Simple diffusion refers to the process by which soluble small molecules move from a high concentration to a low concentration in a cell, and is one of the many forms of biomolecular movement in medical physiology, also known as passive transport. In addition to this, there is also easy diffusion and active transport, which is equivalent to the way water tends to flow from a high place to a low place, resulting in the two sides of an uneven horizontal surface becoming equal in height. This is also true of simple diffusion, the movement of many small molecules in the cell will also flow from high to low concentrations, thus maintaining homeostasis in the cell. The movement of oxygen, carbon dioxide, glycerol, ethanol, urea, and sterols in the cell can be carried out by simple diffusion. Transport in the cell, the higher the cell membrane permeability the larger the molecules that can simply diffuse, is the most basic concept in physiology, and is of great help in modern medical research.