Figure 3.52 The eukaryotic plasma membrane is composed of a lipid bilayer with many embedded or associated
proteins. It contains cholesterol for the maintenance of membrane, as well as glycoproteins and glycolipids that are
important in the recognition other cells or pathogens.
Membrane Transport Mechanisms
The processes of simple diffusion, facilitated diffusion, and active transport are used in both eukaryotic and
prokaryotic cells. However, eukaryotic cells also have the unique ability to perform various types ofendocytosis,
the uptake of matter through plasma membrane invagination and vacuole/vesicle formation (Figure 3.53). A type
of endocytosis involving the engulfment of large particles through membrane invagination is calledphagocytosis,
which means “cell eating.” In phagocytosis, particles (or other cells) are enclosed in a pocket within the membrane,
which then pinches off from the membrane to form a vacuole that completely surrounds the particle. Another type of
endocytosis is calledpinocytosis, which means “cell drinking.” In pinocytosis, small, dissolved materials and liquids
are taken into the cell through small vesicles. Saprophytic fungi, for example, obtain their nutrients from dead and
decaying matter largely through pinocytosis.
Receptor-mediated endocytosisis a type of endocytosis that is initiated by specific molecules called ligands when
they bind to cell surface receptors on the membrane. Receptor-mediated endocytosis is the mechanism that peptide
and amine-derived hormones use to enter cells and is also used by various viruses and bacteria for entry into host
cells.
128 Chapter 3 | The Cell
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