MadSci Network: Cell Biology
Query:

Re: EFFECT OF TEMPERATURE ON THE PERMEABILITY OF CELL SURFACE MEMBRANE

Date: Thu Jan 17 17:41:07 2002
Posted By: Erik von Stedingk, Post-doc/Fellow, Plant and yeast biochemistry, molecular biology and physiology, Physiological Biochemistry
Area of science: Cell Biology
ID: 1011039939.Cb
Message:

Hi Clare,

Let me rephrase my previous response, hoping it will be more to your 
liking. Then I think there is a third way of understanding your question, 
which I will address here.

As long as the temperature does not go beyond what the membrane is 
supposed to withstand, the permeability of the plasma membrane (PM) should 
not be affected. There might be a higher controlled permeability for such 
compounds that are supposed to transit the PM, but it will not break down, 
spewing out red beet colour. So what happens when the temperature goes 
beyond these limits? Water expands, putting pressure on the membranes from 
within. The lipid part of the membrane liquefies, making it more prone to 
leakage. The proteins that span the membrane fall apart, creating holes in 
the fabric. All this combined will allow compounds to exit the cell. Why 
does this happen? That is physics. Higher temperature makes all molecules 
shake and vibrate more. The faster movement disrupts any ordered structure 
there might have been, eventually destroying the structure altogether. So 
much for the rephrasing recap.

Organisms adapt to different temperatures. Plants, as they can't run away 
to a warmer place, do much of their adapting by tweaking their 
biochemistry. I would think that if you took a cold-adapted beetroot and 
heated it while monitoring the temperature, it would give off red colour 
at a lower temperature than if you would take a beetroot that has been 
lying about at room temperature or above. Why? Could this possibly be the 
question I should have answered?

Cells work best with a partially liquefied PM. Different fats have 
different melting temperatures and the cells change the types of fatty 
acids (FAs) it contains in order to keep the membrane at just the right 
plasticity. Olive oil becomes milky in the refrigerator and the butter 
rock-hard: this is the similar. FAs can come with straight chains of 
hydrocarbons, thus:
CH3-CH2- CH2- CH2- CH2- CH2- CH2-etc
Being straight, it allows an easy packing, a bit like matches in a 
matchbox. This means that it will melt at a lower temperature than a FA 
with for instance a  bend. Twisted sticks are not as easily stacked. Bends 
do appear if one introduces double bonds between the carbons:
 CH3-CH2- CH2- CH2= CH2- CH2- CH2-etc
This is called an unsaturation. Indeed, butter, which needs more heat to 
melt, has also more saturated fats than olive-oil! So, what the plant 
does, when it adapts to a lower temperature, is to introduce more and more 
unsaturations. It will also favour shorter FAs, which also stay liquid at 
lower temperatures. But this change is a slow process. If you suddenly 
raise the temperature, then the FAs will be all of the wrong kind and the 
membrane will break at a lower temperature than for a membrane which had 
been hanging around at room temperature.

Does this answer your question? If not, please tell me in what way I have 
missed the point! :-)

Erik vS



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