A Level Biology Practice Exam

What phenomenon does the tension aspect of the cohesion-tension theory relate to?

1. The tension in water columns during evaporation

2. The tension created by soil moisture levels

3. The tension in leaf cells during respiration

4. The tension in the roots during nutrient uptake

The correct answer is: The tension in water columns during evaporation

The tension aspect of the cohesion-tension theory relates specifically to the process of water movement within plants, particularly during evaporation. This theory explains how water is transported from the roots through the xylem up to the leaves, driven by the negative pressure (tension) created when water molecules evaporate from the surfaces of leaf cells during transpiration. As water evaporates, it creates a pull that not only draws in more water from the xylem but also maintains the column of water intact due to the cohesive properties of water molecules. While the phenomenon involves several interactions within the plant, the primary mechanism explaining this tension is directly tied to the loss of water through evaporation on leaf surfaces, which effectively pulls water upward against gravity. Consequently, this makes the first choice particularly relevant to understanding how plants effectively transport water using this principle. The other options do not adequately capture the primary mechanism of tension as it relates to the cohesion-tension theory. Soil moisture levels, leaf cell respiration, and nutrient uptake have different physiological impacts that, while relevant to plant health and functioning, do not directly connect to the tension created by evaporation in the context of water transportation.