A-LevelBiologyPhotosynthesis9700/42 · May/June 2022 · Q215 Marks

(a) Photosynthesis is affected by many environmental factors.

(a)(i)Explain why light intensity can be a limiting factor in photosynthesis.[2]
(a)(ii)The concentration of carbon dioxide (CO₂) can also be a limiting factor. It has an effect on the Calvin cycle in the light-independent stage of photosynthesis. Ribulose bisphosphate (RuBP), triose phosphate (TP) and glycerate 3-phosphate (GP) are three important molecules in the Calvin cycle. [Figure 2.1] shows how the concentration of GP changes when the concentration of CO₂ is reduced from 0.04% (atmospheric) to 0.008%. Complete [Figure 2.1] by sketching the lines for RuBP and TP when the concentration of CO₂ is reduced from 0.04% to 0.008%.[2]
(a)(iii)In very dry conditions, CO₂ concentration can become the main limiting factor of photosynthesis in plants. Explain how very dry conditions cause CO₂ concentration to become the main limiting factor of photosynthesis in plants.[3]
(b)A factor that can limit the rate of photosynthesis is the rate of regeneration of RuBP. Sedoheptulose-1,7-bisphosphatase (SBPase) is an enzyme in the Calvin cycle that controls the rate of regeneration of RuBP. SBPase is coded for by the gene SBPase. In an experiment, wheat plants were genetically modified to make more SBPase by introducing the SBPase gene from another grass species, Brachypodium distachyon. The resulting GM wheat plants were named Sox4. • Wild type plants (not GM) and Sox4 plants were grown in a greenhouse. • Light intensity, CO₂ concentration and temperature were kept constant. • Mature plants were removed and dried to measure the biomass. [Figure 2.2] shows the mean plant biomass for the wild type plants and GM Sox4 plants.
(b)(i)Calculate the percentage change in mean plant biomass when Sox4 plants are grown compared to wild type plants. Show your working. percentage change =[2]
(b)(ii)Suggest and explain why Sox4 plants have a different mean plant biomass than wild type plants.[4]
(b)(iii)Some soils may be deficient in nitrates. Suggest how nitrate deficiency could limit the quantity of SBPase made by Sox4 plants.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

[2] any two from: 1 ref. to light energy / photons ; 2 for, light-dependent stage / photophosphorylation / photolysis / photoexcitation / photoactivation ; 3 to make, reduced NADP / ATP; 4 to open stomata (for CO2 to enter);
[2] correctly showing that TP reduces to a plateau below GP ; correctly showing that RuBP increases to a plateau ;
[3] 1. decrease in / no, CO2, diffusing / moving, to, air spaces / palisade cells / mesophyll cells / chloroplasts / stroma ; any two from: 2 stress response / AW ; 3 ref. to abscisic acid / ABA ; 4 ref. to action of Ca2+ as a second messenger ; 5 stomata close ;
[2] 28 - 20.5 / 20.5 × 100 or 7.5 / 20.5 × 100 ; A 20.4 or 20.6 answer to 1 decimal place 36.6 (%) ; apply ecf
[4] any four from: no mention of increased or more – ecf and max 3 1 Sox4 plants have an extra copy of SBPase gene ; 2 (overall) increased, expression / transcription, of SBPase gene (so more SBPase) ; 3 increased (rate of) / more, regeneration / production, of RuBP ; 4 (so) increased / more, carbon fixation / Calvin cycles / light independent reaction / TP / GP ; 5 more, glucose / sucrose, for respiration / ATP production or more, starch / lipid, for storage or more cellulose for cell walls ; 6 more, amino acids / proteins, for growth; 7 more, cell division / cell elongation / mitosis ; 8 AVP; e.g.(new) SBPase / SBPase gene, more efficient
[2] nitrogen needed to produce: nucleotides for, transcription (of SBPase gene) / expression (of SBPase gene)/mRNA production ; amino acids for, protein synthesis / SBPase production / enzyme production ;