International A Level Biology / Photosynthesis

Absorption and action spectra revision notes

Edexcel International A Level Biology (YBI11), chapter 14: Photosynthesis, specification point 5.6. Free lesson.

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An absorption spectrum shows how much light pigments absorb at each wavelength, while an action spectrum shows the rate of photosynthesis at each wavelength. Comparing them explains which wavelengths drive photosynthesis and how accessory pigments let a leaf use a wider range of light.

What the two spectra show

Both graphs use wavelength of light, measured in nanometres, on the x-axis. Their y-axes differ because they measure different things. An absorption spectrum shows how much light a pigment absorbs at each wavelength. Its y-axis shows absorbance. It is measured using a pigment extract in a spectrophotometer. An action spectrum shows the rate of photosynthesis at each wavelength. Its y-axis shows a rate, such as the volume of oxygen produced per minute. It is measured using a whole plant or alga, tested with one wavelength at a time.

How to read the pigment absorption spectra

The three main types of chloroplast pigment absorb different wavelengths. To describe a spectrum, read the wavelength of each peak from the axis and say whether there is more than one. Chlorophyll a has two peaks, at about 430 nm in the blue-violet and 662 nm in the red. Chlorophyll b also absorbs strongly in the blue and red, with peaks at about 450 nm and 640 nm. Carotenoids, which are yellow or orange, absorb blue-violet light from about 400 to 500 nm. They peak near 450 nm and 480 nm, and absorb almost no red light. Green light, from about 520 to 560 nm, is hardly absorbed by any…

Why the action spectrum follows absorption

The two spectra have similar shapes because light must be absorbed by a pigment before its energy can be used in photosynthesis. When absorption is high, the rate is high too. Photosynthesis has high rates in the blue-violet and red, where the pigments absorb strongly. The rate is lowest in green light because the pigments absorb very little of it. It is not zero, because some green light is still absorbed. When comparing the graph shapes, each spectrum is shown as a percentage of its own highest value.

How accessory pigments widen the range

Chlorophyll a is the main pigment. Accessory pigments, including chlorophyll b and carotenoids, absorb wavelengths that chlorophyll a absorbs poorly. They pass the energy they absorb to chlorophyll a at the reaction centre, where it is used. Using several pigments means a wider range of wavelengths is absorbed, so more light energy is available for photosynthesis. Most green light is reflected or passes through leaves. This is why leaves look green.

Explaining the benefit of a pigment

For an exam question about a pigment, state what it absorbs and then explain the consequence. Merely describing the graph does not explain how the pigment helps. For example, a red seaweed pigment that absorbs green light absorbs wavelengths chlorophyll cannot use well. This widens the range of wavelengths absorbed and makes more light energy available for photosynthesis.