Question

Each of the following molecules move across membranes by simple diffusion except

A) oxygen.
B) fatty acids.
C) water.
D) glucose.
E) carbon dioxide

Answer 1

Answer: Glucose

Explanation: It is too big.

Answer 2

Glucose does not move across cell membranes by simple diffusion because it is a water-soluble, polar molecule and requires transport proteins to cross the hydrophobic lipid bilayer.

Step-by-step explanation:

The movement of molecules across cell membranes can occur by various mechanisms, one of which is simple diffusion. This process allows small, uncharged, and hydrophobic molecules such as oxygen (O2) and carbon dioxide (CO2) to move across the cell membrane without the assistance of transport proteins or energy expenditure, down their concentration gradient. Fatty acids, being hydrophobic, can also pass through the membrane by simple diffusion. On the other hand, polar and larger molecules such as glucose, which is water-soluble, cannot freely pass through the hydrophobic core of the lipid bilayer. Glucose requires the help of transport proteins to cross the cell membrane, a process known as facilitated diffusion. Therefore, the molecule from the list provided that does not move across membranes by simple diffusion is glucose (D).