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Cell : The Unit of Life

Prokaryotic cell

What is a Cell?

A fundamental, structural and functional unit of all living organisms

Anton von Leeuwenhoek first described the live cells.

Cell theory:

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

Prokaryotic Cells

Represented by bacteria, blue-green algae, PPLO and Mycoplasma

Smaller and rapidly multiplying

Vary greatly in shape and size

Characteristic features:

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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