Final answer:
Nonpolar and lipid-soluble substances can freely move between the phospholipids in a membrane, while water can move through protein channels or between the lipid tails of the membrane itself. This is known as osmosis.
Step-by-step explanation:
Molecules that can freely move between the phospholipids in a membrane are nonpolar and lipid-soluble. Examples of such substances include other lipids, oxygen, carbon dioxide, and alcohol. These molecules can easily pass through the hydrophobic core of the phospholipid bilayer due to their low molecular weight and lack of charge. On the other hand, water-soluble materials like glucose, amino acids, and electrolytes need assistance to cross the membrane as they are repelled by the hydrophobic tails of the phospholipids. They typically cross through protein channels or specialized transport mechanisms in a process called facilitated diffusion.
Water, being a polar molecule, can move freely across the cell membrane either through protein channels or by slipping between the lipid tails of the membrane itself. This process is called osmosis, which is the diffusion of water through a semipermeable membrane.