Cell: The Unit of Life

Cell Size and Shape

• Mycoplasmas, the smallest cells, are only 0.3 µm in length.
• Bacteria can be 3 to 5 µm in length.
• The largest isolated single cell is the egg of an ostrich.
• Red blood cells are about 7.0 µm in diameter.
• Cells vary greatly in shape, including disc-like, polygonal, columnar, cuboid, thread-like, or irregular shapes.

Prokaryotic Cells

• Prokaryotic cells include bacteria, blue-green algae, mycoplasma, and PPLO.
• They are generally smaller and multiply more rapidly than eukaryotic cells.
• Prokaryotes exhibit a wide variety of shapes and functions.
• They have a cell wall surrounding the cell membrane, except in mycoplasma.
• The fluid matrix filling the cell is the cytoplasm.
• There is no well-defined nucleus in prokaryotes.
• Genetic material is basically naked, not enveloped by a nuclear membrane.
• Prokaryotes have plasmids, which are small circular DNA outside the genomic DNA, conferring unique phenotypic characters to bacteria.

Cell Envelope and Modifications

• The cell envelope consists of a tightly bound three-layered structure: glycocalyx, cell wall, and plasma membrane.
• Each layer of the envelope performs distinct functions, but they act together as a single protective unit.
• Bacteria can be classified into two groups based on the differences in the cell envelopes and the manner in which they respond to the staining procedure developed by Gram.
• Gram-positive bacteria take up the gram stain, while Gram-negative bacteria do not.