PIGMENTS INVOLVED IN PHOTOSYNTHESIS

PIGMENTS INVOLVED IN PHOTOSYNTHESIS:

Introduction:

·         Photosynthesis in plants is dependent upon capturing light energy in the pigment chlorophyll or chlorophyll a. The chlorophyll resides mostly in the chloroplast and gives leaves a green colour. The range of light absorption in leaves is extended by some accessory pigments such as the carotenoids but does not cover the entire visible range. Some plants and plant like organism have developed other pigments to compensate for low light or poor use of light.

·         Cyanobacteria and red algae have phycocyanin and allophycocyanin as accessory pigments to absorb orange light. They also have a red pigment called phycoerythrin that absorbs green light and extends the range of photosynthesis. The red pigment lycopene is found in vegetables. Some red algae are nearly black, so that increases their photosynthetic effeciency. Brown algae have the pigment fucoxanthin in addition to chlorophyll to widen their absorption range. These red and brown algae grow to depths around 270nm where the light is less than 1% of surface light.

·         Pigments are molecules that absorb specific wavelength (energies) of light and reflect all others. Pigments are coloured, the colour we see is the net effects of all the light reflecting back to us. Flowers, coral and even animals skin contain pigments which make them coloured. More important than their reflection of light is the ability of pigments to absorb certain wave length because they interact with light to absorb certain wavelength of light.

·         There are many forms of chlorophyll: chlorophyll a, chlorophyll b, chlorophyll c1, chlorophyll c2, chlorophyll d and divinyl chlorophyll-a, all reflect light waves in the green spectrum.

·         Due to minor differences in their particular molecule make up, they each reflect shades of green (yellow-green, lime green, forest green, blue green etc.).

·         All of these various forms of chlorophyll, except chlorophyll a are considered as accessory pigments because they unlike chlorophyll a cannot convert photons of light into energy, they assist chlorophyll a in the energy absorption process and then passed their absorbed energy on to chlorophyll a for energy production. This makes chlorophyll a alpha dog in the process of photosynthesis because without chlorophyll a, plants could not actually across the light energy they absorb.

·         Molecules of chlorophyll a, chlorophyll b, carotene and xanthophylls are situated in the thylakoid membrane.

·         Plants use more than one photosynthetic pigment to absorb light. This maximizes the use of energy from the sun. These pigments includes; chlorophyll a. Chlorophyll b and xanthophylls.

·         In plants, algae and cyanobacteria, pigments are the means by which the energy of sunlight is captured for photosynthesis. However, since each pigments reacts with only a narrow range of the spectrum. There is usually a need to produce several kinds of pigments, each with a different colour to capture more of the sun's energy.

·         In plants there are two types of chlorophyll; chlorophyll a and chlorophyll b. Chlorophyll molecule looks like a tadpole with porphyrin head, two phytol tails. Chlorophyll a has methyl group (-CH3) and aldehyde (-CHO group) in chlorophyll b.

·         Chlorophyll a is the major pigment used in photosynthesis, but there are several types of chlorophyll and numerous other pigments that respond to light, including red, brown and blue pigments.

·         These other pigments may help channel light energy to chlorophyll a or protect the cell from photo damage. For example, photosynthetic protists called dinoflagellates, which are responsible for the “red tides” often prompt warnings against eating shell fish, contain a variety of light sensitive pigments including both chlorophyll and the red pigments responsible for their dramatic colouration.

 

              Main Pigments:

        I.            Chlorophyll a:

·         This is the most abundant pigment in plants.

·         Chlorophyll a absorbs light with wave length of 430nm (blue) and 662nm (red).

·         Chlorophyll a is the major pigment involved in trapping light energy and converting it into electrical and chemical energy. It acts as a reaction centre.

·         They are greenish pigments. It reflects green light strongly so it appears green to us.

