Theme №2. Modern methods of studying of the structure and functions of biological membranes.
Purpose of the lesson:
To check and consolidate the knowledge about the main properties of biological membranes and their functions. To master the methods of problems solving on this theme.
Teaching problems:
- physical and chemical peculiarities of the structure of membrane structures and mechanisms of their functioning;
- solving of problems on the theme.
Main questions of the theme:
General concepts about biological membranes.
Modern concepts about the structure of membrane.
Model of Davson and Danielli, mosaic model, liquid-crystal model and etc.
Main functions of biological membranes.
Types and functions of membrane proteins.
Phasic transitions.
Cell membrane
Illustration of a Eukaryotic cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment.[1] The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells.[2] It basically protects the cell from outside forces. It consists of the lipid bilayer with embedded proteins. Cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signaling and serve as the attachment surface for several extracellular structures, including the cell wall, glycocalyx, and intracellularcytoskeleton. Cell membranes can be artificially reassembled.[3][4][5]
Function
The cell membrane or plasma membrane surrounds the cytoplasm of animal and plant cells, physically separating the intracellular components from the extracellular environment.Fungi, bacteria and plants also have the cell wall which provides a mechanical support for the cell and precludes the passage of larger molecules. The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix and other cells to help group cells together to form tissues.
The membrane is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either "passive", occurring without the input of cellular energy, or active, requiring the cell to expend energy in transporting it. The membrane also maintains the cell potential. The cell membrane thus works as a selective filter that allows only certain things to come inside or go outside the cell. Cell employs a number of transport mechanisms that involve biological membranes:
1. Passive diffusion and osmosis: Some substances (small molecules, ions) such as carbon dioxide (CO2), oxygen (O2), and water, can move across the plasma membrane by diffusion, which is a passive transport process. Because the membrane acts as a barrier for certain molecules and ions, they can occur in different concentrations on the two sides of the membrane. Such a concentration gradient across a semipermeable membrane sets up an osmotic flow for the water.
2. Transmembrane protein channels and transporters: Nutrients, such as sugars or amino acids, must enter the cell, and certain products of metabolism must leave the cell. Such molecules are pumped across the membrane by transmembrane transporters or diffuse through protein channels. These proteins, also called permeases, are usually quite specific, recognizing and transporting only a limited group of chemical substances, often even only a single substance.
3. Endocytosis: Endocytosis is the process in which cells absorb molecules by engulfing them. The plasma membrane creates a small deformation inward, called an invagination, in which the substance to be transported is captured. The deformation then pinches off from the membrane on the inside of the cell, creating a vesicle containing the captured substance. Endocytosis is a pathway for internalizing solid particles (cell eating or phagocytosis), small molecules and ions (cell drinking or pinocytosis), and macromolecules. Endocytosis requires energy and is thus a form of active transport.
4. Exocytosis: Just as material can be brought into the cell by invagination and formation of a vesicle, the membrane of a vesicle can be fused with the plasma membrane, extruding its contents to the surrounding medium. This is the process of exocytosis. Exocytosis occurs in various cells to remove undigested residues of substances brought in by endocytosis, to secrete substances such as hormones and enzymes, and to transport a substance completely across a cellular barrier. In the process of exocytosis, the undigested waste-containing food vacuole or the secretory vesicle budded from Golgi apparatus, is first moved by cytoskeleton from the interior of the cell to the surface. The vesicle membrane comes in contact with the plasma membrane. The lipid molecules of the two bilayers rearrange themselves and the two membranes are, thus, fused. A passage is formed in the fused membrane and the vesicles discharges its contents outside the cell.
Prokaryotes
Gram-negative bacteria have a plasma membrane and an outer membrane separated by a periplasmic space. Other prokaryotes have only a plasma membrane. Prokaryotic cells are also surrounded by a cell wall composed of peptidoglycan (amino acid and sugar and chloroplasts). Some eukaryotic cells also have cells walls, but none that are made of peptidoglycan.
Structure
Fluid mosaic model
According to the fluid mosaic model of S.J. Singer and G.L. Nicolson (1972), which replaced the earlier model of Davson and Danielli, biological membranes can be considered as a two-dimensional liquid in which lipid and protein molecules diffuse more or less easily.[6] Although the lipid bilayers that form the basis of the membranes do indeed form two-dimensional liquids by themselves, the plasma membrane also contains a large quantity of proteins, which provide more structure. Examples of such structures are protein-protein complexes, pickets and fences formed by the actin-based cytoskeleton, and potentially lipid rafts.