Final answer:
Biological membranes are selectively permeable, allowing certain molecules to pass through while blocking others. The phospholipid bilayer in the membrane determines which molecules can cross, with hydrophobic molecules having an easier time than ions and polar molecules.
Step-by-step explanation:
Biological membranes are selectively permeable, meaning they allow certain molecules to pass through while blocking others. The molecular make-up of the phospholipid bilayer in the membrane determines which molecules can cross. Hydrophobic molecules like carbon dioxide and oxygen can easily pass through the lipid bilayer, while ions and polar molecules cannot. Transport proteins in the membrane assist in the movement of larger molecules. Overall, membranes' selective permeability is a key concept in biology, chemistry, and physics.