Microbiology, 2021

120 3 • The Cell
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 of
endocytosis, the uptake of matter through plasma membrane invagination and vacuole/vesicle formation
(Figure 3.55). A type of endocytosis involving the engulfment of large particles through membrane
invagination is called phagocytosis, 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 called pinocytosis, 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 endocytosis is 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.
Figure 3.55
Three variations of endocytosis are shown. (a) In phagocytosis, the cell membrane surrounds the particle and pinches off to
form an intracellular vacuole. (b) In pinocytosis, the cell membrane surrounds a small volume of fluid and pinches off, forming a vesicle. (c)
In receptor-mediated endocytosis, the uptake of substances is targeted to a specific substance (a ligand) that binds at the receptor on the
external cell membrane. (credit: modification of work by Mariana Ruiz Villarreal)
The process by which secretory vesicles release their contents to the cell’s exterior is called exocytosis.
Vesicles move toward the plasma membrane and then meld with the membrane, ejecting their contents out of
the cell. Exocytosis is used by cells to remove waste products and may also be used to release chemical signals
that can be taken up by other cells.
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