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Cell: The Fundamental Unit of Life

Cell: The Fundamental Unit of Life Synopsis

Synopsis

  • Cell is the basic structural and functional unit of all living organisms. It is the smallest part of the body of an organism capable of independent existence and able to perform the essential functions of life. 

Cell Theory 

  • In 1838, Matthias Schleiden and Theodor Schwann proposed a basic cell theory. In 1858, another scientist, Virchow made an addition to the existing cell theory.
  • The postulates of the modern cell theory are
    • The cell is the smallest unit of structure of all living things.
    • The cell is the unit of function of all living things.
    • All cells arise from pre-existing cells.

Contribution of Scientists in the Study of Cells

Variety in Cells 

On the Basis of Number of Cells 

  • Single-celled: Organisms made of a single cell. 
    Examples: Bacteria, yeast, Chlamydomonas, Amoeba, Paramoecium
  • Few-celled: Organisms made of few hundred or few thousand cells. 
    Examples: Spirogyra, Volvox 
  • Multi-celled: Organisms made of millions and billions of cells.
    Examples: Man, cow, mango tree, crow

On the Basis of Size of Cells 

  • Smallest cell: Bacteria (0.3–5.0 µm), red blood cells (7 µm)
  • Longest cell: Nerve cell in the neck of a giraffe (˃3 m long) 
  • Largest cell: Ostrich egg (170 mm × 130 mm)

On the Basis of Shape of Cells 

  • Columnar - Epithelial cells, Spherical - Human ovum, Oval - Fat cells
  • Spherical, biconcave - Red blood cells, Rectangular - Spirogyra
  • Spiral - Sperm cell, Rod-shaped – Bacteria, C-shaped - Cartilage cells
  • Branched - Nerve cells, Spindle-shaped - Smooth muscle cells

Prokaryotic and Eukaryotic Cells 

Prokaryotic Cell 
  • Prokaryotic cells may vary greatly in their shape and size, but their cell organisation remains similar.
Structure of Prokaryotic Cell
  • Cell wall: The cell wall of bacteria is made of peptidoglycan. It is absent in mycoplasma and PPLO.
  • Cytoplasm: It is the matrix present in the cell.
  • Nuclear material: DNA is naked, i.e. it is not enclosed by the nuclear envelop. In addition to DNA, a single molecule of circular DNA called a plasmid is present.
  • Mesosome: It is the extension of the plasma membrane present in the cytoplasm in the form of vesicles, tubules or lamellae.
  • Cell organelles: Membrane-bound cell organelles are absent.
  • Flagella: In motile bacteria, flagella are present as extensions of their cell walls.
  • Fimbriae: In non-motile bacteria, fimbriae or pili are present.
  • Ribosomes: 70S ribosomes are present which are made of 50S and 30S subunits.
  • Inclusion bodies: They are suspended in the cytoplasm. They store reserve food material.
  • Gas vacuoles: Gas vacuoles are found in cyanobacteria, purple bacteria and green photosynthetic bacteria.
Eukaryotic Cells
  • Eukaryotic cells include all protists, fungi, plants and animals.
  • These cells show the presence of membrane-bound cell organelles and a well-developed organised nucleus.