Question

For each of the following molecules describe whether or not it can freely diffuse across a cell membrane. Justify your answers

RNA, Cholesterol, Caron Dioxide (CO₂), Sodium ions (Na⁺),

Also, describe what the cell membrane is made of and compare it to the plasma membrane.

Answer 1

a) RNA- No

This is because RNA is negatively charged molecule because of its phosphate groups. The lipid bilayer is composed of hydrophobic molecules which will repel the RNA and thus cannot allow it to pass through diffusion.

b) Cholesterol- Yes

Cholesterol is a nonpolar molecule that contains four fused rings of carbon and hydrogen atoms. Cholesterol can pass through the cell membrane or it can stay in the cell membrane and become part of its structure.

c) Carbon dioxide - Yes

The structure of the lipid bilayer has hydrophobic tail which allows small, uncharged substances such as oxygen and carbon dioxide, and hydrophobic molecules such as lipids, to pass through the cell membrane by diffusion.

d) Sodium ions- No

This is because the interior of the lipid bilayer is hydrophobic. Ions cannot form favorable interactions with hydrophobic molecules and thus they won't leave the aqueous phase to enter and pass through the membrane by diffusion.

Cell membranes including plasma membranes are made of glycerophospholipids, molecules composed of glycerol, a phosphate group, and two fatty acid chains. The cell membranes have phospholipid bilayer and also proteins which acts as transporters or channels for polar molecules.