Chloroplast—the Site of Photosynthesis
Chloroplasts are green plastids which help in the synthesis of organic food. The chloroplasts are distributed uniformly in the cytoplasm of plant cells. They are well developed in all the green plants. However, chloroplasts are absent 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 higher plants, their number varies from 20 to 100. In higher plants, the size of discoid chloroplasts ranges from 4-10 micrometer 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 structure called chloroplast envelope. Each membrane of envelope is a unit membrane like plasma membrane. The space separating the two membranes is about 100-200 Angstrom thick. Proteinaceaous matrix in the chloroplast is called matrix or stroma. Matrix is colloidal in nature and contains all the enzymes needed for photosynthesis, osmiophilic droplets, 70S ribosomes, DNA and RNA. The DNA of chloroplast (Ct-DNA) is naked and circular, which is also termed as plastidome.
A massive membrane system of lamellae or the thylakoids which run parallel to each other is found in the stroma. In higher plants chloroplast possess two types of thylakoids—large 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 are closely packed at places to form grana hence called grana thylakoids. The grana thylakoids fuse with the large stroma thylakoids 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 of thylakoids is to perform the light reaction of photosynthesis. It has been reported that certain coupling factors as ATP-ase and photosynthetic pigments (PS-I and PS-II) are located in the thylakoid membrane. It is now believed that stroma 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. These thylakoids or lamellae lie freely in the cytoplasm but without specific envelope. Here, pigments are found evenly distributed on or inside thylakoid membranes. Pigments found also differ from other eukaryotic photosynthesizing plants.
In chloroplasts porphyrin heads of chlorophyll molecules are associated with mono-nuclear protein and their phytol tails are present in lipid layers,
The carotenoids (carotenes and xanthophylls) are sandwiched between the phytol tails and phospholipids molecules.
The presence of nucleic acids makes the chloroplasts genetically semiautonomous, self duplicating organelles.
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 molecules are present between two layers in mono-molecular film oriented in a specific manner. Other pigments like carotenoids etc. are also present in the lipid layer.
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