Once the concentration gradient no longer exists, particles will continue to diffuse due to random motion.
Explanation:
When there is a permeable membrane between two solutions with different concentrations, the concentration gradient will go from the compartment with more particles to the one with fewer particles to valance the concentrations on both sides of the membrane. Once the concentrations are equal, particles will still randomly move from one side to the other, but the number of particles that move to one side is almost the same as the number of particles that move to the other, so there is still valance and the same concentration in both sides.
As light intensity increases (distance between lamp and plant decreases) the volume of oxygen (or the rate of bubble production) increases. This indicates that the rate of photosynthesis increases with light intensity. However, at sufficiently high levels of light intensity, the rate oxygen evolution remains constant.
