When moving across a selectively permeable membrane, ions, polar and large molecules cannot dissolve easily in the lipid membrabe. Instead, transport must occor by way of protiens that act as carriers or channels to allow such molecules to cross. There are two types of facilitated transport;1) Facilitated diffusionSubstances move down the concentration gradient from high to low. Includes protiens such as ion channel that provide a water filled pore for our hydrophillic ions or carriers for larger molecules such as glucose. Examples:-Channels: The sodium channel transports Na+- Carriers: The GLUT transporters transport glucose2) Active transportSubstances are transported against the concentration gradient from low to high. Protiens act as carriers that use energy from ATP directly (primary transport) or indirectly (co-transport via other ion gradients) to power transport.- Primary: The Na+/K+ ATPase transports 2 potassium and 3 sodium in opposite direcxtions- Secondary: the SLGT transports glucose against concentration gradient using energy released when sodium move down concentration gradient.
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