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Generally in biology books, it is written that the cell membrane is selectively permeable and that it has a protective function, as in here

It says that the cell membrane protects the cell from its surroundings. However the selective permeability is only for the hydrophilic substances, and not for any foreign antigen which may be dangerous for a cell. So how does this count as protective? Also for hydrophilic substances, carrier proteins are present, ready to take the ionic substances inside the cell.

Hence my only question here is how is the cell membrane protective?

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  • $\begingroup$ Not sure but by selectively allowing the non polar molecules, it helps maintaining the electrical charges and/or acid-base neutrality and thus aiding in protection of the cell? $\endgroup$
    – Ojasvi
    Aug 25 '20 at 13:35
  • $\begingroup$ carrier proteins don't let just anything in, that is why viruses need to have mechanisms to penetrate the membrane without a membrane the dna/rna could just float in and start changing things $\endgroup$
    – John
    Aug 25 '20 at 22:25
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You are right that the membrane doesn't stop everything. Its main purpose is to separate the inside of the cell from the outside so that you can accumulate resources in it. Your potassium ions, proteins, sugars, RNA (and DNA, behind another membrane) have to stay inside the cell because they can't dissolve through it. There are indeed channels and transporters from some hydrophilic substances as you say, but they are usually very specific about what chemicals they will transport.*

The membrane is actually very delicate but it is reinforced by a cytoskeleton, and proteins are embedded in it to transmit force to features outside the membrane (such as desmosomes and hemidesmosomes connecting to other cells or extracellular matrix) so that the membrane itself is never put under strain. Think of it like a thin plastic bag that can hold water because you put it inside a much stronger container - even though the stronger container is not waterproof.

Many harmful chemicals would not be harmful if they could not get into your cells, so you should not be surprised if they are often hydrophobic and dissolve freely through the membrane. Some cells have other defense mechanisms (pumps to try to expel the toxins or enzymes to try to break them down), but ultimately any true toxin is defined as the one for which nothing works well enough.

Many pathogens have evolved methods of crossing the cell membrane. Bacteriophages famously break a hole in the membrane with a special injection apparatus, while many human viruses fuse a cell membrane section they carry around themselves with the victim cell membrane, using proteins that have evolved the right hydrophobic features. It is hard to defend against such things because our cellular membranes need to be able to fuse with themselves for purposes such as releasing vesicles of neurotransmitter. The same "no true pathogen..." argument applies - we only get sick from the pathogens that can infect us. If we lacked these protections, we would be even more vulnerable than we are.

  • Exceptions, perforins and granzymes, prove the rule: they are meant to kill the cell affected.
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    $\begingroup$ A nice answer, but exceptions only prove a rule when there is no other evidence than the granting of the exception that the rule exists. Check this out on the Internet. So, they are just exceptions with explanations. Nothing to do with rules. $\endgroup$
    – David
    Aug 9 '20 at 18:06
  • $\begingroup$ However this does not answer op's question that how cell membrane protects the cell? From your answer it seems that cell membrane is not protective. $\endgroup$
    – Ojasvi
    Aug 9 '20 at 21:48

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