Biology 9700 · AS & A Level · Photosynthesis as an energy transfer process

Photosynthesis as an energy transfer process — practice question

Plants contain a range of photosynthetic pigments inside their chloroplasts. A student separated and identified the chloroplast pigments in a leaf extract from a spinach plant by using two slightly different methods. Method A: a form of chromatography called thin layer chromatography (TLC) was used to separate the pigments; a solvent made from ether and cyclohexane was used in TLC. Method B: the student repeated TLC but first treated the spinach leaf extract with a chemical; this chemical causes a magnesium ion in a pigment to be substituted by two hydrogen ions. The student used a leaf from the same spinach plant, and the same solvent as in method A. The student worked out $R_f$ values and compared them with reference values to identify the pigments. Fig. 3.1 shows the outcomes for method A and method B.
(a(i))[2]

Calculate the $R_f$ value of β-carotene in Fig. 3.1.

(a(ii))[2]

Suggest two explanations for the differences in appearance of chromatograms A and B in Fig. 3.1.

(a(iii))[1]

A student found some different $R_f$ values for the chloroplast pigments of spinach in a scientific paper. The values in the scientific paper were not the same as the reference values that the student first used to identify the pigments on chromatograms A and B in Fig. 3.1. Suggest one reason, other than measurement error, for the different $R_f$ values.

(b)[4]

Fig. 3.2 shows the absorption spectra of some chloroplast pigments. Use Fig. 3.2 to compare the similarities and differences between the absorption spectra of chlorophyll a and carotenoids.

(c)[4]

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.

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