2015 AP Biology free response 5

Phototropism in plants is a response in which a plant shoot grows toward a light source. The results of five different experimental treatments from classic investigations of phototropism are shown above.

Part A: Give support for the claim that the cells located in the tip of the plant shoot detect the light by comparing the results from treatment group one with the results from treatment group two and treatment group three.

So, we want to compare these first three treatment groups. You see, when you have an unaltered tip, the whole plant is going to bend in the direction of the light. When you remove the tip, it no longer bends in the direction of the light. Or if you were to cover the tip with some type of an opaque cap so the light can't go through it, it's still not going to bend.

So, this is pretty good evidence that there's something about the tip of the plant that allows the phototropism to occur. It allows the plant to know which direction the light is and then somehow causes it to bend in that direction.

So, I'll just write that down: give support for the claim. We can say that phototropism only occurred when there is an unaltered tip that has access to light. No bending occurred when either the tip was removed (that was treatment two) or the tip had an opaque cover (that was treatment three).

So, that's all there is to it. It only occurred when there was an unaltered tip, so that was treatment one, that has access to light. No bending occurred when either the tip was removed in treatment two or treatment three.

So, it's a pretty good indication that you need that tip, and it needs to have access to the light. Something about the tip lets the plant know what direction to start bending in.

All right, let's do Part B now.

Part B: In treatment groups four and five, the tips of the plants are removed and placed back onto the chute on either a permeable or impermeable barrier. Using the results from treatment groups four and five, describe two additional characteristics of the phototropism response.

Well, when the tip was placed on top of a permeable barrier, the phototropism still happens; the plant still bends. So, even though the tip is disconnected, because this barrier is permeable, the tip must be able to sense the light and release some type of substance or hormone or chemical or something that's able to go through this permeable barrier.

Some type of substance goes through this permeable barrier and signals to the rest of the plant to bend in the right direction. Because when you put an impermeable barrier right over here, the tip might be trying to release those same substances, but they can't get through. So, the rest of the plant doesn't know to bend or in what direction to bend.

So, we could say the tip must release some type of hormone or chemical that signals to the rest of the plant to bend and in which direction. We know this because only treatment four, where the barrier is permeable to that substance, shows phototropism, not treatment five.

We could say this because of the two treatments four and five; only treatment four, where the barrier is permeable to that substance, shows phototropism. That's the only explanation here. It's not doing it through somehow doing it through radio waves or releasing substances that somehow float in the air to the rest of the plant.

Some type of substance is going through directly from the tip to the rest of the plant, and when you block it, it's not able to send those chemical signals. That's the best explanation that I can think of for this behavior that we see between treatments four and five.