1. Chapter 7: Membrane Structure & Function Plasma membrane Composition: primarily lipids (phospholipids) & proteins with some carbohydrates (glycolipids or glycoproteins for cell recognition) Composition: primarily lipids (phospholipids) & proteins with some carbohydrates (glycolipids or glycoproteins for cell recognition) Arranged in a fluid mosaic Arranged in a fluid mosaic Lipid bi-layer with embedded proteins Lipid bi-layer with embedded proteins

2. Discovery of plasma membrane structure 1915- Red blood cell membranes analyzed; lipid & protein composition discovered 1915- Red blood cell membranes analyzed; lipid & protein composition discovered 1925- Gorter & Grendel suggest membrane is phospholipid bi-layer 1925- Gorter & Grendel suggest membrane is phospholipid bi-layer 1935- Davson & Danielli suggest proteins sandwich phospholipids (FALSE) 1935- Davson & Danielli suggest proteins sandwich phospholipids (FALSE) 1950s- Electron Microscopes used to study membrane structure 1950s- Electron Microscopes used to study membrane structure 1972- Singer & Nicolson suggest proteins are dispersed (“float”) within the lipid bi-layer (further shown by freeze-fracture electron microscopy 1972- Singer & Nicolson suggest proteins are dispersed (“float”) within the lipid bi-layer (further shown by freeze-fracture electron microscopy

5. Fluidity of membranes Membrane held together by weak hydrophobic interactions; most lipids & some proteins can drift within their layer of the membrane Membrane held together by weak hydrophobic interactions; most lipids & some proteins can drift within their layer of the membrane Protein movement/non-movement may be dependant on the proteins connection/lack of connection to the cytoskeleton Protein movement/non-movement may be dependant on the proteins connection/lack of connection to the cytoskeleton Temperature affects level of fluidity Temperature affects level of fluidity Fluidity affects permeability Fluidity affects permeability **cholesterol & unsaturated fats increase fluidity (added to membrane to prep for cooler temps) **cholesterol & unsaturated fats increase fluidity (added to membrane to prep for cooler temps)

8. Concept Check What would happen to the fluidity of the membrane in the following scenarios? What would happen to the fluidity of the membrane in the following scenarios? Increase in unsaturated phospholipids? Increase in unsaturated phospholipids? Increase in saturated phospholipids? Increase in saturated phospholipids? A decrease in temperature? A decrease in temperature? An increase in cholesterol levels? An increase in cholesterol levels?

9. Membrane proteins Determine most of the membrane’s specific functions Determine most of the membrane’s specific functions Types: Types: Integral proteins: penetrate through the hydrophobic core of the lipid bi-layer (transmembrane proteins) Integral proteins: penetrate through the hydrophobic core of the lipid bi-layer (transmembrane proteins) Peripheral proteins: not embedded in bi-layer; attached to the surface of the membrane Peripheral proteins: not embedded in bi-layer; attached to the surface of the membrane

10. Functions of membrane proteins Transport Transport Enzyme activity Enzyme activity Signal transduction Signal transduction Cell-cell recognition Cell-cell recognition Intercellular joining Intercellular joining Attachment to the cytoskeleton & extracellular matrix (ECM) Attachment to the cytoskeleton & extracellular matrix (ECM)

13. Carbohydrates & the membrane Carbohydrates in the membrane < 15 sugar units Carbohydrates in the membrane < 15 sugar units Types: Types: Glycolipids Glycolipids Glycoproteins Glycoproteins Function: cell-cell recognition Function: cell-cell recognition

14. Synthesis of membranes See text book figure 7.10 See text book figure 7.10 1. synthesis of membrane proteins & lipids in the ER; Carbohydrate added to make glycoproteins 1. synthesis of membrane proteins & lipids in the ER; Carbohydrate added to make glycoproteins 2. Inside Golgi apparatus glycolipids are made and glycoproteins are modified 2. Inside Golgi apparatus glycolipids are made and glycoproteins are modified 3. Transmembrane proteins, glycolipids, & secretory proteins are transported in vessicles 3. Transmembrane proteins, glycolipids, & secretory proteins are transported in vessicles 4. Vessicles fuse with the membrane releasing secretory proteins & placing glycoproteins & glycolipids on the outside of the membrane 4. Vessicles fuse with the membrane releasing secretory proteins & placing glycoproteins & glycolipids on the outside of the membrane

15. **Outside of plasma membrane is made from the inside of the ER, & Golgi vessicle membranes **Outside of plasma membrane is made from the inside of the ER, & Golgi vessicle membranes (When vessicles formed in ER & Golgi fuse with membrane to release material they become part of the membrane)

16. Concept Check On which side of the membrane are carbohydrates found? How is this location useful to the carbohydrate function in the membrane? On which side of the membrane are carbohydrates found? How is this location useful to the carbohydrate function in the membrane?

