Biology 1113 Chapter 7 Notes
Biology 1113 Chapter 7 Notes Biology 1113
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Date Created: 09/19/16
Plasma Membrane: The Moat of the Cell Wednesday, September 14, 2016 9:20 AM • Concepts 7.1 - 7.5, pg. 125-138 • Learning Outcomes ○ Successful students will be able to: Explain why the plasma membrane is referred to as a fluid mosaic Explain the various functions of membrane proteins Explain the differences between passive, facilitated and active transport Predict what will happen to a cell when it is placed in solutions of different tonicities Ch.7 Lecture Notes Wednesday, September 14, 2016 9:22 AM Plasma Membrane: What's it made of? • Lipids and Proteins ○ Phospholipids What property allows them to easily form membranes? □ Hydrophobic and hydrophilic ends • Fluid Mosaic model ○ Fluid structure with a varietyof proteins embedded or attached to it Membrane Proteins • Different types of cells contain different membrane proteins • 2 Major types: ○ Integral: embedded in the membrane Nonpolar portion of protein extends into the hydrophobic interior of the bilayer ○ Peripheral: looselybound to surface of membrane Often interact with exposed surface of integral proteins • Functions: ○ Transport ○ Enzymatic activity ○ Signal transduction ○ Cell-cell recognition ○ Intercellular joining ○ Attachment to the cytoskeletonand extracellular matrix **Don't need to memorizethese** Question: Which of the following is true of integral membraneproteins? Answer: They are usually transmembraneproteins (a portion inside and outside) ○ All proteins have a tertiary structure ○ They are mobile within the bilayer Question: In order for a protein to be embedded in the cell membraneit would have to be… Answer: Both hydrophobic and hydrophilic Importance of Carbohydrates • Necessary for cell-cell recognition ○ How liver cells distinguish themselvesfrom muscle cells ○ How your body recognizes foreign invaders This is why you can't just give someoneblood or a kidney • Role of MembraneProteins in HIV Infection: ○ HIV can infect a cell that has a co-receptor(CCR5) on its surface, as in most people. ○ HIV cannot infect a cell lacking CCR5 on its surface, as in resistant individuals ○ Berlin patient - had leukemia and HIV, when he underwent a bone marrow transplant, the HIV was gone. This lead to the experimentalresearch of modificationof cells and the CCR5. How is traffic across the membrane regulated? • All moleculescan't just pass through whenever… ○ The membrane is selectivelypermeable Nonpolar molecules(hydrocarbons, CO2, O2) can cross easily Ions and polar molecules are impeded by the hydrophobic interior of the membrane □ Use transport proteins to cross membrane MovementAcross the Membrane • Diffusion (Passive Transport) ○ Movementof any molecule Down its concentration gradient (from HIGH to LOW) Each substance movesdown its OWN  gradient ○ Each substance movesdown its OWN  gradient Unaffected by  of other substances ○ Spontaneous (requires no energy) ○ Osmosis = diffusion of WATER across a selectivelypermeable membrane ○ Tonicity ○ Ability of a surrounding solution to cause a cell to gain/lose water Isotonic = environmentsame as the cell □ No net movement,cell is stable Hypertonic = more solutes in environment □ Water moves out of cell, cell shrivels Hypotonic = less solutes in environment □ Water moves into cell, cell swells (and may burst) • Facilitated Diffusion ○ Compositionof the lipid bilayer impedes the diffusion polar molecules and ions ○ Transport proteins help them diffuse passively across the membrane ○ No energy required (High  to Low ) ○ Channel Proteins ○ Provide door way through membrane ○ Aquaporins transport 3 billion water moleculesper second ○ Carrier Proteins ○ Change shape to shuttle moleculeacross the membrane ○ Cystinuria - defect in carrier protein that transports cysteine across membraneof kidney cells • Active Transport ○ Movementof moleculesagainst their  gradient requires energy ○ Incredibly important for nervous system ○ Uses carrier proteins ○ Ion pumps ○ Crucial for maintaining electrochemicalgradients and membrane potential for nerve impulses ○ Cotransport ○ Uses the  gradient created by an ATP-poweredpump to power the transport of a molecule against it  gradient • Bulk Transport ○ Allows large moleculesand particles to enter the cell ○ Requires energy ○ Endocytosis Phagocytosis Pinocytosis Receptor-mediateendocytosis ________________________________________________________________________ Top Hat Questions Monday, September 19, 2016 9:51 AM Which of the following would likely movethrough the lipid bilayer of a plasma membrane mostrapidly? a. CO2 - would move through the quickest b. K+ - it has a polar charge on it so it will not be able to pass through easily c. Starch - the rest are very large moleculesso they would need a transporter d. Glucose e. An amino acid Which of the following statementsabout osmosisis correct? a. If a cell is placed in an isotonic solution, more water will enter the cell than leaves the cell b. Osmoticmovementof water in