Active Transport

Active transport differs from facilitated diffusion in that it uses energy to move a substance uphill across a membrane—that is, against the substance's electrochemical gradient (Figure 6-10). As with facilitated diffusion, active transport requires binding of a substance to the transporter in the membrane. Because these transporters move the substance uphill, they are often referred to as "pumps." As with facilitated-diffusion transporters, active-transport transporters exhibit specificity and sat-uration—that is, the flux via the transporter is maximal when all transporter binding sites are saturated.

Low concentration

High concentration

Membrane

High concentration

Membrane

Diffusion

Diffusion

Facilitated diffusion

Facilitated diffusion

Active transport

FIGURE 6-10

Direction of net solute flux crossing a membrane by: (1) diffusion (high to low concentration), (2) facilitated diffusion (high to low concentration), and active transport (low to high concentration).

Active transport

FIGURE 6-10

Direction of net solute flux crossing a membrane by: (1) diffusion (high to low concentration), (2) facilitated diffusion (high to low concentration), and active transport (low to high concentration).

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

I. Basic Cell Functions

6. Movement of Molecules Across Cell Membranes

© The McGraw-Hill Companies, 2001

Movement of Molecules Across Cell Membranes CHAPTER SIX

The net movement from lower to higher concentration and the maintenance of a higher steady-state concentration on one side of a membrane can be achieved only by the continuous input of energy into the active-transport process. This energy can (1) alter the affinity of the binding site on the transporter such that it has a higher affinity when facing one side of the membrane than when facing the other side; or (2) alter the rates at which the binding site on the transporter is shifted from one surface to the other.

To repeat, in order to move molecules from a lower concentration (lower energy state) to a higher concentration (higher energy state), energy must be added. Therefore, active transport must be coupled to the simultaneous flow of some energy source from a higher energy level to a lower energy level. Two means of coupling an energy flow to transporters are known: (1) the direct use of ATP in primary active transport, and (2) the use of an ion concentration difference across a membrane to drive the process in secondary active transport.

Primary Active Transport The hydrolysis of ATP by a transporter provides the energy for primary active transport. The transporter is an enzyme (an ATPase) that catalyzes the breakdown of ATP and, in the process, phosphorylates itself. Phosphorylation of the transporter protein (covalent modulation) changes the affinity of the transporter's solute binding site. Figure 6-11 illustrates the sequence of events leading to the active transport (that is, transport from low to higher concentration) of a solute into a cell. (1) Initially, the binding site for the transported solute is exposed to the extracellular fluid and has a high affinity because the protein has been phosphorylated on its intracellular surface by ATP. This phosphorylation occurs only when the transporter is in the conformation shown on the left side of the figure. (2) The transported solute in the extracellular fluid binds to the high-affinity binding site. Random thermal oscillations repeatedly expose the binding site to one side of the membrane, then to the other, independent of the protein's phosphorylation. (3) Removal of the phosphate group from the transporter decreases the affinity of the binding site, leading to (4) the release of the transported solute into the intracellular fluid. When the low-affinity site is returned to the extracellular face of the membrane by the random oscillation of the transporter (5), it is in a conformation which again permits phosphorylation, and the cycle can be repeated.

To see why this will lead to movement from low to higher concentration (that is, uphill movement), consider the flow of solute through the transporter at a point in time when the concentration is equal on the two sides of the membrane. More solute will be bound to the high-affinity site at the extracellular surface of the membrane than to the low-affinity site on the in-tracellular surface. Thus more solute will move in than out when the transporter oscillates between sides.

The major primary active-transport proteins found in most cells are (1) Na,K-ATPase; (2) Ca-ATPase; (3) H-ATPase; and (4) H,K-ATPase.

Na,K-ATPase is present in all plasma membranes. The pumping activity of this primary active-transport protein leads to the characteristic distribution of high intracellular potassium and low intracellular sodium

Transporter ^^ protein

Transported solute

Transporter ^^ protein

Transported solute

Intracellular fluid

Binding site

Intracellular fluid

Extracellular fluid

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  • manuela
    What active transport function in the body?
    8 years ago

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