Video Transcript
How do ions and relatively large
polar molecules pass through cell membranes? (A) They pass through the
hydrophobic tails found in the cell membranes. (B) They pass through proteins
found in the cell membranes. (C) They pass through the
hydrophilic heads found in the cell membranes. (D) They pass through the fatty
acids found in the cell membranes.
Cell membranes consist mainly of a
phospholipid bilayer, cholesterol molecules, and proteins. Each phospholipid molecule has a
hydrophilic, or water-loving, phosphate head and a hydrophobic, or water-repelling,
fatty acid tail. The hydrophobic fatty acid tail is
nonpolar and doesn’t have an electric charge, so it doesn’t mix well with polar
molecules. This makes it difficult for certain
molecules to pass through the membrane. So polar molecules cannot diffuse
directly across the membrane. Knowing this, we can already
eliminate option (A) and option (D).
Option (C) is also incorrect
because these molecules may be able to interact with the hydrophilic heads. But they still can’t pass through
the hydrophobic tail region. Transmembrane proteins are embedded
in the cell membrane and can allow the passage of hydrophilic molecules. So the correct choice is option
(B). They pass through proteins found in
the cell membranes.
