Absorption & Action Spectra: Skills (SL IB Biology)

What is an action spectrum?

• An action spectrum is a graph that shows the rate of photosynthesis at different wavelengths of light
• The rate of photosynthesis is highest at the blue-violet and red regions of the light spectrum, as these are the wavelengths of light that plants can absorb (i.e. the wavelengths of light that chlorophylls and carotenoids can absorb)

Diagram to show the action spectrum of chlorophyll pigments

The photosynthetic action spectrum shows the rate of photosynthesis at different wavelengths of light

Comparing action and absorption spectra

• There is a strong correlation between the cumulative absorption spectra of all pigments and the action spectrum:
  • Both graphs have two main peaks – at the blue-violet region and the red region of the light spectrum which supports the idea that the most light energy is absorbed at these wavelengths leading to the fastest rate of photosynthesis
  • Both graphs have a trough in the green-yellow region of the light spectrum which supports the idea that the least light energy is absorbed at these wavelengths leading to the slowest rate of photosynthesis

Diagram to show the correlation between action and absorption spectra

An overlay of the photosynthetic absorption and action spectra shows there is a strong cumulative correlation

Determining the rate of photosynthesis

• The rate of photosynthesis can be determined by measuring the volume of oxygen produced or the carbon dioxide consumption at different wavelengths of light
• An experiment can be set up similar to the one investigating the effect of light intensity on photosynthesis
  • Remember that the lamp should be kept the same distance from the pondweed as we are investigating the effect of different wavelengths of light only
  • Place different colour filters (covering the full light spectrum) in front of the lamp to change the colour of light the pondweed is exposed to
  • Measure the volume of oxygen produced or the number of bubbles released from the pondweed per minute for each colour
    • Include an experiment with no filter in front of the lamp to investigate the effect of white light on the rate of photosynthesis
  • Repeat the experiment several times to obtain reliable results

Drawing an action spectrum for photosynthesis

Step 1: Draw and label the axes

• Draw an x-axis
• Label the axis wavelength
• Add the units / nm
• Make 400 the smallest value and 700 the largest value
  • Label 500 and 600 nm on the x-axis

• Draw a y-axis
• Label it Rate of photosynthesis / % of maximum rate
• Make 0 the lowest value and 100 the highest value
  • No units are required because the y-axis is showing a percentage scale

Step 1 : Draw and label the axes

Step 2: Draw the plot

• There should be two peaks of rate of photosynthesis
  • One peak at either end, in the blue and red areas of the spectrum
  • And a trough in the middle, which represents green light
  • As below, with a smooth curve

Step 2: Sketch the Curve. An action spectrum for photosynthesis (colour range labels are not required)