In facilitated diffusion, molecules only move with the aid of a protein in the membrane.In simple diffusion, molecules move down the concentration gradient but in facilitated diffusion molecules move up the concentration gradient. Simple diffusion is passive but facilitated diffusion is an active process that uses energy. Simple diffusion requires molecules to move through special doorways in the cell membrane.
Water is a small molecule that easily diffuses through a cell membrane despite the lipid tails. Water diffusion is called osmosis. Oxygen is a small molecule and it’s nonpolar, so it easily passes through a cell membrane. Carbon dioxide, the byproduct of cell respiration, is small enough to readily diffuse out of a cell. Small uncharged lipid molecules can pass through the lipid innards of the membrane. Larger or charged molecules might be able to slowly diffuse across the membrane. The charge on a molecule might help or hinder its diffusion, based on the relative charges on either side of the membrane.
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