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The cell is the structural and functional unit of all living organisms, and is sometimes called the "building block of life." Some organisms, such as bacteria, are unicellular, consisting of a single cell.
The cytoplasm of a eukaryoticcell is surrounded by a plasma membrane.
This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of lipids (hydrophobic fat-like molecules) and hydrophilic phosphorous molecules.
The smallest cell, a type of bacterium known as a mycoplasma, measures 0.0001 mm (0.000004 in) in diameter; 10,000 mycoplasmas in a row are only as wide as the diameter of a human hair.
Prokaryotic cells are among the tiniest of all cells, ranging in size from 0.0001 to 0.003 mm (0.000004 to 0.0001 in) in diameter.
The extraordinary biochemical diversity of prokaryotic cells is manifested in the wide-ranging lifestyles of the archaebacteria and the bacteria, whose habitats include polar ice, deserts, and hydrothermal vents—deep regions of the ocean under great pressure where hot water geysers erupt from cracks in the ocean floor.