Electromagnetic energy travels in wavelengths and is measured in nanometres (nm). Pigments within matter, such as plants, absorb varying wavelengths of light. The absorption spectrum can be defined as a graphical representation of a pigment’s ability to absorb light against varying wavelengths (nm). The pigments light absorbing ability is measured using a spectrophotometer.
An action spectrum can be defined as the graphical representation of the rate of photosynthesis against wavelengths of light. The graph shows how effective the different wavelengths of light are at enabling photosynthesis, and this can be determined by measuring either the carbon dioxide consumed or by the amount of oxygen released. Chlorophyll is a pigment found in the chloroplasts of organisms. In plants, it is the chlorophyll that absorbs the light energy and drives photosynthesis.
On the absorption spectrum, chlorophyll a and b seem to be effective within the violet to blue (approximately 400 – 500nm) region and the red region. This indicates that these types of light are most effectively absorbed by the chlorophyll and it is suggested that these types of lights work best in photosynthesis. The absorption spectrum shows that green is the least effectively absorbed. In comparison, the action spectrum for photosynthesis does not match the absorption spectrum.
The action spectrum suggests that a greater range of wavelengths are able to facilitate photosynthesis. This is because it does not exclusively take into account the contribution of chlorophyll a and b in photosynthesis. The action spectrum also takes into account the work of the accessory pigments in the plant, which absorb other types of light from a greater range of wavelengths, and therefore assist in driving photosynthesis.
