Transport through the Cell Membrane Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 30, 2011

Active Transport Systems

Active transport systems actively split ATP, unleashing free energy to power the transportation process.

Active transport, itself, is the ATP-requiring active pumping of particles from an area where their concentration is low, to an area where their concentration is high. Like facilitated diffusion, active transport uses a protein carrier molecule within the plasma membrane. Unlike facilitated diffusion, however, active transport runs “uphill” (energetically speaking), in that particles are moved from an area of low concentration “up” to an area of high concentration.

For example, sodium (Na + ) ions are removed from the intracellular fluid of many human and animal cells by an active transport system or “ATP pump.” Sodium is at a much higher concentration in the extracellular fluid, compared to the intracellular fluid. Sodium ions rapidly pass through the plasma membrane and enter the intracellular fluid of nerve cells when a person is stimulated or excited. A sodium “ATP pump,” consisting of a special carrier protein in the membrane, combines with the Na + ions that leaked into the cell, then splits ATP. This ATP-splitting generates enough free energy to actively carry Na + particles from the intracellular fluid of the nerve cells, back out into the extracellular fluid. A resting or nonstimulated state of the nerve cell membrane is thus re-established.


Practice problems for these concepts can be found at: Cells: The “Little Chambers” In Plants And Animals Test

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