What specific structures and or characteristics make biological membranes differentially permeable?

Answer 1

The phospholipids let small and hydrophobic molecules through, this is called passive transport and concerns O2 and H2O.

Since that's not all a cell needs, it builds special transporter proteins between the phospholipids to let hydrophilic and big molecules pass. This transport process is driven by concentration difference or active by ATP-cost and brings glucose and the like into the cell.

Answer 2

Biological membranes are differentially permeable due to their phospholipid bilayer structure, which allows small, uncharged molecules to pass through easily while restricting the movement of larger molecules and charged ions. Membrane proteins, such as channels and transporters, also play a key role in regulating the passage of specific molecules across the membrane. Additionally, the presence of cholesterol in animal cell membranes helps maintain membrane fluidity and permeability.