Ch. 3: Plasma Membrane Structure and Function

1. Ch. 3: Plasma Membrane Structure and Function

2. Biochemistry: The unique properties of water δ + Hydrogen bonding is when the partial + charge on Hydrogen is attracted to the partial – charge of another compound. δ + δ - Water molecules have polar covalent bonds.

3. Properties of Phospholipids Phospholipids • Glycerol with Phosphate Head + 2 Fatty Acid Chains • Amphiphilic (“Both” “lover”) • Hydrophilic head • Hydrophobic tail • Forms 2 layers in water (negatively charged phosphate head attracted to + end of polar water molecules) • Makes up cell membranes P.Membrane (PM) held together by weak hydrophobic interactions Phosphate Glycerol Fatty Acids

4. Fluid Mosaic Model Fluidity: (not rigid) • P.Membrane (PM) held together by weak hydrophobic interactions (bi-lipid tails face each other, away from water) • Lateral drifting ability of lipids • Temperature Dependent

5. Why is this rare? Cholesterol puts gaps between phospholipids, increasing fluidity Unsaturated tails prevents packed phospho-lipid rafts

6. “Mosaic” PM is made up of a mosaic or “collage” of: • Phospholipids • cholesterol • integral and peripheral proteins • glycolipids • glycoproteins Hey Sugar! – Let’s “stick” Hey Suga-! glycocalyx

7. Integral or Transmembrane Proteins • Penetrate hydrophobic core of membrane Surface or Peripheral Proteins • Loosely bound to surface • Some attaches to cyto-skeleton or ECM (Extracellular matrix)

8. Review: What organelles are responsible for creating membrane proteins?

10. Membrane Transport • Cells NEED to be able to: • remove waste • take in necessary nutrients from interstitial fluids • Send out signals to other cells • Receive signals from other cells • Transport Classified as: • Passive • Active

11. Selective Permeability of Plasma Membrane General rule: like dissolves like • Non-polar, hydrophobic solutes dissolve in lipid • Ions, hydrophillic, or polar solutes dissolve in water

12. Selective Permeability of Plasma Membrane Selective Permeability: some substances can pass through lipid core or membrane more easily than others • CO2, O2, non-polar molecules, and other lipids, are hydrophobic and can pass hydrophobic lipid membrane core easily • Water, sugars, charged ions, or polar molecules cannot pass lipid core easily  so must use hydrophillic transport proteins to pass (ex. Aquaporins) • Small molecules are more permeable than larger ones

13. Passive Transport • Molecules move down [gradient] (from high to low concentration) until equilibrium is reached • Spontaneous process • No ATP needed; uses Kinetic energy (KE) or Hydrostatic Pressure as E source • Types of Passive Transport: • Simple Diffusion • Facilitated Diffusion • Osmosis • Filtration

14. Simple Diffusion • Diffusion – molecules of any substance moves down [gradient], unassisted • Ex. O2 in blood, CO2 in cells Back to Types of PT

15. Back to Types of PT Facilitated Diffusion • Assisted diffusion of molecules with help from channels or carriers Channels specific for particular molecule, like sugars, amino acids Carriers move substances like ions, water. Selective by size and charge

16. Water always moves from hypotonic to hypertonic Osmosis • Diffusion of WATER across the membrane • Tonicity dependent • Isotonic solution: solution in equilibrium to another solution across the membrane • Hypotonic: solution with less dissolved [solute], higher [water] compared to another solution • Hypertonic: solution with more dissolved [solute], lower [water] compared to another solution Back to Types of PT

17. Filtration • “Forcing” of water and solutes through membrane by hydrostatic pressure • Selective only by SIZE • Ex. only blood cells/proteins too large to pass are held back

18. Active Transport • Molecules move up or against [gradient] (from low to high) to create an electrochemical gradient • Nonspontaneous • Requires ATP as E source • Types: • Primary Active Transport (T) • Secondary Active (T) • Clathrin-coated Vesicular (T) • Endocytosis • Exocytosis

19. Active Transport generates an electrochemical gradient: charge difference (disequilibrium) between both sides of the membrane Back to Types of AT

20. Back to Types of AT Primary Active Transport Uses ATP E directly Ex 1: Sodium-Potassium Pump 3-D overview • Pump keeps Na+ moving out of the cell, against its gradient, building its concentration (disequilibrium) • Pump keeps K+ moving into the cellagainst its gradient, building its concentration • Na+:K+ ratio 3 out : 2 in • Na+:K+ pump uses high concentration gradients to store PE for future cellular work or for secondary AT • Ex 2: Pumping H+ ions into lysosome to create acidic env’t for cellular digestion

21. Back to Types of AT H+ H+ H+ H+ H+ H+ Secondary Active Transport(Coupled Transport) Symport: two transported substances move in the same direction Antiport: two transported substances move in opposite direction (“wave” to each other) Ex. Hydrogen gradient creating PE • Involves the transport of a substance against a concentration gradient powered indirectly by an ATP powered pump ATP ADP + Pi Sucrose transported against gradient into cell, using KE stored from H gradient, falling back down gradient

22. Clathrin: protein coat on PM helps with 1) specifying cargo 2) membrane deformation Endocytosis • The engulfing of substances by pseudopods extensions of the plasma membrane • Three types: • Phagocytosis (cell eating – lg. particles engulfed) • Pinocytosis (cell drinking – sm. ions and liquids engulfed) • Receptor Mediated Endocytosis (use of surface proteins to engulf a specific substrate) Often hijacked by pathogens that mimic a needed substance by the cell

23. Exocytosis Fusing of vesicles to the plama membrane, thus releasing its contents

24. Function of Membrane Proteins • TRANSPORT • protein channels or carriers for passive transport • protein pumps for active transport • Clathrin-lined membrane for vesicular transport • SIGNAL TRANSDUCTION • substrates bind to protein surface  sends a signal within the cell to start a chemical chain reaction or cell response • INTERCELLULAR JOINING • Ex. Gap Junctions, Tight Junctions, Desmosome • CELL to CELL RECOGNITION • Sugar on glycoproteins or glycolipids act as “name tags” for cells. Recognition of invaders, helps with cell communication and coordination • ENZYMATIC • Catalysis of Chemical Reactions at the Membrane Surface • CYTOSKELETON and ECM ATTACHMENT • Maintenance of Cell Shape End of Slide Show

25. Signal Transduction 3 Stages of Signal Transduction • Reception: A ligand or substrate binds to receptor protein. Receptor proteins can be on the cell surface, but not always. Receptor protein changes shape • Transduction: Amplifies and sends the signal through chemical relay • Cell Response: Specific response is triggered

26. Examples of Signal Transduction Why is this hormone-receptor protein not found on the surface of the plasma membrane? Steroids and Hormones are types of lipids, which can pass through phospholipid membranes easily. Back to Function of Membrane Proteins

27. Back to Function of Membrane Proteins Cell Junctions Tight Junctions: -Integral proteins of neighboring cells fuse -prevents leakage btwn cells into extracellular space (ex. Digestive tract) Gap Junctions: communicating junction Hollow, transmembrane protein cylinders, connexons, that provide cytoplasmic channels btwn cells • Desmosomes: • -“anchoring” junctions • Intermediate filaments extend from disc-shaped plaque to reduce tearing due mechanical stress to prevent separation • Disc-shaped plaque w/ linker protein fibers “zipping” tissues together to prevent separation Transports ions, simple sugars, small molecules Abundant in ion-dependent excitable cells (ex. neurons)