In botany, a light curve shows the photosynthetic response of a leaf or algae to the brightness of a light. The shape of the curve shows the principle of limiting factors. In low light, the rate of photosynthesis is limited by the amount of chlorophyll and the efficiency of the light-dependent reactions. In higher light levels it is limited by the efficiency of RuBisCO (an enzyme) and the amount of carbon dioxide. The point on the graph where these two differing lines meet is called the light saturation point. This is where the light-dependent reactions are making more ATP and NADPH than can be used by the light-independent reactions. Since photosynthesis is also limited by ambient carbon dioxide levels, light curves are often repeated at several different constant carbon dioxide concentrations.