Free diffusion

Biological terminology

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Simple diffusion, a biological term, is one of the main ways for substances to enter cells. It refers to the passage of substances from the side with high concentration cell membrane Transport to the side with low concentration, such selected substances are Liposolubility Materials, they do not need carriers to enter and exit cells, and do not consume additional energy. for example O2 、 CO2 、 N2 、 glycerol 、 ethanol , benzene, water and other substances can be transported from the side with high concentration to the side with low concentration. This substance Entrance and exit cell This is called free diffusion. Free diffusion does not need to consume the energy released by metabolism in cells, which is simple type of shipping 。 In this way Active transportation (Active transport), compared with co diffusion, is called Passive transportation (passive transport)。

Chinese name: Free diffusion
Foreign name: Simple diffusion

Definition: material Entrance and exit cell mode
Classification: Passive transportation
Features: No energy consumption

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meaning

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Free diffusion refers to being Selective absorption From the high concentration side through the cell membrane to the low concentration side Entrance and exit cell Mode. No carrier and no energy consumption are required. Free diffusion is a simple mode of transportation. In this way Active transportation Compared to, called Passive transportation 。

Common substances

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Most of them are gas

1 Gas: C O2 、O2、 NO2

two lipid Class: glycerol , fatty acid（ Similitude compatibility ）, sterol

3 Small molecule organic compound ： ethanol , benzene urea .

major factor

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Oil/water partition coefficient The higher the coefficient, the easier the diffusion. Substance to membrane permeability (P) According to its partition coefficient (K) in oil and water and diffusion coefficient (D) Calculation of film thickness (t): P=KD/t

Small molecule than macromolecule Easy transmembrane, Nonpolar molecule than Polar molecule Easier transmembrane, and charged ions need higher transmembrane free energy 。^[1]

Main differences

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Free diffusion is the passage of matter plasma membrane One of the ways to get in and out of cells

The substance passes through the side with high concentration plasma membrane The diffusion to the low concentration side does not require a carrier or consume energy. Freedom of matter Diffusion velocity , and physical Liposolubility Degree, solute concentration difference on both sides of the membrane, solute Molecular size It is related to the nature of electric charge. because lipid double Molecular layer It forms the basic skeleton of plasma membrane, and fat soluble substances can be dissolved in Membranous lipid Therefore, it can preferentially enter and exit the cell through the cell membrane. A large number of experiments have shown that the diffusion rate of many substances through the plasma membrane is proportional to their fat solubility. water Almost insoluble in fat, water molecules can enter and exit cells freely through pores on the plasma membrane. However, the direction and speed of movement of water and fat soluble substances depend on concentration gradient 。 The diffusion of matter depends on the concentration gradient, solute concentration The greater the difference, the faster the diffusion speed. In addition, some water soluble Inorganic ion They can also enter and exit cells through free diffusion, which is not only affected by the concentration gradient, but also restricted by the charge property. There are some high concentrations in the cell that cannot diffuse out of the cell anion , therefore, with positive charge The ions of negative charge It is more difficult for ions of.

Imbibition yes plant cell A way of absorbing water

have vacuole The cells of Infiltration Absorb water. Permeation is the passage of solvent molecules Semipermeable membrane Of Diffusion 。 stay Long necked funnel A semi permeable membrane is tightly tied at the mouth, and the funnel is equipped with sucrose Solution, put the funnel upside down in the Beaker At the beginning, the liquid level inside and outside the funnel is 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 to the funnel through the semipermeable membrane, and the liquid level in the funnel will rise. When the solution rises to a certain height, it will not rise again. This phenomenon is infiltration. Therefore, osmosis is a phenomenon that water moves from the system with high water potential to the system with low water potential through the semipermeable membrane. There are two conditions for osmosis: one is semi permeable membrane, and the other is water potential difference on both sides of semi permeable membrane. Mature cells contain large vacuoles with a certain concentration of Cytosol , outside the vacuole Protoplast When discussing cell water absorption, we often regard it as a semi permeable membrane. When such a cell is placed in water or solution, if there is a water potential difference on both sides of the protoplasm layer, it is ready for Infiltration Conditions. When the water potential of the external solution is higher than that of the cell solution, the cell absorbs water through osmosis. Normally, Soil solution The water potential of is relatively high, so the root can absorb water from the soil. However, when the water potential of the external solution is lower than that of the cell solution (such as excessive fertilizer application at one time), the cell can not absorb water from the outside, but will make the water in the cell flow out, thus causing adverse effects on plants. Because the whole body cells of a plant are mostly Mature cell , so plants absorb water to Imbibition Mainly.

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