We explain what the cell membrane is and some of its characteristics. Furthermore, its function and structure of this lipid layer.
What is the cell membrane?
It is called cell membrane, plasma membrane, plasmalemma or cytoplasmic membrane. a double layer of phospholipids that surrounds and delimits cells separating the inside from the outside and allowing physical and chemical balance between the environment and the cytoplasm of the cell. This is the outermost part of the cell.
This membrane is not visible under the optical microscope (it is visible under the electronic microscope), since has an average thickness of 8 nm (1 nm = 10-9 m) and it is located, in plant and fungal cells, below the cell wall.
The primary characteristic of the cell membrane is its selective permeability, that is, its ability to allow or reject the entry of certain molecules into the interior of the cell, thus regulating the passage of water, nutrients or ionic salts, so that the cytoplasm is always in its optimal conditions of electrochemical potential (negatively charged), pH or concentration.
See also: Amoeba
Cell membrane function
The cell membrane fulfills the following functions:
- Delimitation. It mechanically defines and protects the cell, distinguishing the outside from the inside, and one cell from another. In addition, it is the first defense barrier against other invading agents.
- Administration. Its selectivity allows it to give way to the desired substances in the cell and deny entry to the unwanted ones, serving as communication between the outside and the inside while regulating said transit.
- Preservation. Through the exchange of fluids and substances, the membrane allows the concentration of water and other solutes in the cytoplasm to be kept stable, its pH level and its electrochemical charge constant.
- Communication. The membrane can react to stimuli coming from the outside, transmitting information to the interior of the cell and launching certain processes such as cell division, cell movement or the segregation of biochemical substances.
Cell membrane structure
The cell membrane It is composed of two layers of amphipathic lipids whose hydrophilic polar heads (affinity for water) are oriented inward and outward of the cell, keeping its hydrophobic parts (which reject water) in contact, similar to a sandwich. These lipids are primarily cholesterol, but also phosphoglycerides and sphingolipids.
Also It has 20% integral and peripheral proteins which fulfill functions of connection, transport, reception and catalysis. Integral membrane proteins are embedded in the bilayer with their hydrophilic surfaces exposed to the aqueous environment and their hydrophobic surfaces in contact with the hydrophobic interior of the bilayer.
Transmembrane proteins are integral proteins that completely cross the thickness of the membrane. Peripheral membrane proteins associate with the surface of the bilayer, typically bind to exposed regions of integral proteins, and are easily separated without disturbing the membrane structure. Thanks to them, cellular recognition also occurs, a form of biochemical communication.
Finally, the cell membrane has carbohydrate components (sugars), whether polysaccharides or oligosaccharides, which are found on the outside of the membrane forming a glycocalyx. These sugars represent only 8% of the dry weight of the membrane and serve as support material, as identifiers in intercellular communication and as protection of the cell surface from mechanical and chemical attacks.
Active transport and passive transport
Membranes form compartments within eukaryotic cells that allow for a variety of separate functions. Additionally, they serve as surfaces for biochemical reactions.
Many ions and small molecules move across biological membranes by transport. passive (without energy expenditure) and by transport asset (with energy expenditure).
Diffusion is the net movement of a substance down its concentration gradient from a region of higher concentration to one of lower concentration.
Passive transport across the lipid bilayer is called simple diffusion and the one that is carried out through ion channels and membrane proteins is called facilitated diffusion
The osmosis It is a type of diffusion in which water molecules pass through a semipermeable membrane from a region with a higher effective concentration of water to a region where its effective concentration is lower.
In active transport, the cell expends metabolic energy to move ions or molecules across a membrane, against a concentration gradient.
He primary active transport also called direct active transport, uses metabolic energy directly to transport molecules across the membrane. For example, the sodium-potassium pump uses ATP to pump sodium ions out of the cell and potassium ions into the cell.
In it co-transport Also called indirect active transport, two solutes are transported at the same time. A powered ATP pump maintains a concentration gradient. So a carrier protein cotransports two solutes. It moves a solute down its concentration gradient and uses the energy released to move another solute against its concentration gradient.
Endocytosis and exocytosis
Some of the larger materials, such as large molecules, food particles, or even small cells, also move in or out of cells. They are transferred by exocytosis and endocytosis. Like active transport, these processes require an expenditure of energy directly from the cell. This occurs through the formation of vesicles in the cell membrane that, depending on whether they enter or leave, allow the desired material to dissolve in the cytoplasm or, on the contrary, in the environment.
- In exocytosis. A cell expels waste substances or secretion products (such as hormones) by fusing a vesicle with the plasma membrane.
- In endocytosis. The materials are incorporated into the cell. Several types of endocytosis mechanisms operate in biological systems, including phagocytosis, pinocytosis, and receptor-mediated endocytosis.
- In pinocytosis (“drinking cells”). The cell takes up the dissolved materials.
- In receptor-mediated endocytosis. Specific molecules combine with receptor proteins on the plasma membrane. Receptor-mediated endocytosis is the primary mechanism by which eukaryotic cells take up macromolecules.
- In phagocytosis (literally, “eating cells”). The cell ingests large particles of solids such as food or bacteria. The latter is vital in the case of certain cells and single-celled organisms that phagocytize (wrap in their membrane) the material for nutrition.
Continue with: Mitochondria
References
- Cellular and Molecular Biology. From Robertis Edward, Hib José. (2012) 16th ed. Promed Editorial.
- Biology: Solomon E., Berg L., Martin D. (2013) 9th Edition. Cengage Learning Publishing.
