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Question Video: Defining the Attractive Forces between Water Molecules Biology

The cohesion–tension theory of water movement aims to explain how water is transported through the xylem. Which of the following best explains what is meant by cohesion in water movement? [A] Cohesion refers to the forces of attraction that occur between the water molecules and the walls of the xylem. [B] Cohesion refers to the forces of attraction between the molecules of water inside the xylem. [C] Cohesion refers to the pull of water through the xylem, as water vapor diffuses from the stomata. [D] Cohesion refers to the movement of water from the soil into the root, from an area of low solute concentration to an area of high solute concentration.

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Video Transcript

The cohesion–tension theory of water movement aims to explain how water is transported through the xylem. Which of the following best explains what is meant by cohesion in water movement? (A) Cohesion refers to the forces of attraction that occur between the water molecules and the walls of the xylem. (B) Cohesion refers to the forces of attraction between the molecules of water inside the xylem. (C) Cohesion refers to the pull of water through the xylem, as water vapor diffuses from the stomata. Or (D) cohesion refers to the movement of water from the soil into the root, from an area of low solute concentration to an area of high solute concentration.

Do you know why water transport is so important for plants? One reason that plants need water is that water is a reactant in the chemical reaction with which plants produce their own food, photosynthesis. Photosynthesis generally happens in the leaves of the plant. Water therefore has to be somehow transported from the roots, where the plant takes up the water, to the leaves of the plant. So how does this happen?

The question states that the cohesion–tension theory of water movement aims to explain this water transportation. This theory suggests that water is pulled up from the roots as it evaporates off the surface of leaves through a process called transpiration. Transpiration is the process through which water is lost by evaporation into the atmosphere. This loss of water causes a so-called transpiration pull that pulls the water from the roots through the xylem vessels to the leaves. You can imagine that the transpiration pull is caused by a person pulling on a string at the top of a plant, thereby moving the string through the plant.

Now, you might argue that water is a liquid and therefore does not behave like a solid string. Interestingly, water in the plant’s xylem vessel does exactly that. The reason for this phenomenon are cohesive and adhesive forces. Cohesion describes forces between two or more water molecules. Adhesion, on the other hand, describes the attraction between water molecules and the surrounding walls of the xylem. Both of these forces cause the water to form a continuous water column, which, like a string, can be pulled up the xylem.

Now that we have reviewed the main components of water transport and its importance for photosynthesis, let’s take a look at the provided answer choices.

Answer choice (A) is incorrect because it is adhesion that describes the forces of attraction that occur between the water molecules and the walls of the xylem.

Answer option (B) states that cohesion refers to the forces of attraction between the molecules of water inside the xylem. This seems to correctly describe cohesion. But let’s take a look at the remaining answer choices just in case.

Answer choice (C) is incorrect since this actually describes the process of the transpiration pull. Finally, we can exclude answer option (D). It is osmosis that refers to the movement of water from the soil into the root, from an area of low solute concentration to an area of high solute concentration.

After reviewing each answer choice, we determined that the best answer to our question, “Which of the following best explains what is meant by cohesion in water movement?,” is given by answer choice (B). Cohesion refers to the forces of attraction between the molecules of water inside the xylem.

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