C1.3 Photosynthesis. Practice questions with markscheme.

• B: 800 units lies outside the range investigated, so the straight-line relationship is being extrapolated; at high light intensity carbon dioxide concentration or temperature is likely to become limiting and the rate will plateau, so the predicted value will be too high;

Option B offers the correct-sounding neighbour, interpolation, for what is in fact extrapolation.

• D: water is split using light energy (photolysis), providing hydrogen for the reduction of CO₂ to glucose and releasing oxygen as a waste product;

• B: Rf depends on how the pigment is shared between the solvent (mobile phase) and the paper (stationary phase), so a different solvent changes it; a longer run moves the spot and the front in proportion, so Rf stays 74 ÷ 80 ≈ 0.93 (B); the amount of extract (C) and the width of the paper (D) do not change the ratio;

• A: FACE experiments raise the CO₂ concentration around intact outdoor vegetation, to predict how rising atmospheric CO₂ will affect photosynthesis, growth and ecosystems;

• C: light energy is transformed into chemical energy in carbon compounds (using CO₂ and water, releasing O₂);

• A: O₂ release (bubble counts/volume collected) per unit time gives the rate; CO₂ uptake (pH change) is an alternative;

• light energy is transformed into chemical energy;
• the chemical energy is stored in carbon compounds / glucose (made from carbon dioxide and water);
• (almost) all of the chemical energy used by organisms in an ecosystem was originally captured by photosynthesis / by producers, OWTTE;

Beyond this subtopic. Award if given; the marks above can be earned without it.

• C4.2 the energy in carbon compounds passes to consumers and decomposers along food chains;
• C1.2 the energy is released (by cell respiration) for life processes such as growth, movement and active transport;

Do not accept 'energy is made / created' in photosynthesis. Accept 'solar energy' for light energy.

• axes labelled: absorption (of light) against wavelength (400–700 nm / violet to red);
• peaks drawn in the blue (~430–450 nm) and red (~660–680 nm) regions;
• trough/low absorption in the green region (~550 nm), annotated (green reflected);

Accept curves for chlorophyll a alone or a and b; both peaks required.

• (a) [1]
  • 150 (%);

  Working: (50 − 20) ÷ 20 × 100 = 150 %. Accept answers from 140 % to 160 % to allow for reading the graph.

• (b) [2]
  • (i) temperature, because raising the light intensity five times made almost no difference to the rate at 10 °C (12 compared with 11) OR because at both light intensities the rate rose when the temperature was raised from 10 °C to 15 °C / 20 °C (12 to 20 and 11 to 13);
  • (ii) light intensity, because raising the temperature from 20 °C to 35 °C did not increase the rate (stayed at about 14) OR because at the same temperatures the rate was much higher in high light (e.g. 42 compared with 14 at 25 °C);

  Do not award the mark for naming the factor without a reason from the data. Do not accept carbon dioxide concentration in either case: it was not varied, and the rate responds to the factor named.

• (c) [3 max]
  • (supports) in high light intensity the rate rose as temperature rose from 10 °C to 35 °C / from 12 to 52;
  • (does not support) in low light intensity the rate hardly changed between 15 °C and 35 °C / stayed at about 13 to 15;
  • (because) temperature affects the rate only when it is the limiting factor / not when light intensity is limiting, OWTTE;
  • (does not support) above 35 °C the rate fell at both light intensities / fell steeply in high light from 52 to 10;
  • the hypothesis needs to be revised / is provisional, e.g. the rate rises with temperature up to an optimum (about 35 °C) when light is not limiting, OWTTE;
  • the temperature steps of 5 °C mean the true peak could lie anywhere from about 30 °C to 40 °C OR only means are shown, with no measure of variation, so small differences such as 50 and 52 cannot be judged OR only two light intensities were tested;

  Award [2 max] for answers that only support or only challenge the hypothesis.

• (d) [3 max]
  • light is absorbed by pigments such as chlorophyll (in the chloroplasts of the algal cells);
  • absorbed light energy excites electrons in the pigment molecules / raises electrons to a higher energy level;
  • the energy of the excited electrons is transformed into chemical energy (in carbon compounds such as glucose), OWTTE;
  • at a higher light intensity more light / more photons reach the pigments per second, so more pigment molecules are excited per second / more energy is absorbed per unit time;
  • so more water can be split per second, supplying hydrogen to convert more carbon dioxide to glucose per second;
  • this applies only while light intensity is the limiting factor / until another factor such as temperature limits the rate (as at 10 °C);
  • only light of the wavelengths the pigments absorb (mainly red and blue) can be used, OWTTE;

  Do not accept 'light gives the algae more energy' without reference to absorption by pigments or excitation of electrons for the first two marking points.

