Whether molecules can pass through phospholipid bilayer and the passing rate are related to their molecular size and solubility. Non-polar small molecules (such as oxygen molecules and carbon dioxide molecules) can dissolve in the phospholipid bilayer and permeate quickly, which belongs to "free diffusion". Personally, I don't like the term "free diffusion". The solubility of polar molecules in phospholipid bilayer is small, and only molecules with small molecular weight (such as water molecules and ethanol molecules) can pass through the bilayer smoothly, while glycerol molecules (92 Da) pass slightly slowly and glucose molecules (180 Da) can hardly pass through. All ions and a large number of important polar molecules cannot pass through the artificial phospholipid bilayer smoothly, so the cell membrane needs membrane transporters to transport these molecules. Sometimes, for some polar molecules (such as water molecules) that can "freely" diffuse through the cell membrane, cells need membrane transporters (such as aquaporins) to achieve reverse concentration gradient diffusion.