06 - Transport of ions and small molecules across the membrane Flashcards
A bilayer is permeable to small hydrophobic molecules and small uncharged polar molecules,
slightly permeable to water and urea, and essentially impermeable to ions and to large polar molecules
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If a transported substance carries a net charge, its move-ment is influenced by both its concentration gradient and the membrane potential, the electric potential (voltage) across the membrane. The combination of these two forces, called the electrochemical gradient, determines the energetically favorable direction of transport of a charged molecule across a membrane
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Transport of most molecules into and out of cells requires the assistance of specialized membrane proteins. Even transport of molecules with a relatively large partition coefficient (e.g., water and urea) is frequently accelerated by specific proteins because their transport by passive diffusion usually is not sufficiently rapid to meet cellular needs
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If a transported substance carries a net charge, its move-ment is influenced by both its concentration gradient and the membrane potential, the electric potential (voltage) across the membrane. The combination of these two forces, called the electrochemical gradient, determines the energetically fa-vorable direction of transport of a charged molecule across a membrane. The electric potential that exists across most cellular membranes results from a small imbalance in the concentration of positively and negatively charged ions on the two sides of the membrane.
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Gases, such as O2 and CO2, and small, uncharged polar mol-ecules, such as urea and ethanol, can readily move by passive (simple) diffusion across an artificial membrane composed of pure phospholipid or of phospholipid and cholesterol
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ATP-powered pumps (or simply pumps) are ATPases that use the energy of ATP hydrolysis to move ions or small molecules across a membrane against a chemical concentra-tion gradient or electric potential or both : active transport
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