Page 51 - Anatomy and Physiology of Farm Animals, 8th Edition

36 / Anatomy and Physiology of Farm Animals

Intercellular space becomes equally concentrated throughout
the medium. Diffusion occurs because all
VetBooks.ir Cell molecules and ions have kinetic energy.

They collide with each other and bounce
membrane
away, becoming so dispersed in the sol­
vent that an equal concentration appears
throughout. In solutions, diffusion proceeds
from a region of greater concentration of
Paired particles to a region of lower concentra­
proteins tion, so the net diffusion is said to occur
down a concentration gradient. The rate
at which transport occurs is also deter­
mined by the mechanism of transport.
Passive mechanisms that utilize movement
of particles down a concentration gradient
typically have the fastest rate of transport
Channels
(10 to 10 ions/second) as compared to
7
8
active transport (10 to 10 ions/second) or
0
3
facilitated transport (10 to 10 molecules/
2
4
second).
Only a few substances, such as oxygen,
carbon dioxide, and alcohol, are mem-
brane soluble, that is, capable of diffusing
freely through the lipid bilayer of plasma
Figure 2-10. Two cell membranes connected at a
gap junction by proteins that extend between them. membranes. Such molecules must be
lipid soluble. Certain drugs, such as
barbiturates, a class of anesthetics, are
Transport Across Cell Membranes membrane soluble. If a substance cannot
diffuse freely through the lipid bilayer of
The plasma membrane and the membranes the cell membrane, its ability to diffuse in
of intracellular organelles have an impor­ or out of a cell depends on some other
tant function in determining what enters means of crossing the membrane.
and leaves the cell or its organelles. Our One way lipid‐insoluble substances
very life and that of animals depend on this cross the cell membrane is via a trans­
ability to control what enters and leaves a membrane protein or proteins that form a
cell. It is therefore important to understand channel, or passageway, through the mem­
and appreciate the processes of membrane brane (Fig. 2‐8). If a channel is present that
transport before discussing the func­ permits the passage of a given molecule,
tions of the animal’s organs and systems. then membrane is said to be permeable to
Transport into and out of cells may occur the molecule. The degree of permeability
by simple and facilitated diffusion, osmosis, of an individual channel may also be sub­
active transport, endocytosis, or exocytosis. ject to regulation by factors such as the
(Endocytosis and exocytosis are described electrical potential across the membrane.
at the beginning of this chapter.) Channels whose permeability varies with
the electrical potential across the mem­
brane are said to be electrically gated or
Simple and Facilitated Diffusion voltage gated. A change in the membrane
potential can result in a change in the con­
Diffusion is a passive mechanism. It is formation or configuration of the mem­
simply the distribution of a substance in a brane protein, which results in opening or
solvent medium, usually water, so that it closing of the channel and the subsequent