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Chloroplast

Chloroplastthe Site of Photosynthesis

Chloroplasts are green plastids which help in the synthesis of organic food. The chloroplasts are distributeduniformly in the cytoplasm of plant cells. They are well developed in all the green plants. However, chloroplasts areabsent in blue-green algae and photosynthetic bacteria. These are also absent in non-photosynthetic bacteria and

fungi. They are of different shapes and sizes. Shape varies considerably in algal chloroplasts. In green cells of higherplants, their number varies from 20 to 100. In higher plants, the size of discoid chloroplasts ranges from 4-10micrometer in length and 2-4 micrometer in breadth. The size is constant for a given cell type.A chloroplast consists of proteins 50-60%, lipids 25-30%, chlorophyll 5-10%, carotenoids and other pigments 1-2%,RNA 2-3%, DNA up to 0.5%, vitamins and certain metal ions in traces.

An electron microscopic structure reveals that chloroplast is covered by a double membranous structurecalled chloroplast envelope. Each membrane of envelope is a unit membrane like plasma membrane. The spaceseparating the two membranes is about 100-200 Angstrom thick. Proteinaceaous matrix in the chloroplast is calledmatrix or stroma. Matrix is colloidal in nature and contains all the enzymes needed for photosynthesis, osmiophilicdroplets, 70S ribosomes, DNA and RNA. The DNA of chloroplast (Ct-DNA) is naked and circular, which is alsotermed as plastidome.

A massive membrane system of lamellae or the thylakoids, which run parallel to each other, is found in thestroma. In higher plants chloroplast possess two types of thylakoidslarge and small. The large are known as stroma

thylakoids, extend from one end to the other end of chloroplasts whereas small are disc shaped thylakoids which areclosely packed at places to form grana hence called grana thylakoids. The grana thylakoids fuse with the large stromathylakoids in the region of grana.

Each granum is a stack of 2-100 thylakoids laid in piles once on the top of another. The main function ofthylakoids is to perform the light reaction of photosynthesis. It has been reported that certain coupling factors asATPase and photosynthetic pigments (PS-I and PS-II) are located in the thylakoid membrane. It is now believed thatstroma lamellae contain PS-I, while grana lamellae have both PS-I and PS-II pigments.

Blue green algae and cyanobacteria do not have chloroplasts. However, they bear thylakoids. Thesethylakoids or lamellae lie freely in the cytoplasm but without specific envelope. Here, pigments are found evenlydistributed on or inside thylakoid membranes. Pigments found also differ from other eukaryotic photosynthesizingplants.In chloroplasts porphyrin heads of chlorophyll molecules are associated with mononuclear protein and theirphytol tails are present in lipid layers,

The carotenoids (carotenes and xanthophylls) are sandwiched between the phytol tails and phospholipidsmolecules.The presence of nucleic acids makes the chloroplasts genetically semiautonomous, self-duplicatingorganelles.Molecular model of chloroplast was proposed by Frey and Wyssling (1953), which exhibits the intergranum portion of the lamellae made up of alternate layers of aqueous proteins and lipids. The chlorophyll moleculesare present between two layers in monomolecular film oriented in a specific manner. Other pigments like carotenoidetc. are also present in the lipid layer.

The chloroplast of higher plants are double discoid and ellipsoidal in shape, 4-6 in length and 1-2 thick,. The chloroplast is bounded by two membrane each aprox. 5 thick and consisting of lipid bilayer and protein

Fig; Chloroplast

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Internally chloroplast filled up with matrix called as stoma in which are embedded in grana. Each granum hasdiameter of 0.25-0.8 thicknesses and consist of 5-25 disk shaped grana lamellae placed one above the other likestack of coins. (1 A = 10-2 mm, 1 = 1000m and 1mm= 1000 )Chlorophylls and other photosynthetic pigment are found in the form of protein complex pigments complex in grana.Beside the necessary enzymes, some ribosome and DNA have also been found in chloroplast, which give them partialgenetic identity.Chloroplast pigments

The pigments involved in the process of photosynthesis are known as photosynthetic pigments. Pigments like

chlorophylls, carotenoids (carotenes and xanthophylls), anthocyanin, phycobilins (phycocyanin and phycoerythrin)may be found in a plant cell.a) Chlorophylls: The chlorophylls, the green pigments in chloroplast are of seven types i.e. chlorophyll a, b,

c, d, e, bacteriochlorophyll and bacterioviridin. Chlorophyll molecule in made up of a square tetrapyrrolic ring knownas head and a phytol alcohol called tail. The magnesium atom is present in the central position of tetrapyrrolic ring.The four-pyrrole rings of porphyrin head are linked together by methane (CH=) groups forming a ring system. Eachpyrrole ring is made up of four carbon and one nitrogen. The porphyrin head bears many characteristic side groups atmany points. Different side groups are indicative of various types of chlorophylls.Phytol tail is made up to 20-carbon alcohol attached to carbon 7 position of pyrrole ring IV with a propionic acid esterbond. The name chlorophyll was given to the green pigment by Pelletier and Caventou in 1817. In majority of greenplants chlorophyll a and b are present.

Fig: Chlorophyll a moleculeChlorophyll a C35H72O5N4MgChlorophyll b C55H70O6N4Mg

Chlorophyll a is blue black while chlorophyll b is green black. Both are soluble in organic solvents likealcohol, acetone etc. Chlorophyll a appears red in reflected light and bright green in transmitted light as compared tochlorophyll b, which looks brownish red in reflected light and yellow green in transmitted light. Chlorophyll is agreen colored pigment because it does not absorb green light. Chlorophyll a possesses CH3 (methyl group), which isreplaced byCHO (an aldehyde) group in chlorophyll b.(b) Carotenoid pigments

Carotenoids are the lipids and can be classified into two groups:(i) Carotenes (ii) Xanthophylls.

Carotenes are orange red in color. The have general formula C40H56. They were isolated from carrot. On theother hand, xanthophylls are yellow in color. Xanthophylls contain oxygen also along with carbon and hydrogen

(C40H56O2). All the carotenoid pigments have 40 carbon atoms. Lutein a widely distributed xanthophyll isresponsible for yellow color autumn foliage. Fucoxanthin is another important xanthophyll present inPhaecophycease (Brown algae).Carotenoids absorb light energy and transfer it to chlorophyll a and thus act as accessory pigments. They protect thechlorophyll molecules from photo-oxidation by picking up nascent oxygen and converting it into harmless molecularstage.(C) Phycobilins

These pigments are mainly found in blue-green algae (cyanobacteria) and red algae. These pigments haveopen tetrapyrrolic in structure and do not bear magnesium and phytol chains.Blue-green algae have more quantity of phycocyanin and red algae have more phycoertythrin together formphycobilins. These water-soluble pigments are thought to be associated with small granules attached.