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Question Video: Identifying the Correct Statements about the Transpiration Pull If the Xylem Contains Air Bubbles Biology

A student is studying the movement of water through xylem tissues in a stick of celery. They accidentally introduce some air bubbles into the water moving through the xylem. What is most likely to happen?

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Video Transcript

A student is studying the movement of water through xylem tissues in a stick of celery. They accidentally introduce some air bubbles into the water moving through the xylem. What is most likely to happen? (A) Water will continue to move up the xylem as normal, and when it reaches the leaves, the air bubble will diffuse out of the stomata. (B) Water will begin to diffuse out of the stomata located along the stem. (C) Water will no longer move up the xylem as a continuous column, as the transpiration pull will not occur if there are air bubbles. Or (D) water will no longer be pulled up the stem, but it will be pushed up the stem due to root pressure.

This question asks us about an experiment to study the movement of water in plants.

If you’ve ever taken care of a plant, you might know that they need a lot of water. One reason for this is because water is required for a process called photosynthesis. This is how plants are able to make their own food using carbon dioxide, water, and light energy. Photosynthesis occurs in the leaves of plants. But you might have learned that water is actually taken in through the roots which lie under the soil. How does water get up to the leaves then?

Water actually moves through special vessels called xylem vessels. These are made up of individual cells that are stacked on top of each other to form long, hollow tubes. A so-called transpirational pull pulls the water from the roots through the xylem vessels to the leaves. You can imagine that the transpirational pull is 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, in a plant’s xylem vessel, it does exactly that, thanks to 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 form a continuous water column which can be pulled up the xylem. Any disruption to this created water column leads to a problem. The water cannot be pulled up the xylem vessel anymore. Why is that? Let’s briefly refocus on the person pulling on a piece of string again.

Having a disruption in the water column is like if you cut through that string that the person was pulling upward. Once it is cut, the lower part of the string cannot be pulled up anymore. The introduction of gas or air into the column acts like a cut in a string that was pulled upward. Water is inhibited from moving upwards the way it did when it was a continuous column. A break in the xylem could have the same effect.

Now that we have reviewed an important part of the process that allows the transpiration of water from the roots to the leaves, let’s take another look at our question. We are asked about a situation where air bubbles are introduced into the water moving through the xylem of a stick of celery. We just learned that the introduction of gas will disrupt cohesive and adhesive forces in the xylem, thus limiting the effects of the transpiration pull.

Therefore, the best answer to our question is (C). Water will no longer move up the xylem as a continuous column, as the transpiration pull will not occur if there are air bubbles.

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