why glycerol has no effect on membrane permeability?

I hope I can address your problem in a simple ways.

Qn.1) why a 40% glycerol solution has no effect on membrane permeability?

Ans. 1)
First of all, a cell membrane is a fluid mosaic of lipids, proteins, and carbohydrates. The main component is lipids, in the form of triacylglycerol in which is made by 1 glycerol bonding to 3 fatty acids and forming a lipid bilayers seperating cell from outside environment. Membrane permeability is defined as how quick a solute/molecule is diffused/penetrated throu the membrane and it is affected by molecular size of the solute (permeability generally decreases with increasing size), lipid solubility (permeability usually increases with increasing fat or oil solubility), and degree of ionization (permeability generally decreases with increased ionization).
It is known that the " average " composition of membrane 52% protein, 40% lipid, 8% carbohydrate and each lipid contain similar percentage of glycerol, and hence, your 40% glycerol solution is of similar composition of the cell membrane. Therefore, it won't affect its membrane permeability.

Qn. 2) why AgNO3 can damage the membrane and make it freely permeable?

Ans. 2) AgNO3, Silver Nitrate Silver nitrate, the mechanism of the action of
silver ions is closely related to their interaction with thiol groups although other target sites remain a possibility. The amino acids such as cysteine and other compounds such as sodium thioglycolate containing thiol groups neutralized the activity of silver nitrate. By contrast, amino acids containing disulfide (SS) bonds, non-sulfurcontaining amino acids, and sulfur-containing compounds such as cystathione, cysteic acid, L-methionine, taurine, sodium bisulfite, and sodium thiosulfate were all unable to neutralize Ag anion activity. The action of Ag anion involves hydrogen bond-breaking for thiol groups, especially for enzymes and some of the protein based ion channels.
We all know, besides lipids, there are many other structures on the surface of membrane affecting the membrane permeability including ion channels (CI anion, K cation....), ATP powered active transports... The incorporation of Ag anions to these structure will total disrupt and even deform them, and hence all the solute molecules can easily diffuse in and out through them.

2007-12-11 11:37:33 補充：
And Me M.Sc. (BICH)

very good...
but...er....isn't the Ag ion in AgNO3 a cation?