a cell would likely occur if the cell accumulates water from its environment c. The presence of aquaporins (proteins that form water channels in the membrane)should speed up the process of osmosis d. If a solution outside the cell is hypertonic compared to the cytoplasm,water will moveinto the cell by osmosis e. Osmosis is the diffusion of water from a region o flower water concentrationto a region of higher water concentration The solution in the arms of a U-tube are separated at the bottom of the tube by a selectivelypermeable membrane. The membraneis permeable to sodium chloride but not to glucose. Side A is filled with a solution of 0.4 M glucose and 0.5 M sodium chloride, and side B is filled with a solution containing 0.8 M glucose and 0.4 M sodium chloride. Initially, the volume in both arms is the same. Refer to the figure to answer the following question. At the beginning of the experiment, a. Side A is hypotonic to side B b. Side A is hypertonic to side B c. Side A is hypertonic to side B with respect to glucose d. Side A is isotonic to side B e. Side A is hypotonic to side B with respect to sodium chloride An animal cell lacking oligosaccharides on the external surface of its plasma membrane would likely be impaired in which function? a. Transporting ions against an electrochemicalgradient b. Cell - cell recognition c. Maintaining fluidity of the phospholipid bilayer d. Attaching to the cytoskeleton e. Establishing the diffusion barrier to charged molecules **What would be observed by live-cell fluorescence microscopyimmediatelyafter HIV entry if HIV is endocytosedfirst, and then later fuses with the endocytoticvesicle membrane? a. A spot of red fluorescence will be visible on the infected cell's plasma membrane,marking the site of membrane fusion and HIV entry b. A spot of red fluorescence will remain outside the cell after delivering the viral capsid c. Fluorescence microscopydoes not have enough resolution to visualize fluorescentlylabeled HIV virus particles d. A spot of red fluorescence will diffuse in the infected cell's cytoplasm e. The red fluorescentdye-labeled lipids will appear in the infected cell's interior Book Overview (pg. 124-138) Tuesday, September 20, 2016 11:01 PM Section 7.1 | Cellular membranes are fluid mosaics of lipids and proteins • Membranes are mostlymade out of lipids (mostabundant is phospholipids due to their ability to form membranes from their amphipathic nature) and proteins (carbohydratesplay a smaller role) ○ Amphipathic - has both a hydrophilic region and a hydrophobic region A phospholipid bilayer can exist as a stable boundary between two aqueous compartments Most membrane proteins are amphipathic ○ Fluid mosaic model - the membrane is a mosaic of protein moleculesbobbing in a fluid bilayer of phospholipids The proteins are in groups that carry out commonfunctions • Fluidity of Membranes ○ Phospholipids move laterally in the membrane, proteins are larger than lipids so they move more slowly ○ The fluidity of a membrane affects permeability and the movementof proteins When membranes solidify, permeability changes and enzymatic proteins become inactive When membranes are too fluid, they cannot support protein function either ○ Membrane Lipid Composition Membranes remain fluid at all temperatures until phospholipids settle into a closely packed arrangement □ Depends on types of phospholipids (like unsaturated and saturated) Example: Fishes that live in extremecold have membraneswith a high proportion of unsaturated hydrocarbon tails which enables them to remain fluid in the cold temperatures Example: Winter wheat increases the percentage of unsaturated phospholipids in autumn so that the membranes do not solidify during winter • Membrane proteins ○ A membraneis like a collage of different proteins that are grouped and embedded in the fluid matrix of the lipid bilayer ○ The proteins determinethe function of the membrane ○ Two major types of proteins: Integral proteins - penetrate the hydrophobic interior of the lipid bilayer □ Most span the membrane,others extend partway into the hydrophobic interior □ Consist of one or more nonpolar amino acid helices in hydrophobic region □ Have hydrophilic channels that allow passage of hydrophilic substances like □ Have hydrophilic channels that allow passage of hydrophilic substances like water, passage through the membrane Peripheral proteins □ Are not embedded in the lipid bilayer, instead they are looselybound to the surface of the membrane On extracellular side, attached to fibers of extracellular matrix On cytoplasmicside, attached to cytoskeleton ○ Functions: Transportation through the cell membrane Enzymatic activity (may carry out the steps of a metabolic pathway) Attaching a cell to a neighbor cell/extracellularmatrix Signal relay to the inside of the cell Cell-cell recognition Intercellular joining ○ Example: A protein on the surface of immune cells called CD4 helps HIV infect cells leading to AIDS. People who have a protein called CCR5 attached to the cell surface is a co-receptor of HIV, so if there is