USA Biology Olympiad (USABO) Practice Exam

Which process primarily drives the movement of water from the soil to the top of a tall tree?

1. Osmosis

2. Capillary action

3. Transpiration

4. Root pressure

The correct answer is: Transpiration

The movement of water from the soil to the top of a tall tree is primarily driven by transpiration, which is the process by which water evaporates from the leaves of the plant. This evaporation creates a negative pressure in the leaf area, which pulls water upward through the xylem vessels from the roots to the leaves. Transpiration serves not only as a mechanism for water transport but also plays a critical role in nutrient uptake and maintaining plant structure. As water vapor exits the stomata of leaves into the atmosphere, a vacuum effect is created that aids in pulling more water molecules up from the roots. This process is particularly efficient in tall trees due to the cohesive properties of water molecules, which allows them to stick together and move as a continuous column through the xylem. While osmosis does contribute to the uptake of water by roots and root pressure can assist in pushing water upward when the plant has high moisture levels, these mechanisms are not the primary drivers of water movement to great heights. Capillary action plays a role in the movement of water through small xylem vessels but is limited compared to the significant pull created by transpiration. Overall, transpiration is the key process that facilitates the upward movement of water in tall trees.