PASSIVE FORCES & ACTIVE TRANSPORT

A. Diffusion (concentration gradient force)

1. statistical "force", based on tendency of any substance to diffuse from regions where its concentration is high to regions where its concentration is low (depends on absolute temperature)

2. diffusion force in membrane transport is proportional to the concentration difference between the two sides of the membrane; e.g., for a cell

DC = Co - Ci

where
Ci = intracellular concentration
Co = extracellular (outside) concentration

3. Each substance diffuses according to its own concentration difference at a rate dependent on its own permeability

B. Electrical Force

1. based on the force on charged particles in an electric field

2. in membrane transport, electrical force depends on particle charge and membrane electrical potential, e.g.

Force ~ particle charge x Vm (membrane potential)

3. Positively charged particles are attracted to the negative side of the membrane, while negative particles are attracted to the positive side of the membrane (opposite charges attract, like charges repel)

Note: The combined effect of the concentration and electrical differences or gradients is called the electrochemical gradient

C. Active Transport

1. Define: Movement of particles coupled to a metabolic energy supply, frequently ATP (adenosine triphosphate), which is degraded to ADP (adenosine diphosphate) in the process)

2. Similar characteristics and mechanism as Facilitated Diffusion

except

3. May transport particles against an electrochemical gradient