CHAPTER 7.4 & 7.5 ACTIVE TRANSPORT ENDOCYTOSIS & EXOCYTOSIS

Active transport: movement of substance across a biological membrane against its concentration or electrochemical gradient with the help of energy. Enables a cell to maintain internal concentration of small molecules that differ from concentrations in its environment. ATP supplies the energy for most active transport. It powers active transport is by transferring its terminal phosphate group directly to the transport protein. One transport system that works this way is the sodium-potassium pump. ACTIVE TRANSPORT

All cells have voltage across membranes. Voltage is electrical potential energy - a separation of opposite changes. The cytoplasm of a cell is negative in charge compared to the extracellular fluid because of an unequal distribution of anions and cations on opposite sides of the membrane. The membrane potential acts like a battery, an energy source that affects the traffic of all charged substances across the membrane. Combination of forces acting on an ion is call electrochemical gradient. Gdkje6pg Gdkje6pg A transport protein that generates voltage across a membrane is called electrogenic pump. The main electrogenic pump of plants, fungi, and bacteria is a proton pump. MAINTENANCE OF MEMBRANE POTENTIAL

A single ATP-powered pump that transport a specific solute can indirectly drive the active transport of several other solutes in a mechanism called cotransport. E5t79nh4 E5t79nh4 0Oe3wJM 0Oe3wJM COTRANSPORT: COUPLES TRANSPORT BY A MEMBRANE PROTEIN

The cell secretes macromolecules by the fusion of vesicles with the plasma membrane; this is called exocytosis. A transport vesicle that has budded from the Golgi apparatus moves along microtubules of the cytoskeleton to the plasma membrane. Many secretory cells use exocytosis to export their products. In endocytosis, the cell takes in macromolecules and particulate matter by forming new vesicles from the plasma membrane. :// Q saHw EXOCYTOSIS VS. ENDOCYTOSIS