Phospholipid molecules form a bilayer of the plasma membrane.

The hydrophilic heads are exposed to the aqueous environments on both side of the membrane.

The hydrophobic tails are on the inside, away from water.

Passive transport moves substances through the lipid bilayer down their concentration gradients:

Active transport moves substances across a membrane against their concentration gradients. Energy for this work is often supplied by ATP.

• Diffusion.

  Hydrophobic molecules and small molecules can pass through a membrane by diffusion.

  The dye diffuses down a concentration gradient from where it is more concentrated to where it is less concentrated.

  Eventually the solution reaches a dynamic equilibrium: the solute molecules continue to cross the membrane, but at equal rates in both directions.

  Review:

• Osmosis determines the             of the environment across a membrane.
  
  • Osmosis is the diffusion of water across a semipermeable membrane.

    Two sugar solutions of different concentrations are separated by a semipermeable membrane, which the solvent (water) can pass through but the solute (sugar) cannot.

    Water molecules diffuses from the solution with less concentrated solute to that with more concentrated solute.

  
  
  • Tonicity.

    In hypertonic environments where solute concentration is greater than it is inside the cell, water tends to move outside the cell. In hypotonic environments where solute concentration is less than it is inside the cell, water tends to move inside the cell. In isotonic environments where solute concentration is the same as it is inside the cell, there is no net water movement.

    Review:

   
  
• Organisms without cell          must have adaptations for                   to maintain water balance.
  
  • Osmotic effect on animal cell. In a hypotonic environment, water moves into a cell, causing it to lyse (burst) unless it can offset the uptake of water. An animal cell fares best in an isotonic environment where there is no net water movement. In a hypertonic environment, water tends to move out of an animal cell, causing it to shrink.
  
  
  • The contractile vacuole of the freshwater protist Paramecium is a mechanism for osmoregulation that offsets osmosis in a hypotonic environment by pumping excess water out of the cell. Video:
   
  
• Plant cells fare best in              environments.

• Osmotic effect on plant cell. In a hypotonic environment, where the uptake of water is balanced by the cell wall pushing back, plant cells are turgid (firm) and are healthiest. In isotonic environments plant cells become flaccid (limp). in hypertonic environments the cell membrane pulls away from the wall and the cell is plasmolyzed. Plasmolysis:

• Facilitated diffusion occurs when material diffuse across the plasma membrane via proteins embedded in the membrane.

• Channel proteins provide corridors that allow a specific particle to cross the membrane.
• Carrier proteins undergo a change in shape that translocates the solute-binding site across the membrane.

  These are passive transport mechanisms that move solutes down their concentration gradient.

  Review:

• Active transport.

  The sodium-potassium pump maintains high concentrations of Na^+ outside the cell and K^+ inside the cell, against their concentration gradients.

• 3 cytoplasmic Na^+ bind to the pump.
• ATP donates a phosphate group for energy.
• The protein changes its shape, expelling Na^+ to the outside.
• 2 extracellular K^+ bind to the pump, releasing the phosphate.
• The pump resumes its original shape.
• K^+ is released inside.

  Review:

• In               , the cell takes in macromolecules by forming vesicles from the plasma membrane via invagination.
  
  • Endocytosis.

    
    

  • phagocytosis
    
  • pinocytosis
    
  • receptor-mediated endocytosis

    Review:

  
  
  • Phagocytosis (cell eating). A cell engulfs a particle by wrapping pseudopodia around it and packaging it within a membrane-enclosed sac large enough to be classified as a vacuole. The particle is digested after the food vacuole fuses with a lysosome for digestion.
  
  
  • Pinocytosis (cell drinking). The cell "gulps" droplets of extracellular fluid, together with molecules dissolved in the droplet, into tiny vesicles. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports.
  
  
  • Receptor-mediated endocytosis.

    Receptor proteins embedded in the membrane can bind to specific extracellular substances (ligands).

    The bound complex forms a pit lined by a layer of coat proteins, which pinches off to form a vesicle.

   
  
• In              , transport             migrate to the plasma membrane, _ANI_7,13 fuse with it, and release their contents.
  
  • Exocytosis.

    Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents outside the cell.

    Many secretory cells use exocytosis to export their products such as hormones. /VId/Johnson6e/ch05/endocytosis_exocytosis.swf

   
  
  Review: