Pigments and the Absorption of Light

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Pigments and the Absorption of Light

At GCSE, you will have used the the following equation:

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This shows that carbon dioxide and water are turned into glucose and oxygen in a plant, using light as energy.

The carbon dioxide enters the plant by stomata (in the leaves) or lenticels (in the stem). The water enters through the roots and moves up through the plant in xylem vessels. Light is required as an energy source for the reaction and chlorophyll is necessary to absorb the light.

The glucose made may be used to make other substances or it may be used in respiration. The oxygen may diffuse out through the stomata or it may be used in respiration.

Light energy is necessary but it must be harvested and trapped by the photosynthetic pigments to be of any use.

Chlorophyll absorbs light from the visible part of the electromagnetic part of the spectrum, but there are several types of chlorophyll...

  • chlorophyll a
  • chlorophyll b
  • chlorophyll c
  • bacteriochlorophyll (found in photosynthetic bacteria!)

There are also other families of pigments, such as the carotenoids.

Not all wavelengths of light are equally absorbed and different chlorophylls absorb more strongly in different parts of the visible spectrum.

Absorption and Action Spectrums

The action spectrum shows the rate of photosynthesis at different wavelengths.

The absorption spectrum shows how strongly the pigments absorb at different wavelengths.

The absorption spectrum and action spectrum show that the wavelengths that are most strongly absorbed (red and blue) are the ones that cause photosynthesis to proceed at the fastest rate. Green is not strongly absorbed; rather it is reflected, causing leaves to look green.

The shorter the wavelength, the more energy it contains. During photosynthesis the light energy is converted into chemical energy. The absorbed light excites electrons in the pigment molecules and the energy can be passed on to be used by the plant.

The pigments are arranged in funnel shaped photosystems that sit on the thylakoid membranes in the chloroplasts.

In each photosystem, several hundred pigment molecules, called accessory pigments, are clustered around a particular pigment molecule, known as the primary pigment.

The various accessory pigments absorb light of different wavelengths and pass the energy down the photosystem. Eventually the energy reaches the primary pigment that acts as a reaction centre.

There are 2 types of photosystem:

  • Photosystem One - PS I:
  • Its primary pigment is a molecule of chlorophyll a with an absorption peak at 700nm. It is called P700
  • Photosystem Two - PS II:
  • Its primary pigment is a molecule of chlorophyll b with an absorption peak at 680nm. It is called P680

PS I are situated next to PS II on the thylakoid membrane.


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