Page 55 - Anatomy and Physiology of Farm Animals, 8th Edition

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and absorption of food, as is discussed in protein to change its shape so that the Na
+
and K are moved to the other side of the
subsequent chapters.
+
VetBooks.ir membrane. There the ions are released,
and the protein returns to its original shape.
Active Transport These movements are summarized in
+
+
+
Figure 2‐14. The Na –K pump, or Na –
Some molecules and ions can move across K –ATPase, is a component of the mem­
+
cell membranes (either in or out of cells) brane of all cells, and it is always arranged in
against concentration or electrical gradients. the membrane so that Na moves out of the
+
The term against, as used here, means that cell and K moves into the cell. The contin­
+
the particles are moving in the direction uous operation of this transport system is a
opposite that of diffusion. This movement major factor in keeping the intracellular
across the cell membrane consumes energy concentration of Na relatively low in all
+
produced by the cell and is called active cells, while the intracellular concentration of
transport. K is relatively high in all cells.
+
The best‐recognized example of a primary Secondary active transport also requires
active transport system is the sodium– a membrane protein carrier and cellular
potassium (Na –K ) pump. This pump is energy, but the carrier proteins are not
+
+
actually a membrane protein that is also an ATPases (enzymes that can use ATP directly).
enzyme. The protein reversibly binds three The uptake of glucose from the lumen of the
Na and two K ions. The enzymatic activity intestine and the lumen of renal tubules by
+
+
of this protein permits it to hydrolyze ATP epithelial cells is an example of secondary
to gain energy. The gain of energy causes the active transport. In both types of epithelial
K +
(A) Extracellular (B) Na + K + Extracellular

ATP ~P

Na + Intracellular ADP Intracellular

(D) Extracellular (C) Extracellular

K +

ATP
ATP K + Intracellular ~P Intracellular

Figure 2-14. A schematic representation of the hypothetical operation of the Na –K pump, Na –K ‐
+
+
+
+
ATPase. The Na icon represents three sodium ions and the K icon represents two potassium ions.
+
+