The apparatus shown in the diagram can be used to measure the change in rate of photosynthesis in pondweed when the lamp is moved closer or further away from the pondweed. What would change to cause an increase or decrease in that rate?

The rate of photosynthesis in pondweed, or any plant, is primarily influenced by three factors: light intensity, carbon dioxide concentration, and temperature. In the context of the experiment described, the variable being manipulated is light intensity, which is controlled by moving the lamp closer or further away from the pondweed.

1. Increase in Light Intensity: When the lamp is moved closer to the pondweed, the light intensity increases. This provides more energy for the photosynthesis process, which in turn increases the rate of photosynthesis. This is because light energy is used in the photosynthesis process to convert carbon dioxide and water into glucose and oxygen.

2. Decrease in Light Intensity: Conversely, when the lamp is moved further away from the pondweed, the light intensity decreases. This provides less energy for the photosynthesis process, which in turn decreases the rate of photosynthesis.

It's important to note that while increasing light intensity generally increases the rate of photosynthesis, there is a limit to this. Beyond a certain light intensity, the rate of photosynthesis will no longer increase because other factors, such as carbon dioxide concentration and temperature, become limiting.