Why do lipids in a cell hydrolyze into glycerol and fatty acids?

Question

Most Lipids in a cell have ester bonds (made up of glycerol and fatty acids).

I have learned in my chemistry class that we can hydrolyze an ester by simply adding water to it.

$$\ce{CH3COOCH2CH3 + H2O <=>[H^+ (aq)] CH3COOH + EtOH}$$

Then why do lipids which have an ester bond don't hydrolyze inside a cell which is 70 percent water? While searching throught web I have found this link which explains of different ways to hydrolyze an ester. In that it explains about how to hydrolyze esters using dilute acids like HCl. In our stomach HCl is present so logically HCl should destroy the ester in stomach lining (which are made up of cells) instantaneonusly, but this is not happening. Why?

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Answer 1

Organisms control rates of chemical reactions in two ways. First, by controlling the conditions, and second, by controlling the location of reagents.

Conditions

Hydrolysis of esters is very slow at normal physiological conditions: 37 ^oC and pH 7.4. A mixture of water and simple esters like ethyl acetate remains stable for years. At higher or lower pH, the reaction becomes faster. As you mention, the HCl in the stomach would definitely increase the rate of hydrolysis. The reaction is also accelerated by enzymes, and organisms use enzymes for all intentional ester hydrolysis.

Location

For a reaction to occur, the water molecule and the acid catalyst needs to get to the ester. In cell membranes, that ester functional group is not on the surface of the membrance; the ionic phosphate part is. The ester group is just below the surface, in the hydrophobic interior of the membrane. Neither water, nor acid can penetrate into this hydrophobic region.

In the stomach, there is an extra layer of protection: mucus, which prevents the stomach acid from reaching the cells.

Answer 2

Stomach wall are made of mucous membrane, that secrets mucus like crazy. Mucous is made of glycoproteins -- sugar+protein complex, that makes diffusion slower, and protein structure make it acid hydrolysis slow. The mucous membrane cells maintains normal physiological pH, and while they are isolated from stomach acid by mucus they secret, they are fine, as hydrolysis is very slow without catalyst.

here is wikipedia article on the question http://en.wikipedia.org/wiki/Gastric_mucosal_barrier

The barrier may be broken by some chemical agents or if mucus secreting cells are suppressed by illness. In latter case stomach damages itself routinely and peptic ulcer decease occurs.