A Level Biology Practice Exam

What is indicated by the water vapour potential gradient?

1. The difference in pressure between water and solutes

2. The concentration of water vapour inside and outside the leaf

3. The rate at which water is lost from the leaf surface

4. The total amount of water vapour in the air

The correct answer is: The concentration of water vapour inside and outside the leaf

The water vapour potential gradient refers to the difference in concentration of water vapour between the inside and outside of a leaf. This gradient is essential in understanding how water moves from an area of higher concentration (within the leaf) to an area of lower concentration (the surrounding air). The greater the difference in concentration, the stronger the gradient, which influences the rate of transpiration—the process through which water is absorbed by plant roots from the soil and subsequently evaporated from the leaf surfaces. In this context, the concentration gradient directly affects the movement of water vapour. When the conditions in the external environment outside the leaf are drier, the potential gradient increases, leading to a higher rate of water loss from the plant's surfaces. While the other options touch on relevant concepts about water and transpiration, they do not specifically define what a water vapour potential gradient entails. For instance, the difference in pressure between water and solutes relates to osmotic pressure rather than vapour potential. The rate at which water is lost from the leaf surface pertains to transpiration but doesn't define the gradient itself. Lastly, the total amount of water vapour in the air does not specify the gradient but rather quantifies the overall moisture present in the atmosphere.