Study Notes on Cell The Unit of Life

160

Study Notes on Cell The Unit of Life

  • Cell theory

Cell theory was formulated by schleiden and Schwann, and was modified by Rudlof Virchow.Cell theory states.

  • All living organisms are composed of cells and products of cells.
  • All cells arises from pre-existing cells.

Prokaryotic cell:

Modification of cell envelope

  • Slime layer: Glycocalyx in form of loose sheath.
  • Capsule: Glycocalyx in form of thick and tough sheath.
  • Mesosomes: Extension of plasma membrane.

These can be in form of vesicles and lamellae.

Function:

  • cell wall formation, DNA replication and distribution to daughter cells, respiration, secretion process, to increase surface area of plasma membrane and enzyme content.
  • Flagella: Extension of cell wall. It is composed of three structure- filament, hook and basal body. It help in motility of bacteria.
  • Pili and fimbriae: Surface structure of some bacteria which attaches them to rocks in streams and to host tissues.

Genetic material:

  • It is not covered by nuclear envelope. In addition to the genomic DNA ( the single chromosome/ circular DNA) , many bacteria have small circular self replicating , double standard DNA which is called as plasmid, plasmid contain genes like antibiotic resistance.

EUKARYOTIC CELLS

  • Possess an organised nucleus with nuclear envelope and have a variety of complex locomotory and cytoskeletal structures.

Cell membrane

  • Singer and Nicolson (1972) gave ‘fluid mosaic model’ .
  • According to this the quasi-fluid nature of lipid enable lateral movement of proteins within the overall bilayer , two types of proteins (peripheral and integral proteins) with cholesterol, glycolipids and glycoprotein.
  • Erythrocyte membrane has 52% protein and 40% lipids.

Function

  • It is semi permeable and transport of molecule across it.

Passive transport

  • Transport of molecules from higher to lower concentration.

Active transport
It do not utilise energy (ATP) .eg., diffusion.

  • Transport of molecules from lower to higher concentration.
  • It utilise energy (ATP).eg. Na+/k+ ATPase Pump.

(fluid mosaic model of plasma membrane)

 

CELL WALL

 

  • Cell wall is non-living rigid structure which gives shape to the cell and protects cell from mechanical damage and infection, which in cell-to-cell interaction and provides barrier to undesirable macromolecules.
  • Cell of algae is made of cellulose, galactose, mannose and mineral like calcium carbonate. •Plant cell wall consists of cellulose, hemicellulose, pectins and proteins.
  • middle lamella is made of calcium pectate which holds neighbouring cells together.
  • Plasmodesmata connect the cytoplasm of neighbouring cells.

Endoplasmic Reticulam(ER)

  • Consists of network of tiny tubular structure.ER divides the intracellular space into two distinct compartments-luminal (inside ER) and extra luminal (cytoplasm).

(1)Rough Endoplasmic Reticulum (RER):

  • Ribosomes attached to outer surface.

Function

  • Involved in Protein synthesis and secretion.

(2) Smooth Endoplasmic Reticulum (SER)

  • Lack ribosomes.

Function

Site for synthesis of lipid.

Golgi apparatus

  • First observed by Camilio Golgi in (1988)
  • Consists of cisternae stacked parallel to each other.
  • Two faces of the organelles are convex/sis or forming face and concave/ trans or maturing face.
  • Function:
    • perform packaging of material, to be delivered either to the intra-cellar targets or secreted outside of the cell.
    • Important site of formation of glycoprotein and glycolipids.

    Lysosomes:

    • Membrane bound vesicular structure formed by the process of packaging in the Golgi apparatus. Contain hydrolysing enzymes(lipase, protease, carbohydrose) Which are active in acidic PH.
    • Also called ‘suicidal Bag’.

    Function:

    • Intracellular digestion.

    Vacuoles:

    • Member bound space found in the cytoplasm.
    • Contain water, sap, excretory products, etc.
    • In plant cell vacuole occupies 90% of the space.

    Function:

    • In plants tonoplast (single membrane of vacuoles ) facilites transport of ions and other substances.
    • Contractile vacuole for excretion in amoeba and food vacuoles formed formed in protists for digestion of food.

    Mitochondria:

    • Doubled membrane structure.
    • Outer membrane smooth and inner membrane forms a number of infolding called cristae.
    • The cristae increase the surface area.

    Function:

    • Sites of aerobic respiration.
    • Called power house of the cell which produce cellular energy in the form of ATP.
    • Matrix possess single circular DNA molecule, a few RNA molecules, ribosomes (70s).
    • It divides by binary fission.



    Plastids:

    • Found in plant cells and euglenoides.
    • Chloroplasts, chromoplasts, leukoplasts are 3 types of Plastids depending on pigments contained.
    • Choloroplast which is green coloured Plastids contain chlorophyll, double standard DNA and 70s ribosomes.
    • Chromoplast which give colour to plant parts .
    • Leucoplast ,amyloplast (starch),Elaioplasts(oil+fat).

    Function:

    • Site of photosynthesis, and imparts colour to fruits and flowers.

    Chloroplasts:-

    • organelles that conduct photosynthesis where the photosynthetic pigment chlorophyll capture

    the energy from sunlight, converts it and store it in the form of ATP.

    Ribosomes:

    • Composed of RNA and proteins; without membrane. Eukaryotic ribosomes are 80s .(s-svedberg’s unit)

    Function:

    • Site of protein synthesis.

    Cytoskeleton:

    • Network of filaments.
    • Proteinaceous structure in cytoplasm made up of microtubules and micro filaments.

    Function:

    • Mechanical support, motility, maintenance of the shape of the cell.

    Cilia and Flagella:

    • Cilia are small structure which work like a oars which help in movement.
    • Flagella are longer and responsible for cell movement. They are covered with a plasma membrane. Core is called axoneme which has 9+2 arrangement of axonemal microtubules.

    Centrosome and centriole

    • Centrosome contain two cylindrical structure called centrioles.
    • Surrounded by amorphous pericentriolar materials.
    • Centrioles from the basal body of cilia or Flagella and spindle fibres for cell division in animal cell.
    • They produces spindle apparatus during cell division.

    Nucleus:

    • with double membrane with perinuclear space and nuclear pores has chromatin ,nuclear matrix and nucleoli (site for RNA synthesis) .
    • Named by Robert Brown in 1831.
    • chromatin is consists of DNA and non histone proteins which is named by Flemming.
    • centrosomes in which primary constriction disc is known as kinetochore.
    • Satellite is some chromosome have non straining secondary construction at a constant location , which gives the appearance of small fragment called satellite.

     

     

REFERENCES

  1. https://ncert.nic.in/ncerts/l/kebo108.pdf