an absence of this protein, the HIV virus cannot enter the cell. Now a drug is being developedthat will target CCR5 and mask the proteins presence so that HIV cannot enter a cell. • Membrane carbohydrates ○ Short, branched chains of fewer than 15 sugar units Glycolipids - when carbohydrates covalentlybond with lipids Glycoproteins - when carbohydrates covalentlybond with proteins ○ Cells recognize other cell by binding to moleculeswith carbohydratein the extracellular surface of the plasma membrane Cell to cell recognition - a cell's ability to distinguish one type of cell from another □ Helps to sort cells into tissue and organs □ Basis for rejecting foreign cells in the immune system Because the carbohydratesvary in location and moleculemake-up, cells are distinguishable • Membranes are made up of two lipid layers (with different lipid compositions)that are inside and outside faces 7.2 | Membrane structure results in selectivepermeability • The permeability of the lipid bilayer ○ Can pass through easily and rapidly: Nonpolar molecules(CO2 and O2) □ Because the moleculesare hydrophobic the can dissolve in the lipid bilayer of the membrane ○ Impedes movement,movesthrough slowly Polar molecules □ Pass through lipid bilayer very slowly Ions □ Are less likely to penetrate the hydrophobic interior of the membrane because they are hydrophilic • Transport proteins ○ Span the membrane and allow hydrophilic substances to avoid contact with lipid bilayer Channel proteins - have a hydrophilic channel that certain moleculesand ions can use as a tunnel through the membrane □ Aquaporins - channel proteins that facilitate water molecules Carrier proteins - hold on to passengers and change shape to shuttle them across membrane ○ Each protein is specific for a certain substance (so it only allows passage for that one type of substance) substance) Ex: A carrier protein in the red blood cells transports glucose through the membrane, but rejects fructose passage 7.3 | Passive transport • Diffusion - the movementof particles of any substance so that they spread out into available space ○ A result of the constant motion of molecules ○ Diffusion is directional ○ Rule of diffusion: a substance will diffuse from where it is moreconcentrated to where it is less concentrated Concentrationgradient - the region along which the density of a chemical substance increases or decreases □ Any substance will diffuse down its concentration gradient ○ Diffusion is a spontaneous process (requires no energy) ○ Ex: Cellular respiration where oxygen diffuses into the cell across the plasma membrane • Passive transport - diffusion of a substance across a membrane with no energy investment ○ The concentrationgradient is like potential energy which drives diffusion • Osmosis and water balance ○ Osmosis - diffusion of water across a selectivelypermeable membrane ○ Sugar moleculesare too big to pass through the pores in the membrane of the figure below Therefore,water moleculescluster around them and the free water molecules diffuse across the membranein order to make the concentrationon both sides equal ○ Tonicity - the ability of a surrounding solution to cause a cell to gain or lose water Depends upon concentration of solutes that cannot cross membrane relative to inside the cell 3 types of solutions: □ Isotonic - there will be no net movementof water across the plasma membrane The volume of an animal cell is stable □ Hypertonic - there will be movementout of the cell The cell will lose water, shrivel, and most likely die ◊ This is why an increase in saltiness of a lake can kill animals □ Hypotonic - water enters a cell faster than it is leaving The cell will swell and lyse (burst) A cell without rigid cell walls cannot tolerate excessiveuptake or loss of water □ Sea water is isotonic to marine invertebratestherefore they can live there □ Extracellular fluid is isotonic to the cells in land animals □ Osmoregulation- the control of solute concentration and water balance Example: paramecium lives in pond water which is hypotonic to the cell, Example: paramecium lives in pond water which is hypotonic to the cell, so the plasma membraneof this organism is much less permeable to water compared to other organism which slows the uptake of water. The cell also has a pump that rapidly pumps water out of the cell. A cell with cell walls like plants, prokaryotes,and fungi are able to maintain their water balance □ Cell walls exert turgor pressure that opposes water uptake which makes the cell turgid (very firm) which is a healthy state for cells Hypotonic solution - make turgid cells Hypertonic solution - make flaccid cells □ Cell walls however cannot protect cells if they are immersedin a hypertonic environment The plant cells will shrivel Plasmolysis- causes the plant to wilt and can lead to plant death • Facilitated diffusion - polar moleculescan pass through plasma membranewith the help of transport proteins ○ Channel proteins - provide corridors for specific molecules/ionsto cross Aquaporins □ Kidney cells have many aquaporins so that they can