Chloroplast: The photosynthetic apparatus or site


The process of photosynthesis takes place inside the chloroplast. The below article will explain to you the structure of chloroplast. You will also find different types of pigments found in the chloroplast. To know about chlorophyll and differences between chlorophyll-a and chlorophyll-b, please read this article.

Introduction

The process of photosynthesis takes place inside the green plastid i.e., chloroplast (chloros=green, plastos=moulded). The chloroplast is popularly known as the photosynthetic apparatus. However, different types of photosynthetic apparatus are found in different groups of plants. For example, in lower plants like photosynthetic bacteria and cyanobacteria, the chloroplast is absent and photosynthesis takes place inside photosynthetic lamellae. In many divisions of algae, photosynthetic lamellae are distributed parallel to each other. Stroma and grana are not differentiated. In such a chloroplast, carotenoids are found in abundance. Therefore, these are known as chromatophores. Inside green cells of bryophytes, pteridophytes, gymnosperms and angiosperms, chloroplasts are well developed. They contain true chloroplasts.

Structure of Chloroplast

chloroplast
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In higher plants, chloroplasts are biconvex or plano-convex in shape. However, different plants cells may have filamentous, spheroid, ovoid or club-shape. The size of chloroplast varies from species to species. Electron microscopic studies have shown that chloroplasts are bounded by two unit membranes like mitochondria. Between two membranes, there is a fluid-filled intermembranous peri-plastidial space. The stroma is the inner matrix of the chloroplast, which fills in the hollow space. The chloroplasts consist of many granular chlorophyll bearing bodies known as grana. Each granum is composed of 10-100 disc like superimposed, membranous components known as thylakoids.

Photosynthetic Pigments

Chloroplast or chromatophores contain pigments which convert light energy into chemical energy during photosynthesis. There are three types of pigments in photosynthetic cells:
  1. Chlorophyll
  2. Carotenoids
  3. phycobilins

Chlorophyll

Chlorophyll is a green pigment, found within chloroplast of all green plants and is involved in photosynthesis. It is made up of five types of elements C, H, O, No and Mg. Chlorophylls are of ten types:
  • (1 to 5) - Chlorophyll a, b, c, d, and e
  • (6 to9) -Bacteriochlorophyll a, b, c and d
  • (10)- Chlorobium chlorophyll (Bacteriovirdin)

Chlorophyll-a: Chlorophyll-a is a blue green coloured pigment which is soluble in petroleum ether. It absorbs red and violet colored light during photosynthesis. The empirical formula of chlorophyll-a is C55H72O5N4Mg. It is found in almost all green plants.

Chlorophyll-b: Chlorophyll-b is a pale green coloured pigment which is soluble in methyl alcohol. During photosynthesis, this pigment also absorbs light of red and violet colour just like chlorophyll-a. It is found in two forms: chlorophyll 640 and chlorophyll-b 650. Its empirical formula is C55H70O6N4Mg.

Chlorophyll-c: Chlorophyll-c is found in the algae belonging to class bacillariophyceae and phaeophyceae. Its empirical formula is C35H32O5N4Mg.

Chlorophyll-d: Chlorophyll-d is found in the algae belonging to calss rhodophyceae. Its empirical formula is C54H70O6N4Mg.

Structure of Chlorophyll

chlorophyll
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Willstatter, Stoll and Fischer (1912) described the structure of chlorophyll for the first time. According to them, chlorophyll is made up of two parts like that of a tadpole larva. Head is made up of pyrrole group and tail is made up of phytol group.

Pyrrole: Chlorophyll is magnesium porphyrin compound. Porphyrin ring consists of four pyrrole rings (tetrapyrrole) joined together by methane bridge (-CH3bridge). It is hydrophobic in nature. The centre of tetrapyrrole is occupied by a bivalent Magnesium (Mg++) which is complexed with nitrogen atoms of four pyrrole rings.

