A-LevelBiologyPhotosynthesis9700/41 · Oct/Nov 2025 · Q313 Marks

Plants have several different photosynthetic pigments in their chloroplasts. (a) A student separated and identified the chloroplast pigments present in a leaf extract from a spinach plant using two slightly different methods. Method A • A type of chromatography known as thin layer chromatography (TLC) was used to separate the pigments. • A mixture of ether and cyclohexane was used as a solvent in TLC. Method B • The student repeated TLC but treated the spinach leaf extract with a chemical. The chemical causes a magnesium ion in a pigment to be replaced by two hydrogen ions. • The student used a leaf from the same spinach plant, and used the same solvent as in method A. The student calculated R values and compared these to reference values to identify the pigments. [Figure 3.1] shows the results for method A and method B.

(a)(i)Calculate the R₁ value of ẞ-carotene in Fig. 3.1. R = [2]
(a)(ii)Suggest two explanations for the differences in appearance of chromatograms A and B in Fig. 3.1.[2]
(a)(iii)The student found some different R values for the chloroplast pigments of spinach in a scientific paper. The values in the scientific paper were different from the reference values that the student originally used to identify the pigments on chromatograms A and B in Fig. 3.1. Suggest one reason, other than measurement error, for the different R₁ values.[1]
(b)Use Fig. 3.2 to compare the similarities and differences between the absorption spectra of chlorophyll a and carotenoids.[4]
(c)In some species of plant, the absorption of light stimulates seed germination. The absorption of light increases the production of gibberellin in the embryo of a seed. Describe the role of gibberellin in the germination of a seed.[4]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

[2] (a)(i) 0.95 (no units) ; ;
[2] (a)(ii) any two from: 1 B, run for less time / stopped early ; 2 chlorophyll, converted / changed, to phaeophytin; 3 AVP;
[1] (a)(iii) any one from: 1 different solvents ; 2 different, TLC plate / chromatogram/stationary phase (material) ; 3 different temperatures ;
[4] (b) any four from: similarities 1 both absorb, blue / between 400 and 475 nm ; 2 both absorb, little / no, light between 525 and, 600 / 630, nm or both absorb, little / no, green / yellow, light; 3 AVP; max three differences 4 chlorophyll a absorbs, at more / a larger range of, wavelengths ; 5 chlorophyll a absorbs, more / better, between 400 and 445 nm ; 6 carotenoids absorb, more / better, between 445 and 525 nm ; 7 chlorophyll a absorbs between, 600 / 640, and, 680 / 690 nm, and carotenoids do not ; 8 chlorophyll a (blue absorption) peak / highest, at 425 / 430 nm and carotenoids (blue absorption) peak / highest, at 460 / 465 nm ; 9 chlorophyll a has blue peak and red peak but carotenoids have two blue peaks and no red peak or chlorophyll a has two peaks but carotenoids have three peaks ;
[4] (c) any four from: 1 gibberellin, moves to / acts on / stimulates, aleurone layer ; 2 binds to, (gibberellin) receptor / GID(1) ; 3 breaks down DELLA (protein) ; 4 releases, PIF / transcription factor ; 5 switches on gene for, amylase / maltase / protease ; 6 amylase, breaks down / hydrolyses, starch to maltose ; 7 sugars / glucose, for, respiration / growth, of embryo ;

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