How substances are transported across cell membranes

As we all know, the cell membrane is one of the important components of the cell and is the "protective layer" of the cell. It has the function of controlling the entry and exit of external substances into the cell. Without the cell membrane, the cell can easily be damaged and lose certain functions. So, what are the characteristics of cell membrane? What impact will its destruction have on the normal functions of cells? What is its method of material transport? Let’s popularize it today.

(a) Simple diffusion: The phenomenon that fat-soluble small molecules or ions move from the high-concentration side to the low-concentration side of the membrane is called simple diffusion. Factors affecting simple diffusion: 1. Concentration difference on both sides of the membrane; 2. Membrane permeability. The characteristics of simple diffusion are: no help from membrane proteins, no consumption of the cell's own metabolic energy, and proceeding along the concentration gradient. Substances that are simply transported by diffusion: fat-soluble small molecules, such as CO2, O2, N2, NO, etc.

(ii) Facilitated diffusion: refers to the transport method in which water-soluble small molecules or ions move from the high-concentration side of the membrane to the low-concentration side with the help of membrane proteins. Types of facilitated diffusion: (1) Carrier transport: refers to facilitated diffusion accomplished with the help of carrier proteins. Characteristics of carrier transport: 1. Specificity; 2. Saturation; 3. Competitive inhibition. Substances transported by carriers: mainly water-soluble small molecule organic substances, such as glucose and amino acids. (2) Channel transport: refers to facilitated diffusion accomplished with the help of channel proteins. Classification of channels: ① voltage-gated channel; ② chemical-gated channel; ③ mechanical-gated channel. Substances transported by channel transport: mainly inorganic salt ions, such as Na+ and K+. Factors affecting facilitated diffusion: 1. Concentration difference or potential difference on both sides of the membrane; 2. The number of carriers and the functional state of the channel. Characteristics of facilitated transport: requires the help of membrane proteins, does not consume the cell's own metabolic energy, and proceeds along the concentration gradient.

(iii) Active transport: refers to the transport of substances from the low-concentration side of the membrane to the high-concentration side through the energy consumption of the cell itself under the action of the biological pump on the cell membrane. Substances transported by active transport: mainly ionic substances, such as Na+, K+, and Ca2+. Characteristics of active transport: requires the action of a biological pump, digests the cell's own metabolic energy, and proceeds against the concentration gradient. Factors affecting active transport: 1. Functional status of biological pumps; 2. Metabolic level of cells

(IV) Exocytosis and endocytosis: The movement of macromolecules from inside the cell to outside the cell is called exocytosis. The movement of macromolecules from outside the cell to inside the cell is called endocytosis. Substances transported into and out of the cell: macromolecules, such as transmitters, hormones, digestive enzymes, bacteria, necrotic tissue fragments, and aging red blood cells. Characteristics of exocytosis and endocytosis: require movement of the cell membrane and consume the cell's own metabolic energy.