4.2 Mediated-Transport Systems

Active Transport

Energy-requiring system that uses transporters to move ions or molecules across a membrane against an electrochemical difference

Cotransport

Form of secondary active transport in which net movement of actively transported substance and “downhill” movement of molecule supplying the energy are in the same direction

Countertransport

Form of secondary active transport in which net movement of actively transported molecule is in direction opposite “downhill” movement of molecule supplying the energy

Facilitated Diffusion

System using a transporter to move molecules from high to low concentration across a membrane; energy not required

Mediated Transport

Movement of molecules across membrane by binding to protein transporter; characterized by specificity, competition, and saturation; includes facilitated diffusion and active transport

Na+/K+-ATPase Pump

Primary active-transport protein that hydrolyzes ATP and releases energy used to transport sodium ions out of cell and potassium ions in

Primary Active Transport

Active transport in which chemical energy is transferred directly from ATP to transporter protein

Secondary Active Transport

Active transport in which energy released during transmembrane movement of one substance from higher to lower concentration is transferred to the simultaneous movement of another substance from lower to higher concentration

Transporters

Integral membrane proteins that mediate passage of molecules through membrane