Transport mechanism

• Plasma membrane protects a cell by acting as a barrier between its living contents and the surrounding environment.
• Every cell has a plasma membrane, a phospholipid bilayer with embedded proteins.
• Structure of Plasma Membrane

  A. Phospholipids arranged as a bilayer

  B. Membrane carbohydrates

  C. Membrane Proteins
• A.Phospholipids arranged as a bilayer


  1.Phosphate head

  • hydrophilic , polar

  2. Fatty acid tails

  • hydrophobic, non- polar
• B. Membrane carbohydrates


  Ability of a cell to distinguish one cell from another.

  Basis for rejection of foreign cells by immune system
• Both phospholipids and proteins can have attached carbohydrate chains. These molecules are called glycolipids and glycoproteins
• C. Membrane Proteins (Classes of membrane proteins)

  A. Peripheral proteins

  B. Integral proteins
• 1. Peripheral proteins - loosely bound to surface of membrane. Example: cell surface identity marker (antigens)
• B. Integral proteins- penetrate lipid bilayer, usually across whole membrane Example: transport proteins Integral proteins can be:


  a. Channel proteins Involved in passage of molecules through the membrane They have a channel that allows a substance to simply move across the membrane b. Carrier proteins They combine with a substance and help it move across a membrane c. Cell recognition proteins

  They are glycoproteins Helps the body recognize when it is being invaded by pathogens d. Receptor proteins Have shape that allows a specific molecule to bind to it
• MOVEMENT OF SUBSTANCES ACROSS THE MEMBRANE

  a. Cells need nutrients and oxygen. b) Cells produce waste product which exit through the plasma membrane. c) The plasma membrane control the types and the amounts of substances needed by the cell at any one time.
• The phospholipid bilayer is permeable to:

  a. Small non-polar (hydrophobic) molecules that are lipid-soluble, such as fatty acids, glycerol, steroid, vitamin A, D, E and K. b. Small uncharged molecules, such as water, oxygen and carbon dioxide. These molecules are small enough to squeeze through between the phospholipid gaps by simple diffusion or osmosis down their respective concentration gradients.
• The phospholipid bilayer is not permeable to:

  a. Large polar molecule, that are not soluble in lipid, such as glucose, amino acids, nucleic acids and polysaccharides. b. Ions (charged), regardless of size, such as: H+, Na+, HCO3-, K+, Ca²+, and Mg²+
• TYPES OF TRANSPORT ACROSS THE PLASMA MEMBRANE

  A. Passive transport No energy needed Movement down concentration gradient B. Active transport Movement against concentration gradient requires ATP
• 1. Diffusion Movement of molecules from a higher to lower concentration down their concentration gradient until equilibrium is achieved and they distribute equally.
• Osmosis is the movement of water molecules from region of high water concentration to low water concentration through a semi-permeable membrane.
• OSMOSIS


  a. ’ ISO ’ means the same as and tonicity refers to the strength (concentration of solute) of the solution. Two solution are isotonic if they have the same solute concentrations.

  b. ‘ HYPER ’ means more than. Solution A is hypertonic to solution B if solution A has a higher solute concentration than solution B.

  c. ‘ HYPO ’ means less than. Solution A is hypotonic to solution B if solution A has a lower solute concentration than solution B.
• 3. Facilitated Diffusion

  Pore protein (channel protein)


  Charge ions (such as Na+, K+, Ca²+, and Mg²+) cannot diffuse across the non-polar center of the phospholipids bilayer.

  Pore proteins open up pores or channel across the membrane to allow entry or exit

  Each pore or channel is specific and will only allow one particular type of ion
• 3. Facilitated Diffusion

  Carrier proteins


  They allow larger polar molecules (such as sugar and amino acids) to pass through. A particular protein attaches itself to the binding site of a carrier protein. Then the carrier protein changes shape and delivers the molecule across the plasma membrane.
• 4. Active Transport


  Active transport is the movement of particles across the plasma membrane against the concentration gradient, that is from a region of low concentration to a region of high concentration.
• 5. Membrane Assisted Transport


  Moving large molecules into & out of cell requires ATP! through vesicles & vacuoles
• 5. Membrane Assisted Transport

  A. Endocytosis

  • phagocytosis = “cellular eating”
  • pinocytosis = “cellular drinking”
  • receptor-mediated endocytosis