Fluid Mosaic Model of the Plasma Membrane

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					             Fluid Mosaic Model of the Plasma Membrane

We know that membranes have certain properties.

  1. They act as a barrier between the cell and its environment,
     allowing a complex organized system to exist inside the cell.
  2. They permit the passage of selected substances into and out of the
  3. They flex, bend and flow to allow the cell to change shape.

                    The Fluid Mosaic Model
 A collage of many different proteins, lipids, and carbohydrates
 Main fabric of the membrane is the phospholipid
   Recall: phospholipid consists of 1 molecules of glycerol, 2 fatty
    acids and 1 phosphate group
   This structure causes hydrophilic and hydrophobic regions of the

Function of Membrane Proteins

     1. Transportation: Proteins that span the membrane provide a
        hydrophilic channel for certain substances (selective) to move
        across the membrane.
     2. Enzymes: Some membrane proteins act as enzymes that help
        carry out steps in a metabolic pathway.
     3. Receptor sites: Proteins exposed to the outside of the cell may
        have receptor sites for different chemical messengers such as
        hormones. This may initiate a chain reaction of chemical
        changes in the cell.
     4. Cell adhesion: Membrane proteins of adjacent cells may be
        hooked together in various protein-protein junctions.
     5. Attachment to the cytoskeleton: Some membrane proteins are
        attached to the cytoskeleton which is important in maintaining
        cell shape and fixes the location of certain membrane proteins.

Two populations of membrane proteins
 Integral proteins: Proteins that insert into the membrane
  (transmembrane proteins)
 Peripheral proteins: Proteins attached to the surface of the cell
  membrane (often to the exposed parts of the integral proteins). Some
  peripheral proteins are held in place by filaments of the cytoskeleton.

 Membrane carbohyrates are usually branced molecules consisting of
  fewer than 15 sugar units.
 Some membrane carbohydrates are covalently bonded to lipids:
 Most membrane carbohydrates are covalently bonded to proteins:
 Glyco: refers to the presence of carbohydrates

Function of Membrane Carbohydrates

Cell-cell recognition: The ability of a cell to determine if other cells it
encounters are alike or different from itself. Cells recognize other cells
by keying on surface molecules of the plasma membrane. The diversity
of glycoproteins and glycolipids on the membrane are likely candidates
as markers that distinguish one cell from another.

                    The Fluid Quality of Membranes
   Membranes are not static, solid sheet of molecules
   A membrane is held together by hydrophobic attractions
   Most membrane lipids and proteins can drift about laterally in the
    plane of the membrane
   At critical low temperatures, some membranes may solidify
   Problems with solidification:
     Permeability changes
     Enzymatic proteins become inactive
   Membranes with unsaturated tails remain fluid at lower temperatures
    when compared with membranes with saturated tails because
    unsaturated hydrocarbons do not pack together as closely as saturated
   Cholesterol also enhances membrane fluidity. It wedges in between
    phospholipids and hinders the close packing of them. Found in the
    plasma membrane of eukaryotes and animal cells at low temperatures.
                           Through the Cell

Every cell needs to maintain a steady environment in order to efficiently
carry out life functions. This steady state condition is called homestasis.
The cell membrane is a complex structure that allows for homeostasis to
be maintained.

Passive Transport

Diffusion: The movement of molecules from a region of high
concentration to a region of low concentration.

Osmosis: The diffusion of the solvent across a semi-permeable
membrane separating two solutions.

     Isotonic: [water] inside the cell equals the [water] outside the cell
     and equal amounts of water move in and out of the cell.
     Hypotonic: [water] outside the cell is greater than that inside the
     cell, water moves into the cell. This may cause the cell to burst
     known as lysis.
     Hypertonic: [water] inside the cell is greater than outside the cell.
     Water moves out of the cell. This may cause the cell to shrink
     known as plasmolysis.

Facilitated Diffusion: Passive movement of a substance into or out of
the cell by means of carrier proteins or channel proteins imbedded in the
cell membrane.
Carrier proteins: Accept non-charged molecules with a specific shape.

Channel proteins: Accept charged particles to pass through the cell