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
  • 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
• Water moves from solution with lower concentration of dissolved particles to solution with higher concentration of dissolved particles
• 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) remanis 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

• Cellullar [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