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The Cell Membrane
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The Cell Membrane






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The Cell Membrane The Cell Membrane Presentation Transcript

  • Membrane Structure and Function
  • Plasma Membrane
    The membrane at the boundary of every cell.
    Functions as a selective barrier for the passage of materials in and out of cells.
  • Membrane Composition
  • Fluid Mosaic Model
    Refers to the way the lipids and proteins behave in a membrane.
  • “Fluid”
    Refers to the lipid bilayer.
    Molecules are not bonded together, so are free to shift.
    Must remain "fluid" for membranes to function.
  • “Mosaic”
    Proteins: float in a sea of lipids.
    Proteins form a collage or mosaic pattern that shifts over time.
  • Protein Function in Membranes
    Enzymatic activity.
    Receptor sites for signals.
    Cell adhesion.
    Cell-cell recognition.
    Attachment to the cytoskeleton.
  • Types of Membrane Proteins
    Integral - inserted into the lipid bilayer.
    Peripheral - not embedded in the lipid bilayer, but are attached to the membrane surface.
  • Membranes are Bifacial
    The lipid composition of the two layers is different.
    The proteins have specific orientations.
    Carbohydrates are found only on the outer surface.
  • Question
    How do materials get across a cell's membrane?
  • Problems
    Lipid bilayer is hydrophobic. Hydrophilic materials don't cross easily.
    Large molecules don't cross easily. Too big to get through the membrane.
  • Mechanisms
    1. Passive Transport
    2. Active Transport
  • Passive Transport
    Movement across membranes that does NOT require cellular energy.
  • Types of Passive Transport
    1. Diffusion
    2. Osmosis
    3. Facilitated Diffusion
  • Diffusion
    The net movement of atoms, ions or molecules down a concentration gradient.
    Movement is from:
    High Low
  • Equilibrium
    When the concentration is equal on both sides.
    There is no net movement of materials.
  • Factors that Effect Diffusion
    1. Concentration
    2. Temperature
    3. Pressure
    4. Particle size
    5. Mixing
  • Osmosis
    Diffusion of water.
    Water moving from an area if its high concentration to an area of its low concentration.
    No cell energy is used.
  • Tonicity
    The concentration of water relative to a cell.
    1. Isotonic (same)
    2. Hypotonic (below)
    3. Hypertonic (above)
  • Isotonic
    Isosmotic solution.
    Cell and water are equal in solute concentration.
    No net movement of water in or out of the cell.
    No change in cell size.
  • Hypotonic
    Hypoosmotic solution
    Cell's water is lower than the outside water (more solutes).
    Water moves into the cell.
    Cell swells, may burst or the cell is turgid.
  • Hypertonic
    Hyperosmotic solution
    Cell's water is higher than the outside water (less solutes)
    Water moves out of the cell.
    Cell shrinks or plasmolysis occurs.
  • Facilitated Diffusion
    Transport protein that helps materials through the cell membrane.
    Doesn't require energy (ATP).
    Works on a downhill concentration gradient.
  • Active Transport
    Movement across membranes that DOES require cellular energy.
  • Types of Active Transport
    1. Carrier-Mediated
    2. Endocytosis
    3. Exocytosis
  • Carrier-Mediated Transport
    General term for the active transport of materials into cells AGAINST the concentration gradient.
    Movement is:
    low high
  • Examples
    1. Na+- K+ pump
    2. Electrogenic or H+ pumps
    3. Cotransport
  • Exocytosis
    Moves bulk material out of cells.
    Example - secretion of enzymes.
  • Endocytosis
    Moves bulk materials into cells.
    Several types known.
  • Types
    1. Pinocytosis - liquids
    2. Phagocytosis - solids
    3. Receptor Mediated - uses receptors to "catch" specific kinds of molecules.