96
96
Changes in red blood cell
shape due to osmosis.
chapter
4
Movement
of Molecules
Across Cell
Membranes
a
s we have seen, the contents
of a cell are separated from
the surrounding extracellular
fl uid by a thin layer of lipids and
protein—the plasma membrane.
In addition, membranes associated
with mitochondria, endoplasmic
reticulum, lysosomes, the Golgi
apparatus, and the nucleus divide
the intracellular fl uid into several
membrane-bound compartments.
The movements of molecules and
ions both between the various
cell organelles and the cytosol,
and between the cytosol and the
extracellular fl uid, depend on the
properties of these membranes. The
rates at which different substances
move through membranes vary
considerably and in some cases
can be controlled—increased or
decreased—in response to various
signals. This chapter focuses
upon the transport functions of
membranes, with emphasis on the
plasma membrane. There are several
mechanisms by which substances pass
through membranes, and we begin
our discussion of these mechanisms
with the process known as diffusion.
Diffusion
Magnitude and Direction of Diffusion
Diffusion Rate Versus Distance
Diffusion Through Membranes
Mediated-Transport Systems
Facilitated Diffusion
Active Transport
Osmosis
Extracellular Osmolarity and Cell
Volume
Endocytosis and Exocytosis
Endocytosis
Exocytosis
Epithelial Transport
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