A. Passive Transport Passive transport involves the movement of a molecule down a concentration gradient or electrochemical gradient. When a substance moves in this direction, it uses its own kinetic energy which is intrinsic to the particles that are moving. There is no input energy from the cell.
2 types of Passive Transport:
1) Diffusion : The passive movement of molecules down their concentration gradient or electrochemical gradient. This occurs until equilibrium is reached on either side of the membrane. No cellular input energy is required. The greater the concentration gradient, the greater the rate of diffusion. a) Simple Diffusion: The passive process in which substances move freely through the lipid bilayer of cell plasma membranes without the help of membrane proteins. Simple diffusion is important for the movement of CO2 and O2 between blood and body cells. Examples include: b) Facilitated Diffusion: Substances too polar, hydrophilic/lipophobic or highly charged move through the lipid bilayer by facilitated diffusion. In this case, an integral protein assists the substance across the membrane (either an ion channel or carrier). Most ion channels are specific to the ions. Some channels are “Gated” which means they change shape in one way to open the channel and another to close the channel. Examples include:
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