increased, which makes the electrical driving force stronger Electrochemical Driving Forces Total force acting on the ions is the combination of these chemical and electrical driving forces, referred to as the electrochemical driving force o Also can be used in reference to un- charged molecules, which are not influenced by electrical driving forces; in this case, the electrochemical driving force is synonymous with the chemical driving force 4.2 Rate of Transport Rate of a metabolic reaction is important because reactions must proceed at a rate fast enough to meet the body’s metabolic demands o In pulmonary edema (excess fluid in the lungs), diffusion of oxygen (and of carbon dioxide) is impeded o Under these conditions, a person has difficulty exercising because oxygen cannot move across the pulmonary epithelium fast enough o If the condition is severe, oxygen delivery may be impaired so much that the individual feels breathless even at rest 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, which is called the flux o Usually expressed in units of moles per second, or some equivalent o Flux is the number of molecules crossing a membrane per unit time per unit membrane surface area o For simplicity, we omit area from our definition When we say that molecules are transported in one direction or the other (into or out of a cell, for instance), we are referring to a net movement of molecules across the membrane (similar to the net direction of a chemical reaction)
No net transfer of molecules from one side to the other occurs, the concentrations do not change, and the system is in diffusional equilibrium Rate of this movement is the net flux o Molecules are being transported passively or actively in a certain direction, they are always referring to the direction of the net flux 4.3 Passive Transport The Basis for Simple Diffusion Simple diffusion is the passive transport of molecules through a biological membrane’s lipid bilayer, but in fact the mechanism of simple diffusion is not strictly biological Thermal motion is often called random thermal motion because individual molecules move helter-skelter in many different directions because of collisions with other molecules Molecules do not go in any particular direction for long before changing course, and when this change occurs the direction is unpredictable Individual molecules, which move randomly, and a population of molecules, which always moves down its concentration gradient Factors Affecting Rates of Simple Diffusion When a substance is transported passively across a membrane by simple diffusion, the rate at which it is transported depends on three factors: the magnitude of the driving force, the membrane surface area, and the permeability of the membrane, a measure of the ease with which molecules are able to move through it The Magnitude of the Driving Force
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- Fall '08
- Electric charge, Cell Membrane Transport, Ion channel