Photosynthetic Pigments

CHAPTER: 10 (B)

Photosynthetic Pigments

Photosynthetic Pigments:

•      The photosynthetic products are energy-rich organic compounds. The potential chemical energy of these compounds comes from the light energy .

•      The light energy to be effective in photosynthesis must be absorbed by a siutable pigments.

•      This vital role is performed by green pigment, Chlorophyll, in plants.

•      In plants, algae, and cyanobacteria, pigments are the means by which the energy of sunlight is captured for photosynthesis.

•      There are following types of photosynthetic pigments:

            1. Chlorophylls

            2. Carotenoids

            3. Phycobillins

            4. Pteridines

1. Chlorophylls:

•      Chlorophylls are greenish pigments which contain a porphyrin ring. This is a stable ring-shaped molecule around which electrons are free to migrate. Because the electrons move freely, the ring has the potential to gain or lose electrons easily, and thus the potential to provide energized electrons to other molecules. This is the fundamental process by which chlorophyll "captures" the energy of sunlight.

•      Chlorophyll consists of tetrapyrrole skeleton formed into a ring , with an atom of magnesium in the center of the ring.

•      There are several kinds of chlorophyll, the most important being chlorophyll "a". This is the molecule which makes photosynthesis possible, by passing its energized electrons on to molecules which will manufacture sugars. All plants, algae, and cyanobacteria which photosynthesize contain chlorophyll "a".

•      A second kind of chlorophyll is chlorophyll "b", which occurs only in "green algae" and in the plants.

•      A third form of chlorophyll which is common (not surprisingly) called chlorophyll "c", and is found only in the photosynthetic members of the Chromista as well as the dinoflagellates.

2. Carotenoids:

•      Carotenoids are usually red, orange, or yellow pigments, and include the familiar compound carotene, which gives carrots their color.

•      These compounds are composed of two small six-carbon rings connected by a "chain" of carbon atoms. As a result, they do not dissolve in water, and must be attached to membranes within the cell.

•      Carotenoids cannot transfer sunlight energy directly to the photosynthetic pathway, but must pass their absorbed energy to chlorophyll. For this reason, they are called accessory pigments.

•      Light absorbed by carotenoids has been found to result in the flourescence of chlorophyll.

•      These are located in chloroplasts and chromoplasts.

•      These are of two types:

            1. Carotenes

            2. Carotenols (Xanthophylls)

1. Carotenes:

•      Carotenes are orange- yellow in colors consisting of carbon and hydrogen having general formula C₄₀H₅₆.

•      Carotenes are named after carrot in which they are abundant.

•      These are capable of absorbing light in violet and blue green parts of the spectrum.

2. Carotenols (Xanthophylls)

•      Xanthophylls are yellow or brown pigment and contains hydroxyl group. These have general formula C₄₀H₅₆ O_2. 

•      They are capable of absorbing light and help in converting elemental oxygen to molecular form.

3. Phycobillins:

•      Phycobilins are water-soluble pigments, and are therefore found in the cytoplasm, or in the stroma of the chloroplast. They occur only in Cyanobacteria and Rhodophyta.

•      These are the phycoerythrin (red pigment) and phycocyanin (blue pigment) found in the red and the blue green algae.

•      They are protein- linked pigments which are destroyed by heat.

•      These pigments take active parts in photosynthesis.

4. Pteridines:

•      They are generally found in animals and thallophytes but rarely in higher plants.

•      It is yellow pigment and act as photoreceptor

Role of Light in Photosynthesis:

•      It has been observed that full sunlight is inhibitory for photosynthesis. The leaves are oriented in such a manner that the sunlight intensity is effectively reduced to a level when photosynthesis becomes more efficient.

•      In direct sunlight the leaves lie at an acute angle to the rays. In the shade, on the other hand, the leaves lie at right angles to the general direction of rays.

•      A major amount of  incident light 80% is absorbed , 10% is reflected and 10% is transmitted. These three things vary with the wavelengths.

•      It has been stated earlier that the pigments of photosynthetic process absorb light only in certain regions of the spectrum and transmit the remaining wavelengths.

•      The chlorophyll absorbs both the shorter (blue and violet) as well as longer waves (orange and red) . On the other hand carotenoids absorb shorter wavelengths only.

•      The phycoerythrin absorbs blue, green and yellow colors and the phycocyanin absorbs the longer wavelengths. Together they absorb most of the visible light.

•      The photosynthetic bacteria absorb even the infra-red light.

•      Part of the radiant energy absorbed by chlorophyll  is used in producing a chemical change is its photochemical effect. Some part of radiant energy is re-emitted as light, which is called as fluorescence.

•      Chlorophyll-a absorbs photon of light & becomes activated.

•      At this state chlorophyll-a expels one electron & develops a positive charge.

•      The expelled electron contains extra amount of energy which is used during the formation of ATP.

•      During photophosphorylation step ATP is formed. This ATP is utilized for dark reaction to fix carbon.

Absorption Spectra:

•      Light is the visible part of spectrum of magnetic radiation. The radiation travels in the form of waves which have longer and shorter wavelength .

•      Visible light consists of radiation having wavelength from 390nm to 760nm which can be absorbed by photosynthetic pigments. This spectra of wavelength which can be absorbed by photosynthetic pigments during photosynthesis is known as absorption spectra.

•      Photosynthetic pigments absorb light energy only in the visible part of the spectrum. However, certain photosynthetic Bactria absorbs light energy use infrared, light of comparatively shorter wavelength.

•      The effectiveness of different wavelength of light on the photosynthetic activity of an autotrophic plant is called action spectrum. The action spectrum is closely related to the absorption spectrum for the photosynthetic plants.

Quantosome – The Photosynthetic Unit:

•      A photosynthetic unit is the smallest group of pigment molecules which collaborate together to cause a photochemical reaction, i.e., the absorption and migration of a light quantum to a trapping centre where it brings about the release of an electron.

•      Quantasomes are particles found in the thylakoid membrane of chloroplasts in which photosynthesis takes place.

•      There is no actual morphological structure which could be termed as photosynthetic unit.

•      Photosynthesis is now believed to require the coordination  of a series of structures or sub-units distributed throughout the membrane system of the chloroplast.

•      It is assumed that to make a effective photochemical reaction about 250 chlorophyll , 8 molecules of oxidising agents , 8 molecules of Cytochrome F and 16 molecules of cytochrome b are present.

•      Park and Biggins (1964) discovered distinct morphological structures in the thylakoid membrane and many workers considered them to be the photosynthetic units. However this hypothesis has been found to be unconvincing. They are now considered as the system of ATP synthesis only but the complete process of photosynthesis requires the participation of other structures too.