Carrier Mediated Transport

The processes of membrane transport described up to this point do not necessarily require a cell membrane; they can occur just as well through artificial membranes. We now, however, come to processes for which a cell membrane is essential, because they employ transport proteins to get through the membrane. Thus, the next two processes are cases of carrier-mediated transport.

The carriers act like enzymes in some ways: The solute is a ligand that binds to a specific receptor site on the carrier, like a substrate binding to the active site of an enzyme. The carrier exhibits specificity for a certain lig-and, just as an enzyme does for its substrate. A glucose carrier, for example, cannot transport fructose. Carriers also exhibit saturation; as the solute concentration rises, its rate of transport through a membrane increases, but only up to a point. When every carrier is occupied, adding more solute cannot make the process go any faster. The carriers are saturated—no more are available to handle the increased demand, and transport levels off at a rate called the transport maximum (Tm) (fig. 3.17). As we'll see later

Transport Maximum Urinary System
Figure 3.17 Saturation of a Membrane Carrier. Up to a point, increasing the solute concentration increases the rate of transport through a membrane. At the transport maximum (Tm), however, all carrier proteins are busy and cannot transport the solute any faster, even if more solute is added.

in the book, the transport maximum explains why glucose appears in the urine of people with diabetes mellitus. An important difference between a membrane carrier and an enzyme is that carriers do not chemically change their lig-ands; they simply pick them up on one side of the membrane and release them, unchanged, on the other.

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110 Part One Organization of the Body

There are three kinds of carriers: uniports, symports, and antiports. A uniport18 carries only one solute at a time. For example, most cells pump out calcium by means of a uniport, maintaining a low intracellular calcium concentration so that calcium salts don't crystallize in their cytoplasm. A symport19 carries two or more solutes through a membrane simultaneously in the same direction; this process is called cotransport.20 As an example, absorptive cells of the small intestine and kidneys take up sodium and glucose simultaneously by means of a sym-port. An antiport21 carries two or more solutes in opposite directions; this process is called countertransport. Cells everywhere have an antiport called the sodium-potassium pump that continually removes Na+ from the cell and brings in K+.

These carriers employ two mechanisms of transport called facilitated diffusion and active transport. (Any carrier type—uniport, symport, or antiport—can use either of these transport mechanisms.) Facilitated22 diffusion is the carrier-mediated transport of a solute through a membrane down its concentration gradient. It is a passive transport process; that is, it does not consume ATP. It transports solutes such as glucose that cannot pass through the membrane unaided. The solute attaches to a binding site on the carrier, then the carrier changes conformation and releases the solute on the other side of the membrane (fig. 3.18).

Active transport is the carrier-mediated transport of a solute through a membrane up its concentration gradient, using energy provided by ATP. The calcium pumps mentioned previously use active transport. Even though Ca2+ is already more concentrated in the ECF than within the cell, these carriers pump still more of it out of the cell. Active transport also enables cells to absorb amino acids that are already more concentrated in the cytoplasm than in the ECF.

A prominent example of active transport is the sodium-potassium (Na+-K+) pump, also known as Na+-K+ ATPase because the carrier is an enzyme that hydrolyzes ATP. The Na+-K+ pump binds three Na+ simultaneously on the cytoplasmic side of the membrane, releases these to the ECF, binds two K+ simultaneously from the ECF, and releases these into the cell (fig. 3.19). Each cycle of the pump consumes one ATP and exchanges three Na+ for two K+. This keeps the K+ concentration higher and the Na+ concentration lower within the cell than in the ECF. These ions continually leak through the membrane, and the Na+-K+ pump compensates like bailing out a leaky boat.

19sym = together + port = carry

22facil = easy

Extracellular fluid

Pump Theory

Cytoplasm

Figure 3.18 Facilitated Diffusion. (7) A solute particle enters the channel of a membrane protein (carrier). (2) The solute binds to a receptor site on the carrier and the carrier changes conformation. (3) The carrier releases the solute on the other side of the membrane. Note that the solute moves down its concentration gradient.

Sodium Potassium Pump

Figure 3.19 The Sodium-Potassium Pump (Na+-K+ ATPase).

In each cycle of action, this membrane carrier removes three sodium ions from the cell, brings two potassium ions into the cell, and hydrolyzes one molecule of ATP.

Why would the Na+-K+ pump, but not osmosis, cease to function after a cell dies?

Figure 3.19 The Sodium-Potassium Pump (Na+-K+ ATPase).

In each cycle of action, this membrane carrier removes three sodium ions from the cell, brings two potassium ions into the cell, and hydrolyzes one molecule of ATP.

Why would the Na+-K+ pump, but not osmosis, cease to function after a cell dies?

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Chapter 3 Cellular Form and Function 111

Lest you question the importance of the Na+-K+ pump, about half of the calories you consume each day are used for this alone. Beyond compensating for a leaky plasma membrane, the Na+-K+pump has at least four functions:

1. Regulation of cell volume. Certain anions are confined to the cell and cannot penetrate the plasma membrane. These "fixed anions," such as proteins and phosphates, attract and retain cations. If there were nothing to correct for it, the retention of these ions would cause osmotic swelling and possibly lysis of the cell. Cellular swelling, however, stimulates the Na+-K+ pumps. Since each cycle of the pump removes one ion more than it brings in, the pumps are part of a negative feedback loop that reduces ion concentration, osmolarity, and cellular swelling.

2. Secondary active transport. The Na+-K+ pump maintains a steep concentration gradient of Na+ and K+ between one side of the membrane and the other. Like water behind a dam that can be tapped to generate electricity, this gradient has a high potential energy that can drive other processes. Since Na+ has a high concentration outside the cell, it tends to diffuse back in. Some cells exploit this to move other solutes into the cell. In kidney tubules, for example, the cells have Na+-K+ pumps in the basal membrane that remove Na+ from the cytoplasm and maintain a low intracellular Na+ concentration. In the apical membrane, the cells have a facilitated diffusion carrier, the sodium-glucose transport protein (SGLT), which simultaneously binds Na+ and glucose and carries both into the cell at once (fig. 3.20). By exploiting the tendency of Na+ to diffuse down its concentration gradient into these cells, the SGLT absorbs glucose and prevents it from being wasted in the urine. The SGLT in itself does not consume ATP, but it does depend on the ATP-consuming Na+-K+ pumps at the base of the cell. We say that glucose is absorbed by secondary active transport, as opposed to the primary active transport carried out by the Na+-K+ pump.

3. Heat production. When the weather turns chilly, we not only turn up the furnace in our home but also the "furnace" in our body. Thyroid hormone stimulates cells to produce more Na+-K+ pumps. As these pumps consume ATP, they release heat, compensating for the body heat we lose to the cold air around us.

4. Maintenance of a membrane potential. All living cells have an electrical charge difference called the resting membrane potential across the plasma membrane. Like the two poles of a battery, the inside of the membrane is negatively charged and the outside is positively charged. This difference stems from the unequal distribution of ions on the two sides of the membrane, maintained by the Na+-K+ pump. The membrane potential is essential to the function of nerve and muscle cells, as we will study in later chapters.

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Responses

  • vivaldo
    Why would the Na K pump, but not osmosis, cease to function after a cell dies?
    8 years ago
  • tapio paasio
    What is it called when a membrane carrier is busy?
    8 years ago
  • simone
    When all transport carriers for a solute are occupied, the rate of transport levels off at the?
    8 years ago
  • laila
    Why would the sodium potassium pump, but not osmosis, cease to function after a cell dies?
    8 years ago
  • Hending
    When all membrane carriers are busy its called?
    8 years ago

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