Section Two

• Solutes: Substances dissolved in a liquid
• Solvent: The liquid in which solutes are dissolved in
• Solution: Solutes dissolve in a solvent
• Passive Diffusion: movement of molecules from one location to another as the result of their random thermal motion
• Passive diffusion requires no energy/no ATP
• Molecules diffuse from an area of higher concentration to an area of lower concentration
• Passive diffusion requires no outside energy
• There is net movement until the concentration is equal, known as Dynamic equilibrium
• Diffusion is rapid over short distances and slower over long distances
• Diffusion is directly related to temperature, with molecules moving faster at higher temperatures
• Diffusion is inversely related to molecule size, where larger molecules move slower
• Diffusion can take place in an open space or across a partition that separates 2 systems
• Diffusion depends on the molecular properties to cross the lipid bilayer, considering polar vs nonpolar molecules
• The rate of diffusion is directly proportional to the surface area of the membrane, meaning a larger surface area allows more molecules to diffuse per unit time
• The rate of diffusion is inversely proportional to the thickness of the membrane, resulting in a slower rate of diffusion in thicker membranes
• Small molecules (CO2 and O2) and nonpolar molecules have high permeability across membranes
• Charged molecules (Na+, Cl-) and large polar molecules (glucose) cannot diffuse through lipid bilayers
• Small polar molecules (H2O, ethanol, urea) have certain permeability across membranes
• Facilitated diffusion involves specialized protein molecules, but they are still going down (high to low) their concentration gradients
• Facilitated diffusion does not require energy
• Facilitated diffusion contains two specialized proteins: Channels and Carrier-mediated that are both integral
• Some channels in facilitated diffusion can include leak channels (always open) and ion channels (allow ions to diffuse across the membrane)
• Selectivity filter: Permeation restricted according to size and charge
• Gates: Can fluctuate between open and closed state
• Ligand: Any molecule or ion that is bound to a protein
• Binding Site: The region of a channel protein where the ligand binds
• The binding of a ligand to the binding site changes the conformation of the channel protein
• Chemical Specificity: The ability of a channel protein binding site to bind a specific ligand
• The strength of ligand-protein binding is a property of the binding site known as affinity
• Affinity determines how likely it is that a bound ligand will leave the protein surface and return the channel to an unbound state
• Binding sites that tightly bind a ligand are high affinity sites whereas binding sites that weakly bind are low affinity binding sites
• Carrier-mediated (facilitated diffusion): movement of substances through a membrane by means of a conformation change of transporters. An example of this is a glucose transporter
• Four factors that determine rate of diffusion of a substance

  1. The solute concentration
  2. The affinity of the transporter for the solute
  3. The number of transporters in the membrane
  4. The rate at which the conformational change in the carrier protein occurs
• Saturation: An equilibrium is reached between unbound solutes/ligands in a solution and their corresponding protein binding sites
• Percent saturation of binding sites increases with increases solute/ligand concentration until all sites are occupied
• Cystic fibrosis: disease of facilitated diffusion
• The Cystic Fibrosis Transmembrane Regulator (CFTR channel) is absent or defective in Cystic Fibrosis. Therefore, Cl- transport in both directions across epithelium is impaired
• Cystic fibrosis is a genetically inherited disease. Heterozygous person (some CFTR channels are working) is more likely to survive cholera (impacts CFTR channels, so not as severe diarrhea)
• Active Transport: energy is used to move a substance against (low concentration to high concentration) its concentration gradient
• In active transport, it is required for the substance to bind to a transporter in the membrane (pumps)