Question

When is facilitated diffusion necessary?

Answer 1

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Cells do not have to undergo a state or phase where they "Need" or it is "neccessary" facilitated diffusion to take place. Its a natural process occurring across cell surface membranes from high concentration to low concentration. Facilitated diffusion is quite different than normal diffusion. As the molecules are polar they cannot pass though the phospholipid bilayer directly. These molecules are transported by carrier proteins through channel proteins specific to these molecule e.g Glucose.
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