A Level Biology Practice Exam

What is a primary mechanism by which xylem transports water?

1. Active transport through cell membranes

2. Capillary action and root pressure

3. Diffusion of nutrients

4. Photosynthesis in leaf cells

The correct answer is: Capillary action and root pressure

The primary mechanism by which xylem transports water is capillary action and root pressure. Capillary action occurs when water molecules adhere to the walls of xylem vessels, which are typically narrow tubes, allowing water to rise against gravity due to cohesion between water molecules and adhesion to the vessel walls. This phenomenon helps to move water upward from the roots to the leaves, particularly in taller plants. Root pressure also plays a significant role; it is generated when minerals are actively taken up by root cells, leading to a decrease in water potential within the root. Consequently, water from the soil moves into the roots through osmosis, creating pressure that can push water up into the xylem. Other choices, such as active transport through cell membranes and diffusion of nutrients, do not primarily relate to water transport in xylem. Active transport is more relevant to nutrient uptake rather than the bulk flow of water. Similarly, while diffusion is vital for transporting small molecules, it is not the primary means of long-distance water transport in plants. Photosynthesis is a metabolic process that does not involve the movement of water through the xylem; rather, it occurs in the leaves using water and carbon dioxide to produce glucose and oxygen. Thus, the combination