CHLOROPLAST

 CHLOROPLAST

·   Plastids are Cytoplasmic organelles of Photosynthetic protists and Plants that often possess pigments such as Chlorophylls and Carotenoids, and are the sites of synthesis and storage of food reserves. The most important type of plastid is the Chloroplast.

·     Chloroplasts (Greek “chloros”, meaning “green” and “plast”, meaning “form” or “entity”) are Light harvestings structures present in Algae and Plants and it is the site of Photosynthesis.

·       Chloroplast is an organelle that contains the Photosynthetic pigment Chlorophyll.

·    The size of the Chloroplast usually varies between 4 - 6 µm in diameter and 1 - 3 µm in thickness.

·       Chloroplasts are Oval or Biconvex in shape.

·       Chloroplasts are relatively large and readily visible with the Light microscope, and their number per cell varies among species.

·      Like Mitochondria and the Nucleus, Chloroplasts have two Phospholipid bilayer membranes and DNA.

·     Although Chloroplasts resemble Mitochondria, Chloroplasts are larger, contain special Pigments, and are much more varied in shape. 

·       The pigments of Chloroplasts gather Light energy to produce ATP and form Sugar from Carbon-di-oxide.

Structure of Chloroplast

i) Outer membrane and Inner membrane

·    Chloroplasts are Double-membrane organelle with the presence of Outer membrane and Inner membrane.

·       Both Outer and Inner membranes are made up of Lipid bilayer membrane.

·    The Outer membrane is permeable to small organic molecules, whereas the Inner membrane is less permeable and studded with transport proteins.

·       The Inner membrane separates the Stroma from the Intermembrane space.

ii) Intermembrane space

·       The space between the Outer membrane and Inner membrane is called Intermembrane space.

iii) Thylakoid

·       The third membrane in Chloroplast is folded into small, disclike sacs called Thylakoids.

·       The Chlorophyll pigments are found in the Thylakoid membranes.

·     Thylakoid is the sight for the process of Light-dependent reactions of the Photosynthesis process.

·    The Thylakoid membrane is highly impermeable and its major function is to form the light-driven proton motive force that results in ATP synthesis.

iv) Stroma

·       The fluid between the Thylakoids and the Inner membrane is called the Stroma.

·      Stroma is a colourless, alkaline, aqueous, protein-rich fluid present within the Inner membrane of the Chloroplast.

·    Stroma contains DNA, Ribosomes, Lipid droplets, Starch granules and flattened, membrane-delimited sacs, the Thylakoids.

·       Stroma contains metabolic enzymes and multiple copies of the Chloroplast genome.

·       The Stroma also contains the enzyme Ribulose bisphosphate carboxylase, the key enzyme of the Calvin cycle.

v) Grana

·     The Thylakoids are arranged in Stacks known as Grana and each Granum contains around 10 - 20 Thylakoids.

·       Grana are the sites of conversion of light energy into chemical energy.

vi) Chlorophyll

·       Chlorophyll is a green photosynthetic pigment that helps in the process of Photosynthesis.

Pyrenoids

·    The Chloroplasts of many Algae contain a Pyrenoid, a dense region of Protein surrounded by Starch or another Polysaccharide.

·       Pyrenoids participate in Polysaccharide synthesis.

Functions of Chloroplasts

·       The most important function of the Chloroplast is to synthesize food by the process of Photosynthesis.

·       Absorbs Light energy and converts it into Chemical energy.

·       Chloroplast has a structure called Chlorophyll which functions by trapping the Solar energy and is used for the synthesis of food in all green plants.

·       Produces NADPH and Molecular oxygen (O₂) by Photolysis of water.

·       Produces ATP by the process of Photosynthesis.

·     The Carbon-di-oxide obtained from the air is used to generate Carbon and Sugar during the Calvin Cycle or Dark reaction of Photosynthesis.