CELL THEORY :
- Schieiden and Schwann together formulated the cell theory.
- They proposed that the body of animal and plant are composed of cells and product of cells.
- Rudolf Virchow (1855) he modified and gives the final shape to the cell theory.
- All living organisms are composed of cells and products of cells.
- All cells arise from pre-existing cells.
PROKARYOTIC CELLS :
- Represented by bacteria, blue-green algae, mycoplasma and PPLO.
- Smaller and multiply more rapidly than eukaryotic cells.
- Four basic shape of bacteria are
- Bacillus (rod like)
- Coccus (spherical)
- Vibrio (comma shaped)
- Sprillum (spiral)
- All prokaryotes have a cell wall surrounds the plasma membrane.
- There is no well defined nucleus.
- Chromosome is single and circular, not enveloped by nuclear membrane.
- Presence of many small, circular, extra chromosomal and self replicable DNA called plasmid.
- Plasmid provides unique characteristic to the bacteria.
- One of the characteristic is antibiotic resistant.
- No membrane bound cell organelles.
- A specialized differentiated form of cell membrane called mesosome is present.
Cell envelope and its Modifications :
- Most prokaryotic cell have characteristic complex cell envelope.
- The cell envelops consists of tightly bound three layer structure;
- The outermost glycocalyx.
- Middle cell wall
- Inner plasma membrane.
- According to the nature of the cell wall and behavior towards the stain developed by Gram, bacteria are Gram positive or Gram negative.
- Glycocalyx may for a loose sheath called slime layer.
- Glycocalyx may be thick and tough called the capsule.
- The cell wall determines the shape of the cell, strong structural support to prevent the bacterium from bursting and collapsing.
- Plasma membrane is semi-permeable in nature and interacts with outside world.
- A special membranous structure is the mesosome which is formed by extensions of plasma membrane into the cell.
- These extensions are in the form of vesicles, tubules and lamellae.
- They help in cell wall formation, DNA replication and distribution to the daughter cell.
- They also help in respiration and secretion.
- Some bacteria have filamentous extensions called flagella.
- Bacterial flagellum has three parts – filament, hook and basal body.
- Pili are elongated tubular structures made of a special protein.
- Fimbriae are small bristle like fibres sprouting out of the cell. Play role in adhesion.
Ribosome and inclusion bodies :
- Ribosomes are associated with the plasma membrane of the cell.
- Each ribosome (70 S) has two sub units; smaller (30 S ) and large (50 S).
- Ribosomes are the site of protein synthesis.
- Several ribosomes attached to a single mRNA to form a chain called polyribosome or polysome.
- Reserve materials are stored in the form of inclusion bodies.
- Phosphate granules, cyanophycean granules and glycogen granules are the inclusion bodies.
EUKARYOTIC CELL :
- Eukaryotes include all the Protists, plants, animals and fungi.
- Extensive compartmentalization due to membrane bound cell organelles.
- Possess an organized nucleus with nuclear envelope.
- Have a variety of complex locomotory and cytoskeletal structure.
- Genetic materials are organized into chromosomes.
- The plant cell possess cell walls, plastids and a large central vacuole, which are absent in animal cell.
- Animal cells have centrioles which absent in plant cells.
Cell membrane :
- The cell membrane composed of lipids that arranged in bilayer.
- Lipids are arranged within the membrane with the hydrophilic polar head towards the outer sides and the hydrophobic tails towards the inner part.
- Non polar tail of saturated hydrocarbons is protected from the aqueous environment.
- The lipid component of the membrane mainly consists of phophoglycerides.
- Cell membrane also possesses protein and carbohydrates.
- Ratio of protein and lipids varies from cell to cell.
- Human erythrocyte plasma membrane contains 52 % protein and 40 % lipids.
- Membrane protein may be integral or peripheral.
- Peripheral protein lie on the surface and integral proteins are partially or totally buried in the membrane.
- The improved model of the structure of plasma membrane was proposed by singer and Nicolson (1972) widely accepted as fluid mosaic model.
- According to this the quasi fluid nature of lipid enables the lateral movement of proteins within the overall bilayer.
