Question

Given the following characteristics of an unknown molecule, determine how it will cross intothe cell (across the membrane – think transport type).  Explain your reasoning.

1. molecule A is non-polar, with an external concentration of 10mM and an internal cellular concentration of 10mM.

2. molecule B is an ion with an external concentration of 5mM and an internal concentration of 140mM.

3. molecule C is a small, polar molecule, with an external concentration of 1mM and an internal concentration of 100mM.

Answer 1

1. By Passive Diffusion

Explanation: Molecule A being Non polar and same concentration with that of internal cellular environment (i.e., 10mM) the movement of molecule is probably by passive diffusion as the molecule is lipophilic and there is no movement against the concentration gradient. So, it does not require energy and the concentration gradient drives the molecule A. As the concentration is now equal both internally and externally across cell membrane, it leads to equilibrium.

2. Active transport mechanisms like Ion pair transport (Electrochemical diffusion)

Explanation: Molecule B being an ion with an external concentration of 5mM and internal concentration of 140mM have to move into the cell for which Active transport like Ion pair transport is the most favorable mechanism of transport for the movement of ions (since ions do not easily cross across cell membrane) across the concentration gradient.

3. Pore transport

Explanation: Molecule C being small polar molecule with external concentration of 1mM and internal concentration of 100mM, it is transported by Pore transport. As the the molecule is very small and being polar it will be water soluble (upto molecular weight of less than 100 Daltons) and there is concentration differential, it is favorable transported by pore transport