Which of the following best describes an aspect of a membrane's molecular structure that causes it to be selectively permeable? a. Carrier proteins on either side of the membrane slow the traffic of large molecules into the cell.
b. Nonpolar molecules cannot dissolve in the polar membrane and do not enter the cell easily.
c. The lipid bilayer is hydrophilic and allows polar molecules to enter the cell easily.
d. Transport proteins within the membrane serve as a tunnel for molecules to enter the cell.

d. Transport proteins within the membrane serve as a tunnel for molecules to enter the cell.

Explanation:

Solutes are typically moved across the cell through either passive or active transport. The cells, surrounded by a bilipid layer or plasma membrane is amphiphlic- its polar, hydrophilic lipid heads face outwards, while their non-polar hydrophobic lipid tails face inwards towards each other.

While lipid-soluble molecules move across the layer easily, it is also difficult for charged and also large molecules to move across its surface, into the cell. Transmembrane channels, embedded within the membrane, help to maintain selective permeability as transport proteins, pores and gated channels. Simple diffusion happens as a method of passive transport in cells through plasma membranes.

The solutes travel through the plasma membrane in the process of diffusion from regions of high concentration to regions of low concentration; this occurs without the use of energy. Molecules moving against their concentration require active transport mechanism to cross the membrane.

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