Page 53 - Anatomy and Physiology of Farm Animals, 8th Edition

38 / Anatomy and Physiology of Farm Animals

VetBooks.ir M M

M

Figure 2-12. Facilitated diffusion of solute (labeled M) across a cell membrane by the action of a
transmembrane carrier protein.

electrochemical gradient is used to refer pancreas. Insulin facilitates the entry of
to the combined effects of the concentra­ glucose into skeletal muscle, in part by
tion gradient and electrical gradient on the increasing the number of carrier proteins
diffusion rate of an individual ion. in the cell membrane of these cells.
Facilitated diffusion is the same as
simple or free diffusion in that it operates Osmosis
passively down the concentration or elec­
trochemical gradient. However, facilitated Osmosis is movement of water across
diffusion requires a carrier system in the membranes. Like many solutes, water does
membrane to assist the crossing. The carrier not diffuse freely through the lipid bilayer
system is a transmembrane protein that of cell membranes but rather must diffuse
binds the diffusing molecule or molecules through water channels formed by trans­
on one side of the membrane and then membrane proteins. These proteins are
transfers them to the other side, where aquaporins. If the intracellular fluid
the transported molecules are released within a cell has a higher concentration of
(Fig. 2‐12). The movement or transport undiffusible solutes than the interstitial
across the membrane probably entails a fluid bathing the cell, water will move into
change in the shape of the protein, but it the cell from the interstitial fluid until the
does not require any direct use of ATP concentrations are the same on both sides
for energy as in active transport. of the membrane. As the water moves in,
Sugars, especially glucose, depend on the volume in the cell increases. The driving
facilitated diffusion to enter cells by joining force moving water from the solution on
with carrier proteins upon reaching the the side of the lower solute concentration
lipid bilayer of the membrane. In many cell to the side with the higher solute concen­
types, a glucose carrier complex transports tration is osmotic pressure.
glucose down the glucose concentration The osmotic pressure of an aqueous solu­
gradient to the inside of the cell membrane. tion can be measured by using a U‐tube in
Here the carrier releases the glucose to which the two sides of the tube are sepa­
enter the cell. The carrier remains in the rated by a membrane which is permeable
membrane and reconfigures itself so that only to water (Fig. 2‐13). A solution with a
it is available for more transport. Other high solute concentration is placed in one
substances besides glucose, such as amino side of the U‐tube, and distilled water is
acids, also depend on facilitated diffusion placed in the other. Hydrostatic pressure is
to cross cell membranes. the pressure generated by the fluid as a result
The rate at which facilitated diffusion of the force of gravity. The force of osmosis
occurs also depends on the number of moves water through the membrane from
carrier proteins available in the membrane. the side containing distilled water to the side
In the case of glucose, the speed of entry containing the solution. This movement
into many cells, such as skeletal muscle, is continues until the hydrostatic pressure
greatly increased by the hormone insulin, generated by the increased height of the
which is produced and secreted by the fluid column on the solution side is equal