Download Big Idea: The plasma membrane allows the cell to separate the

Big Idea: The plasma membrane allows the cell to separate the extracellular (outside cell) environment from the inside of the cell while still allowing nutrients in the cell and wastes out of the cell. Plasma Membrane —  A selectively permeable membrane about 8 nm thick, allows certain molecules in/out easier than others. —  Structure: —  Made mostly of phospholipids & other lipids, but also contains many different types of proteins and carbs —  Amphipathic: Most phospholipids and other lipids & proteins in membrane have a hydrophobic & hydrophilic component to them. —  Fluid Mosaic Model: Proteins are embedded in or attached to the phospholipid bilayer. History of Membrane —  The “sandwich model” was proposed by Davson & Danielli in 1935 and was the dominant view until 1970. —  Structure: phospholipid bilayer sandwiched by a protein layer on both sides of membrane. Modern View of Membranes —  Fluid Mosaic model—
the new view of the plasma membrane that electron microscopy and freeze fracture research contributed to —  Accepted view since the 1970’s Membranes are Fluid —  Lipids & proteins can drift laterally, but not flip-­‐flop because a hydrophilic region would have to pass through a hydrophobic region. —  Lipids move rapidly (2 micrometers per second) —  Proteins move more slowly—connected to cytoskeleton where motor proteins move membrane proteins. —  Some proteins are stationary —  http://www.susanahalpine.com/anim/Life/memb.htm Temperature Affects Membrane Fluidity —  Big Idea: Membranes work best when they are fluid because as membranes solidify the permeability changes and enzyme proteins become inactive. —  Unsaturated hydrocarbons of phospholipids increase fluidity at lower temperatures. (VEGETABLE OILS) —  Cholesterol decreases fluidity at body temp, but increase fluidity at lower temps. —  Cold tolerant plants change phospholipids to unsaturated hydrocarbons before cold season —  Ex: Winter wheat changes over in the fall Fluid versus Viscous Membrane Membrane Structure —  Integral proteins: span the membrane, have a hydrophobic & 2 hydrophilic regions. —  Hydrophobic region consists of nonpolar amino acids that make alpha helices. —  Often attached to cytoskeleton. Ex: Integrins —  Peripheral proteins: proteins not embedded in lipid bilayer, typically loosely attached to membrane proteins. Ex: collagen, fibronectin —  Proteins, lipids & carbs are directional—made for inside or outside of cell. —  Made in the ER (inside of ER = outside of plasma membrane) What level of protein structure is alpha helical? Membrane Structure —  Oligosaccharides: Short polysaccharides attached to membrane proteins (glycoproteins) or lipids (glycolipids) —  These allow for diversity in the surface molecules of different organisms or cells within organisms. Oligosaccharides and Blood Type FuncDons of Membrane Proteins figure 8.9 in your book —  Transport (Integral proteins) —  Enzyme Activity —  Signal Transduction (chemical messengers) —  Intercellular Joining —  Cell to Cell Recognition —  Attachment to cytoskeleton or ECM Transport Proteins —  Integral proteins that open hydrophilic channels or hydrolyze ATP to pump ions across membrane. —  What is the structure of the integral proteins? Enzyme AcDvity —  Active site exposed to cytoplasm or ECM where substrate attaches. —  Often occur in metabolic pathways where product from enzyme is reactant for nearby enzyme. Signal TransducDon —  Protein on ECM may have a binding site that changes the shape of the protein which relays message to the inside of the cell. —  Chemical messengers: —  Hormones—insulin, glucagon, TSH, etc Intercellular Joining —  Membrane proteins may hook cells together. —  What are some examples? —  Tight junctions —  Desmosomes Cell to Cell RecogniDon —  Some glycoproteins serve as identification tags specifically recognized by other cells APachment to cytoskeleton & ECM —  Proteins bond to microfilaments or other parts of cytoskeleton to maintain cell shape or coordinate intra/extra-­‐
cellular changes Many individual molecules (proteins, lipids, carbs) work together towards regulating transport. Permeability of the Lipid Bilayer —  Hydrophobic molecules can dissolve in the lipid bilayer and cross w/ ease, such as hydrocarbons, & CO2 —  Hydrophilic molecules like ions & polar molecules (sugars-­‐-­‐glucose, amino acids) are impeded by the hydrophobic core of the membrane —  Can water move in easily? Transport Proteins —  H2O, ions, & other polar molecules can pass through transport proteins that span the membrane. —  Hydrophilic channels are specific to certain molecules —  Proteins can also physically move molecules across —  Example: transport proteins of liver cells move glucose rapidly but exclude the structural isomer fructose —  Why would glucose need to be transported rapidly into liver & muscle cells? Transport Proteins—what drives the movement of molecules? Passive Transport —  Diffusion: the thermal motion or tendency for solutes to spread out evenly into available space. —  Spontaneous process—decreases free energy —  In a population of molecules, directionality occurs from high to low concentration (concentration gradient) until dynamic equilibrium occurs—equal movement of molecule in both directions. —  Regardless of other molecules present, each substance diffuses down its own concentration gradient. Diffusion —  Much of the membrane traffic occurs from diffusion —  Example: In cell respiration, O2 diffuses across the membrane continuously because it is used up quickly. —  No energy is necessary for this process, in fact, potential energy is stored in the concentration gradient, which drives diffusion. Diffusion AnimaDon —  http://highered.mcgraw-­‐hill.com/sites/0072495855/
