AP Biology · Free-response skill

Interpreting data: CO2 and oxygen release

The move. From a data table, read the highest value and describe a trend with numbers, calculate a rate with units, and name the parts of photosynthesis behind the data.

How to do it

  1. Read the table one column at a time. Check what each column measures and under what conditions before you compare numbers.
  2. Describe a trend with a direction and numbers: say what happened, over which range, and quote the values at each end of that range.
  3. For a rate, divide the amount by the time and keep the units. For example, 18 millimeters grown in 6 days is 18 divided by 6, or 3 millimeters per day. Write each step.
  4. To explain data with biology, name the process and the stage that uses or makes the substance in the table, and say where in the cell that stage runs.

Practice prompt Try it

In a fictional study, a student put equal samples of a green alga into five sealed flasks of water, all under the same bright light at 25 degrees C. Each flask started with a different concentration of dissolved CO2. The student measured the oxygen the alga released in each flask over 30 minutes.

Oxygen released in 30 minutes at five starting CO2 concentrations

Starting CO2 (millimolar)Oxygen released (micromoles in 30 minutes)
0.56
1.011
2.018
4.027
8.028

Use the passage and the table to respond to each part.

  1. Give the starting CO2 concentration at which the alga released the most oxygen.
  2. Describe how the oxygen released changed as the starting CO2 went from 0.5 to 4.0 millimolar. Quote the values at 0.5 and at 4.0 millimolar.
  3. Calculate the average rate of oxygen release, per minute, in the flask that started at 2.0 millimolar CO2. Write out the division and give the units.
  4. Name the stage of photosynthesis that uses CO2, and name the part of the chloroplast where that stage runs.
  5. Name the molecule that is split in the light-dependent reactions to release the oxygen the student measured.

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 alga released the most oxygen, 28 micromoles in 30 minutes, at a starting CO2 concentration of 8.0 millimolar.

  • Point 1 Reads the highest value in the oxygen column and gives its CO2 concentration: 8.0 millimolar.

(b) As the starting CO2 went from 0.5 to 4.0 millimolar, the oxygen released rose, from 6 micromoles at 0.5 millimolar to 27 micromoles at 4.0 millimolar.

  • Point 2 Gives the direction, a rise, and quotes the values at both ends: 6 and 27.

(c) Rate = oxygen released divided by time = 18 micromoles divided by 30 minutes = 0.6 micromoles per minute.

  • Point 3 Writes out the division and keeps the units through to the answer.

(d) The Calvin cycle uses CO2, fixing it into sugar with the ATP and NADPH from the light-dependent reactions. The Calvin cycle runs in the stroma of the chloroplast.

  • Point 4 Names the Calvin cycle as the stage that uses CO2.
  • Point 5 Names the stroma as where it runs.

(e) Water is split in the light-dependent reactions. Its electrons replace those lost by chlorophyll, and its oxygen atoms are released as the oxygen gas the student measured.

  • Point 6 Names water as the molecule that is split.