Endocytosis and Exocytosis

Endocytosis and exocytosis The existence of transport proteins on the cell membrane provides assistance for the entry of ions and small molecules through the bilayer membrane, but there are still proteins and macromolecules that are extracellularly needed by the cell and enter through the endocytosis mechanism. Basically, endocytosis occurs by the mechanism of bending part of the cell membrane to form an invagination with the material inside. The mechanism of the endocytosis process occurs when part of the plasma membrane experiences indentation or is called invagination, which is regulated by a group of special proteins found in the cell membrane, including clathrin. The indentations in the cell membrane contain material that comes from extracellular sources to be brought into the cell and become early endosomes. There are three types of endocytosis, namely: 1) phagocytosis, for solid macromolecules, 2) pinocytosis, for liquid materials, and 3) endocytosis in a special way that requires receptor mediation. Endosomes containing extracellular material will combine with lysosomes to be degraded or broken down. Lysosomes are a cell organelle that is located inside the cell with a membrane that surrounds it, thus providing a boundary with the cytosol. This organelle is mainly found in animal cells, and is responsible for breaking down components that are no longer used by the cell or organism. Material obtained through endocytosis or phagocytosis mechanisms may also be broken down in lysosomes. Lysosomes contain a group of enzymes capable of breaking down polymers into monomeric subunits. For example, nuclease enzymes that break down RNA and DNA nucleotides into mononucleotides, proteases that break down many types of proteins and peptides, phosphatases that remove phosphate groups from mononucleotides, phospholipids and other components, as well as other enzymes that break down complex polysaccharides and glycolipids into units. the smaller one. All the enzymes in the lysosomes work efficiently at acidic pH, because the acidic conditions will help denature proteins. On the other hand, enzymes in lysosomes experience low activity if the cell is in a neutral pH condition and most of these conditions are in the extracellular fluid. For this reason, the lysosomal membrane will always pump H+ and Cl- ions from the cytosol through the membrane to maintain acidic conditions in the lysosomal lumen. The disease associated with lysosomal damage is Tay-sachs, where there is damage to a step in enzyme catalysis and causes damage to the central nervous system. The result is an accumulation of glycolipids, especially in nerve cells and as a result, damage to these nerve cells occurs. This disease usually attacks before the child is 1 year old and dies before the sufferer reaches the age of 3 years. Apart from obtaining the materials needed by cells by endocytosis, cells also need a mechanism for removing metabolic products from inside the cell to outside the cell, which is called exocytosis. The mechanism of exocytosis is basically the opposite of endocytosis, where the process of secreting macromolecules is carried out by combining the vesicles that accommodate the secretion results with the plasma membrane. These transport vesicles are part of the Golgi apparatus which are moved by the cytoskeleton to the plasma membrane, one of the secreted proteins of which is the result of the work of the endomembrane system network (explained in chapter 4). Through the exocytosis process, the plasma membrane will be continuously reorganized. This condition occurs because when the vesicle membrane and plasma membrane meet, the two identical lipid molecules will combine and the contents of the vesicle are released into the extracellular fluid. The exocytosis mechanism is supported by the endocytic pathway where vesicles from the plasma membrane also carry membrane proteins and their ligands into the cell. After being inside the cell by the endocytosis mechanism, it will be transported to the lysosomes and then sent back to the cell surface after going through a degradation process.