How we can say that plasma membrane is selectively permeable membrane ??
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Answer:
The plasma membrane is called selectively permeable because it only allows substances that meet a particular criterion to pass through it. This is due to its hydrophobic nature and the bilayer structure as elucidated by fluid mosaic model. This is why energy utilisation is required when there is a need to transport bigger ions, or substances against the concentration gradient across the membrane.
Due to the distinctive nature of the cell membrane, we have three types of intercellular transport. It's much like bribing officers to get things done:
Diffusion- You already have all the necessary documents or eligibility or whatever to get the job done, so you require no help.
Facilitated diffusion- This is when someone influential, like a political party leader gives you a push, since you aren't entirely fit for the job.
Active transport- This is when you offer money, and also get help from that influential guy you know, to get in, since you don't fit any of the requirements.
I have answered the question in its very basic level, since I figured you were a beginner. For more extensive explanations, you can search Wikipedia.
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