The chloroplasts of leaves of tobacco plants, Nicotiana sp., contain chlorophyll a and chlorophyll b. (a) Describe the role of chlorophyll b in photosynthesis. (b) A mutant tobacco plant was found to contain more chlorophyll b than normal tobacco plants. An investigation was carried out to measure the rate of photosynthesis of normal and mutant tobacco plants at increasing light intensities. The rate of production of oxygen was used as a measure of the rate of photosynthesis. Other variables were kept constant. [Figure 9.1] shows the results of this investigation.

Mark scheme

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

1. [2] any two from: 1 accessory pigment; 2 absorbs light wavelengths not absorbed by, reaction centre / primary pigment / chlorophyll a ; 3 so extends the range of wavelengths absorbed ; 4 pass energy to, reaction centre / primary pigment / chlorophyll a ; 5 idea that improves efficiency of light-dependent stage ;
2. [3] 1 rate of photosynthesis / rate of oxygen production, increases then levels off in both (plants) ; 2 rate of mutant plants higher than rate of normal plants ; 3 comparative data quote ;
3. [4] any four from: 1 absorbs more light ; 2 so faster rate of, photosynthesis / light-dependent stage; 3 more ATP and reduced NADP produced ; 4 more turns of Calvin cycle / AW ; 5 more, triose phosophate / TP, produced ; 6 more, hexose/ AW, for respiration; 7 more energy for growth; 8 more synthesis of, amino acids / proteins / lipids / cellulose / starch ;