Question

Each component gives membranes different purposes. For example, the lipid component provides a permeability barrier for the cell – meaning, since the lipid bilayer has a nonpolar region from the hydrophobic lipid tails, any charged molecule will have a difficult time getting through. Therefore, small, uncharged molecules (i.e. nonpolar) can diffuse through the membrane the best due to the membrane properties. Small, partially charged molecules (i.e. polar) can diffuse across the membrane as well, but large polar molecules or fully charged molecules (as in ions) cannot diffuse across the membrane on their own. Which molecule will diffuse the slowest across the pure lipid membrane?

a. a small, nonpolar molecule

b. a medium-sized, nonpolar molecule

c. a small, polar molecule like H2O

d. an ion, like a potassium ion (K+)

Answer 1

The molecule that will diffuse slowest across the pure lipid membrane would be an like a potassium ion. This is because phospholipid is a type of lipid that have hydrophilic head (water attracting) and hydrophobic tails (water repelling). So, ions being polar in nature could easily cross the hydrophilic head but when they encounter hydrophobic tails , they could not go through them because tails are hydrophobic. Hence, correct option is d.

Figure: phospholipid structure having hydrophilic head and hydrophobic tail