Understanding Membrane Permeability and Transport Mechanisms in Cells

1. Chapter 7Membrane Structure & Function

2. I Can’s • Explain why membranes are selectively permeable • Describe the roles of phospholipids, proteins, & carbohydrates in membranes • Outline how water will move if a cell is placed in an isotonic, hypertonic, or hypotonic solution • Identify how electrochemical gradients are formed

3. 7.1 • The cell (plasma) membrane is selectively permeable • Allows some things to cross easier than others • Made primarily of phospholipids & proteins • Held together by weak interactions that cause the membrane to be fluid • The FLUID MOSAIC MODEL describes the the membrane as fluid, with proteins embedded in or associated with the phospholipid bilayer

5. phospholipids • Provide a hydrophobic barrier that separates the cell from its liquid environment • Hydrophilic cannot easily enter the cell • Hydrophobic can enter more easily

6. Proteins • 2 types of proteins: • 1) Integral • Completely embedded in the membrane • Some are transmembrane proteins that span the membrane completely • 2) Peripheral • Loosely bound to the surface of the membrane

7. Six major functions of membrane proteins: • Transport • Enzymatic activity • Signal transduction • Cell-cell recognition • Intercellular joining • Attachment to the cytoskeleton and extracellular matrix (ECM)

9. Carbohydrates • Crucial for cell-cell recognition (for immune function) • Important for developing organisms (tissue differentiation) • Cell surface carbs vary from species to species and are the reason that blood transfusions must be type-specific

10. 7.2 • Membrane structure results in selective permeability • Nonpolar molecules (hydrocarbons, CO2, & O2) • Hydrophobic & can dissolve in the phospholipid bilayer & cross the membrane

11. Hydrophobic core of the membrane • Impedes the passage of ions & polar molecules (hydrophilic) • Hydrophilic substances can avoid the lipid bilayer by passing through transport proteins (transmembrane proteins) • Carrier proteins bind to molecules and change shape to shuttle them across the membrane • Movement of water • Move through special transport proteins called aquaporins by accelerating passage

12. 7.3 • Passive transport is diffusion of a substance across a membrane with NO ENERGY investment • Hydrocarbons, CO2, & O2 exhibit PT • Passive diffusion • Travels from high concentration to a less concentration • Flows down the concentration gradient • Requires NO WORK • Relies on thermal motion energy

13. Osmosis = diffusion of water across a selectively permeable membrane 3 relationships: 1) Isotonic 2) Hypertonic 3) Hypotonic

14. Isotonic • No net movement across the membrane • Water crosses at the same rate in both directions

15. Hypertonic solution • Cell will lose water to its surroundings • More solutes around the cell so water moves to the higher concentration • Cell loses water, shrivels, & dies

16. Hypotonic • Water will enter faster than it leaves • Fewer solutes in the water around the cell • Movement of water follows the higher concentration of solutes • The cell will swell and possibly burst

18. Ions and polar molecules • Cannot easily pass across the membrane • Called facilitated diffusion • Utilizes transport proteins • TP are specific for what they transport

19. How transport proteins work: • 1) provide a hydrophilic channel that molecules can pass through • 2) bind loosely to the molecules and carry them through the membrane

21. 7.4 • Active transport uses energy to move solutes against their gradients • Moved from less concentrated to higher concentrated (think of uphill movement) • Requires energy (usually ATP)

22. Sodium-Potassium Pump • Pumps sodium out of the cell and potassium into the cell • Necessary for proper nerve transmission • Major energy consumer in your body

24. Diffusion of ions • Membrane Potential • The difference in electric charge across a membrane that is expressed in voltage • The inside carries a (-) charge • Leads to an attraction with a Cation such as sodium • This leads to 2 forces called the electrochemical gradient: • 1) a chemical force, which is the ion’s [ ] gradient • 2) a voltage gradient, attracts + ions and repels - ions

25. Electrogenic pump • A transport protein that generates voltage across a membrane • Na-K pump & Proton pump are examples

27. Cotransport • An ATP pump that transports a specific solute indirectly drives the active transport of other substances • The substance that was initially pumped across the membrane can do work as it moves back across the membrane by diffusion & will bring a second compound against its gradient

30. 7.5 • Bulk transport across the membrane occurs by exocytosis & endocytosis • Exocytosis • Vesicles from the cell’s interior fuse with the cell membrane • Expels the contents of the vesicles

31. Endocytosis • Cell forms new vesicles from the membrane (reverse of exo) • Allows the cells to take IN large molecules • 3 types: • 1) Phagocytosis • 2) Pinocytosis • 3) Receptor-mediated endocytosis

32. Phagocytosis – “cellular eating” • Occurs when the cell wraps pseudopodia around a solid particle and brings it into the cell

33. Pinocytosis – “Cellular drinking” • Cell takes in small droplets of extracellular fluid within small vesicles • Not specific, because it takes in anything

34. Receptor-mediated endocytosis • VERY SPECIFIC • Certain substances (ligands) bind to specific receptors on the cell’s surface (clusters) • Causes a vesicle to form around the substance and then pinch off into the cytoplasm