Aim for FiveBiology

Interpreting and evaluating experimental results with graphing

Light intensity and oxygen release

  • Unit 3
  • 9 points
  • About 25 minutes

You can use a calculator on this question, just like on exam day.

A long question with an experiment and a data table that you turn into a graph. On this site you describe the graph you'd draw (type, axes, units, scale, points and error bars), then analyze the data and make and justify a prediction. On the exam: Question 2 of 6 in Section II (90 minutes, 50% of the exam score); about 25 minutes suggested.

The question and its sources

Aquatic plants release O₂ during photosynthesis and use O₂ during cellular respiration. The net rate of O₂ production is the rate of O₂ release from photosynthesis minus the rate of O₂ use in respiration.

Experiment

Researchers placed sprigs of an aquatic plant in sealed, water-filled chambers at 25°C with the same starting concentration of dissolved CO₂. Each chamber was exposed to one of six light intensities, and an O₂ sensor measured the net rate of O₂ production for one hour. There were five chambers at each light intensity.

Source: Hypothetical study

Table 1. Net rate of O₂ production at different light intensities

Light intensity (μmol photons/m²/s)Net rate of O₂ production (μmol O₂/g/h), mean±2SE
0−2.00.4
501.50.5
1004.80.6
2009.60.8
40012.90.9
80013.21.0

Source: Hypothetical data

Suggested time: 25 minutes

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Part (a)

1 point

Describe the source of the O₂ released during photosynthesis.

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Part (b)

4 points

(i) Describe the graph you would construct to represent the data in Table 1. In your description, name the type of graph, give the label and units for each axis, describe the scale for each axis, list the values you would plot, and describe the error bars. (ii) Based on the data, determine whether the net rate of O₂ production at 800 μmol photons/m²/s is significantly different from the rate at 400 μmol photons/m²/s.

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Part (c)

2 points

(i) Explain why the net rate of O₂ production is negative at a light intensity of 0. (ii) Assume that the rate of cellular respiration is the same at every light intensity. Calculate the gross rate of O₂ release from photosynthesis at 200 μmol photons/m²/s. Include units.

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Part (d)

2 points

The researchers add a chemical that blocks the transfer of electrons from photosystem II to the rest of the electron transport chain in the chloroplast. They then measure the net rate of O₂ production at 800 μmol photons/m²/s. (i) Predict the net rate of O₂ production with the chemical. (ii) Justify your prediction.

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