Transport function of biofilm

For each living cell to maintain its normal life activities, it must absorb nutrients from the outside through the cell membrane in time, and at the same time it must continuously discharge its metabolites. The ways in which these nutrients and metabolites enter and exit the biomembrane can be divided into two types: simple diffusion and membrane protein-mediated transmembrane transport, depending on whether membrane protein is required. According to the metabolic energy consumption during transportation, the latter is divided into passive transportation and active transportation.

1. Selective permeability and simple diffusion of membrane

Some substances do not need the help of membrane proteins, and can diffuse freely along the concentration gradient. The lipid bilayer through the membrane. This form of transmembrane transport is called simple diffusion, also known as passive diffusion. It does not require energy consumption and is a substance. The simplest form of transmembrane transportation. Generally speaking, non-polar molecules with small molecular weight and strong fat solubility can quickly pass through the lipid bilayer membrane, and small molecules without charge are also more permeable, such as CO2, O2, ethanol and urea can quickly diffuse through the lipid double Floor. Because H2O has small molecules, it has no charge, and it has a bipolar structure, so it can easily pass through the membrane. Some charged molecules such as Na +, K +, Cl-, etc., although small, are often difficult to pass through the hydrophobic area of ​​the lipid bilayer due to the hydrated layer formed around them. Uncharged glucose, because of its large molecules, can hardly diffuse freely through the membrane.

2. Membrane protein-mediated transmembrane transport

Some relatively large polar or charged molecules, such as glucose, amino acids and ions, cannot pass through the membrane freely. The transport of these substances needs to be mediated by membrane proteins, which are called membrane transport proteins. According to the form of membrane protein-mediated substance transport, it can be divided into two types of carrier protein-mediated and channel protein-mediated.

In eukaryotic cells, some large molecules such as proteins, polysaccharides, and peptides are transported across the membrane through the deformation of the plasma membrane. This is called endocytosis and extracellular vomiting.

(1) Endocytosis (dog)

Endocytosis, also known as endocytosis, is the process of transporting extracellular substances into cells through the deformation of the plasma membrane. According to the different size of the cellular material and the mechanism of cellularization, endocytosis can be divided into three types: phagocytosis, phagocytosis, and receptor-mediated endocytosis.

1. Phagocytosis refers to the process of ingesting particulate matter with a diameter greater than 1 μm. When ingesting particulate matter, the cells are partially deformed, causing the plasma membrane to sag or forming pseudopods to wrap the particles into human cells. The extension of the pseudopod is involved in actin. If drugs that inhibit actin polymerization, such as cytochalasin, can inhibit cell phagocytosis.

2. Pinocytosis is the process by which cells take up solutes or liquids. When the cells are swallowed, the local plasma membrane sinks to form a small pit, surrounding the liquid substance, and then the small pit leaves the plasma membrane to form a vesicle and enters the cell. The effect of swallowing is divided into liquid endocytosis and adsorption endocytosis. The former way is for non-specific cells to take extracellular fluid and soluble substances into the cells. In the latter method, extracellular macromolecules or particulate matter are first adsorbed on the cell surface in a certain way and then taken up into the cell. For example, cationic ferritin is first adsorbed on the surface of negatively charged cells by electrostatic action and then taken up by the cells. Adsorption endocytosis has a certain specificity.

3. Receptor mediated endocytosis (receptor mediated endocytosis) is a process in which cells rely on receptors on the cell surface to specifically take up extracellular proteins or other compounds. The receptors on the cell surface are highly specific and combine with corresponding ligands (molecules that are endocytosed) to form a complex, and then this part of the plasma membrane is recessed to form a pit, and the pit is separated from the plasma membrane to form a vesicle. Ingest extracellular material into cells. After the vesicles enter the cell, they take off their coats and combine with the vesicles of the endosome to form a large endosome, whose content is acidic, so that the receptor is separated from the ligand. Part of the membrane structure with receptors germinates and falls off, and then merges with the plasma membrane, and the receptor returns to the plasma membrane to complete the recycling of the receptor.

During endocytosis, there are some proteins attached to the cytoplasmic surface of the specific binding site of the receptor and ligand on the plasma membrane (the coat will be formed by vesicles): â‘  clathrin is one of the most important proteins. It is a fibrin and forms a structural unit coated with vesicles with another smaller peptide, a three-legged protein complex. The three-legged protein complex includes three clathrins and three smaller peptides. Many three-legged protein complexes are polymerized to form a pentagonal or hexagonal grid-like structure, covering the cytoplasmic surface with vesicles or pits. The coat assembled from clathrin provides the mechanical force that pulls the plasma membrane, resulting in the depression of the pit, and also helps to capture the specific receptors on the membrane and the transported molecules bound to it; â‘¡ Modulin is Another important type of protein that is covered by vesicles is a complex of multiple subunits, which can recognize specific transmembrane protein receptors and connect it to the three-legged protein complex to selectively mediate effect. The tail of the cytoplasmic peptide chain of the transmembrane receptor protein often turns in a region composed of four amino acid residues, forming an endocytic signal, which is recognized by the regulator. So modulators can mediate different types of receptors, allowing cells to capture different types of substances.

(2) Exocytosis

Exocytosis, also known as exocytosis, is a process opposite to endocytosis. The secretion of intracellular substances, the release of molecules such as viruses and undigested residues outside the cell are all extracellular vomiting processes.

1. Intrinsic secretion (constitutive pathway of secretion) is a newly synthesized molecule loaded into transport vesicles in the Golgi complex, which is soon taken to the plasma membrane and continuously secreted by cells. It is ubiquitous in all cells . The "SNARE hypothesis" believes that in intrinsic secretion, V-SNARE and t-SNARE recognize and bind to each other to form a 7S complex, which is the specific binding of secretory particles to the target membrane, and then NSF is mediated by SNAP and 7S complex Combined to form a 20S complex, the SNAP in this complex can activate the ATPase activity of NSF, NSF hydrolyzes ATP to provide energy to depolymerize the 20S complex, then membrane fusion occurs automatically, and the intragranular material is secreted out of the cell.

2. Regulated pathway of secretion is the synthesis of macromolecules in cells that are stored in special vesicles such as secretory granules. Only when the cells receive the action of extracellular signal substances, it causes a series of biochemical changes in the cells. The secreted particles are fused with the plasma membrane and vomiting occurs. Modulated secretion mainly exists in specialized secretory cells, such as endocrine and exocrine cells, nerve cells, etc. They can specifically secrete their products quickly as needed, such as hormones, digestive enzymes, neurotransmitters and so on.

(3) The circulation and movement of the plasma membrane The area of ​​the plasma membrane changes continuously during the process of endocytosis and exocytosis. During endocytosis, transport vesicles bring the plasma membrane into human cells, such as macrophages can bring 30% of the plasma membrane into the cell every minute during phagocytosis; during the process of extracellular vomiting, the area of ​​the plasma membrane increases, such as When some exocrine cells secrete digestive enzymes, they can increase the plasma membrane at the top of the cell by 30 times. However, the volume and surface area of ​​the cell will not increase or decrease, which means that the process of endocytosis and vomiting are two complementary processes, that is, a certain amount of plasma membrane is reduced by endocytosis, there will be a corresponding amount The plasma membrane is replenished through the process of vomiting to keep the cell membrane area constant. This is a way of circulating the plasma membrane. At the same time, during this cycle, the plasma membrane is also moving, and its composition and distribution have flowed. This flow will be conducive to the execution of cell functions.