USA Biology Olympiad (USABO) Practice Exam

What primarily drives the movement of water through xylem in plants?

1. Capillary action

2. Transpiration

3. Root pressure

4. Phloem transport

The correct answer is: Transpiration

The primary driver of water movement through xylem in plants is transpiration. This process involves the evaporation of water from the stomata, the tiny openings on the leaves. As water vapor escapes into the atmosphere, it creates a negative pressure in the xylem vessels, which pulls more water up from the roots through the plant. This movement is essential for the plant’s nutrient transport and overall hydration. Transpiration not only helps maintain the flow of water from the roots to the leaves but also aids in regulating temperature and optimizing photosynthesis by facilitating the uptake of carbon dioxide. While capillary action, root pressure, and interactions in phloem transport play roles in plant water and nutrient dynamics, they do not drive water movement through the xylem to the same extent or in the same manner as transpiration. In summary, transpiration is a critical mechanism that enables the upward movement of water, making it the key process driving the movement of water through xylem in plants.