Photosynthetic pigments

Pigments:

• All photosynthetic organisms have at least one pigment which can absorb light

• Pigments are found in the thylakoid membranes in protein complexes called photosystems I and II

Two main classes of photosynthetic pigments:

• Chlorophylls

• Carotenoids

Chlorophyll A:

Absorbs red and blue-violet light (660

and 430 nm)

Appears green in colour

Has a flat light-absorbing head end

(porphyrin group) which contains a

magnesium ion its centre.

Chlorophyll B

Chlorophyll a is the most abundant form (2:1)

Chlorophyll b has the same structure but a CH3 in the porphyrin group is

replaced by an aldehyde group (-CHO)

Chlorophyll b absorbs at 450nm and 640nm (appears blue-green)

Carotenoids — ß-carotene and Xanthophyll

•Carotenoids are yellow, orange, red or brown pigments that absorb strongly in the blue-violet range (400-500nm).

•They are called accessory pigments (along with chlorophyll b) because they pass the light

energy they absorb on to chlorophyll a.

•They are usually masked by the green chlorophylls but can be seen in the autumn just

before leaf-fall because chlorophylls break down first.

•Beta-carotene is the most widespread and important carotenoid. It provides the orange

pigment in carrots. It is broken down during digestion to form 2 molecules of vitamin A.

Absorption spectra:

An absorption spectrum

shows the percentage of

lights of each wavelength

that is absorbed by

photosynthetic pigments

Light wavelengths that are

not absorbed are reflected,

giving the pigment its colour

Action spectra

An action spectrum shows the rate of

photosynthesis of a particular species of plant

for different wavelengths of light

We can compare the action spectrum against

the absorption spectrum of a pigment to help

identify the pigments present in a plant

species

Importance of accessory pigments?

• Expand the range of light absorption: They absorb light wavelengths that chlorophyll (the primary pigment) cannot efficiently absorb. This allows plants and photosynthetic organisms to capture more sunlight for energy.

• Enhance photosynthesis efficiency: By capturing additional light energy and transferring it to chlorophyll, accessory pigments help maximize the energy available for photosynthesis.

• Protect against photo-damage: Some accessory pigments, like carotenoids, help protect the plant cells from damage caused by excess light or harmful reactive oxygen species.

• Aid in specific environments: In some algae and bacteria, accessory pigments enable photosynthesis in deeper or shaded waters by absorbing different light wavelengths.