Re: What is the effect of temperature on membrane integrity?

1.  Example:  Membranes are composed of lipids.  Lipids in everyday terms 
are simply fat.  Lipids compose the bulk of membrane mass.  So, what 
happens when you heat fat up?  Think of frying bacon or sausage or of 
melting butter.  Each of those is either fat or very hit in fat.  When you 
heat them they melt.  Likewise with a membrane.  When it melts, it loses 
its structure.

There are other factors also.  

2.  Membranes obviously have lots of proteins in them also.  Lipids are 
small molecules and are generally stable over a wide range of temperatures 
(you can melt butter, then cool it, then melt it again, etc.).  However, 
proteins are much less stable over a range of temperatures.  That's one 
reason that many organisms live only at certain ranges of temperature and 
why organisms like us maintain a constant body temperature, so proteins 
will be stable.  Instead of melting and remelting butter, think of frying 
an egg,  the white of the egg is pretty much pure protein.  You can't fry 
it, make it liquid again, and refry it.

3.  In addition to proteins, the membrane is in part "held" together by 
interaction of proteins and lipids in the membrane with the cell's 
cytoskeleton (microtubules and filaments).  These fall apart also as 
temperature is raised.  It's like a bridge.  The bridge's roadway is 
reasonably stable by itself and you can think of it as a membrane, but all 
those cross pieces beneath it are the cytoskeleton.  What happens to the 
bridge if all those crosspieces are taken away or weakened?  At some point 
the bridge will collapse.

4.  Why the membrane becomes permeable to many things as temperature 
increases is because it is falling apart.  There's no semi-rigid structure 
packed tightly together like there is in an intact membrane.  Consider a 
stick of butter.  Try blending in sugar or flour.  It's hard and basically 
can't be done without physically messing up the stick of butter.  Now melt 
the butter, much easier to mix in flour or sugar.

5.  If all of the above is true, then how do some organisms manage to live 
a very high temperatures?  For example, there are lots of kinds of 
bacteria (called thermophiles or hyperthermophiles) that live in hot 
springs where the temperature may reach boiling.  They survive.  They 
survive for (at least) two reasons.  First their proteins have adapted and 
are stable at high temperatures.  We don't fully understand what makes a 
protein stable at a high temperature.  But in addition, the membranes of 
cells that live at high temperature have different kinds of lipids.  Your 
and my cell have small lipids that easily melt.  Think of comparing olive 
oil and butter.  Olive oil is liquid at room temperature whereas butter is 
not.  Both are lipid.  Thermophiles have lipids that tend to be larger and 
most times for very long chains and usually for cyclic molecules.  These 
are much more stable at high temperature (but wouldn't work well at our 
temperatures because they'd be like butter and quite solid).  If you want 
to learn more about these kinds of organisms, do a search on the Internet 
for Carl Woese (USA), Karl Stetter (Germany) or Thomas Brock (USA).  
Alternatively search for the combination of "bacteria" and "Yellowstone" 
(National Park, USA)

6.  The following URLs might be of help. http://esg-www.mit.edu:8001/esgbio/7001main.html
       A hypertextbook on the web from Mass. Inst. Tech. http://www.life.uiuc.edu/crofts/bioph354/index.html
       A Biophysics course about cellular energy metabolism.  Has a lot of 
membrane material scattered throughout.  Chapter 11 has a molecular model 
of a lipid bilayer (membrane) http://www.life.uiuc.edu/crofts/bioph354/bergman/eanfang.htm
        This is a link from the above web site to a site in Germany on 
structures of membrane proteins.