AP Biology · Free-response skill

Design an experiment: a growth hormone

The move. Read the plan of a cell-signaling experiment: name what is changed, what is measured, the control and a fixed condition, then predict and explain the effect of a blocked receptor.

How to do it

  1. Say exactly what is measured, in the study's own terms: what is counted or read, in what units and when. Not just plant growth, but the mean height of the seedlings in centimeters after 10 days.
  2. The factor changed on purpose is what differs between the groups by design. Everything else is kept the same, so a difference in the result can be credited to that factor.
  3. A control group shows the result without the factor being tested, so the treated groups have something to be compared with.
  4. State a prediction as a direction: more or fewer, higher or lower, than a named group. Then explain it by naming the step the change affects.

Practice prompt Try it

In a fictional study, a researcher thinks that hormone F makes cultured liver cells divide by binding receptor F on the cell surface. Every dish starts with the same number of cells (10,000), in the same nutrient medium, at the same temperature (37 degrees C). Group 1 gets no hormone F. Group 2 gets hormone F. Group 3 gets hormone F together with drug B, a drug that sits on receptor F and blocks it. After 3 days, the researcher counts the cells in each dish.

Use the passage to respond to each part.

  1. Group 1 and group 2 differ in only one way. Name that one difference (leave group 3 aside).
  2. Name what the researcher counts in each dish after 3 days.
  3. One group is given no hormone F at all and serves as the control. Name that group.
  4. Give one condition that the passage says is the same in every dish.
  5. If the researcher's idea is right, predict whether group 3 will have more cells or fewer cells than group 2 after 3 days.
  6. In group 3, drug B is blocking receptor F. Explain what this means for whether hormone F can bind the receptor.

Write each part, then check yourself against the checklist. Nothing you type here is saved or graded.

Check yourself and see a model answer

Checklist

Our own checklist of what a full answer does, one line per point. Tick each one your response earns.

0 of 6 points

Model answer, annotated

(a) The factor that differs on purpose is hormone F: group 1 gets no hormone F and group 2 gets hormone F.

  • Point 1 Names hormone F as the factor changed on purpose.

(b) The researcher counts the number of cells in each dish after 3 days.

  • Point 2 Names the measured result: the number of cells in each dish.

(c) Group 1 is given no hormone F at all, so it is the control.

  • Point 3 Names group 1, the group with no hormone F.

(d) Every dish is kept at the same temperature, 37 degrees C.

  • Point 4 Gives one condition the passage holds the same: the temperature.

(e) If the idea is right, group 3 will have fewer cells than group 2 after 3 days.

  • Point 5 Predicts the direction: fewer cells in group 3 than in group 2.

(f) Drug B sits on receptor F and blocks it, so hormone F cannot bind the receptor. A hormone acts on a cell only by binding its receptor, so with the receptor blocked, hormone F cannot start the signal that makes the cells divide.

  • Point 6 Explains that drug B blocks receptor F, so hormone F cannot bind it.