Alcohol and Yeast cells

[patzi]

Can alcohol get into yeast cells? if so how does it does this, is the yeast cell wall permeable to it??

[NetSquirrel]

Ethanol (the type of alcohol that people drink) is actually made inside yeast cells, and then diffuses out through the cell membrane and cell wall into the surrounding (liquid) medium.  Yeast can respire aerobically (with oxygen) and also anaerobically (without oxygen) but the latter yields less energy and is therefore not preferred.  Ethanol is a waste product of this anaerobic respiration and is highly toxic to yeast (it inhibits certain vital enzymes), so it slowly poisons itself under these conditions.  Ethanol will diffuse out of the cells if the ethanol concentration inside them exceeds that of outside.  Yeast cells die if the concentration of ethanol within them (i.e. outside them as well) exceeds about 14%.

P.S. It is unsurprising that yeast's cell wall is permeable to a small molecule like ethanol, as the wall is there primarily to give the cell support and rigidity.  There is a diagram of the yeast cell surface membrane and cell wall here http://bio71.gba.insa-tlse.fr/eurocellwall/wall.html

And a skeletal diagram of the long chitin molecules that make up the wall can be found here: http://www.psrc.usm.edu/macrog/sea/images/chitin1.gif

Compare that to cellulose (found in plant cell walls) http://www.psrc.usm.edu/macrog/images/cell05.gif and you can see that both are long, thin polymers that leave large "gaps" when performing their structural role.  The gaps are big enough for small molecules to fit through.  http://www.biologie.uni-hamburg.de/b-online/fo26/03.jpg