Diffusion, Osmosis, and Movement Across a Membrane

Diffusion

• Spontaneous movement of particles from an area of high concentration to an area of low concentration
• Does not require energy (exergonic)
• Occurs via random kinetic movement
• Net diffusion stops when concentration on both sides equal (if crossing a membrane) or when there is a uniform distribution of particles
  • Equilibrium is reached
  • Molecules continue to move, but no net change in concentration (hence the phase "net diffusion" above
  • Diffusion of one compound is independent to diffusion of other compounds

Factors Affecting Diffusion Across a Plasma Membrane

• Diffusion directly through lipid bilayer
  • The greater the lipid solubility of the diffusing particle, the more permeable the membrane will be
  • All else being equal, smaller particles will diffuse more rapidly than larger particles
  • O₂, H₂O, CO₂ rapidly diffuse across lipid bilayer
• Diffusion of Hydrophilic Molecules Across a Plasma Membrane
  • Plasma membrane is semipermeable
  • Water, while polar, is small enough to freely move across the plasma membrane
  • Larger hydrophilic uncharged molecules, such as sugars, do not freely diffuse (they can diffuse, but it is very slow)
  • Charged molecules cannot diffuse through lipid bilayer
  • Ion channels and specific transporters are required for charged molecules and larger, uncharged molecules

Osmosis, the Passive Transport of Water

• Osmosis = the diffusion of water across a semi-permeable membrane
• Plasma membrane permeable to water but not to solute
  • Solute = dissolved particle
  • Solvent = liquid medium in which particles may be dissolved
  • If a solute does not readily cross a plasma membrane, it is called an osmotically active solute
• Water moves from solution with lower concentration of osmotically active solutes (i.e. the concentration of water is high)  to solution with higher concentration of osmotically active solutes (i.e. the concentration of water is low)
• Water moves from dilute solution to concentrated solution
• Osmotic potential is the total of all dissolved particles

How Will Water Move Across Semi-Permeable Membrane?

• Solution A has a glucose concentration of 100 mM
• Solution B has a fructose concentration of 100 mM
• How will the water molecules move? Answer

• Solution A has a glucose concentration of 100 mM
• Solution B has a fructose concentration of 75 mM
• How will the water molecules move? Answer

• Solution A has a glucose concentration of 100 mM
• Solution B has a NaCl concentration of 100 mM
• How will the water molecules move? Answer

Solution Types Relative to Cell

• Hypertonic Solution: Solute concentration of solution higher than cell
  • More dissolved particles outside of cell than inside of cell
  • Hyper = more (think hyperactive); Tonic = dissolved particles
  • Water moves out of cell into solution
  • Cell shrinks
• Hypotonic Solution: Solute concentration of solution lower than cell
  • Less dissolved particles outside of cell than inside of cell
  • Hypo = less, under (think hypodermic, hypothermia); Tonic = dissolved particles
  • Water moves into cell from solution
  • Cell expands (and may burst)
• Isotonic Solution: Solute concentration of solution equal to that of cell
  • No net water movement

Osmosis Produces a Physical Force

• Movement of water into a cell can put pressure on plasma membrane
• Animal cells will expand and may burst
  • Some cells, such as Paramecium have organelles called contractile vacuoles which are basically little pumps which pump excess water out of cell
  • You can alter the rate of contractile vacuole pumping by placing it in increasingly hypotonic solutions
• Organisms with a cell wall, such as plants, do not burst
  • Cell membrane pushes against cell wall
  • The rigid cell wall resists due to its own structural integrity
  • These opposing forces create turgidity, which keeps plants upright
  • If you don't water a plant, it wilts (this is called plasmolysis). Water it, the leaves will come back up do to the reestablishment of turgidity.
    • What part of the plant is responsible for drawing water into the plant cell?

