Figure 2a shows the changes in atmospheric carbon dioxide (CO2) concentration measured in a biomass chamber containing a stand of soybean plants. The plants were subjected to cyclic periods of artificial lighting and darkness. During the periods of illumination, small amounts of CO2 were injected into the chamber to prevent the concentration of this gas falling below 1.0-2.5 mmol mol-1(because in a real situation, this would have implications for human health). Figure 2b shows the changes in ambient oxygen concentration measured in a separate experiment using a second biomass chamber containing a similar stand of soybean plants

Figure 2 Gas exchange in a stand of soybean plants measured in a biomass chamber. (a) Changes in atmospheric concentration of carbon dioxide during periods of artificial lighting (non-shaded) and darkness (indicated by grey shading). (b) Changes in oxygen concentration for a stand of soybean plants grown in a biomass chamber (periods of lighting and darkness as indicated in (a)). Note that a different scale for the x-axis has been used in each graph.

(i) Using your knowledge of plant metabolism, write a short account explaining how it produces the changing pattern of CO2 concentration?