Final answer:
Water and lipids can move freely between the phospholipids in a membrane through simple diffusion.
Step-by-step explanation:
The substances that can move freely between the phospholipids in a membrane are water (H2O) and other lipids.
Lipid soluble molecules, such as other lipids, oxygen, carbon dioxide, and alcohol, can pass through the lipid bilayer of the membrane by simple diffusion.