Photosynthesis

Q1. [3] $6CO_2 + 6H_2O \xrightarrow[\text{chlorophyll}]{\text{light}} C_6H_{12}O_6 + 6O_2$

• Correct reactants (6CO₂ + 6H₂O) [1]
• Correct products (C₆H₁₂O₆ + 6O₂) [1]
• Conditions (light + chlorophyll) shown on arrow [1] Why: Balancing requires 6 of each on the left to yield one glucose plus 6 oxygen; light and chlorophyll are the essential conditions, not consumed reactants.

Q2. [4] (1 mark each)

• (a) Thylakoid: flattened membrane sac containing chlorophyll/photosystems; site of light-dependent reactions. [1]
• (b) Granum: a stack of thylakoids; increases surface area for light absorption. [1]
• (c) Stroma: fluid matrix surrounding grana; site of the Calvin cycle (light-independent reactions). [1]
• (d) Envelope: double membrane controlling entry/exit of substances. [1]

Q3. [4] (a) Blue-violet (~420–450 nm) [1] and red (~640–680 nm) light. [1] (b) Absorption spectrum = graph of the amount of light absorbed by a pigment at each wavelength [1]; action spectrum = graph of the rate of photosynthesis at each wavelength. [1] Why: The close match of the two spectra shows chlorophyll pigments drive photosynthesis.

Q4. [3] (1 each)

• (a) Light-dependent reactions: thylakoid membranes [1]
• (b) Photolysis of water: inside the thylakoid (lumen) / on thylakoid membrane at PSII [1]
• (c) Calvin cycle: stroma [1]

Q5. [4] (a) $2H_2O \xrightarrow{\text{light}} 4H^+ + 4e^- + O_2$ [2] (products correct [1], balanced [1]) (b) Fates: [2] (any two, ½ each rounded up)

• Electrons (e⁻) replace those lost from PSII (chlorophyll).
• Protons (H⁺) used to reduce NADP → reduced NADP (and drive chemiosmosis).
• Oxygen released as a by-product / waste gas.

Q6. [5]

• PSII + PSI vs PSI only [2]
• Water split in non-cyclic, not in cyclic [1]
• Non-cyclic products (ATP + reduced NADP + O₂) [1]
• Cyclic product (ATP only) [1]

Q7. [6] (a) RuBisCO (ribulose bisphosphate carboxylase/oxygenase) [1] (b) Acceptor = ribulose bisphosphate (RuBP) [1]; first product = glycerate-3-phosphate (GP / 3-PGA) [1] (c) ATP provides energy [1]; reduced NADP provides hydrogen/reducing power to convert GP → triose phosphate (TP) [1] (d) Most TP is used to regenerate RuBP (only a small fraction leaves to form glucose/organic products) [1]

Q8. [4] (a) RuBisCO also acts as an oxygenase; at high O₂/low CO₂ it fixes O₂ instead of CO₂ [1], producing a 2-carbon compound that must be recycled (wasteful) [1]. (b) High temperature / high O₂ concentration / low CO₂ concentration (any one) [1] (c) Reduces net carbon fixation / wastes energy / lowers photosynthetic yield (any one) [1]

Q9. [3] (1 mark per correctly distinguished plant type)

• C3: CO₂ fixed directly by RuBisCO in mesophyll cells; e.g. wheat/rice. [1]
• C4: CO₂ first fixed in mesophyll (into C4 acid), then released in bundle-sheath cells; e.g. maize/sugarcane. [1]
• CAM: CO₂ fixed at night (stomata open), stored as acid, used by day; e.g. cacti/pineapple. [1]

Q10. [4] Three limiting factors (1 each, any three): light intensity, carbon dioxide concentration, temperature (also accept water availability / chlorophyll) [3] Definition: A limiting factor is the factor in shortest supply that directly limits/controls the rate of a process when other factors are in excess. [1]

Total: 40 marks

[
  {"claim": "Photosynthesis equation is balanced: 6CO2+6H2O -> C6H12O6+6O2 (C,H,O atoms match)", "code": "C_left=6; H_left=6*2; O_left=6*2+6*1; C_right=6; H_right=12; O_right=6+6*2; result = (C_left==C_right) and (H_left==H_right) and (O_left==O_right)"},
  {"claim": "Photolysis equation 2H2O -> 4H+ + 4e- + O2 balances H, O and charge", "code": "H_left=2*2; O_left=2*1; H_right=4; O_right=2; charge_left=0; charge_right=4*(+1)+4*(-1); result = (H_left==H_right) and (O_left==O_right) and (charge_left==charge_right)"},
  {"claim": "Non-cyclic photophosphorylation uses 2 photosystems, cyclic uses 1", "code": "noncyclic_PS=2; cyclic_PS=1; result = (noncyclic_PS==2) and (cyclic_PS==1)"}
]