Transport by Simple Diffusion
Water, oxygen, carbon dioxide, ethanol and urea are examples of molecules that readily cross cell membranes by simple diffusion. They pass either directly through the lipid bilayer or through pores created by certain integral membrane proteins. The relative rate of diffusion is roughly proportional to the concentration gradient across the membrane. For example, oxygen concentrations are always higher outside than inside the cell and oxygen therefore diffuses down its concentration gradient into the cell; the opposite is true for carbon dioxide.
The rate limiting step for simple diffusion across a cell membrane is movement of the molecule from the aqueous environment outside or inside the cell into the lipid bilayer of the membrane. Rate of transport for a particular molecule is therefore proportional to the lipid solubility or hydrophobicity of that molecule. Oxygen, carbon dioxide and ethanol are highly lipid soluble and therefore diffuse across the bilayer almost as if it were not there.
So what about water? We know that water crosses cell membranes very readily - the amount of water that diffuses back and forth across the membrane of the red blood cell every second is roughly equivalent to 100 times the volume of the whole cell. Water, however, is not at all lipid soluble (almost by definition) and is polar. However, it seems that water is small enough and has sufficient kinetic energy that it can diffuse through the lipid bilayer with minimal interference. Certain other small, uncharged, hydrophilic molecules, urea being an example, diffuse across lipid bilayers with relative ease.
Ions and charged molecules diffuse cross the lipid bilayer of cell membranes very, very poorly. Their charge, either positive or negative, causes them to be repelled from like charges in the cell membrane. Additionally, their charge causes them to electrically bind water molecules, causing them to be hydrated and effectively quite large.
To summarize, many small and uncharged molecules diffuse across the plasma membrane by virtue of their kinetic energy of motion. In most cases, such molecules are hydrophobic, nonpolar and can dissolve in and out of membrane lipid. Bearing a charge or having a large size virtually abolishes an ability to be transported across lipid membranes by simple diffusion, but do not fear - membranes have other means to facilitate such transport.
Next Topic for Passive Transport: Facilitated diffusion
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Last updated on May 21, 1997
Comments: rbowen@lamar.colostate.edu