absorption spectrum to deduce which pigment(s) are responsible for the observed photosynthetic activity.

Sample graph description for AO2

A line graph shows photosynthetic O₂ evolution (y‑axis) versus wavelength (x‑axis, 400–700 nm). Two peaks are evident at ~440 nm and ~660 nm, matching the absorption maxima of chlorophyll a. Students could be asked to label the peaks, explain why the curve falls off at >700 nm, and discuss the contribution of accessory pigments.

3. Light‑Dependent Reactions (Photophosphorylation)

3.1 Overall Z‑Scheme (AO2)

Electrons travel from water → PS II → plastoquinone (PQ) → cytochrome b₆f → plastocyanin (PC) → PS I → ferredoxin (Fd) → NADP⁺. The flow is “Z‑shaped” when drawn with the two photosystems at opposite ends.

3.2 Non‑Cyclic Photophosphorylation (the main pathway)

1. Photon absorption by PS II (P680) – Light (~680 nm) excites P680, ejecting an electron to the primary electron acceptor.
2. Water splitting (photolysis) – The oxygen‑evolving complex extracts electrons from H₂O:

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

   Electrons replace those lost from P680; O₂ is released to the atmosphere.

3. Electron transport & energy release – The high‑energy electron passes through a series of carriers (PQ → cytochrome b₆f → PC). At each step the electron drops to a lower redox potential; the released free‑energy is used to pump protons from the stroma into the thylakoid lumen.
4. Proton gradient formation – For every pair of electrons transferred:

   • 2 H⁺ come from the water‑splitting reaction,
   • 2 H⁺ are taken up by PQ from the stroma,
   • Cytochrome b₆f uses the energy to pump an additional 2 H⁺ into the lumen.

   Result: ≈ 4 H⁺ accumulate in the lumen per 2 e⁻, creating an electrochemical gradient (ΔpH ≈ 2–3; Δψ ≈ 120 mV).

5. Photon absorption by PS I (P700) – Light (~700 nm) excites P700, raising another electron to a higher energy level.
6. Final electron acceptor – NADP⁺ reduction – The high‑energy electron is transferred to ferredoxin, then to NADP⁺‑reductase:

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

7. ATP synthesis (photophosphorylation) – The proton‑motive force drives protons back through ATP synthase (CF₁CF₀). The rotary motor couples the flow of ≈ 3 H⁺ per ATP formed:

   \$\text{ADP} + \text{P}i + n\text{H}^+{\text{lumen}} \;\xrightarrow{\text{ATP synthase}}\; \text{ATP} + n\text{H}^+_{\text{stroma}}\$

   (Typically n ≈ 3–4.)

3.3 Cyclic Photophosphorylation (supplementary ATP production)

• Occurs only in PS I and is activated when NADP⁺ is scarce.
• Excited electrons from P700 are transferred to ferredoxin, then back to the plastoquinone pool via the cytochrome b₆f complex.
• The electron returns to PS I, completing a closed loop.
• Energy released at cytochrome b₆f pumps additional protons (≈ 2 H⁺ per electron pair), generating extra ATP but no NADPH or O₂.

3.4 Energy Summary (per 8 photons = 4 × PSII + 4 × PSI)

4. Light‑Independent Reactions – Calvin Cycle (AO1 + AO2)

1. Carbon fixation (carboxylation) – Rubisco catalyses:

   \$\text{RuBP (5‑C)} + \text{CO}_2 \;\xrightarrow{\text{Rubisco}}\; 2\;\text{3‑PGA (3‑C)}\$

2. Reduction phase

   • ATP phosphorylates 3‑PGA to 1,3‑bisphosphoglycerate (1,3‑BPGA).
   • NADPH reduces 1,3‑BPGA to glyceraldehyde‑3‑phosphate (G3P):

     \$\text{1,3‑BPGA} + \text{NADPH} \rightarrow \text{G3P} + \text{NADP}^+ + \text{P}_i\$

3. Regeneration of RuBP – Five of the six G3P molecules are recycled, consuming 3 ATP:

   \$5\;\text{G3P} + 3\;\text{ATP} \;\rightarrow\; 3\;\text{RuBP} + 3\;\text{ADP} + 3\;\text{P}_i\$

4. Net equation (per CO₂ fixed):

   \$\text{CO}2 + 2\;\text{NADPH} + 3\;\text{ATP} \;\longrightarrow\; \text{G3P} + 2\;\text{NADP}^+ + 3\;\text{ADP} + 3\;\text{P}i\$

5. Six turns of the cycle fix six CO₂, producing two G3P; one G3P can leave the cycle to form glucose and other carbohydrates.

5. Factors Limiting the Rate of Photosynthesis (AO2)

6. Integrated Summary of Photosynthesis

• Light energy excites electrons in PS II (P680) and PS I (P700).
• Water splitting supplies electrons, protons and O₂.
• Energetic electrons lose potential energy as they travel through the ETC; this energy pumps H⁺ into the thylakoid lumen, creating a proton‑motive force.
• Protons flow back through ATP synthase, driving synthesis of ATP (photophosphorylation).
• The terminal electron acceptor, NADP⁺, is reduced to NADPH.
• ATP and NADPH power the Calvin cycle in the stroma, fixing CO₂ into G3P and ultimately glucose.
• Any limiting factor (light, CO₂, temperature, water) reduces the overall output, which is reflected in the rate‑limiting step of the whole process.

Key Points to Remember (Exam Checklist)

• The ETC does not store energy; it transfers energy from high‑energy electrons to a proton gradient.
• Photophosphorylation is a specific case of chemiosmosis – the same principle that drives mitochondrial oxidative phosphorylation.
• Non‑cyclic photophosphorylation produces O₂, ATP and NADPH; cyclic photophosphorylation produces only ATP.
• Rubisco can act as a carboxylase (fixes CO₂) or an oxygenase (initiates photorespiration – useful for AO3 discussion).
• When answering AO2 questions, always link experimental data (e.g., action spectra, TLC Rf values, gas‑exchange graphs) to the underlying processes described above.

Suggested Diagram Set (for revision)

1. Labelled cross‑section of a chloroplast showing outer/inner membranes, stroma, grana, lamellae, thylakoid lumen, and the locations of PS II and PS I.
2. Absorption spectra of chlorophyll a, chlorophyll b, and carotenoids, together with a typical action spectrum for O₂ evolution.
3. Complete Z‑scheme with arrows indicating electron flow, proton pumping, and sites of ATP synthesis.
4. Calvin cycle diagram showing the use of ATP and NADPH, the regeneration of RuBP and the export of G3P.