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
- BRINN
- Electric charge, Cell Membrane Transport, Ion channel