The Structure of Cells
The plasma membrane encloses the cell, creates defined boundaries, and maintains differences between the cytosol and the external environment. It is comprised of a lipid bilayer that contains proteins and cholesterol, and provides a fluid structure relatively impermeable to water-soluble molecules.
Image left: TEM of microvilli, shown above as they appear on cells lining the small intestine, increase the surface area available for absorption.
These microvilli are found only on areas of the plasma membrane facing the cavity from which substances will be absorbed.
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