Question

Cell membrane or plasma membrane is described as a selectively permeable membrane. Which of the following statements justify the fact?
A) cell membrane allows the entry and exit of some molecules into and out of the cell.
B) it prevents movement of some other molecules. Hence, it is called a selectively permeable membrane.
C) both a and b.
D) none of the above.

Answer 1

Hint:About Cell membrane or plasma membrane we know that this is the outermost covering of the cell. The word selectively permeable means that it allows only some of the material across it passively from either side of the membrane.

Complete answer:
Cell membrane is the outermost covering of the cell. The function of the plasma membrane is the transport of molecules across it. The membrane is said to be selectively permeable to some molecules present on the other side of it. Many molecules can move actively across the membrane without any requirement of energy and this type of transport is called passive transport. For the movement of the neutral solute across the membrane is by the process of simple diffusion that means, it is along the concentration gradient that is from the higher concentration to the lower concentration. Water may also move across this membrane from higher to lower concentration, and the movement of water by diffusion is known as osmosis. Now, as the polar molecules cannot pass through the non-polar lipid bilayer, they require a carrier protein of the membrane to facilitate their transport across the membrane. A few ions or molecules are transported across the membrane against their concentration gradient that means from lower to the higher concentration. This type of transport that requires energy and is involved in the energy-dependent process, ATP is utilized and it is called active transport. For example, Sodium and potassium pump.

Here, in the entire process of transport, the passive movement across the membrane are those molecules that favour the nature of plasma membrane Plasma membrane selectively allows some substance to pass through it and prevent the movement of some other molecules. This passive movement is a selectively permeable function of the plasma membrane.

So, from the above points we can say that plasma membrane or the cell membrane allows the entry and exit of some molecules into and out of the cell selectively and also it prevents movement of some other molecules and hence it is called as selectively permeable. So, both points are correct.

Hence the correct answer is option C.

Note: Nature of plasma membrane is of the lipid bilayer and proteins. Quasi-fluid nature of lipids enables the lateral movement of proteins within the overall bilayer.