Study Notes on Cell The Unit of Life

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