(a(i))[2]
Suggest why, in all four experiments, the photosynthesis rate stayed constant as the carbon dioxide concentration increased.
(a(ii))[3]
Using Fig. 4.1, describe how the photosynthesis rate differs, at $10^\circ\text{C}$, between barley and sugar cane.
(a(iii))[4]
Sugar cane is a C4 plant. Barley is not a C4 plant. Explain why, at $25^{\circ}\text{C}$, sugar cane had a higher photosynthesis rate than barley.
(b)[4]
Carbon dioxide for photosynthesis enters leaves through open stomata. Stomata are not open all the time. Environmental changes can make stomata close. Describe these conditions and explain how stomatal closure helps the plant.