Photosynthesis is a process where organic compounds are produced in cells using light energy, carbon dioxide and water. A product of photosynthesis is oxygen gas. Hence, the rate of photosynthesis can be measured by the rate of carbon dioxide gas uptake or by the rate of oxygen gas production (Choice A).
At x, this is known as the optimum temperature. The rate of photosynthesis is the highest here. In other words, x is the temperature where the maximum rate of carbon dioxide absorption and oxygen production occurs.
At y, light intensity and carbon dioxide concentration are not the limiting factors because, beyond the optimum temperature, the graph does not plateau but instead, it slopes downwards (Choice D). At y, the rate of photosynthesis decreases because the temperature is affecting the enzymes involved in photosynthesis (Choice B). Hence, the rate of photosynthesis does not just increase with temperature, it also decreases above the optimum temperature.
