Question

A team of biologists develop a new drug, and one team member hypothesizes that the drug is incapable of freely passing across the plasma membrane and requires the help of membrane proteins to enter cells. Alternatively, another biologist on the team hypothesizes that the drug can diffuse passively across the plasma membrane like oxygen and carbon dioxide can.
Which of the following, if true about the drug, best supports the alternative hypothesis that the new drug will exhibit simple diffusion across plasma membranes?
A. The drug is a small nonpolar molecule.
B. The drug is a small charged molecule.
C. The drug is a large polar molecule.
D. The drug is a large charged molecule.

Answer 1

Answer:

B: The drug is a small charged molecule

Explanation:

Substances can passively diffuse in and out of the cell via the cell membrane in two ways;

• Simple diffusion
• Facilitated diffusion

The cell membrane allows small molecules or ions to freely diffuse across it in response to concentration difference between the inner and outer parts of the cell membrane. This is known as simple diffusion.

In facilitated diffusion, special proteins in the cell membrane, known as channel/carrier proteins binds with molecules and facilitates their diffusion across the cell membrane by carrying them through special channels in the membrane.

Hence, to support the alternative hypothesis that the new drug will exhibit simple diffusion across the plasma membrane, the drug should be a small charged molecule.

Correct option: B

Answer 2

Answer: A. The drug is a small nonpolar molecule.

Explanation:

The plasma membrane is a lipid bilayer that delimits the entire cell. It is a semi-permeable membrane, also called selectively permeable membrane that will allow certain molecules or ions to pass through it by diffusion, and occasionally specializes in "facilitated diffusion". The rate of passage depends on the osmotic pressure, concentration, electrochemical gradient, and temperature of the molecules or solutes on either side, as well as the permeability of the membrane for each solute.  Depending on the membrane and solute, permeability may depend on the size of the solute, solubility characteristics, or chemistry.

Simple diffusion is the process by which molecules flow through a permeable membrane without any external input of molecular energy. This process, which is ultimately due to a difference in molecular concentration between the two media separated by the membrane, does not require an input of energy because its main driving force is the increase in the total entropy of the system.

The plasma membrane consists of hydrophobic and hydrophillic characteristics. Towards the outsides, they are hydrophillic, so they can create bonds with water. The insides are hydrophobic, allowing no water inside and keeping them tight together due to the polar forces.

In the case of living cells, the simple diffusion process is established across the cell membrane. Since the velocity-limiting process is diffusion through the separating medium, a model can be simplified where the particle flow depends on the concentration difference between the two sides of the separating medium and the type of interactions that the molecule that is going to cross the membrane with that medium has.

The molecules that can easily pass through the cell membranes due to this phenomenon are gases, hydrophobic molecules and small nonpolar molecules. On the other hand, large polar molecules such as glucose, amino acids and charged molecules or ions interact too strongly with the aqueous environment outside the lipid bilayer, and it is therefore very unfavourable from an energy point of view to break these interactions to cross the central hydrophobic region. As a consequence, biological membranes are practically impermeable to this type of molecules, so they require other transport mechanisms.

So, a non-polar particle (if small), can pass through this membrane because it does not interfere with the hydrophobic/hydrophillic (polar) nature of the plasma membrane. However, polar particles would not have the opportunity to move in, because the insides (hydrophobic) are literally afraid of water, or charges, don't allow polar substances to pass through.  So only hydrophobic (nonpolar), gases, and small particles (nonpolar) can pass through.