Absorption & Action Spectrums
- An absorption spectrum (for a particular pigment) shows how much light of different wavelengths is absorbed by the pigment
- An action spectrum shows how each wavelength of light affects the rate of photosynthesis that it can power
- These are two graphs that use the wavelength of light on the x-axis
- Violet light is the lowest wavelength of the visible spectrum, at 400 nanometres (nm)
- Red light is the highest wavelength of the visible spectrum, at 700 nanometres (nm)
Drawing an absorption spectrum for chlorophyll
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 % absorption
- 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 absorbance peaks
- 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 absorbance spectrum for photosynthesis (colour range labels are not required)
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 absorbance spectrum for photosynthesis (colour range labels are not required)
Examiner Tip
Remember – the pigments themselves have a distinctive colour. This is different from the colours of light that they absorb.Key points to remember:
- Label 400 - 700nm on the x-axis, in 100nm increments
- Use a % scale on the y-axis
- Smooth curve
- Peaks at either end
- Trough in the middle for green light