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animation__how_diffusion_works.html Osmosis—Passive Transport of H2O —  Hypertonic—high solute concentration —  “Hyper” means “more” —  Hypotonic—low solute concentration (“Hypo” = “less”) —  The cell will have a hyper & hypotonic side (comparison) —  Isotonic—solutions that have equal solute concentrations on both sides of a membrane. —  Direction of osmosis: water moves from hypo to hypertonic side —  A nurse hooks a patient up to an IV, the solution is hypertonic to the patients body. What happens? How about if it is hypotonic to patient? —  A roadside plant is shriveled up after a winter of salting the roads. Why? Cells & Balancing Water Uptake —  What do animal cells do in isotonic solutions? —  No net movement of water in/out —  What do animal cells do in hypertonic solutions? —  Net movement of water out —  Cell typically shrivels and likely dies —  Ex: blood cell & dehydration, salinity change affects homeostasis (Great Salt Lake) —  Most marine animals have cells that are isotonic to seawater Plant & Animal Cells Cell Walls & Water Transport —  Become turgid (firm) in hypotonic solution because water will come in only until pressure from the cell wall (turgor pressure) pushes back —  Isotonic environ: plant becomes flaccid (limp) —  Hypertonic environ: Plasmolysis—lethal —  Non-­‐woody plants rely on firmness (turgidity) to stand upright. OsmoregulaDon —  Control of water balance in organisms w/out cell walls that live in hypotonic environments —  Paramecium uses a contractile vacuole to deal with water uptake Facilitated Diffusion —  NO ENERGY REQUIRED —  Uses concentration gradient to drive diffusion of polar molecules and ions that cannot cross membrane. —  Proteins behave like enzymes: —  Protein is specific to a solute (like enzyme/substrate) —  Protein may have a binding site (like active site) —  Limited proteins specific to a solute in the membrane, so there is a maximum speed of transfer (reaction rate) —  Imposter solute can compete w/ normal solute (competitive inhibitor) —  Difference is: Physical vs Chemical process Channel Proteins —  Corridor for ions or small molecules—
hydrophilic channel —  Aquaporins: allow fast movement of lots of H2O —  Gated channels: open/
close based on a stimulus. —  Ex: Nerve cells & NT, allow Na+ ions into cell Aquaporins Shape Changers —  Solute binds to binding site, changing shape of transport protein & it unloads solute inside —  Changes back after —  Disorder—cystinuria —  Doesn’t allow for reabsorption of cystine & other amino acids—
makes kidney stones Facilitated Diffusion AnimaDon —  http://highered.mcgraw-­‐hill.com/sites/0072495855/
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animation__how_facilitated_diffusion_works.html AcDve Transport —  The pumping of a solute against the concentration gradient (low to high) —  An “uphill” process; not spontaneous; requires work —  NEEDS ENERGY INPUT —  Allows cell to increase or decrease solutes in comparison to the outside of the cell. —  Performed by specific proteins in membrane in conjunction w/ ATP—transfers a Pi to the protein, causing a shape change, brings solute across the membrane Sodium-­‐Potassium Pump —  Na+ in cytoplasm attaches to binding sites which stimulates phosphorylation —  Pi causes the protein to change shape—expels Na+ out —  Na+ leaving opens K+ binding site; K+ attaches causing Pi to release —  Pi loss restores the original shape, causing release of K+ inside of the cell. —  An exchange of Na+ for K+ against both concentration gradients Na+/K+ Pump —  http://highered.mcgraw-­‐hill.com/sites/0072495855/
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ks.html Voltage Across Membranes —  Membrane Potential: cytoplasm is more negative than extracellular fluid—resulting in voltage across the membrane. —  Which way will cations & anions like to travel? —  Anions out & cations in —  Electrochemical gradient: combination of forces acting on ion’s movement —  Chemical force: ion’s concentration gradient —  Electrical force: effect of membrane potential Voltage Across Membrane —  Electrogenic Pump: Na+/K+ is the major type in animal cells where 3Na+ ions go out & 2K+ ions move in. This makes the inside negative. —  STORES ELECTRICAL PE FOR CELL WORK —  Proton Pump: main electrogenic pump of plants, bacteria, & fungi —  Transfer H+ ions out of cell—stores electrical PE Proton Pump AnimaDon —  http://highered.mcgraw-­‐hill.com/olcweb/cgi/
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%20Synthesis Transport of Large Molecules —  Exocytosis: secretion of macromolecules by fusion of vesicles w/ the plasma membrane —  Vesicle moves from Golgi to membrane. —  How does it move from Golgi to membrane?? —  Examples: —  Pancreatic cells release insulin into blood —  Neuron releasing neurotransmitters to stimulate other neurons or muscle cells. —  In plants, carbs released to help build cell walls Endocytosis —  Vesicle forming from plasma membrane pinching inward taking in intended contents. —  Types: —  Phagocytosis—how does cell accomplish this? —  Pinocytosis—cell drinking (forms tiny vesicles) —  Receptor mediated endocytosis: —  Specific receptor sites in membrane bind to target molecules —  Form in “coated pits” where proteins on cytoplasmic side have a fuzzy layer of protein in the groove —  Allows cell to bring in unconcentrated substances from extracellular fluid Endo/Exocytosis AnimaDon —  http://highered.mcgraw-­‐hill.com/olcweb/cgi/
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