8.Plazma (cytoplasmic) membrane. Structure. Functions. Destruction of the plasma membrane by antimicrobial agents

The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells.  The basic function of the cell membrane is to protect the cell from its surroundings. The bacterial cytoplasmic membrane is composed of a phospholipid bilayer and thus has all of the general functions of a cell membrane such as acting as a permeability barrier for most molecules and serving as the location for the transport of molecules into the cell.

The proteins in plasmatic membrane has a liquid-mosaic model of atom. Proteins divided to surface proteins, peripheral and integral proteins according to connection with  phospholipid bilayer. The plasma membrane, or cytolemma among various cell membrane has a special place. This is just a peripheral structure that limits the cell from the outside, which makes it a direct connection to the extracellular environment and, consequently, with all substances and stimuli acting on the cell . In structure of the membrane of bacterial, animal and plant cells are very similar. This gives rise to an universal "unit membrane»

The plasma membrane plays an important role in metabolism. It serves as an osmotic barrier cells and controls both the flow of substances into the cell, as well as access to the outside. In the membrane mechanisms are active transport and the substrate-specific permeases. Apparently, the lipid film unit membrane permeated the "bridge" (or channels) of the proteins, and these proteins are the pores through which the regulated transport of substances.

Destruction of the plasma membrane by antimicrobial agents. The cytoplasmic membrane is selectively permeable barrier, a violation of its function leads to the exit of the cell proteins, purine and pyrimidine nucleotides, ions with subsequent death.

Products in this group include polymyxins, polyene antibiotics ,gramicidin. Polymyxins - a complex of polypeptides synthesized by Bacillus polymyxa and some other bacteria. Polymyxins break osmotic resistance MTC. Like cationic detergents, polymyxins act on the cell membrane of bacteria-rich phosphatidylethanolamine. Their spectrum bactericidal gram includes microflora (Escherichia, Shigella, Proteus, Klebsiella, Pseudomonas, etc.). Toxic drugs in clinical practice using a polymyxin B and E (colistin) locally.

Gramicidin is polypeptide antibiotics produced by Bacillus brevis. They violate the integrity of the MTC; gramicidin partially used as a bacteriostatic agent for infections caused by gram-positive cocci and bacilli.

Ethanol is a solvent so will affect the permebility of the cell. It will break down the phospholipid bilayer and will denature the protein by breakin the hydrogen bonds.

9.Movement of materials across membranes. Simple diffusion. Facilitated diffusion.Osmosis.

The term simple diffusion refers to a process whereby a substance passes through a membrane lipid bilayer without the aid of an intermediary such as a integral membrane protein.

The force that drives the substance from one side of the membrane to the other is the force of diffusion. In order for substances to pass through a cell membrane by simple diffusion it must penetrate the hydrophobic core of the phospholipid bilayer.

The types of molecules that can do this are themselves substantially hydrophobic in nature such as carbon dioxide, oxygen or ethanol. Hydrophilic molecules cannot participate in simple diffusion because they  would move into the hydrophobic region of the membrane and be rejected.

Facilitated diffusion is a type of passive transport that allows substances to cross membranes with the assistance of special transport proteins. Some molecules and ions such as glucose, sodium ions and chloride ions are unable to pass through the lipid bilayer of cell membranes. Through the use of ion channel proteins and carrier proteins that are embedded in the cell membrane these substance can be transported into the cell. Ion channel proteins allow specific ions to pass through the protein channel. The ion channels are regulated by the cell and are either open or closed to control the passage of substances into the cell. Carrier proteins bind to specific molecules, change shape and then deposit the molecules across the membrane. Once the transaction is complete the proteins return to their original position.

Osmosis is the spontaneous net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides. Net movement of solvent is from the less concentrated (hypotonic) to the more concentrated (hypertonic) solution, which tends to reduce the difference in concentrations

Osmosis can occur when there is a partially permeable membrane, such as a cell membrane. When a cell is submerged in water, the water molecules pass through the cell membrane from an area of low solute concentration to high solute concentration (e.g. if the cell is submerged in saltwater, water molecules move out; if it is submerged in freshwater, however, water molecules move in); this is called osmosis. Cellular membrane permeable and only delays the mikromoleculas. This passive transfer of substances does not require energy input, and proceeds to a uniform concentration with the external solution.

In the cells included in the reaction substance constructive and energy metabolism, the concentration of some of them will be lower than in the medium and delivery of these substances is possible to complete exhaustion of their substrate.

If the microorganism enters the substrate, the osmotic pressure is higher than the cell cytoplasm that sends the water to the environment. Nutrients do not enter the cell, the cell contents and decreases in volume behind protoplast cell membrane. This phenomenon is called Plasmolysis cells.

When excessive low osmotic pressure of the environment can occur plazmoptis cells - the phenomenon of reverse plasmolysis, when due to the high osmotic pressure difference between the cytoplasm is crowded with water and leads to the rupture of the cell membrane.