Advanced Placement (AP) Biology Practice Exam

What allows water to move up a tree despite gravity?

1. Transpiration

2. Evaporation

3. Cohesion

4. Adhesion

The correct answer is: Cohesion

The movement of water up a tree against the force of gravity is primarily facilitated by cohesion. Cohesion refers to the attraction between water molecules due to hydrogen bonding, which allows them to stick together as they move through the plant's vascular system, particularly in the xylem. This property is critical for the formation of a continuous water column, helping to minimize the risk of the water column breaking under tension. Additionally, as water evaporates from the leaves through the process of transpiration, it creates negative pressure within the xylem. Cohesion allows the water molecules to pull each other upward from the roots through the stem and into the leaves. This combination of negative pressure from transpiration and cohesion is what drives the substantial upward movement of water against gravity in trees. While evaporation and adhesion also play roles in the overall process, they do not drive the upward movement of water as directly as cohesion does. Adhesion refers to the attraction between water molecules and the plant cell walls, aiding in the initial adherence of water to the xylem vessels but being less impactful in maintaining the water column under significant tension. Thus, cohesion is the key factor enabling trees to transport water to great heights despite the opposing force of gravity.