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Chapter 5

Chapter 5 - Active Transport, Phagocytosis, Endocytosis, Exocytosis.docx

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Matthew Smith

Active Transport, Exocytosis and Endocytosis Active Transport  Active transport requires a direct or indirect input of energy derived from ATP hydrolysis  Moves substances against their concentration gradients; requires cells to expend energy  Depends on membrane transport proteins  Specific for certain substances  Can be saturated  Two kinds of Active transport o Primary active transport  The same protein that transports substance also hydrolyses ATP to power transport directly o Secondary active transport  Transport indirectly driven by ATP hydrolysis  Transport proteins do not break down ATP  Instead use a favorable concentration gradient of substance, built up by primary active transport, as their energy source Primary Active Transport  Moves positively charged ions across membrane o H+ pumps (proton pumps)  Cells lining stomach  In the plasma membrane pushes hydrogen ions from the cytoplasm to the cellular exterior  Temporarily bind a phosphate group from ATP during the pumping cycle o Ca2+ pump  Maintain low intracellular Ca2+ concentration  Pushes Ca2+ from the cytoplasm to the cell exterior and from the cytosol into the vesicles of the endoplasmic reticulum.  Ca2+ is typically high outside the cells and inside ER vesicles.  A regulatory control of cellular activities as diverse ass secretion, microtubule assembly, and muscle concentration o Na+/K+ pump  3 Na+ out, 2 K+ in for every pump cycle  creates negative membrane potential  electrochemical gradient across membrane Secondary Active Transport  uses the concentration gradient of an ion established by a primary pump as their energy source.  Occurs by two mechanisms: o Symport  Cotransported solute moves through membrane channel in same direction as driving ion. A phenomenon known as cotransport.  Sugars such as glucose and amino acids are examples of molecules actively transported into cells by symport. o Antiport  Driving ion moves through membrane channel in one direction, providing energy for active transport
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