The Cell Membrane

1. The Cell Membrane

2. Cell Membranes: • Fluid-like composition… like soap bubbles • Composed of: • Lipids in a bilayer • Proteins embedded in lipid layer (called transmembrane proteins) • And, Proteins floating within the lipid sea (called integral proteins) • And Proteins associated outside the lipid bilayer (peripheral).

3. Membrane Lipids • Composed largely of phospholipids • Phospholipids composed of….glycerol and two fatty acids + PO4 group • P-Lipids are amphipathic molecules, with polar and nonpolar regions… phosphate (hydrophilic) polar glycerol fatty acids (hydrophobic) nonpolar

4. Membrane Proteins • Integral: embedded within bilayer • Peripheral: reside outside hydrophobic region of lipids Text pg 80

5. Fluid Mosaic Membrane

6. Fluid Mosaic Membrane Fluid Mosaic Model - lipids arranged in bilayer with proteins embedded or associated with the lipids.

7. Evidence for the Fluid Mosaic Model Frey and Edidin

8. Membrane Permeability • Biological membranes are physical barriers..but which allow small uncharged molecules to pass… • And, lipid soluble molecules pass through • Big molecules and charged ones do NOT pass through • Semi-permeable / selectively permeable • There are two ways that the molecules typically move through the membrane: • passive transport and active transport • Active transport requires that the cell use energy that it has obtained from food to move the molecules (or larger particles) through the cell membrane. • Passive transport does not require such an energy expenditure, and occurs spontaneously

9. Membrane Transport MechanismsI. Passive Transport Diffusion- simple movement from regions of high concentration to low concentration by random motion of particles caused by internal thermal energy. Osmosis- diffusion of water across a semi-permeable membrane Facilitated diffusion- protein transporters which assist in diffusion

10. Diffusion Rates • Factors affecting diffusion rate through a membrane • temperature -  temp.,  motion of particles • molecular weight - larger molecules move slower • steepness of concentrated gradient - difference,  rate • membrane surface area -  area,  rate • membrane permeability -  permeability,  rate

11. Concentration Gradient of Ions across a Membrane and the Resulting Electrochemical Potential

12. Tonicity • Tonicity - ability of a solution to affect fluid volume and pressure within a cell • depends on solute concentration and permeability • Hypotonic solution • low concentration of nonpermeating solutes (high water concentration) • cells absorb water, swell and may burst (lyse) • Hypertonic solution • has high concentration of nonpermeating solutes (low water concentration) • cells lose water + shrivel (crenate) • Isotonic solution = normal saline

13. Osmosis Movement of water across a semi-permeable barrier. Example: Salt in water, cell membrane is barrier. Salt will NOT move across membrane, water will. How Osmosis Works

14. cell Osmosis in Hypertonic medium • Hypertonic solutions- shrink cells

15. Osmosis in Hypotonic medium • Hypotonic solutions- swell cells • “Hypos make hippos”

16. For more animations view: http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm

18. For Osmosis in Action View frozen frogs at: http://www.pbs.org/wgbh/nova/sciencenow/3209/05.html How did the frog use the principles of osmosis and diffusion to survive the winter? Make sure you use the following terms appropriately in your description: hypertonic, hypotonic, solute, solvent, diffusion, osmosis, cytolysis, crenation, isotonic and semi-permeable membrane.

19. Osmosis & Food Preservation • Food can be preserved by causing any microorganism that comes in contact with it to become plasmolysed and, therefore, shrivel and die. To do this food is placed in a high salt or sugar medium. The salt or sugar concentration is higher than the cytoplasm of bacteria or fungi. Bacteria or fungi, that contaminate the food, will lose water by osmosis and their metabolism will decline. Many will die but some bacteria may survive by forming dormant resistant endospores. Meat and fish are often preserved in salt. Fruit is commonly preserved in sugar as in jam or syrup.

20. Membrane Transport:Active Transport(Direct & Indirect) Movement from region of low free energy(low concentration) to regions of high free energy (high conc.) Requires energy input • Cotransport • Sodium-Potassium Exchange Pump • Endocytosis and Exocytosis • Proton Pump

21. Endocytosis • Part of the membrane engulfs the particle and folds inward to “bud off.” • Phagocytosis • Pinocytosis • Receptor Mediated Endocytosis

22. Exocytosis: Cellular Secretion • Vesicles (lysosomes, other secretory vesicles) can fuse with the membrane and open up the the outside…

23. Vesicular Transport(Active Transport) Endocytosis Phagocytosis, 2 Pinocytosis Receptor Mediated Endocytosis Exocytosis

