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Measure and graph photosynthesis rate

Bioenergetics • Photosynthesis

Key concepts

What you'll likely be quizzed about

  • Pondweed, such as Cabomba or Elodea, is submerged in a boiling tube filled with water, with a paper clip to weigh the stem and a lamp as the light source.
  • The setup includes a ruler and beaker of water to vary light intensity while minimizing direct heating to the plant.
  • Counting oxygen bubbles emitted from the pondweed's cut tip over one minute serves as a quantitative measure of photosynthetic rate.
  • Repeating measurements at each distance and computing a mean rate enhances reliability.

Flashcards

Test your knowledge with interactive flashcards

Why is a beaker of water placed between the lamp and pondweed?

Click to reveal answer

The water absorbs heat from the lamp and reduces direct heating of the pondweed, preventing temperature confounding.

Key notes

Important points to keep in mind

Count oxygen bubbles for a fixed time and repeat each measurement at least three times to calculate a mean.

Convert lamp distance to relative light intensity using 1/distance² for more accurate plotting of intensity.

Place light intensity on the x-axis and photosynthesis rate on the y-axis when plotting.

Label axes clearly and include units for all plotted quantities.

Use most of the graph grid by choosing regular, evenly spaced scales that suit the data range.

Plot data points first, then draw a line of best fit or smooth curve to show the trend.

Investigate and repeat any anomalous data points before excluding them.

Maintain temperature, CO₂ level, and pondweed size as control variables to ensure a fair test.

A plateau in the graph indicates a limiting factor other than light.

Use a gas syringe or oxygen probe for higher accuracy than bubble counting when available.

Wait a short equilibration period after changing light distance before recording rates.

Include error bars to display measurement variability and precision.

Recognise that extremely high light can cause heating that reduces rate, so separate light and temperature effects.

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