Photosynthetic Pigments: Definition, Meaning Types, Examples, Functions

Definition Of Photosynthetic pigments

These are the light-absorbing molecules used to coordinate the photosynthetic process. Photosynthetic pigments include, mainly, chlorophyll a, chlorophyll b, carotenoids, and phycobilins, contained in the chloroplast of plant cells and other photosynthetic organisms. They capture light energy at specific wavelengths and convert it into chemical energy used to power the synthesis of glucose from carbon dioxide and water.

Photosynthetic pigments are a requirement for high photosynthetic efficiency since they capture light energy and trigger the process by which it is converted into chemical energy. Chlorophyll a is the main photosynthetic pigment capturing light energy and passing it on in a form that the plant can use. Carotenoids and chlorophyll b broaden the wavelength range absorbed and protect against photodamage. These are the pigments that if missing, plants would not be able to adequately harness the energy of sunlight; therefore, the energy production and growth would be reduced.

Types Of Photosynthetic Pigments

Chlorophylls are the major photosynthetic pigments involved in the absorption of light energy. They participate in light-dependent photosynthetic reactions, converting light energy into chemical energy.

Chlorophylls

The different types are:

Chlorophyll a

This is the most abundant and important pigment of photosynthesis. Chlorophyll mainly absorbs light in the blue-violet and red portions of the visible spectrum. It participates directly in the conversion of light energy into chemical energy and is present in all photosynthetic organisms.

Chlorophyll b

This is a pigment that complements the light absorbed by chlorophyll in the blue and red-orange regions, thus broadening the spectrum of light that can be used. It is mainly found in green plants and green algae; its function appears to pass the captured energy on to chlorophyll a.

Chlorophyll c, d, and e

These forms of chlorophyll are found in certain algae and cyanobacteria. Chlorophyll c is present in brown algae and the diatoms; chlorophyll d is found in red algae, and chlorophyll e in some green algae. They perform the same functions as chlorophyll but are adjusted to other light conditions.

Carotenoids

Carotenoids are explained below:

Carotenes

These include a class of carotenoids that appear orange, including β-carotene, which is a precursor to vitamin A in animals. Carotenes help in light absorption and photoprotection.

Xanthophylls

These are yellow carotenoids which take part in protecting the photosynthetic apparatus from too much light energy. They take part in dissipating excess energy as heat. Examples include the pigments lutein and zeaxanthin.

Phycobilins

These are explained below:

Phycocyanin

This is a blue pigment that absorbs orange and red light, hence aiding photosynthesis in cyanobacteria and some algae. It augments light absorption in low-light conditions.

Phycoerythrin

This is a red pigment; it absorbs blue and green light and provides good complementarity with phycocyanin. At depths below 10 meters in water, by the time it reaches this depth, most of the other light has been already absorbed, so blue-green light penetrates far better; hence, it becomes useful.

The Function Of Major Photosynthetic Pigments

The major functions of photosynthetic pigments are given below:

Chlorophyll a

Chlorophyll forms the reaction centre, involving the main pigment of photosynthesis and light energy capture. This pigment is responsible for absorbing light centrally in the reaction centre of photosystems, more precisely in the blue-violet and red regions of the spectrum. It turns the light energy it absorbs into chemical energy by facilitating electron transfer in the light-dependent reaction that forms ATP and NADPH

Chlorophyll b

Chlorophyll b helps broaden the light-absorption spectrum for photosynthesis. It mainly absorbs in the blue and red-orange parts of the visible spectrum, thereby "filling in" the absorption spectrum of chlorophyll a.

Since it captures more light energy, chlorophyll b itself transfers that energy to chlorophyll a, hence significantly improving the efficiency of photosynthesis. It is essential in modifying plants to diverse conditions of light, mainly shady conditions.

Carotenoids

Carotenoids are the accessory pigments that protect chlorophyll from photooxidation and excess light energy. They can absorb light in regions of the blue and green areas of the spectrum that chlorophyll does not absorb efficiently, then transfer the energy along to chlorophyll.

In addition to their light-gathering functions, carotenoids have a protective role in absorbing excess light energy and then releasing it as heat, which prevents damage to the photosynthetic apparatus and minimises the risk of oxidative stress.

Phycobilins

Therefore, phycobilins are of huge significance in terms of light absorption by cyanobacteria and red algae. They are capable of capturing the light under conditions where there is a low availability of light. While phycocyanin absorbs orange and red light, phycoerythrin does so for green and blue light.

This enables the organisms to photosynthesise at greater depths or in shaded conditions. With the extended range into the realm of light absorption, phycobilins present photosynthetic organisms with an opportunity to grow in a wide range of diverse and low-light conditions.

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