17. Selective permeability Fluid mosaic model explains how membrane can regulate passage of materials Fluid mosaic model explains how membrane can regulate passage of materials Hydrophobic (non-polar) molecules can diffuse through lipid bi-layer easily Hydrophobic (non-polar) molecules can diffuse through lipid bi-layer easily Polar molecules & ions which are impeded by the lipid bi-layer pass through specific transport proteins Polar molecules & ions which are impeded by the lipid bi-layer pass through specific transport proteins

18. Passive Transport No energy required No energy required Diffusion Diffusion Molecules will move from high to low concentration Molecules will move from high to low concentration Diffusion of molecules is unaffected by the concentration of other substances Diffusion of molecules is unaffected by the concentration of other substances Rate is determined by membrane permeability to the molecule Rate is determined by membrane permeability to the molecule

20. Passive Transport (cont’d.) Osmosis Osmosis Diffusion of water across a selectively permeable membrane Diffusion of water across a selectively permeable membrane Tonicity=ability of a solution to cause a cell to gain or lose water Tonicity=ability of a solution to cause a cell to gain or lose water isotonic: no net movement of water isotonic: no net movement of water Hypertonic: net loss of water Hypertonic: net loss of water Hypotonic: net gain of water Hypotonic: net gain of water

23. Passive Transport (cont’d.) Facilitated diffusion Facilitated diffusion Movement of molecules down their concentration gradient with the assistance of specific transport proteins in the membrane Movement of molecules down their concentration gradient with the assistance of specific transport proteins in the membrane Types of transport proteins: Types of transport proteins: Channel proteins: “corridors” for passage of specific ion or molecule Channel proteins: “corridors” for passage of specific ion or molecule Aquaporins (water channel proteins) Aquaporins (water channel proteins) Ion channels/gated channels (electrical or chemical signal causes opening or closing Ion channels/gated channels (electrical or chemical signal causes opening or closing Carrier proteins: change shape to translocate substances across the membrane Carrier proteins: change shape to translocate substances across the membrane

25. Concept Check What would happen to a Paramecium that swam from a hypotonic environment to an isotonic one? What would happen to a Paramecium that swam from a hypotonic environment to an isotonic one? Why do water molecules need aquaporins to cross the membrane? Why don’t substances like oxygen and carbon dioxide require transport proteins? Why do water molecules need aquaporins to cross the membrane? Why don’t substances like oxygen and carbon dioxide require transport proteins?

26. Active Transport Molecules move against the concentration gradient (low to high) Molecules move against the concentration gradient (low to high) Energy required Energy required Uses carrier transport proteins Uses carrier transport proteins

27. Active transport: sodium-potassium pump

28. Na+ in cell binds to protein Na+ in cell binds to protein ATP binds to protein ATP binds to protein Protein changes shape Protein changes shape Na+ moves out of cell Na+ moves out of cell K+ outside cell binds to protein K+ outside cell binds to protein P from ATP is removed (dephosphorylation) P from ATP is removed (dephosphorylation) Original protein shape is restored Original protein shape is restored

29. Active transport: electrogenic pump

30. H+ pumped out through protein with the help of ATP H+ pumped out through protein with the help of ATP Outside cell becomes +, inside – Outside cell becomes +, inside – Charge difference across the membrane is used to do work Charge difference across the membrane is used to do work

31. Active transport: Cotransport

32. Same as electrogenic pump but… when H+ moves back into cell by diffusion it carries another molecule with it (i.e. sucrose) Same as electrogenic pump but… when H+ moves back into cell by diffusion it carries another molecule with it (i.e. sucrose)

33. Passive vs. Active Transport

34. Concept Check Why is the sodium-potassium pump not considered a cotransporter? Why is the sodium-potassium pump not considered a cotransporter? Which solute(s) will exhibit net diffusion into the cell? Which solute(s) will exhibit net diffusion into the cell? Which solution “cell” or environment is hypertonic? Which solution “cell” or environment is hypertonic? In which direction will there be a net osmotic movement of water? In which direction will there be a net osmotic movement of water? After the cell was placed in the beaker did it become for flaccid, or more turgid? After the cell was placed in the beaker did it become for flaccid, or more turgid?

35. Bulk Transport Exocytosis: cell secretes macromolecules through the fusion of vesicles with the plasma membrane Exocytosis: cell secretes macromolecules through the fusion of vesicles with the plasma membrane Endocytosis: cell takes in macromolecules by forming new vesicles from the plasma membrane Endocytosis: cell takes in macromolecules by forming new vesicles from the plasma membrane Phagocytosis (“cellular eating”) Phagocytosis (“cellular eating”) Pinocytosis (non-specific “cellular drinking”) Pinocytosis (non-specific “cellular drinking”) Receptor-mediated endocytosis (specific uptake) Receptor-mediated endocytosis (specific uptake)

36. Concept Check As a cell grows, its plasma membrane expands. Is this a result of exocytosis or endocytosis? Explain. As a cell grows, its plasma membrane expands. Is this a result of exocytosis or endocytosis? Explain. After a neuron has been stimulated by neurotransmitters from a neighboring neuron, the neuron takes in the neurotransmitters by endocytosis. Is it by pinocytosis or receptor- mediated endocytosis? Explain. After a neuron has been stimulated by neurotransmitters from a neighboring neuron, the neuron takes in the neurotransmitters by endocytosis. Is it by pinocytosis or receptor- mediated endocytosis? Explain.