·         Chlorophyll a contains a porphyrin ring.  This is stable ring shaped molecule around which electrons are free to migrate because ring has the potential to gain or lose electrons easily and provide potential to the energised electrons of another molecule. This is the fundamental process by which chlorophyll capture the energy from sunlight.

·          It contains a hydrophobic (fat soluble) phytol chains that allow it to be lipid membrane. The rest of the structure called tetra-pyrolic ring rests outside of the membrane. This part of the pigment absorbs the energy from light. The metal at the centre of the structure is Mg, which can have variable oxidation states. This means that it can accept and donate electron rapidly depending on the situation. It is flexible which is very important for the function of molecule.

·         The absorption of light energy and its conversion into chemical energy occurs in multiprotein complexes called photosystem, located in the thylakoid membrane. A photosystem has two closely linked components, antennae containing light absorbing pigments and reaction centre comprising a complex of proteins and two chlorophyll a molecules. Each antenna contains one or more light harvesting complexes (CHCs). The energy of the light captured by CHCs is funnelled to the two chlorophyll molecule in the reaction centre, where the primary events of photosynthesis occur.

·         Chlorophyll a is found in all photosynthetic organisms, both eukaryotes and prokaryotes.

      II.            Chlorophyll c:

·         It is found only in the photosynthetic membrane of the Chromista as well as the Dinoflagellates.

Accessory Pigments:

1.       Carotenoids:

·         This is a class of accessory pigments that occur in all photosynthetic organisms.

·         Carotenoids are usually red, orange or yellow pigments and include the familiar compound carotene, which gives carrot their colour.

·         They are completely hydrophobic (fat soluble) and exist in lipid membrane.

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

·          Carotenoids absorb light maximally between 460nm and 550nm and appear red orange and yellow to us. 

·         Carotenoids cannot transfer sunlight energy directly to the photosynthetic pathways but must pass their absorbed energy to chlorophyll, for this reason they are known as accessory pigments.

·         Most important function of carotenoids is to protect the plant from radicals formed from UV rays or other reaction. Free radicals are dangerous because they contain extra odd electrons they don't really want to have.

2.       Chlorophyll b:

·         It acts as an accessory pigment.

·         It helps in broaden the spectrum of light absorbed during photosynthesis.

·         It absorbs light of 453nm and 642nm maximally. On absorbing light it become excited and transfer its electron to chlorophyll a molecule.

·         It is not as abundant as chlorophyll a.

·         Chlorophyll b occurs only in green algae and in the plants.

·         It helps in increasing the range of light a plant can use for energy.

·         Chlorophyll b constitutes about 1/4th of the total chlorophyll content.

3.       Fucoxanthin:

·         It is a brown colour pigment.

·         Its colour helps other brown algae as well as diatoms.

 

4.       Phycobilins:

·         They are water soluble pigments which are found in the cytoplasm or in the stroma of the chloroplast.

·         They occur only in the cyanobacteria and Rhodophyta.

·         Both phycobilin and phycoerythrin fluoresces at a particular wave length i.e. when they are exposed to strong light, they absorb the light energy and released it by emitting light of a narrow range of wave length.

·         The light produced by this fluorescence is so distinctive and reliable that phycobilin is used as chemical lags.

·         The pigments are chemically bonded to antibody which is then put into a solution of cells, when the solution is sprayed as a stream of fine droplets past a laser and computer sensor, a machine can identify whether the cells in the droplets have been tagged by the antibody. This has found extensive use in cancer research.

5.       Anthocyanin:

·         Anthocyanin are water soluble pigments produced via the flavanoid pathway to the cytoplasm of the coloured plant cell.

·         The attachment of the sugar molecule makes them particularly soluble in the sap of the vacuole, where these molecule are stored.

·         These are responsible for the pink- red colour of the most flower petals of most red fruits (like apples) and almost all red leaves during the autumn.

·         Anthocyanin absorbs light in the blue green wave lengths, allowing the red wavelength to be scattered by the plant tissues to make these organ visible to us as red.

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