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Chapter 7: Cell Membrane and the Movement Across It!
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Cell Membrane…more than just a barrier!
The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.
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Plasma Membrane Boundary that separates living cell from its environment 8 nm thick Selectively permeable Controls traffic in and out Sandwich like Middle = phospholipids Outer = proteins
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Plasma Membrane Phospholipid bilayer Proteins Carbohydrates
Integral (transport) Peripheral (surface) Carbohydrates Used in cell to cell recognition Sorts cells in developing embryos Rejection of foreign cells Amphipathic
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Davson-Danielli Model
1st Model Phospholipid bilayer between two protein layers Two problems Not all membranes are identical If entire outside were proteins it would be too unstable
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Figure 7.2 Two generations of membrane models
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Fluid Mosaic Model Not sandwich like as in first model
Molecules drift laterally Must be fluid to function Cholesterol maintains membrane fluidity unsaturated phospholipids increase in low temperatures
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Selectively Permeable
Property of biological membranes which allows some substances to cross more easily than others Nonpolar (hydrophobic) cross easily hydrocarbons, O2, CO2 Polar (hydrophilic) pass w/ less ease Cannot dissolve - hydrophobic region Water and ethanol (small) Glucose Ions
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Selectively Permeable
Transport proteins Integral membrane proteins that transpose specific molecules or ions across biological membranes Key role in regulating transport Similar to a tunnel May bind
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Figure 8.9 Some functions of membrane proteins
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Cell (plasma) membrane
Cells need an inside & an outside… separate cell from its environment cell membrane is the boundary Can it be an impenetrable boundary? NO! OUT waste ammonia salts CO2 H2O products IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O OUT IN cell needs materials in & products or waste out
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Lipids of cell membrane
Membrane is made of phospholipids phospholipid bilayer phosphate hydrophilic inside cell outside cell lipid hydrophobic
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Phospholipid bilayer What molecules can get through directly?
fats & other lipids can slip directly through the phospholipid cell membrane, but… what about other stuff? lipid inside cell outside cell salt NH3 sugar aa H2O
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Membrane Proteins Proteins determine most of membrane’s specific functions cell membrane & organelle membranes each have unique collections of proteins Membrane proteins: peripheral proteins = loosely bound to surface of membrane integral proteins = penetrate into lipid bilayer, often completely spanning the membrane = transmembrane proteins Integral – enzyme activity, intercellular joining, Peripheral – protein hormones
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Membrane Protein Types
Channel proteins – wide open passage Ion channels – gated Aquaporins – water only, kidney and plant root only Carrier proteins – change shape Transport proteins – require ATP Recognition proteins - glycoproteins Adhesion proteins – anchors Receptor proteins - hormones
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A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.
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Membrane Carbohydrates
Play a key role in cell-cell recognition ability of a cell to distinguish neighboring cells from another important in organ & tissue development basis for rejection of foreign cells by immune system The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.
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Cholesterol Provides stability in animal cells
Replaced with sterols in plant cells
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Getting through cell membrane
Passive transport No energy needed Movement down concentration gradient Active transport Movement against concentration gradient low high requires ATP
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Diffusion 2nd Law of Thermodynamics - Universe tends towards disorder
Organization tends to disorder. Molecules move to disarray. Diffusion movement from high low concentration
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Simple diffusion across membrane
Which way will lipid move? lipid lipid lipid inside cell lipid lipid lipid low high lipid outside cell lipid lipid lipid lipid lipid lipid lipid
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Diffusion of 2 solutes Each substance diffuses down its own concentration gradient, independent of concentration gradients of other substances Things tend to get mixed up evenly.
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Facilitated diffusion
Move from HIGH to LOW concentration through a protein channel passive transport no energy needed facilitated = with help
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Gated channels Some channel proteins open only in presence of stimulus (signal) stimulus usually different from transported molecule ex: ion-gated channels when neurotransmitters bind to a specific gated channels on a neuron, these channels open = allows Na+ ions to enter nerve cell ex: voltage-gated channels change in electrical charge across nerve cell membrane opens Na+ & K+ channels When the neurotransmitters are not present, the channels are closed.
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Na+/K+ pump in nerve cell membranes
Active transport Cells may need molecules to move against concentration situation need to pump against concentration protein pump requires energy ATP Plants have nitrate & phosphate pumps in their roots. Why? Nitrate for amino acids Phosphate for DNA & membranes Not coincidentally these are the main constituents of fertilizer. Na+/K+ pump in nerve cell membranes
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Transport summary
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How about large molecules?
Moving large molecules into & out of cell requires ATP! through vesicles & vacuoles endocytosis phagocytosis = “cellular eating” pinocytosis = “cellular drinking” receptor-mediated endocytosis exocytosis exocytosis
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receptor-mediated endocytosis
fuse with lysosome for digestion phagocytosis non-specific process pinocytosis triggered by ligand signal receptor-mediated endocytosis
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The Special Case of Water Movement of water across the cell membrane
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Osmosis is diffusion of water
Diffusion of water from high concentration of water to low concentration of water across a semi-permeable membrane
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Concentration of water
Direction of osmosis is determined by comparing total solute concentrations Hypertonic - more solute, less water Hypotonic - less solute, more water Isotonic - equal solute, equal water hypotonic hypertonic water net movement of water
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Managing water balance
Cell survival depends on balancing water uptake & loss freshwater balanced saltwater
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Hypotonicity animal cell in hypotonic solution will gain water, swell & possibly burst (cytolysis) Paramecium vs. pond water Paramecium is hypertonic H2O continually enters cell to solve problem, specialized organelle, contractile vacuole pumps H2O out of cell = ATP plant cell Turgid (turgor pressure) Cell wall
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Hypertonicity animal cell in hypertonic solution will lose water, shrivel & probably die salt water organisms are hypotonic compared to their environment they have to take up water & pump out salt plant cells plasmolysis = wilt
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Osmosis… .05 M .03 M Cell (compared to beaker) hypertonic or hypotonic Beaker (compared to cell) hypertonic or hypotonic Which way does the water flow? in or out of cell
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Water Potential Water moves from a place of greater water potential to a place of lesser water potential (net). As the concentration of a solute increases in a solution, the water potential will decrease accordingly. Which has the greater water potential: .2M or .8M? 20% or 80% water?
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