Cell Membrane:
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Maintains homeostasis by controlling what substance may enter or leave cells.
Passive Transport: Substances crossing cell membrane without energy input
by the cell, from high to low concentration. “Down” the concentration
gradient
1) Diffusion (no facilitation)
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movement of molecules from high to low concentration
concentration gradient: difference in concentration of molecules across a distance
Driven by molecules’ kinetic energy
Molecules in constant motion, move randomly, in straight line
End result: causing the concentration of solution the same throughout the space =
Equilibrium
Random movement of molecules continues even in equilibrium
Diffusion across membranes = simple diffusion
- depends on size, type of molecules
- depends on the chemical nature of the membrane
- Phospho-lipid bilayer
- certain proteins can form pores in the membranes
(e.g. molecules can dissolve in lipids may pass directly thruCO2 and O2 are non-polar
and can dissolve in lipids.
(e.g. molecules that are very small, but not soluble in lipids may diffuse across the
membrane by moving thru the poles in the membrane)