1. Chapter 7Membrane Structure & Function

2. 7.1 Life at the Edge Plasma Membrane is a thick film About 8 nm thick Controls traffic in and out of the cell Exhibits selective permeability Allows some substances into the cell, and otherscannot enter Ability to discriminate between certain materialscan be life or death 3. Fluid Mosaic Model Lipids and proteins are staple materials inthe membrane Most abundant lipids are phospholipids Amphipathic molecule both hydrophilic region and hydrophobic region 4. Fluid Mosaic Model Fluid Mosaic Model Membrane is a fluid structure with a mosaic ofvarious proteins 5. The Fluidity of Membranes Membranes are held together byhydrophobic interactions It is possible for the lipids and proteins to shiftaround within the membrane itself Adjacent phospholipids switch places about 107times per second Proteins are larger and move a lot slower 6. The Fluidity of Membranes A membrane remains fluid as temperaturedecreases Eventually, the lipids get packed together and themembrane solidifies Similar to bacon grease turning into lard If the membrane is rich in unsaturatedhydrocarbons, the kinks in their tails make itharder to pack together So, the membrane will remain fluid at lower temperatures 7. The Fluidity of Membranes Cholesterol Steroid lodged between phospholipid moleculesin the membrane At higher temperatures, it makes the membraneless fluid by restraining phospholipid movement Lowers temperature for membrane to solidify 8. The Fluidity of Membranes Membranes must be fluid in order to functionproperly When it solidifies, the permeability changes Enzymatic proteins may become inactive if theycannot move within the membrane If too fluid, however, it cannot support proteinfunction 9. Evolution of Differences inMembrane Lipid Composition Variations in the cell membrane lipid composition of many species are speculated to be caused by evolutionary adaptation Fishes that live in extreme cold have a higher amount of unsaturated hydrocarbon tails which allows their membrane to stay fluid at these temperatures At high temperature extremes, some bacteria and archaea have unusual lipids that may prevent excess fluidity at these temperatures 10. Evolution of Differences inMembrane Lipid Composition The ability to change the lipid composition based off the current temperature has also been shown as an adaptation Some plants that live in cold, such as winter wheat, adjust the amount off unsaturated phospholipids in autumn to keep the membranes from solidifying in the winter 11. Membrane Proteins and their Functions A membrane is a collage of different proteins that are often grouped and embedded into the fluid matrix of the lipid bilayer More than 50 unique proteins have been discovered in the plasma membrane of the red blood cell Phospholipids make the material or structure of the membrane, and proteins determine the function 12. Membrane Proteins and their Functions Two major types of proteins Integral Peripheral Integral Proteins Penetrate the hydrophobic interior of the bilayer They are integrated into the membrane Mostly transmembrane proteins Peripheral Proteins Not embedded; on outside of the membrane Often exposed to parts of integral proteins 13. Membrane Proteins and their Functions On the cytoplasmic side of the cell, some proteins are held in place by being attached to the cytoskeleton On the extracellular side, certain membrane proteins are attached to fibers of the extracellular matrix These attachments give a stronger structure than the membrane could provide by itself 14. Membrane Proteins and their Functions Proteins on the surface of the cell are important in the medical field Some can help outside agents invade the cell HIV (human immunodeficiency virus) infects the cells through the help of a surface protein Also important in medicine being transferred tothe cells, etc. 15. The Role of Membrane Carbohydratesin Cell-Cell Recognition Cell-cell recognition Ability to distinguish one type of neighboring cell from another Important in defense and making sure everything is working properly Glycolipids Short, branched chains of sugars that bond to lipids Glycoproteins Short, branched chains of sugars that bond to proteins 16. Selective Permeability The fluid mosaic model helps explain S.P. A steady traffic of small molecules and ionsmoves across the plasma membrane in bothdirections Cell regulates inorganic ions too Na+, K+, Ca2+, Cl- Nonpolar molecules such as hydrocarbons,carbon dioxide, and oxygen, are hydrophobic They can dissolve easily in the lipid bilayer and cross easily Passive transport 17. Transport Proteins Transport Proteins Proteins that allow hydrophilic substances to passthrough without making contact with lipid bilayer Channel Proteins Hydrophilic channel that specific ions and materials use to pass into/out of the cell Aquaporins Water can pass through these at 3 billion H2O molecules/second 18. Permeability Diffusion The movement of molecules of any substance sothat they spread out evenly into the availablespace A higher concentration of material X will diffuse into whatever solution has a lower concentration of material X Concentration Gradient The region along which the concentration of achemical substance increases or decreases No work must be done to make this happen 19. Passive Transport Passive transport requires no energy Like following the flow of a river 20. Tonicity Isotonic The cell and its surrounding solution are the sameconcentration Hypertonic The cell is hypotonic, so the surrounding solutionis hypertonic and the contents of the cell willdiffuse out of the cell and into the solution Hypotonic The cell is hypertonic, so the surrounding solutionis hypotonic and the solution around the cell willdiffuse into the cell 21. Water Balance Without Walls Osmoregulation Control of solute concentrations and water balance Paramecium caudatum Live in hypotonic pond water To prevent water from rushing in, it has adapted to thewater by developing a membrane that is much lesspermeable to water than other organisms 22. Water Balance of Cells with Walls Cells of plants, prokaryotes, fungi, and some protists are surrounded by walls The walls help to maintain water balance After being pushed so far, the walls inelasticity will cause the wall to rebound, demonstrating turgor pressure Opposes further water uptake 23. Water Balance of Cells with Walls The cell will become Turgid Very firm, and healthy state for a lot of these cells This is what allows non-woody plants to stand upright Flaccid If the cell is placed in an isotonic solution, there is no pushback of water; so, the cells become limp, or flaccid Plasmolysis If placed into a hypertonic solution, the membrane will pull from wall, causing wilting in plants & eventual death 24. Facilitated Diffusion Facilitated Diffusion Passive transport aided by proteins Polar molecules can diffuse through proteins in the membrane and get into/out of the cell Ion Channels Channel proteins that transport ions Gated Channels Ion channels that open/close in response to stimuli 25. Active Transport Active transport To pump a solute across a membrane, against itsconcentration gradient Proteins that help do this are called carrierproteins 26. How Ion Pumps Maintain Membrane Potential Membrane Potential The voltage across a membrane Acts like a battery, an energy source that affectsthe traffic of all charged substances across themembrane Electrochemical Gradient Two forces (electrical, chemical) driving ionsacross the membrane 27. How Ion Pumps Maintain Membrane Potential Electrogenic Pump A transport protein that generates electricalvoltage across a membrane Proton Pump The main electrogenic pump of plants, fungi, andbacteria 28. Cotransport Cotransport When H+ may enter through into the membraneand carry another material with it Amino acids, sugars, etc 29. Bulk Transportation Exocytosis The transport of a bulk amount of materialsthrough vesicles containing the substance fusingwith the membrane Endocytosis The transport of a bulk amount of materials bypushing into the membrane, and as it deepens, iteventually pinches off into a vesicle that travelsinto the cell