MadSci Network: Cell Biology |
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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