Lecture 6 - Membrane Transport.docx

**[Lecture 6: Membrane Transport]** Membrane Transport - The notion of membrane permeable, impermeability, selective permeablity. - Permeability: (Oxygen) - Impermeability: (Ions, large particles) - Selective Permeability: - What properties influence/decides whether a...

**[Lecture 6: Membrane Transport]** Membrane Transport - The notion of membrane permeable, impermeability, selective permeablity. - Permeability: (Oxygen) - Impermeability: (Ions, large particles) - Selective Permeability: - What properties influence/decides whether a substance is permeable or not? 1. Relative Solubility of the particle in lipid (because plasma membrane is protected by phospholipids) - Uncharged or Non-Polar molecules → Highly lipid-soluble = Easily permeate plasma membrane. - Charged or polar molecules → Low lipid-soluble, water-soluble = Cannot permeate or need to be selected - Ions = Need Channels - Proteins & Glucose (too large) = Assisted Transport 2. Size of the particle How does lipid-soluble molecules or particles that require channels go through membrane? - Force is required - **Active Force:** Requires expenditure of energy. - **Passive Force:** Doesn't require energy. Unassisted or Passive Transport - Penetrate Plasma Membrane on their own due to - Diffusion down a concentration gradient - **Diffusion:** random movement - **Concentration gradient:** the difference of particles moving from an area with a high number of particles to a lower number. - Movement along electrical gradient - **Electrical Gradient:** Additional charge from one side to the other. 1. **Passive Diffusion of Particles** 1. Molecules are in constant, random motion 2. Come to a steady state over time (Equilibrium/Steady state) 3. Diffusion 1. Concentration Gradient 2. Net Diffusion: There is equal distribution from one side to another. (Doesn't mean the molecules are no longer movement) What happens when there's a Plasma Membrane involved?/ Reminder: - Hydrophilic, Charged, Water-soluble, ions, proteins, Polar, Large molecules, Amino Acid = cannot cross - Hydrophobic, uncharged, small molecules, non-polar = easy to cross plasma membrane. - **Down the Concentration Gradient:** From High to Low Concentration. - Substance can permeate the membrane = Diffusion Occurs, passive (force, but no energy required) - Example: Oxygen trans across lung membrane - Substance membrane is impermeable = No Diffusion What controls speed at which molecules cross? Fick's Laws of Diffusion - Collective influence of these factors affect the speed (net rate of net diffusion) of a substance: 1. Concentration Gradient Level 1. Higher level of concentration of molecules on one side will result in quicker rate of net diffusion. 2. Surface Area of Membrane 2. Larger surface are means Molecules can pass faster due to more space to move 3. Lipid Solubility 3. Ability to 4. Weight of the Substance 4. Larger molecules take more time or cannot cross 5. Distance traveled 5. Depends on the thickness of Plasma membrane 1. **Passive Diffusion of Ions** - Ions are electrically charged, which affects their movement - Alike charges = repel - Opposite charges = attract - Negatively charged = cations, Positively charged = anions - Difference in charge creates **Electrical Gradient** - Combination of Concentraion & Electrical Gradient effect = Electrochemical Gradient - The concentration and electrical gradient can move in opposite or same direction (moving inside or outside cell) Osmosis - **Osmosis:** Diffusion of water. **What drives water to move to the other side?** - Concentration Gradient → water wants to move to where there is less of it. - Solute wants to move to where there is less of it - They want to reach equilibrium - Molecules continue to move to both sides even at equilibrium **Water is Hydrophilic, how does it cross the Plasma Membrane?** - Due to it being a small molecule and not highly charged, so it can slip through lipids - **Aquaporins:** Channels that specifically allow water to cross though - Concentration usually refers to the density of the solute in a given volume of water. - ***At Equilibrium, molecules are still moving across the membrane, however there is no net movement.*** \*Osmotic Pressure (pulling pressure): a measure of the tendency for water to move into that solution because of the concentration of non-penetrating solutes and water. - Water rushing into the other side to reach steady state due to the solutes inability to permeate the membrane. \*Hydrostatic (fluid) Pressure: pressure excrete by a standing or stationary, fluid on an object (the membrane). - Water wanting to leave due to the high influx of water coming onto the other side.