Cell : The Unit of Life

Cell envelope; Cell membrane, Cell wall

  • Plasma membrane is thin, elastic, regenerative and selectively permeable in nature which externally covers every living cell.
  • The lipid of plasmamembrane are of three types namely, phospholipids, glycolipids and sterols.
  • Carbohydrate of plasma membrane are coralently linked to both lipid and protein components. (2% - 10%)
  • The most accepted model of the structure of cell membrane was proposed by singer and Nicolson (1972)
  • According to this model, the membrane is quasifluid in nature and made up of a phospholipid bilayer, enables the movement of intrinsic and extrinsic proteins.
  • Lipids are amphipathic, they are structurally asymmetric with polar hydrophilic head and non polar hydrophobic tail.
  • Sugar chains are attached on the outer side of some of the lipids, particularly to this polar heads. So these are known as glycolipids.
  • Bulk transport of plasma membrane includes pinocytosis (cell drinking) and phagocytosis (cell eating).
  • Pinocytasis is bulk intake of fluid, ions and molecules through development of small endocytic resides of 100-200 nm in diameter.
  • Phagocytosis is ingestion of large particles by living cells, eg: WBC (neutrophils, monocytes), kupffer's cells of liver.
  • Functions of cell membrane are, it provides mechanical strength as well as act as protecting layer.
  • Plasma membrane is responsible for the transportation of materials, molecules, ions etc.
  • Diffusion of gases (O2 & CO2) takes place through plasma membrane by simple and facilitated diffusion.
  • Cell wall is the outermost, rigid, protective, non living and supportive layer, present in all plant cell, bacteria, cyanobacteria, and some protists, It is not found in animal cell.
  • In young plant, primary cell wall is capable of growth, which gradually diminishes as the cell matures and the secondary wall is formed on inner (towards membrane) side of the cell.
  • Middle lamella is the outermost region which is found as a common cementing layer between two cells.
  • Plasmodesmata (or) cytoplasmic bridge are present in pits through which the cytoplasm of one cell is in contact with another.
  • Growth of cell wall occurs by intussuception (or) by apposition.
  • Composition of cell wall, in plants - cellulose + Hemicellulose, in fungi - chitin, in bacteria & mycoplasma - peptidoglycan.
  • Plants cell can withstand lot of variations in the surrounding medium as compared to animal cells because of the presence of cell wall.


View this video for the topic from 27:54 to 45:14


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