There are four modes of cell membrane substance transport, including free diffusion, assisted diffusion, active transport, and cytotoxicity, of which free diffusion and assisted diffusion are collectively known as passive transport. 1. Free diffusion: It means that substances enter the cell from the high concentration side to the low concentration side of the membrane by simple diffusion without the need of energy consumption and carrier assistance. Common free diffusing substances are fat-soluble substances, oxygen, carbon dioxide and water. 2. Assisted diffusion: such as large molecules of glucose and some non-lipid-soluble substances in the carrier protein assistance, without energy consumption to comply with the concentration difference or potential difference in the cell, so the type and number of carrier proteins also affect the efficiency of diffusion. 3. Active transport: when ions or small molecules of nutrients or metabolic wastes and harmful substances need to be transported across the membrane against the concentration difference or potential difference, the body will use the energy released by the cell and complete the process with the assistance of carrier proteins. 4. cytophagy cytotoxicity: the use of cell membrane fluidity will be large molecules or clumps of material wrapped and swallowed into the cell process called cytotoxicity, such as small intestinal epithelial cells on the absorption of nutrients; cytotoxicity is through the fusion of secretory vesicles wrapped in metabolic wastes to the extracellular transport of substances. Both processes are energy-consuming.