Video: Identifying the Effect of Auxin on Plant Growth and Response to Light

The diagram shows the effect of auxin on a plant shoot. What effect does auxin have on the cells of a plant shoot? On what side of the shoot is the concentration of auxin the highest? What is the response of the plant shoot?

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Video Transcript

The diagram shows the effect of auxin on a plant shoot. What effect does auxin have on the cells of a plant shoot? Our answer choices are: it causes cell walls to break down, it inhibits or stops growth, or it stimulates or causes cell elongation.

This question is asking us to examine the diagram provided and to use it in our own knowledge of plant physiology to draw some conclusions about the effects of auxins on the cells and behavior of plants. So let’s start by examining the diagram just a little bit more closely. In the first frame, we see a close-up diagram of a green plant shoot. In the tip, there are orange dots. The orange dots represent auxins.

In the second frame, we see that the light source is to the right of the plant shoot, the auxins have gathered on the opposite side to the left of the plant shoot, and the plant shoot is growing taller. And in the third and final frame, in the third frame, we see the plant shoot begins to bend toward the light source. We could deduce that this is related to the effect of the auxin on the plant shoot.

Now, we’re ready to return to our question. What effect does auxin have on the cells of a plant shoot? It causes cell walls to break down. If the cell walls of the plant shoot were being broken down by the auxin, the plant would begin to lose its internal structure. This is not a good explanation for the shoot growing taller and bending towards the light source. It inhibits or stops growth. Since we can see that the plant shoot is actually growing taller in the presence of the auxins, we know that the auxins cannot be stopping the growth of the plant shoot.

It stimulates or causes cell elongation. If the cells along one side of the plant shoot were to grow longer, this would cause the shoot to begin to curve. This choice best explains the effect that we see auxins have on the plant shoot in our diagram. So the effect that auxins have on the cells of a plant shoot is that auxin stimulates or causes cell elongation.

On what side of the shoot is the concentration of auxin the highest? Our choices are: the sunlit side, the shaded side, or there are equal amounts of auxins on all sides.

To answer this question, we’ll return to our diagram and determine where we see the most auxin or the highest concentration. Since we know that light source is coming from the right, the left side of the plant must be shaded. We can see evidence of this in frame two and three of our diagram. Now, let’s return to our answer choices. On what side of the shoot is the concentration of auxin the highest? The sunlit side. Looking in frame two and three of our diagram, we can see that the auxin tends to move away from the sunlit side of the plant shoot.

The shaded side. Once again in frame two and three of our diagram, we can see that most of the auxin is found away from the light source towards the shaded side of the plant shoot. There are equal amounts of auxins on all sides. While if we only looked at frame one, we may think that this option is correct. The evidence in frame two and three suggest that the concentration of auxin is not always equal on all sides of the plant shoot. So the side of the shoot where the concentration of auxin is the highest is the shaded side.

What is the response of the plant shoot? Our choices are: it grows away from the light, it grows straight upward, and it grows toward the light.

This question is asking us to examine the diagram and draw a conclusion about how the plant shoot reacts to the presence of the auxin. Well, what does the plant shoot actually do? It grows away from the light. In frame three of our diagram, we can clearly see that the plant shoot actually bends towards the light. This statement is simply not true.

It grows straight upward. While the plant shoot does grow straight upward for some time, we can see that the plant shoot respond to the presence of the auxin by bending away from them not by continuing to grow straight upward. It grows toward the light. In frame three of our diagram, we can see the plant shoot responding to the presence of the auxins by bending toward the light source. This statement does describe the response of the plant shoot that we see in the diagram. So, the response of the plant shoot to the presence of the auxin is to grow toward the light.

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