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Yue Li

CELL MEMBRANE TRANSPORT Factors Affecting the Direction of Transport PASSIVE TRANSPORT VS. ACTIVE TRANSPORT  Active transport: transport across a membrane requiring energy; Passive transport: without the use of energy.  Simple diffusion: passive transport, uses thermal motion to move molecules into or out of cells;  Pumps: transport proteins used in active transport.  Spontaneous (passive; down the concentration gradient) movement from high to low energy; the opposite requires an input of energy. Energy increases as solute concentration increases. DRIVING FORCES ACTING ON MOLECULES  Any differences in energy existing across a membrane acts as a driving force that tends to push molecules in one direction or another (higher to lower energy).  CHEMICAL DRIVING FORCES o When a substance is present in different concentrations on either side of a membrane, a concentration gradient exists. The rate at which concentration changes with distance. o Refer to concentration gradient as a chemical driving force, the direction is down the concentration gradient. Magnitude increases when concentration increases.  ELECTRICAL DRIVING FORCES o Electrical driving forces in addition to chemical driving forces influence ions. o They arise due to the membrane potential, a difference in electrical potential or voltage that exists across the membrane. MEMBRANE POTENTIAL o In intra. or extra. fluid, cations and anions are unequal, therefore not electrically neutral. o Intra. fluids have more anions than cations (negative charge). Extra. fluids contain more cations than anions (positive charge). Separation of charge across the membrane. o The greater the difference in charge between the two sides of a membrane, the larger the membrane potential. o By convention sign of membrane potential is taken to be the sign of the net charge inside the cell relative to outside. HOW THE MEMBRANE POTENTIAL CREATES AN ELECTRICAL DRIVING FORCE THAT ACTS ON IONS o Electrical force has the potential to cause charged particles to move. FACTORS AFFECTING THE DIRECTION AND MAGNITUDE OF THE ELECTRICAL DRIVING FORCE o The separation of charge is a potential energy for current to flow across the membrane. o The direction of the electrical driving force on a positively charged ion is inward. On a negatively charged ion is outward. o The magnitude of the electrical driving force on an ion depends on the size of the membrane potential and the quantity of the charge carried by the ion. ELECTROCHEMICAL DRIVING FORCES o The direction of the electrochemical driving force acting on an ion depends on the net direction of the electrical chemical driving forces. o Equilibrium potential: a hypothetical value for the membrane potential at which the electrical driving force is equal and opposite to the chemical driving force, producing an electrochemical driving force of zero. Rate of Transport  The rate at which a substance is transported across a membrane refers to the number of molecules that cross the membrane in a given length of time (flux).  The number of molecules crossing the membrane per unit time in just one direction is called the one-way flux (unidirectional flux).  When concentrations do not change, the system is diffusional equilibrium. CELL MEMBRANE TRANSPORT Passive Transport SIMPLE DIFFUSION: PASSIVE TRANSPORT THROUGH THE LIPID BILAYER  THE BASIS FOR SIMPLE DIFFUSION o Diffusion: movement of molecules from one location to another by their thermal motion. o Thermal motion is called random thermal motion because individual molecules move in various directions because of collisions with other molecules. o Individual molecules move randomly, population of molecules move down its concentration gradient. THE MAGNITUDE OF THE DRIVING FORCE o In simple diffusion, the net flux of a substance is directly proportional to the size of the concentration gradient. MEMBRANE SURFACE AREA o The rate at which molecules are transported across a membrane varies in direct proportion to the membrane’s surface area. MEMBRANE PERMEABILITY o Depends on the nature of the transported substance and the various properties of the membrane that influence the ease with which molecules are able to penetrate it. o Lipid solubility of the diffusing substance: hydrophobic substances are the most lipid soluble; they enter the bilayer more readily. o The size and shape of diffusing molecules: larger molecules and those with more irregular shapes move through the bilayer more slowly. o Temperature: molecules move faster at higher temperatures; relatively constant. o Membrane thickness: a thicker membrane has a lower permeability. FACILITATED DIFFUSION: PASSIVE TRANSPORT THROUGH MEMBRANE PROTEINS  Mediated transport: molecules passively transported through proteins; facilitated diffusion.  CARRIERS IN FACILITATED DIFFUSION o Carrier: transmembrane protein that binds molecules on one side of a membrane and transport them to the other side by conformational change; have binding site(s) that are specific to molecules. o The net flux of facilitated diffusion depends on the frequency of solute binding to the carrier molecule (down the concentration gradient); net transport occurs from the side with greater frequency of binding. o Two factors that affect the binding of a solute to a carrier: affinity of binding site on the carrier, and concentration (electrochemical) gradient. Affinity is the same in intra. and extra fluid. Difference in binding is due to concentration gradient.  FACTORS AFFECTING THE RATE OF FACILITATED DIFFUSION o The rate is determined by: 1. Transport rate of individual carriers 2. Number of carriers
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