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 – store oils and fats.
  • Aleuroplast – 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.
• ‘S’ 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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