24. Membrane Permeability • lipid soluble solutes go through faster • smaller molecules go faster • uncharged & weakly charged go faster • Channels or pores may also exist in membrane to allow transport 1 2

25. Functional Roles of Membrane Protein Molecules

26. Types of Protein Transporters: Ion Channels • Work fast: No conformational changes needed • Not simple pores in membrane: • specific to different ions (Na, K, Ca...) • gates control opening • Toxins, drugs may affect channels • saxitoxin, tetrodotoxin • cystic fibrosis • work by facilitated diffusion No E! • deal with small molecules... ions • open pores are “gated”- Can change shape. • important in cell communication Receptors Linked to a Channel Protein

27. Ion Channels • Channel proteins or carrier proteins allow the facilitated diffusion of solutes down their concentration gradients or electrochemical gradients • Carrier proteins allow the active transport of solutes up their concentration gradients or electrochemical gradients.

28. Cystic Fibrosis • Proteins for diffusion of salt into the airways don't work.  • Less salt in the airways means less water in the airways.  • Less water in the airways means mucus layer is very sticky (viscous). • Sticky mucus cannot be easily moved to clear particles from the lungs.     • Sticky mucus traps bacteria and causes more lung infections.

29. Transport ProteinsFacilitated Diffusion & Active Transport • move solutes faster across membrane • highly specific to specific solutes • can be inhibited by drugs

30. Facilitated Diffusion: Glucose Transporters • Transport of glucose into cells mediated by proteins in the GLUT (GLUcose Transport) family of transporters. • All GLUT proteins share a set of similar structural features and are all about 500 amino acids in length (giving them a predicted molecular weight of about 55,000 Daltons) • Glucose uptake shows saturation and glucose uptake can be inhibited by drugs • These are uniporters, different from the Sodium-dependent glucose cotransporters (SGLT)

31. Glucose Transporter: How it works.. • glucose binds to outside of transporter (exterior side with higher glucose conc.) • glucose binding causes a conform. change in protein • glucose drops off inside cell • protein reassumes 1st configuration

32. Active Transport:Sodium-Potassium Pump Balance of the two ions goes hand-in-hand ATP required for maintenance of the pump Na+ low Na+ high K+ low K+ high How it Works

33. Sodium-Potassium Pump • 3 Na+ bind to inner region of protein • Na+ binding triggers phosphorylation of protein. ATP ADP + Pi • Phosphorylation causes conformation change and Na+ binding site faces outside • 3 Na+ released to outside • 2 K+ ions on outside are able to bind • K+ binding causes dephosphorylation and new conformation change • 2K+ ions exposed to inside and released • Cyclic process uses ATP energy to drive Na & K ion transport against conc. Gradient

35. Cell Junctions • Allow specific types of cells to stay together to perform special jobs • Layers of these types of cells… • Line body cavities • Cover body surfaces • Tight Junctions • Desmosomes • Gap Junctions • Plasmodesmata

36. Intercellular Junctions

37. Tight Junctions • intimate physical connections linking cells that line the inner or outer surface of organs or body cavities • Leak proof barriers that prevent the movement of molecules through the spaces located b/t cells, must diffuse to get by and are therefore subject to the precise control mechanisms inherent to transport through cell mem • e.g. bladder

38. Desmosomes • junctions exhibiting mechanical strength • found in organs/tissues exposed to mechanical forces that subject cells to much stretching and distortion • maintains integrity of cell Pemphigus is an autoimmune disease in which the patient has developed antibodies against proteins (cadherins) in desmosomes. The loosening of the adhesion between adjacent epithelial cells causes blistering. Carcinomas are cancers of epithelia. However, the cells of carcinomas no longer have desmosomes. This may partially account for their ability to metastasize.

39. Gap Junctions • permit small molecules to move b/t cells w/o passing thru mem • six dumbbell shaped protein units in mem, adjacent in the cells The action potential in cardiac muscle provides the rhythmic contraction of the heartbeat. At some electrical synapses in the brain, gap junctions permit the arrival of an action potential at the synaptic terminals to be transmitted across to the postsynaptic cell without the delay needed for release of a neurotransmitter. As the time of birth approaches, gap junctions between the smooth muscle cells of the uterus enable coordinated, powerful contractions to begin.

40. Plasmodesmata • similar to gap junctions, but in plant cells • allows continuous flow of cytoplasm through cells

41. Resources • Directory of Animations: • Anatomy & Physiology Chapter 3 Animations • Cell Membrane: Just Passing Through • Absorption in the Small Intestine • GLUT4 & Diabetes (Monogenetic disorder)