Cell : The Unit of Life
What is a Cell?
A fundamental, structural and functional unit of all living organisms
Anton von Leeuwenhoek first described the live cells.
All living organisms are composed of cells and products of cells.
All cells arise from pre-existing cells.
Smallest cell: Mycoplasma (0.3 μm)
Largest cell: Ostrich egg
Longest cell: Nerve cell
Represented by bacteria, blue-green algae, PPLO and Mycoplasma
Smaller and rapidly multiplying
Vary greatly in shape and size
Have cell wall surrounding the cell membrane
Absence of a well-defined nucleus
May have plasmids − small, circular, extra-chromosomal DNAs present in addition to the genomic DNAs; this confers characteristics like antibiotic resistance to bacteria, and help in bacterial transformation with foreign DNA.
Absence of organelles (only ribosomes are found in prokaryotes)
Have mesosomes (specialised differentiated cell membranes); these are infoldings of the cell membrane
Have inclusion bodies
Cell Envelope in Prokaryotes
Cell Envelope − Three-Layered Structure
Glycocalyx (Outermost): May be present in the form of loose sheath called slime layer in some bacteria, or as a thick and tough capsule in others
Cell wall (middle): Determines the shape of a cell and provides a strong structural support
Plasma Membrane (innermost): Semi-permeable and structurally similar to that of eukaryotes
Formed by the extens…
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