• B: Rf = distance moved by the pigment ÷ distance moved by the solvent front = 48 ÷ 80 = 0.60;

• D: correlation between absorption and rate shows chlorophyll captures the light energy used; accessory pigments explain the differences;

• D: FACE experiments raise the CO₂ concentration around intact outdoor vegetation, to predict how rising atmospheric CO₂ will affect photosynthesis, growth and ecosystems;

• C: amino acids contain nitrogen; carbon skeletons derived from the Calvin cycle are combined with nitrogen from absorbed nitrate/ammonium to synthesize amino acids;

• A: Rubisco has a low affinity for carbon dioxide and works slowly, so at low carbon dioxide concentrations the rate at which it carboxylates RuBP falls; A invents product inhibition (glycerate 3-phosphate is consumed, not accumulated); B reverses cause and effect, since a lack of carbon dioxide affects photosystem II only indirectly and RuBP regeneration needs ATP, not photolysis; C confuses low substrate concentration with denaturation, which is caused by heat or pH;

• A: without CO₂ no glycerate 3-phosphate is made, so NADPH is not oxidised; NADP⁺ runs out, electrons cannot leave photosystem I, the chain backs up and photosystem II can no longer pass on electrons, so photolysis stops; the oxygen released comes from water (A); Rubisco fixes CO₂ and does not split water (B); CO₂ does not bind to chlorophyll (D);

• B: GP is converted to triose phosphate using both ATP and NADPH (the NADPH supplies the hydrogen/electrons for the reduction), and tube 4 is the only tube with both; light is not needed once ATP and NADPH are supplied, and the extract has no thylakoids to use it; tube 1 has only oxidised NADP⁺, which cannot reduce GP; tubes 2 and 3 each lack one of ATP and NADPH;

• C: 6 CO₂ → 12 GP → 12 triose phosphate; 2 TP (6C total) exit to make hexose, while 10 TP (30C) rebuild 6 RuBP (30C) using ATP, the cycle's carbon bookkeeping;

• A: antenna/accessory pigments harvest photons over a range of wavelengths and funnel the energy to the reaction-centre chlorophyll a, from which an excited electron is emitted;

• (a) [2]
  • 10⁶ ÷ 417 = 2398 / about 2400 chlorophyll molecules per molecule of oxygen OR 417 × 8 = 3336 reaction centres per 10⁶ chlorophyll molecules;
  • 2400 ÷ 8 OR 10⁶ ÷ 3336 = 300 (chlorophyll molecules per reaction centre);

  Accept 299 to 300. Award [2] for a correct final answer with or without working. ECF from the first step.

• (b) [2]
  • at high intensity every reaction centre receives enough energy to emit an electron in each flash;
  • no reaction centre can emit a second electron during a flash, so the yield is limited by the number of reaction centres, not by light, OWTTE;
  • extra light energy absorbed by the pigments cannot be used (and is lost as heat);
  • at low intensities only some reaction centres receive energy, so the yield rises with intensity;
• (c) [2]
  • energy absorbed by all the pigment molecules is passed / funnelled to one reaction centre;
  • so the reaction centre is excited far more often / works close to its maximum rate, OWTTE;
  • even in dim light enough energy reaches the reaction centre for electrons to be emitted;
  • fewer reaction centres / electron carriers are needed for the same light capture, saving materials;
  • the different pigments in the array absorb a wider range of wavelengths than chlorophyll a alone;

• oxygen production stops, because electrons are not being drained from photosystem II, so photolysis of water is not needed / halts;
• no electrons pass from photosystem II to photosystem I, so NADP is not reduced / no NADPH is made (and non-cyclic ATP production stops);
• so the Calvin cycle stops, as reduction of glycerate 3-phosphate needs NADPH (as well as ATP), OWTTE;
• (accept) some ATP can still be made using electrons emitted by photosystem I that return to it, but without NADPH the cycle cannot continue;

• CO₂ is a substrate (for carbon fixation), so low concentration limits the rate;
• light provides the energy (for the light-dependent reactions), so low intensity limits the rate;
• temperature affects enzyme(-catalysed) reactions, so rate falls when too cold (and above the optimum);
• (at any moment) the factor in shortest supply / furthest below saturation determines the rate, OWTTE;