Facilitated Diffusion

• Allows diffusion of large, membrane insoluble compounds such as sugars and amino acids
• Does not require energy (passive)
• Highly Selective
• Substance binds to membrane-spanning transport protein
• Binding alters protein conformation, exposing the other surface
• Fully reversible - molecules may enter the cell and leave the cell through the transport protein.
• Particles move from areas of high concentration to areas of low concentration.
• Movement rate of particles will saturate
  • Maximum rate limited by number of transporters
  • Once all transporters are operating at 100%, an increase in concentration will not increase rate

How to Cheat - Glucose Enters the Cell by Facilitated Diffusion

• Glucose binds to transport protein
• Transporter changers conformation and glucose is released into cell
• Intracellular glucose is immediately phosphorylated
  • phosphorylated glucose does not diffuse out (remember that the transport protein is very specific)
  • internal glucose (unphosphorylated) concentration remains low providing large concentration difference for entry

Regulation of Glucose Uptake by Insulin

• Insulin stimulates increase in number of glucose transporters at membrane surface
  • Increase number of transporters increases diffusion rate
  • Driving force (phosphorylation) remans the same
• Low insulin levels decrease the number of glucose transporters at membrane surface
  • Portions of membrane with transporters endocytose, trapping the transport protein in a vesicle
  • Vesicle cannot refuse with membrane until insulin levels increase

Diabetes

• Type I - Juvenile Diabetes - cannot make insulin
  • Autoimmune disease
  • Insulin-secreting pancreatic cells destroyed
• Type II - Adult Onset Diabetes - loss of ability to respond to insulin
  • Lack of membrane receptors for insulin
  • Therefore, cannot mobilize enough facilitative transport proteins to surface

Active Transport

• Movement across membrane with an energy cost (usually against concentration or electrochemical gradient, but not always)
• Used to pump specific compounds in or out of the cell
• Requires energy to overcome the concentration and electrochemical gradient or to allow a large or charged particle to cross membrane
• Requires specific integral membrane proteins
  • Can be saturated like facilitated diffusion proteins
  • The energy requirement distinguishes active transport from facilitated diffusion

The K⁺ / Na⁺ Pump: An Example of Active Transport

• Cellular [K⁺] is low and [Na⁺] is high - must pump K⁺ in and pump Na⁺ out
• K⁺ and Na⁺ transport require ATP energy
• Experimental evidence has shown that this pump will only work if [K⁺] is high on outside and [Na⁺] is high on inside.
• This pump works independent of concentration gradient
• The pump is an integral membrane protein
• Binds 3 Na⁺ inside cell
• ATP is hydrolyzed and phosphate group transferred to protein
• when the pump is phosphorylated, its configuration changes and it opens up the Na⁺ to the outside of the cell
• The Na⁺ are released (the altered configuration does not favor the binding of Na⁺)
• Two K⁺'s from the outside now bind to the altered protein
• The binding of the K⁺ causes the protein to lose its phosphate group
• Now that the phosphate group is gone, the altered protein reverts back to its original shape, which was open to the inside of the cell
• The original shape does not favor the binding of K⁺, so these are released. Na⁺ then binds to the protein and the process is repeated

The The K⁺ / Na⁺ Pump

Other Active and Transport Mechanisms - The H⁺ / Sucrose Pump

• H⁺ is actively pumped out by hydrolyzing ATP
• H⁺ accumulated outside the membrane, generating a concentration and electrochemical gradient
  • This is a common means to store energy in cells
  • Used in mitochondria & chloroplasts
• The H⁺ cannot cross the membrane, but there is a carrier protein.
• H⁺ binds to carrier protein, but sucrose must also bind. When both are bound, the configuration changes and the protein opens to the membrane interior.
  • This is known as cotransport as two molecules are pumped across a membrane, one "downhill" (with its gradient) coupled with one "uphill" (against its gradient)
  • It is also known as a symport as both molecules are crossing in the same direction
  • If the molecules are moving in opposite directions it is known as an antiport