Phytol: Phytol is hydrophilic in nature and is made up of alcohol phytol (C28H39OH). Chl-a and b differ in because in Chl-b there is a -CHO group instead of a -CH3 group at the third carbon atom in II pyrrole ring. The phytol chain is responsible for lipoidal solubility of the chlorophylls.

Functions of chlorophyll:The functions of chlorophyll are as below:
  • Chlorophyll absorbs radiant energy of sunlight for photosynthesis.
  • Chlorophyll helps in the production of ATP.
  • Chlorophyll plays an active role in the transport of electrons during photophosphorylation.
  • Chlorophyll is the most essential requirement for the production of organic food during photosynthesis.

Carotenoids

Carotenoids are yellow or orange coloured pigments, usually found in close association with chlorophylls. They occur in thylakoids and act as an accessory pigment of photosynthesis. Carotenoids absorb light energy in the mid-region of the visible spectrum and transfer their absorbed energy to chlorophyll molecules. They pick up nascent O2 released during photo-oxidation of water and change them into the molecular state. Thus, carotenoids protect the chlorophyll molecules from photo-oxidation. Carotenoids are also found in the chromoplasts in flower and fruits and make them attractive for cross pollination and dispersal. There are two major groups of carotenoids the carotenes and xanthophylls:

Carotenes: Carotenes are hydrocarbons with a general molecular formula C40H56 and are orange-red in colour. Carotenes absorb blue and green light. The most common carotene is b-carotene which is converted into vitamin-A by animals and human beings.

Xanthophylls: Xanthophylls are yellow coloured carotenoids with a general formula C40H56O2. E.g., Zeaxanthin and Lutein. These are mainly found in photosystem II (PS-II). Fucoxanthin is also a type of xanthophyll which is found in many algae.

Functions of carotenoids: The functions of carotenoids are as follows:
  • Carotenoids protect chlorophyll from photo-oxidation.
  • Carotenoids function as accessory photosynthetic pigments and absorb light energy which is transferred to cholorophyll-a.

Phycobilins

Phycobilins pigments are found inside phycobilisomes and mainly found in brown and red algae. Phycobilins are soluble in water. Phycobilins are always found in association with proteins. Phycobilins may be destroyed at a higher temperature. Like chlorophyll molecules, phycobilins are also open-chain tetrapyrroles but phycobilins do not contain magnesium atom. Phycobilins molecules also absorb light energy and transfer it to chlorophyll-a. Phycobilins are of two types:

Phycoerythrin: Phycoerythrin is red in colour and found in blue-green algae as c-phycoerythrin and in red algae as r-phycoerythrin.

Phycocyanin: Phycocyanin is blue in colour and found in blue green-algae as c-phycocyanin and in red algae as r-phycocyanin.

In blue-green algae and red-green algae, phycobilins are found in phycobilisomes which are attached with thylakoids. Phycobilins are useful in chromatic adaptation.

Differences between Chlorophyll-a and Chlorophyll-b

The differences between chlorophyll-a and chlorophyll-b are as follows:
  1. Chlorophyll-a is blue-green in colour whereas Chlorophyll-b is pale-green coloured.
  2. Chlorophyll-a contains a methyl group with third carbon of second pyrrol ring but Chlorophyll-b has aldehyde(-CHO) group at this position.
  3. Chlorophyll-a is a primary light harvesting molecule whereas Chlorophyll-b is an accessory light harvesting molecule.
  4. Chlorophyll-a releases elecftron during cyclic and non-cyclic photophyosphorylation but Chlorophyll-b transfer the absorbed radiation energy tochlorophyll molecules.
  5. Chlorophyll-a is soluble in petroleum ether whereas Chlorophyll-b is soluble in methyl alcohol.
  6. Chlorophyll-a is found in all photosynthetic organisms except photosynthetic bacteria but Chlorophyll-b is found in higher plants and green algae.
  7. Chlorophyll-a mainly absorbs red coulored light whereas Chlorophyll-b mainly absorbs violet coloured light.


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