Is the plasma membrane semi permeable or selective permeable?

Question

Can it be both? I've tried searching it up but keep seeing different answers? Also is the plasma separate from the phospholipid bilayer? I would really appreciate it if someone can answer this question and ease my confusion. Thank you.

Answer 1

Yes, it can be both. Semipermeable suggests that the membrane allows some substances to pass through, but not all, and this is generally by DIFFUSION. Selectively permeable suggests that a specific substance can pass through and is much more limited, and is usually by a carrier mediated process such as facilitated diffusion or active transport.  So, as an example, a membrane might allow small ions like sodium and potassium and chloride to pass through, but not allow other small molecules to pass.  This would be the semipermeable aspect.  Then a membrane may be SELECTIVELY permeable to glucose, but not allow other monosaccharides to pass.  This would suggest a specific carrier protein for glucose that cannot be used by other monosaccharides. This would be the selectively permeable aspect.

Answer 2

Carli,

 

Semi permeable implies to property of plasma-membrane where it permits transport of certain molecules but not all. Permissible molecules are selected for the best survival of the cell and therefore, plasma membrane is selectively permeable!

 

Phospholipid bilayer is the structural component of all cell membranes, it contributes to selective permeability of plasma-membrane through ion-gated channels as well as other mechanisms. 

 

 

Hope this ic clear and you can look at figures of phospholipid bilayer in your text book.