CELL: THE UNIT OF LIFE

 

Plastids :

  • Plastids are found in all plant cells and in Euglenoids.
  • They bear some specific pigment, impart specific colour to the plants.
  • Based on the type of pigments plastids can be classified into
    • Chloroplast.
    • Chromoplast
    • Leucoplast.

  • The chloroplasts contain chlorophyll and carotenoid pigments, traps solar energy for photosynthesis.
  • In the chromoplasts fat soluble carotenoid pigments like carotene, xanthophylls are present.
  • The leucoplasts are colourless plastids of varied shapes and size with stored nutrients.
    • Amyloplast - store carbohydrates.
    • Elaioplasts ā€“ store oils and fats.
    • Aleuroplast ā€“ store proteins and minerals.

  • Chloroplasts are found in the mesophyll cells of the leaves.
  • These are oval, spherical, discoid or even ribbon like organelles.
  • Chloroplast is a double membrane organelle.
  • The space limited by inner membrane is called stroma.
  • A number of organized flattened membranous sacs called thylakoid are present in the stroma.
  • Thylakoids are arranged like stakes of coins to form grana.
  • There are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana.
  • They thylakoids enclose a space called lumen.
  • Chlorophyll pigments are located in the thylakoids.
  • Chloroplast contains enzymes required for the synthesis of carbohydrates and proteins.
  • Stroma contains small circular DNA and ribosomes.

Ribosomes :

  • Ribosomes are granular structure first observed by George Palade (1953).
  • Composed of ribonucleic acid (RNA) and proteins.
  • Non-membranous cell organelles.
  • Eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S.
  • ā€˜Sā€™ stands for sedimentation coefficient; measure of density and size.
  • Both 70S and 80S ribosomes consists of two subunits.
  • Primary function is protein synthesis hence called protein factory of the cell.

Cytoskeleton :

  • An elaborate network of filamentous proteinaceous structures present in the cytoplasm is collectively known as cytoskeleton.
  • Cytoskeleton involved in many function such as mechanical support, motility, maintenance of the shape of the cell.

Cilia and Flagella :

  • Cilia and flagella are hair-like outgrowths of the cell membrane.
  • Cilia are small help in the movement of cell or surrounding fluid.
  • Flagella are longer and responsible for cell movement.
  • Cilia and flagella covered by plasma membrane.
  • Their core called axoneme, possess a number microtubules running parallel to the long axis.
  • The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules and a pair of centrally located microtubules.
  • The central tubules are connected by bridges and are also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublet by radial spoke.
  • The peripheral doublets are also interconnected by linkers.
  • Both cilia and flagella emerge from centrioles-like structure called basal bodies.

Centrosome and centrioles :

  • Centrosome is an organelle usually containing two cylindrical structures called centrioles.
  • They are surrounded by amorphous pericentriolar materials.
  • Both centrioles in a Centrosome lie perpendicular to each other.
  • Each centriole has an organization like the cartwheel.
  • They are made of nine evenly spaced peripheral fibrils of tubulin.
  • Each of the peripheral fibril is a triplet.
  • The adjacent triplets are also linked to each others.
  • The central part of the centriole is called hub.
  • The hub connected to peripheral triplets by radial spokes.
  • The centriole forms the basal body for cilia, flagella and form spindle fibres during cell division.

Nucleus :

  • Nucleus as a cell organelle was first described by Robert Brown in 1831.
  • Materials inside the nucleus was stained by Flemming and named as chromatin.
  • The interphase nucleus has highly extended and elaborates nucleoprotein fibres called chromatin.
  • The nucleus also contains nuclear matrix and one or two spherical bodies called nucleoli.
  • Nuclear envelope consists of two membranes with perinuclear space (10- 50 nm).
  • The outer membrane remains continuous with endoplasmic reticulum.
  • Presence of nuclear pore due to fusion of two membranes.
  • Nuclear pores allow the movement of RNA and protein in both directions.
  • The nuclear matrix or nucleoplasm contains nucleolus and chromatin.
  • Nucleolus is the site for active ribosomal RNA synthesis.
  • During cell division the chromatins condensed to form chromosomes.
  • Chromatin contains DNA and some basic proteins called histones, non-histone proteins and some RNA.
  • A single human cell contains approximately two meter long thread of DNA in 46 chromosomes.
  • Each chromosome essentially has a primary constriction or the centromere.
  • On each side of centromere there is disc shaped structures called kinetochores.

  • Based on the position of the centromere chromosomes are classified into four types:-
    • Metacentric: centromere at the middle with two equal arms.
    • Sub-Metacentric: one short arm and one long arm.
    • Acrocentric: with extremely short arm and a very long arm.
    • Telocentric: with terminal centromere.

  • A few chromosomes have non-staining constrictions at a constant location. This gives the appearance of a small fragment called the satellite.

 

CBSE Biology (Chapter Wise) Class XI ( By Mr. Hare Krushna Giri ) 
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