Carbon dioxide is the source of carbon for the sugars made in photosynthesis. In the light-independent reaction, the enzyme rubisco combines carbon dioxide with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). If carbon dioxide is scarce, this step is slow, and so is the whole Calvin cycle.Carbon dioxide enters leaves by diffusion through open stomata. Air contains only about 0.04% carbon dioxide, so on a bright, warm day carbon dioxide concentration is often the limiting factor.On a graph of rate against carbon dioxide concentration, the rate rises as the concentration increases, then levels off when light intensity or temperature becomes limiting. Raising the concentration to around 0.1% can increase the rate considerably, which is why some growers add carbon dioxide to glasshouses.
Worked example: At 0.04% carbon dioxide a plant produces 12 cm3 of oxygen per hour. At 0.08% it produces 20 cm3 per hour, and at 0.12% it still produces 20 cm3 per hour. Carbon dioxide was limiting between 0.04% and 0.08%, because increasing it raised the rate. Between 0.08% and 0.12% the rate did not change, so another factor, such as light intensity, was limiting.In water, carbon dioxide can be supplied as hydrogencarbonate ions, for example by adding sodium hydrogencarbonate to the water around pondweed.
Worked example: At 0.04% carbon dioxide a plant produces 12 cm3 of oxygen per hour. At 0.08% it produces 20 cm3 per hour, and at 0.12% it still produces 20 cm3 per hour. Carbon dioxide was limiting between 0.04% and 0.08%, because increasing it raised the rate. Between 0.08% and 0.12% the rate did not change, so another factor, such as light intensity, was limiting.In water, carbon dioxide can be supplied as hydrogencarbonate ions, for example by adding sodium hydrogencarbonate to the water around pondweed.