AP Biology · Free-response skill

Experimental design: phosphate and duckweed

The move. Plan an ecology experiment: name the variables and the control, say why repeated trials help, state the result that would support an idea, and trace a disruption through a pond.

How to do it

  1. Find the one thing the researcher changes on purpose and the one thing she counts or measures. Name each in words that match the setup.
  2. The control is the group that gets none of the factor being tested. Say what it shows: what happens without that factor, for comparison.
  3. Repeating each treatment in several tanks or plots means one unusual tank, or chance, has less effect on the result.
  4. A result that supports an idea says which group would end with more or less of the measured thing. A prediction about a disruption follows each step in turn, naming the organisms or process involved.

Practice prompt Try it

A student wants to test whether the amount of phosphate in pond water limits the growth of a fictional duckweed, a small floating plant. She fills 12 identical tanks with the same pond water and places 10 duckweed plants in each. She adds no phosphate to 4 tanks, a small amount of phosphate to 4 tanks and a large amount to 4 tanks. All tanks get the same light and temperature. After 3 weeks she counts the duckweed plants in every tank.

Use the passage to respond to each part.

  1. Name the variable the student changes on purpose, and name the variable she measures.
  2. Say which tanks are the control group and what they show.
  3. The student used 4 tanks for each amount of phosphate instead of 1. Say how this makes her results more trustworthy.
  4. Suppose phosphate does limit duckweed growth. Say whether the average plant count in the tanks with no added phosphate would be higher or lower than in the tanks that got phosphate (small or large amount).
  5. In a real pond, phosphate runoff causes a thick mat of duckweed that later dies and sinks. Say whether the dissolved oxygen in the deep water should rise or fall as the dead duckweed is broken down. Then name the kind of organism that breaks it down and the cell process those organisms use that changes the oxygen.

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 8 points

Model answer, annotated

(a) The student changes the amount of phosphate added to each tank: none, a small amount or a large amount. She measures the number of duckweed plants in each tank after 3 weeks.

  • Point 1 Names the amount of phosphate, with its three levels, as the changed variable.
  • Point 2 Names the plant count after 3 weeks as the measured variable.

(b) The 4 tanks with no phosphate added are the control group. They show how the duckweed grows in this pond water without added phosphate, so the other tanks can be compared with them.

  • Point 3 Names the no-phosphate tanks and says what they show and why that matters.

(c) With 4 tanks for each amount, one unusual tank, or chance differences between tanks, has much less effect on the result, because the student can average the 4 counts and see whether they agree.

  • Point 4 Says that repeating each amount lowers the effect of chance or one odd tank.

(d) The tanks with no added phosphate would have a lower average plant count than the tanks that got phosphate, because without extra phosphate the duckweed could not grow as much.

  • Point 5 Gives the direction of the comparison: lower in the no-phosphate tanks.

(e) The dissolved oxygen in the deep water should fall. Decomposers, mainly bacteria, break down the dead duckweed, and they use up oxygen in cellular respiration as they do it.

  • Point 6 States the direction first: oxygen falls.
  • Point 7 Names the organisms: decomposers, mainly bacteria.
  • Point 8 Names the process: cellular respiration, which uses up the oxygen.