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:

  • Membrane Fluidity

    • The mosaic nature of the membrane, its phospholipid chemistry, and the presence of cholesterol contribute to membrane fluidity.
    • There are multiple factors that lead to membrane fluidity .
    • First, the mosaic characteristic of the membrane helps the plasma membrane remain fluid.
    • It lies alongside the phospholipids in the membrane and tends to dampen the effects of temperature on the membrane.
    • Explain the function of membrane fluidity in the structure of cells

  • 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.

  • Components of Plasma Membranes

    • The plasma membrane protects the cell from its external environment, mediates cellular transport, and transmits cellular signals.
    • The plasma membrane (also known as the cell membrane or cytoplasmic membrane) is a biological membrane that separates the interior of a cell from its outside environment.
    • The primary function of the plasma membrane is to protect the cell from its surroundings.
    • The membrane also maintains the cell potential.
    • The cell employs a number of transport mechanisms that involve biological membranes:

  • Selective Permeability

    • Plasma membranes are asymmetric: the interior of the membrane is not identical to the exterior of the membrane.
    • On the interior of the membrane, some proteins serve to anchor the membrane to fibers of the cytoskeleton.
    • This adds considerably to the selective nature of plasma membranes.
    • Polar substances present problems for the membrane.
    • The exterior surface of the plasma membrane is not identical to the interior surface of the same membrane.

  • Osmosis

    • Osmosis is the movement of water through a semipermeable membrane according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes.
    • Semipermeable membranes, also termed selectively permeable membranes or partially permeable membranes, allow certain molecules or ions to pass through by diffusion.
    • While diffusion transports materials across membranes and within cells, osmosis transports only water across a membrane.
    • The semipermeable membrane limits the diffusion of solutes in the water.
    • On both sides of the membrane the water level is the same, but there are different concentrations of a dissolved substance, or solute, that cannot cross the membrane (otherwise the concentrations on each side would be balanced by the solute crossing the membrane).

  • The Plasma Membrane and the Cytoplasm

    • Despite differences in structure and function, all living cells in multicellular organisms have a surrounding plasma membrane (also known as the cell membrane).
    • Cholesterol, also present, contributes to the fluidity of the membrane.
    • This structure causes the membrane to be selectively permeable.
    • In the case of the plasma membrane, only relatively small, non-polar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar).
    • Explain the structure and purpose of the plasma membrane of a cell

  • Membrane Potentials as Signals

    • Membrane potential (also transmembrane potential or membrane voltage) is the difference in electrical potential between the interior and the exterior of a biological cell.
    • The membrane serves as both an insulator and a diffusion barrier to the movement of ions.
    • Ion transporter/pump proteins actively push ions across the membrane to establish concentration gradients across the membrane, and ion channels allow ions to move across the membrane down those concentration gradients, a process known as facilitated diffusion.
    • The membrane potential has two basic functions.
    • Signals are generated by opening or closing of ion channels at one point in the membrane, producing a local change in the membrane potential that causes electric current to flow rapidly to other points in the membrane.

  • Fluid Mosaic Model

    • Nicolson in 1972 to explain the structure of the plasma membrane.
    • Plasma membranes range from 5 to 10 nm in thickness.
    • Proteins make up the second major component of plasma membranes.
    • The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins.
    • Integral membrane proteins may have one or more alpha-helices that span the membrane (examples 1 and 2), or they may have beta-sheets that span the membrane (example 3).

  • Epithelial Membranes

    • The mucous membranes are linings of ectodermal origin.
    • These mucus membranes are involved in absorption and secretion.  
    • These membranes exist in the  hollow organs of the digestive, respiratory, and urogenital tracts.
    • The term "mucous membrane" refers to where they are found in the body; not every mucous membrane secretes mucus.
    • Most mucous membranes contain stratified squamous or simple columnar epithelial tissue.

  • Molecules in Biological Membranes