Carbon dioxide is a substrate of the light-independent reactions (the Calvin cycle) in the stroma. The enzyme RuBisCO fixes carbon dioxide by combining it with the 5-carbon compound RuBP. The unstable 6-carbon product immediately splits into two molecules of the 3-carbon compound GP, which is then reduced to GALP using ATP and reduced NADP.RuBP + CO2 → 2 GP Carbon dioxide diffuses into the leaf through the stomata. The atmosphere contains only about 0.04% carbon dioxide, so on a bright, warm day carbon dioxide concentration is often the limiting factor. Increasing its concentration increases the rate of photosynthesis until another factor, such as light intensity or temperature, becomes limiting.
When carbon dioxide concentration is low, RuBP cannot be fixed as fast. Less GP is made, so the amounts of GP and GALP fall. RuBP is still being regenerated from the GALP already present, so RuBP accumulates.Growers make use of this. Commercial glasshouses can raise the carbon dioxide concentration to about 0.1%, for example by burning paraffin or releasing bottled carbon dioxide. This increases yield as long as light and temperature are not limiting.In the lab, carbon dioxide concentration can be varied in pondweed experiments by dissolving different concentrations of sodium hydrogencarbonate in the water. The rate is measured as the volume of oxygen given off per minute, with light intensity and temperature kept constant.
When carbon dioxide concentration is low, RuBP cannot be fixed as fast. Less GP is made, so the amounts of GP and GALP fall. RuBP is still being regenerated from the GALP already present, so RuBP accumulates.Growers make use of this. Commercial glasshouses can raise the carbon dioxide concentration to about 0.1%, for example by burning paraffin or releasing bottled carbon dioxide. This increases yield as long as light and temperature are not limiting.In the lab, carbon dioxide concentration can be varied in pondweed experiments by dissolving different concentrations of sodium hydrogencarbonate in the water. The rate is measured as the volume of oxygen given off per minute, with light intensity and temperature kept constant.