reclaim water from urine before it is excreted Ion channels - transport ions □ Act as gated channels which open and close in response to electrical stimulus Example: In a nerve cell, an ion channel opens in response to an electric stimulus which allows a stream of potassium ions to leave the cell □ Other gated channels open and close when a specific substance binds to the channel ○ Carrier proteins - change shape which is triggered b the binding and release of the transported molecule 7.4 | Active transport • Active transport - uses energy to move solutes against their gradients ○ The transport proteins that move solutes against the gradient are all carrier proteins Instead of picking them up though, they help them to diffuse ○ Allows a cell to maintain internal concentrationsof small solutes that differ from the environment Example: the plasma membranepumps Na+ out of the cell and K+ into the cell despite the lower concentration of Na+ ions in an animal cell ○ ATP powers active transport by transferring the terminal phosphate group directly to the transport proteins which allows the protein to change shape Example: The sodium phosphate pump exchanges Na+ for K+ across the plasma membrane of animal cells • Ion pumps ○ Membrane potential - the voltage across a membrane Is an energy source that affects the traffic of all charged substances across the membrane Electrochemicalgradient - the membranepotential favors the passive transport of cations into the cell and anions out of the cell because the inside of the cell is negative compared to the outside □ An ion diffuses down its "electrochemicalgradient" not concentrationgradient Some membrane proteins that actively transport ions contribute to the membrane potential ○ Electrogenic pump - transport protein that generates voltage across a membrane The sodium-potassium pump is the main electrogenic pump of animal cells The proton pump is the main electrogenicpump of plant, fungi, and bacteria cells The proton pump is the main electrogenicpump of plant, fungi, and bacteria cells □ Actively transports protons out of the cell Electrogenic pumps help store energy that is used for cellular work • Cotransport ○ A transports protein can couple the diffusion of the solute to the transport of a second substance against its own concentration gradient Example: a plant cell uses H+ generated by ATP-poweredproton pumps to drive the active transport of amino acids, sugars, and several other nutrients into the cell Since we know about cotransportproteins in animal cells, diarrhea in developing countries where the colon cannot reabsorb sodium which can cause death, is now being treated by drinking a solution that contains high concentrationsof salt and glucose. The solutes are taken up by sodium-glucosecotransporterson the surface of intestinal cells and passed through the cells into the blood. 7.5 | Exocytosis and endocytosis • Since small solutes and water enter and leave the cell by diffusion, how do large molecules like proteins and polysaccharides travel across the membrane? ○ They travel across the membranein bulk, packaged vesicles (bulk transport) through exocytosisand endocytosis • Exocytosis- the cell secretescertain molecules by the fusion of vesicles with the plasma membrane ○ When the membrane of a vesicle and the plasma membranecome into contact, proteins arrange the lipid moleculesof the bilayer so that the membranesfuse, and the contents of the vesicle will spill out of the cell and the vesicle becomesa part of the plasma membrane ○ Secretorycells use exocytosisto export products Example: pancreas secretesinsulin into the extracellular fluid Example: nerve cells release neurotransmittersthat signal other neurons or muscle cells Example: when plant cells are making cell walls, proteins and carbohydrates are delivered from the Golgi vesicle to the outside of the cell • Endocytosis- the cell takes in moleculesand particulate matter by forming new vesicle from the plasma membrane ○ A small area of the plasma membranewill sink inward to form a pocket that will pinch in to form a vesicle that contains material that was outside the cell Phagocytosis - a cell engulfs a particle by extending pseudopodia around it and packaging it within a membranoussac called a food vacuole. The particle will be packaging it within a membranoussac called a food vacuole. The particle will be digested after the food vacuole fuses with a lysosome Pinocytosis - a cell continuously gulps droplets of extracellular fluid into tiny vesicles formed by infoldings of the plasma membrane. The cell will then obtain the molecules dissolved in the droplets. Receptor-mediatedendocytosis- a specialized type of pinocytosis that enables the cell to acquire the bulk quantities of specific substances, even those these substances may not be very concentratedin the extracellular fluid. Specific solutes bind to receptor sites within proteins. The receptor proteins then cluster in the coated pits and the pits form a vesicle contain the bound molecules.
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