1 Membranes. 2 Outline Phospholipid Bilayer Fluid Mosaic Model Membrane Proteins Diffusion...
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Transcript of 1 Membranes. 2 Outline Phospholipid Bilayer Fluid Mosaic Model Membrane Proteins Diffusion...
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Membranes
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Outline
• Phospholipid Bilayer• Fluid Mosaic Model• Membrane Proteins• Diffusion• Facilitated Diffusion• Osmosis
– Osmotic Balance• Bulk Transport• Active Transport
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Phospholipid Bilayer
• Phospholipid has two fatty-acid chains attached to its backbone.
– One end is strongly nonpolar while the other end is strongly polar.
polar head oriented toward water and nonpolar tails oriented away from water
bilayer is stable because water’s affinity for hydrogen bonding never stops
Fatty acidPhosphorylated
alcohol
Polar(hydrophilic) region
Nonpolar (hydrophobic) region
Fatty acidGLYCEROL
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Fluid Mosaic Model
• Plasma membrane is composed of both lipids and globular proteins.
– Membrane proteins are not very soluble in water.
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Cell Membrane
• Cell membrane components– phospholipid bilayer– transmembrane proteins– interior protein network– cell surface markers– extra cellular matrix (ECM)
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Membrane Proteins
• transporters• enzymes• cell surface identity markers• cell adhesion proteins• attachments to cytoskeleton
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Plasma Membrane Proteins
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Membrane Protein Structure• Anchoring proteins in the bilayer• Transmembrane proteins
Cytoplasmic sideof cell membrane
Cytoskeletalproteins
Junctionalcomplex
100 nm
Actin
Spectrin
Linkerproteincomplex
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Membrane Protein Structure• Transmembrane proteins
– single-pass anchors– multiple-pass channels and carriers– pores
Phospholipids
Polar areasof protein
Cholesterol
Nonpolar areas ofprotein
NH2
H+
COOH
Cytoplasm
Retinalchromophore
Nonpolar(hydrophobic)-helices in thecell membrane
H+
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Solutions
• Intra cellular fluid (ICF) - within cells• Extra cellular Fluid (ECF) - outside cells
– Inter cellular = tissue fluid = interstitial fluid– Plasma = fluid portion of blood
• Composition of fluids change as substances move between compartments
– nutrients, oxygen, ions and wastes move in both directions across capillary walls and cell membranes
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Selective Permeability of Membrane
• Lipid bilayer– permeable to nonpolar, uncharged molecules --
oxygen, CO2, steroids
• Transmembrane proteins act as specific channels– small and medium polar & charged particles
• Macromolecules unable to pass through the membrane
– vesicular transport
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Passive Transport
• Diffusion– Random motion causes a net movement
of substances from regions of high concentration to regions of low concentration – down their concentration gradient. AnimationAnimation
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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
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Selective Membrane Transport
• Cell transport channels are selectively permeable, as only certain molecules are allowed to enter.
– Ions cannot move between the cytoplasm of a cell and the extracellular fluid without the assistance of membrane transport proteins.
ion channels
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Facilitated Diffusion
• Carriers transport ions and other solutes across the plasma membrane.
– Facilitate movement by physically binding molecules on one side of the membrane, and releasing them on the other down their concentration gradient. animationanimation
essential characteristicsspecificpassive (no ATP) transport saturation
Outside of cell
Inside of cell
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Osmosis• In an aqueous solution, both water and solutes diffuse down
concentration gradients.• Net movement of water through a selectively permeable
membrane from an area of high water concentration to an area of lower water concentration
• Only occurs if membrane is permeable to water but not to certain solutes
Aquaporins = channel proteins specialized for osmosis
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Osmotic Pressure
• Amount of hydrostatic pressure required to stop osmosis
• Osmosis slows due to filtration of water back across membrane due to increased hydrostatic pressure
• AnimationAnimation
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Tonicity
• Tonicity - ability of a solution to affect fluid volume and pressure within a cell
– depends on concentration and permeability of solute• Hypotonic (hypoosmotic) solution
– low concentration of nonpermeating solutes (high water concentration)
– cells absorb water, swell and may burst (lyse)• Hypertonic solution (hyperosmotic)
– has high concentration of nonpermeating solutes (low water concentration)
– cells lose water + shrink (crenate)• Isotonic (isoosmotic) solution = normal saline
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Maintaining Osmotic Balance
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Osmotic Pressure
• Hydrostatic pressure - pressure of the cytoplasm pushing out against the cell membrane
• Osmotic pressure - pressure that must be applied to stop the osmotic movement of water across a membrane
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Bulk Transport
• Endocytosis - enveloping food
– phagocytosis - material taken in is in particulate form
– pinocytosis - material taken in is in liquid form
– receptor-mediated - transport of specific molecules
• Exocytosis - discharge of material from vesicles at the cell surface
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Endocytosis / Exocytosis
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Active Transport
• Active transport involves the expenditure of energy to move substance against their concentration gradient.
– involves highly selective protein carriers within the membrane
sodium-potassium pump• Secondary active transport
– coupled transport - using energy stored in a gradient of a different molecule
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Sodium-Potassium Pump
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Secondary Active Transport
• Uniporter– carries only one solute at a time
• Symporter– carries 2 or more solutes simultaneously in same
direction (cotransport)• Antiporter
– carries 2 or more solutes in opposite directions (countertransport) (animationanimation)
sodium-potassium pump brings in K+ and removes Na+ from cell
• Any carrier type can use either facilitated diffusion or active transport (primary or secondary)
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Digitalis
• Slows the sodium pump, which lets more Na+ accumulate in the heart muscle cells.
• Less Na+ concentration gradient across the membrane
• Na+/Ca+2 antiporters slow down so more Ca+2 remains inside the cardiac cells
• Strengthening the force of contraction• Balance between concentration of Na+ and
Ca+2 in cytosol & extracellular fluid is important
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Summary
• Phospholipid Bilayer• Fluid Mosaic Model• Membrane Proteins• Diffusion• Facilitated Diffusion• Osmosis
– Osmotic Balance• Bulk Transport• Active Transport
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