A Level Biology Practice Exam

What does the cohesion-tension theory explain?

1. The storage of glucose in plants

2. The movement of water through a plant during transpiration

3. The process of photosynthesis in leaf cells

4. The transport of nutrients in the soil

The correct answer is: The movement of water through a plant during transpiration

The cohesion-tension theory provides a fundamental explanation for how water moves through a plant, particularly during the process of transpiration. According to this theory, water molecules are held together by cohesive forces due to hydrogen bonding. When water evaporates from the leaves during transpiration, it creates a negative pressure (tension) that pulls more water upward through the xylem vessels from the roots. This upward movement is facilitated by the cohesive properties of water, allowing a continuous column of water to be maintained from the roots to the leaves. This theory addresses the mechanism of water transport, showing how the loss of water vapor creates a pressure gradient that enables the ascent of water without the need for energy expenditure from the plant itself. Understanding this process is crucial for appreciating how plants maintain hydration and facilitate nutrient transport within their vascular systems.