Page 56 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy and Physiology of the Cell / 41

cells, a membrane protein that acts as a car­ −100 mV. In many nerve and muscle cells,
it is about −85 mV. This means that the
rier is found in the portion of cell membrane
VetBooks.ir facing the lumen. This protein is capable of inside of the membrane is 85 mV more
negative than the outside.
binding Na and glucose simultaneously
+
when both are in the lumen. After both Na The concentrations of various cations
+
and glucose are bound, the protein changes and anions throughout the intracellular and
its shape so that both are moved to the oppo­ extracellular fluids are maintained relatively
site side of the membrane. There they are constant in normal, healthy animals. As a
released into the interior of the cell. This result, two features of the membrane pri­
transport can move glucose against its con­ marily determine the magnitude of the
centration gradient because of the potential membrane potential. These are the transport
energy of the concentration gradient for Na . mechanisms available to move cations and
+
Recall that the low intracellular Na concen­ anions across the membrane and permea­
+
tration in all cells is maintained by the con­ bility of the membrane to the different ions.
tinuous operation of the Na –K –ATPase. Recall that all cell membranes contain
+
+
Thus, ATP is used directly in maintenance of the Na –K pump or Na –K –ATPase sys­
+
+
+
+
the low intracellular concentration of Na , tem (Fig. 2‐14). The net effect of the Na –
+
+
and this energy is used indirectly or second­ K –ATPase system is constant movement
+
arily to transport glucose. of Na out of the cell and K into the cell.
+
+
An important characteristic of primary The system actually moves out three Na
+
and secondary active transport systems is ions for every two K ions that move in. This
+
their degree of specificity. In most cases, a difference contributes to the net negative
given transport protein transports only charge found on the inside of the membrane.
specific ions or molecules. For example, In resting conditions, cell membranes
the Na –K pump transports only Na and are relatively impermeable to Na (and
+
+
+
+
K . Other electrolytes are not transported proteins, which tend to be anionic) but are
+
by this system. quite permeable to K . Even though some
+
Na tends to leak back into the cell down
+
its concentration gradient, the relatively
Membrane Potentials low membrane permeability to Na and the
+
and Excitable Cells continuous operation of the Na –K pump
+
+
maintain the intracellular concentration
Resting Membrane Potential of Na (10 mEq/L) less than that in the
+
extracellular fluid (140 mEq/L).
There is a relatively small difference in the In contrast, the intracellular potas­
amounts of charged ions locally on the sium concentration (140 mEq/L) is much
opposite sides of the outer cell membrane greater than its extracellular concentration
of all animal cells. In most conditions (i.e., (5 mEq/L). Because the cell membrane is
resting conditions), the outside of the cell quite permeable to K , it can freely diffuse
+
membrane has a small excess of positive out of the cell down the concentration
ions (cations), and the inside of the cell gradient. This exit of a positively charged
membrane has a small excess of negative cation is a major contributor to the relative
ions (anions). The excess negative and excess of negatively charged ions on the
positive charges tend to attract each other, inside of the membrane. The importance
so they line up on each side of the mem­ of this exit of K is illustrated by the
+
brane, creating an electrical potential effects of changes in the concentration
across the membrane. The measurable of extracellular K on the electrical
+
voltage difference across the membrane activity of the heart. Abnormal increases
is the membrane potential (Fig. 2‐11). The in extracellular K concentration, such
+
size of the membrane potential varies among as may occur with kidney disease, are
types of cells from −10 mV (millivolts) to often associated with abnormal electrical