Photosynthesis

Question 1 (10 marks)

(a) $6\text{CO}_2 + 12\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 + 6\text{H}_2\text{O}$ — 1 mark for correct reactants/products, 1 mark for showing 12 H₂O in (net 6 produced). (Simplified $6\text{CO}_2+6\text{H}_2\text{O}\to \text{C}_6\text{H}_{12}\text{O}_6+6\text{O}_2$ accepted for 1.)

(b) The $\text{O}_2$ will be labelled with $^{18}\text{O}$ (1). Reason: oxygen gas comes from the photolysis of water in the light-dependent reactions ($2\text{H}_2\text{O}\to 4\text{H}^+ + 4e^- + \text{O}_2$) (1), so oxygen atoms in O₂ originate from water, not CO₂ (1).

(c) The $^{18}\text{O}$ appears in the carbohydrate (and in the H₂O produced) (1). CO₂ is fixed in the Calvin cycle by RuBisCO; its oxygen atoms are incorporated into the 3-carbon acids/sugars (1), and some appear in the water released during the cycle — never in O₂ gas (1).

(d) Twelve water molecules are consumed (as the electron/H⁺ source in photolysis) and six are regenerated during carbon fixation/condensation (1); writing both sides shows water is both a reactant and a product, which the isotope experiment (b/c) confirms (1).

Question 2 (10 marks)

(a) Highest rates at ~430 nm (blue) and ~670 nm (red) (1); these correspond to the two absorption peaks of chlorophyll a (and b) (1); light absorbed here drives the light reactions and O₂ release most efficiently (1).

(b) Between 500–550 nm (green/yellow) the small non-zero rate is due to accessory pigments (carotenoids, chlorophyll b) absorbing some of this light and passing energy to the reaction centre (2). (Also acceptable: not all wavelengths perfectly monochromatic / scattering.)

(c) Rate at 430 and 670 nm would remain high (chlorophyll a still present) (1); the rate in the green–yellow (500–550) region would fall closer to zero because the accessory pigments harvesting that light are absent (1); overall the graph becomes more sharply "two-peaked" with a deeper trough (1).

(d) An absorption spectrum shows light absorbed by all pigments, some of which is lost as heat/fluorescence or not transferred efficiently, whereas the action spectrum shows only wavelengths that actually drive photosynthesis (1); hence they broadly match (both peak in blue/red) but differ in the green region and peak heights (1).

Question 3 (10 marks)

(a) Glycerate-3-phosphate (GP / 3-PGA) (1); enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase) (1).

(b) CO₂ combines with the 5-C acceptor RuBP to form an unstable 6-carbon intermediate (1) which immediately splits into two 3-carbon molecules of GP (1); the 6-C compound is too unstable to accumulate, so the 3-C GP is the first stable product (1).

(c) In darkness, the light reactions stop, so ATP and NADPH are no longer made (1). GP cannot be reduced to triose phosphate, and RuBP cannot be regenerated (1). Therefore GP (the 3-C compound) rises/accumulates and RuBP falls (it keeps being consumed by continuing carboxylation but is not remade) (1).

(d) 6 turns of the cycle (1); requiring 18 ATP and 12 NADPH per glucose (1).

Question 4 (12 marks)

(a) At high light and temperature the C3 grass suffers high photorespiration because RuBisCO fixes O₂ when CO₂ is low (stomata partly closed in heat) (1). The C4 grass fixes CO₂ first with PEP carboxylase into a 4-C acid in mesophyll cells (1), which has no oxygenase activity and high CO₂ affinity (1); CO₂ is then concentrated in bundle-sheath cells so RuBisCO works efficiently with little photorespiration → higher net gain (1).

(b) The CAM succulent opens stomata at night to fix CO₂ (via PEP carboxylase) into malic acid stored in vacuoles, keeping stomata closed by day to reduce transpiration (1) for reducing water loss (1). By day the malate releases CO₂ for the Calvin cycle behind closed stomata (1). Key difference: CAM separates the two carboxylations in time (night vs day), whereas C4 separates them in space (mesophyll vs bundle-sheath cells) (1).

(c) In cool, moist conditions photorespiration is low and water is not limiting, so the extra ATP cost of the C4/CAM CO₂-concentrating mechanisms is not justified (1); the simpler, energetically cheaper C3 pathway then gives faster growth (1).

(d) RuBisCO can bind either CO₂ or O₂; at midday high temperature and closed stomata lower internal CO₂ and raise O₂, favouring the oxygenase reaction (photorespiration) in the C3 grass (1). The C4 grass keeps CO₂ concentration high around RuBisCO in bundle-sheath cells, so oxygenation is suppressed (1).

Question 5 (8 marks)

(a) Raising light (1→2) raises rate from 10 to 22 (1); therefore light was the limiting factor at low light — increasing it increased the rate while other factors stayed constant (1).

(b) Comparing condition 2→3, only CO₂ was increased (light already high) and rate rose 22→40, so CO₂ concentration was now limiting (1); because at high light the plant could use extra CO₂, showing light was no longer the limit (1); rate is now capped by CO₂ availability (1).

(c) Between 30 °C and 40 °C the rate collapses (65→12) (1); the high temperature denatures the enzymes (e.g. RuBisCO, ATP synthase) (1), so their active sites change shape and catalysis fails despite ample light and CO₂ (1).

[
  {"claim":"Photosynthesis equation is balanced (12 H2O form): C=6, H=24, O=30 both sides",
   "code":"C_l=6; H_l=24; O_l=2*6+12; C_r=6; H_r=12+2*6; O_r=6+6+2*6; result=(C_l==C_r) and (H_l==H_r) and (O_l==O_r)"},
  {"claim":"Six turns of Calvin cycle needed per glucose",
   "code":"carbons_per_turn=1; carbons_needed=6; turns=carbons_needed/carbons_per_turn; result=(turns==6)"},
  {"claim":"18 ATP and 12 NADPH required per glucose (3 ATP + 2 NADPH per turn * 6 turns)",
   "code":"turns=6; ATP=3*turns; NADPH=2*turns; result=(ATP==18) and (NADPH==12)"},
  {"claim":"Light is limiting between conditions 1 and 2 (only light changed, rate rose)",
   "code":"rate1=10; rate2=22; only_light_changed=True; result=only_light_changed and (rate2>rate1)"}
]