Photosynthesis (AP)

$6CO_2 + 6H_2O \xrightarrow{\text{light energy}} C_6H_{12}O_6 + 6O_2$

• Location: Chloroplasts (in mesophyll cells of leaves)
• Two stages: Light reactions (thylakoid membranes) and Calvin cycle (stroma)
• Inputs: CO₂, H₂O, light energy
• Outputs: glucose (G3P), O₂

• Outer membrane: permeable to small molecules
• Inner membrane: more selective
• Thylakoid membrane: internal membrane system; contains photosystems, ETC, ATP synthase
  • Grana: stacks of thylakoids
  • Thylakoid lumen: space inside thylakoids (H⁺ accumulates here)
• Stroma: fluid outside thylakoids; site of Calvin cycle; contains enzymes, DNA, ribosomes

Photosystem II (PS II) — Absorbs light at 680 nm (P680)

1. Light energy excites electrons in chlorophyll P680
2. Excited electrons pass to the primary electron acceptor
3. Lost electrons are replaced by splitting water (photolysis): $2H_2O \rightarrow 4H^+ + 4e^- + O_2$
4. Oxygen is released as a byproduct; $H^+$ ions contribute to the proton gradient

Electron Transport Chain (ETC)

5. Electrons pass through carriers (plastoquinone → cytochrome complex → plastocyanin)
6. Energy released is used to pump $H^+$ from stroma into the thylakoid lumen (chemiosmosis)
7. This creates a proton gradient (high $[H^+]$ in lumen, low in stroma)

Photosystem I (PS I) — Absorbs light at 700 nm (P700)

8. Light re-energises the electrons in P700
9. Electrons pass to ferredoxin
10. NADP⁺ reductase uses electrons and $H^+$ to produce NADPH: $NADP^+ + 2e^- + H^+ \rightarrow NADPH$

ATP Synthesis (Chemiosmosis)

11. $H^+$ ions flow down their gradient through ATP synthase (from lumen to stroma)
12. This drives photophosphorylation: $ADP + P_i \rightarrow ATP$

Summary of Light Reactions

Inputs: H₂O, light, NADP⁺, ADP + Pᵢ Outputs: O₂, ATP, NADPH

The Calvin cycle uses ATP and NADPH from the light reactions to fix CO₂ into organic molecules.

Three Phases (per 3 CO₂ fixed):

1. Carbon Fixation

• RuBisCO enzyme catalyses: $CO_2 + RuBP (5C) \rightarrow 2 \times 3\text{-PGA} (3C)$
• RuBisCO is the most abundant enzyme on Earth

2. Reduction

• 3-PGA is phosphorylated by ATP and reduced by NADPH → G3P (glyceraldehyde-3-phosphate)
• For every 3 CO₂: 6 G3P produced; 1 G3P exits the cycle (net gain)
• 5 G3P are recycled

3. Regeneration of RuBP

• The 5 remaining G3P are rearranged using ATP to regenerate 3 RuBP
• This allows the cycle to continue

For 1 glucose: 6 turns of the Calvin cycle (6 CO₂ fixed)

• Uses: 18 ATP + 12 NADPH
• 2 G3P molecules combine to form glucose

Limiting Factors

The factor in shortest supply determines the rate — the law of limiting factors.

Photorespiration

• On hot, dry days, stomata close → CO₂ drops, O₂ rises inside the leaf
• RuBisCO binds O₂ instead of CO₂ → produces a 2C compound that must be recycled → wastes energy, produces no sugar

C4 Plants (e.g., corn, sugarcane)

• Fix CO₂ first into a 4-carbon compound (using PEP carboxylase, which has no affinity for O₂)
• Concentrate CO₂ in bundle sheath cells where the Calvin cycle occurs
• Minimises photorespiration in hot environments

CAM Plants (e.g., cacti, pineapple)

• Open stomata only at night (fix CO₂ into organic acids)
• During the day, stomata close (conserve water); stored CO₂ is released for the Calvin cycle
• Temporal separation of CO₂ fixation and Calvin cycle

Question: Explain how the light reactions produce a proton gradient and how this gradient is used to synthesise ATP. (4 points)

Solution:

During the light reactions, water is split (photolysis) in the thylakoid lumen, releasing $H^+$ ions, electrons, and O₂. As electrons pass through the electron transport chain between PS II and PS I, energy is used to actively pump $H^+$ ions from the stroma into the thylakoid lumen via the cytochrome complex. This creates a high concentration of $H^+$ in the lumen relative to the stroma — a proton-motive force (both concentration gradient and charge difference). $H^+$ ions then flow down this gradient through ATP synthase (embedded in the thylakoid membrane) back into the stroma. The flow of protons drives the rotational catalysis of ATP synthase, which phosphorylates ADP to produce ATP (photophosphorylation/chemiosmosis).

• Light reactions (thylakoids): H₂O split → electrons through ETC → proton gradient → ATP (chemiosmosis) + NADPH; O₂ released.
• Calvin cycle (stroma): CO₂ fixed by RuBisCO → 3-PGA → G3P (using ATP + NADPH) → RuBP regenerated.
• Limiting factors: light intensity, CO₂ concentration, temperature.
• Photorespiration: RuBisCO binds O₂ → wasteful; addressed by C4 (spatial separation) and CAM (temporal separation) plants.
• 6 turns of the Calvin cycle fix 6 CO₂ → 1 glucose; requires 18 ATP + 12 NADPH.