What Are the Differences Between Diffusion and Osmosis?

Free diffusion (simple diffusion), a biological term, refers to the transport of substances from the high concentration side through the cell membrane to the low concentration side. For example, O2, CO2, N2, glycerol, alcohol, benzene, water and other substances can be transferred from the concentration The higher side is transported to the lower concentration side. The way this substance gets in and out of cells is called free diffusion. Free diffusion does not require the energy released by the metabolism in the cell and is a simple means of transport. Compared with active transport, this method is called passive transport.

Free diffusion

free

Mostly gas

1 gas: CO2, O2, NO2

2

Oil / water

Free diffusion is one of the ways that substances enter and exit cells through the plasma membrane

Diffusion of substances from the high-concentration side through the plasma membrane to the low-concentration side requires no carrier and consumes no energy. The free diffusion rate of a substance is related to the degree of fat solubility of the substance, the difference in the concentration of solutes on both sides of the membrane, the size of the solute molecules, and the charge properties. Since the lipid bilayer constitutes the basic framework of the plasma membrane, fat-soluble substances can be dissolved in the membrane lipid, so they can preferentially enter and exit cells through the cell membrane. A large number of experiments have shown that the diffusion rate of many substances through the plasma membrane is directly proportional to their degree of fat solubility. Water is almost insoluble in lipids, and water molecules can enter and exit cells freely through small holes in the plasma membrane. However, the moving direction and speed of water and fat-soluble substance molecules depend on the concentration gradient. The diffusion of a substance depends on the concentration gradient. The greater the difference in solute concentration, the faster the diffusion rate. In addition, some water-soluble inorganic ions can also enter and exit the cell by free diffusion. They are not only affected by the concentration gradient, but also restricted by the charge properties. There are some anions in the cell that are high in concentration and cannot diffuse out of the cell. Therefore, positively charged ions are more likely to enter the cell, and negatively charged ions are more difficult to enter the cell.

Osmotic absorption is a way for plant cells to absorb water

Cells with vacuoles absorb water mainly by osmosis. Osmosis is the diffusion of solvent molecules through a semi-permeable membrane. A semi-permeable membrane is tightly tied to the mouth of the long-necked funnel. The funnel contains a sucrose solution. The funnel is inverted into a beaker containing pure water, and the liquid level inside and outside the funnel starts to be equal. Because the water potential of pure water is high and the water potential of sucrose solution is low, the water in the beaker will move into the funnel through the semi-permeable membrane, and the liquid level in the funnel will rise. After the solution rises to a certain height, it no longer rises. This phenomenon is called osmosis. Therefore, osmosis is a phenomenon in which water moves from a system with a high water potential through a semi-permeable membrane to a system with a low water potential. Two conditions are required for osmosis to occur: one is a semi-permeable membrane, and the other is a potential difference between the two sides of the semi-permeable membrane. Mature cells contain large vacuoles, with a certain concentration of cellular fluid in the vacuoles. The outer layer of the vacuoles is the protoplast layer. When discussing the absorption of water by cells, it is often regarded as a semipermeable membrane. When such a cell is placed in water or a solution, if there is a water potential difference on both sides of the protoplast layer, it has the conditions for infiltration. When the water potential of the external solution is higher than the water potential of the cell fluid, the cells absorb water through osmosis. Under normal circumstances, the water potential of the soil solution is relatively high, so the root system can absorb water from the soil. However, when the water potential of the external solution is lower than the water potential of the cytosol (such as excessive application of chemical fertilizers at one time), the cells not only cannot absorb water from the outside, but will cause the water in the cells to flow out, which will adversely affect the plants. Since the whole body cells of a plant are mostly mature cells, the plant absorbs water, mainly by osmotic absorption.