membrane

(noun)

A flexible enclosing or separating tissue forming a plane or film and separating two environments (usually in a plant or animal).

Related Terms

Examples of membrane in the following topics:

  • Injuring the Plasma Membrane

    • Several types of antimicrobial drugs function by disrupting or injuring the plasma membrane.
    • The plasma membrane or cell membrane is a biological membrane that separates the interior of all cells from the outside environment.
    • The plasma membrane is selectively permeable to ions and organic molecules.
    • The membrane basically protects the cell from outside forces.
    • Diagram of a typical gram-negative bacterium, with the thin cell wall sandwiched between the red outer membrane and the thin green plasma membrane.

  • Gram-Negative Outer Membrane

    • The Gram-negative cell wall is composed of an outer membrane, a peptidoglygan layer, and a periplasm.
    • In the Gram-negative Bacteria the cell wall is composed of a single layer of peptidoglycan surrounded by a membranous structure called the outer membrane.
    • In Gram-negative bacteria the outer membrane is usually thought of as part of the outer leaflet of the membrane structure and is relatively permeable.
    • Sandwiched between the outer membrane and the plasma membrane, a concentrated gel-like matrix (the periplasm) is found in the periplasmic space.
    • Together, the plasma membrane and the cell wall (outer membrane, peptidoglycan layer, and periplasm) constitute the gram-negative envelope.

  • Alteration of Membrane Permeability

    • The lipid portion of the outer membrane is impermeable to charged molecules.
    • As a phospholipid bilayer, the lipid portion of the outer membrane is impermeable to charged molecules.
    • However, channels called porins are present in the outer membrane that allow for passive transport, across the outer membrane, of many ions, sugars, and amino acids.
    • These molecules are present in the periplasm, the region between the cytoplasmic and outer membranes.
    • After binding to lipopolysaccharide (LPS) in the outer membrane of gram-negative bacteria, polymyxins disrupt both the outer and inner membranes .

  • Proton Reduction

    • Anaerobic respiration utilizes highly reduced species - such as a proton gradient - to establish electrochemical membrane gradients.
    • First, the electrical component is caused by a charge difference across the lipid membrane.
    • Second, a chemical component is caused by a differential concentration of ions across the membrane.
    • The electrochemical potential difference between the two sides of the membrane in mitochondria, chloroplasts, bacteria, and other membranous compartments that engage in active transport involving proton pumps, is at times called a chemiosmotic potential or proton motive force.
    • The membrane in question is the inner mitochondrial membrane in eukaryotes and the cell membrane in prokaryotes.

  • Filtration

    • Fluids that would be damaged by heat, irradiation, or chemical sterilization can be sterilized by microfiltration using membrane filters.
    • The typical microfiltration membrane pore size range is 0.1-10 µm, with the most commonly used being 0.2 µm ; and 0.45 µm is sufficient to eliminate bacteria and fungi.
    • To achieve higher total throughput or avoid premature blockage, pre-filters might be used to protect small pore membrane filters.
    • Membrane filters used in production processes are commonly made from materials such as mixed cellulose ester or polyethersulfone.
    • To ensure proper functioning of the filter, the membrane filters are integrity tested post-use or sometimes pre-use.

  • Chemical Analysis of Microbial Cytoplasm

    • Microbial cytoplasm is contained within the cytoplasmic membrane and includes the cytosol, macromolecules, and inclusions.
    • The bacterial cytoplasmic membrane plays a role in permeability and energy conservation in microbial cell structure.
    • The membranes however, may contain compounds called hopanoids and various fatty acids as well.
    • The cytoplasm itself is enclosed within the membrane.
    • These lipid droplets store molecules such as fatty acids which are present in the cytoplasmic membrane of prokaryotes.

  • Spirochaetes

    • Spirochaetes are characterized by the presence of a double-membrane and long, spiral-shaped cells that are chemoheterotrophic.
    • The spirochaetes belong to a phylum of distinctive double-membrane bacteria that are characterized by their long, spiral-shaped cells.
    • The flagella, in spirochaetes, runs lengthwise between the inner and outer membranes in the periplasmic space.
    • Leptospira, the cause of leptospirosis - leptospirosis is transmitted to humans from animals and a common form of transmission is by allowing contaminated water to come in contact with unhealed breaks in the skin, eyes and mucous membranes.

  • Osmotic Pressure

    • Osmotic pressure is the pressure which must be applied to a solution to prevent the inward flow of water across a semipermeable membrane.
    • Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane.
    • It is also defined as the minimum pressure needed to nullify osmosis.The phenomenon of osmotic pressure arises from the tendency of a pure solvent to move through a semi-permeable membrane and into a solution containing a solute to which the membrane is impermeable.
    • As a result, when a cell is placed in a hypotonic solution, water rushes into the membrane, increasing its volume.
    • Eventually, the cell's membrane is enlarged such that it pushes against the cell's rigid wall.

  • Attachment and Entry to the Host Cell

    • HA proteins bind to cells with sialic acid on the membranes, such as cells in the upper respiratory tract or erythrocytes.
    • Second, once bound, it facilitates the entry of the viral genome into the target cells by causing the fusion of the host endosomal membrane with the viral membrane.
    • The cell membrane then engulfs the virus and the portion of the membrane that encloses it pinches off to form a new membrane-bound compartment within the cell called an endosome, which contains the engulfed virus.
    • This so-called "fusion peptide" acts like a molecular grappling hook by inserting itself into the endosomal membrane and locking on.
    • Then, when the rest of the HA molecule refolds into a new structure (which is more stable at the lower pH), it "retracts the grappling hook" and pulls the endosomal membrane right up next to the virus particle's own membrane, causing the two to fuse together.

  • Group Translocation

    • Bacteria may have a single plasma membrane (Gram-positive bacteria) or an inner membrane plus an outer membrane separated by the periplasm (Gram-negative bacteria).
    • Proteins may be incorporated into the plasma membrane.
    • The basic mechanism at the plasma membrane is similar to the eukaryotic one.
    • In addition, bacteria may target proteins into or across the outer membrane.
    • Systems for secreting proteins across the bacterial outer membrane may be quite complex.