MARCH 2020 P12

15 Which description of cell surface membrane permeability is correct?


A An increase in the concentration of cholesterol molecules in the cell surface membrane can increase its permeability to hydrophilic substances. 

B Cell surface membrane permeability to large hydrophilic molecules is high and can be increased by membrane transport proteins involved in facilitated diffusion. 

C The permeability of the cell surface membrane to ions increases as the proportion of saturated fatty acid chains in the phospholipids increases. 

D Without the presence of carrier and channel membrane proteins, the cell surface membrane has a low permeability to large polar molecules. 

ANSWER D  

In this question, we need to keep in mind that cell surface membrane has a low permeability to hydrophilic substances if transport proteins are not present since the phospholipid bilateral itself is hydrophobic in nature.

Therefore, in A an increase in cholesterol molecules would not increase its permeability to hydrophilic substances, however, the membrane would become more efficient in regulating the fluidity of the membrane. Cholesterol acts as a hydrophobic plug - thus it influences membrane permeability to hydrophobic substances.

Aforementioned, the membrane permeability to hydrophilic substances is low though the other part of the statement is true. Thus, B is incorrect due to the first statement.

In C, an increase in saturated fatty acids would reduce the membrane fluidity.

D is the correct option. Since the core of the membrane is the phospholipid bilayer, transport proteins are required to ensure that membrane has a higher permeability to large,hydrophilic substances which are otherwise not allowed by the hydrophobic phospholipid bilayer.


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