Biology 9700 · AS & A Level · Photosynthesis as an energy transfer process
Photosynthesis as an energy transfer process — practice question
Chloroplasts contain several different photosynthetic pigments in plants. A student separated and identified the chloroplast pigments in a spinach leaf extract by using two slightly different procedures. In Method A, thin layer chromatography (TLC) was used to separate the pigments, with a solvent mixture of ether and cyclohexane in the TLC. In Method B, the student carried out TLC again after treating the spinach leaf extract with a chemical. This chemical replaces a magnesium ion in a pigment with two hydrogen ions. The student used a leaf from the same spinach plant and the same solvent as in Method A. After calculating $R_f$ values and comparing them with reference values, the student identified the pigments. Fig. 3.1 displays the outcomes for Method A and Method B.
(a(i))[2]
Calculate the $R_f$ value shown by $\beta$-carotene in Fig. 3.1.
(a(ii))[2]
Suggest two reasons for the differences in how chromatograms A and B appear in Fig. 3.1.
(a(iii))[1]
The 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 plant species, the absorption of light stimulates seed germination. Light absorption increases the production of gibberellin in the embryo of a seed. Describe the role of gibberellin in the germination of a seed.
Worked solution & mark scheme
This 13-mark question has a full step-by-step worked solution and mark scheme. One marking point: “0.95, with no units.” …