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CELL MEMBRANE Structure and Function
Chapter 5 CELL MEMBRANE Structure and Function
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Chapter 5 At a Glance 5.1 How Is the Structure of the Cell
Membrane Related to Its Function? 5.2 How Do Substances Move Across Membranes? 5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate? Case Study Investigation: Y:\Biology\D Cell Membrane and Transport\Transport CSI Reading.pdf
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5.1 How is the Structure of the Cell Membrane Related to its Function?
All cell membranes have a similar basic structure Proteins suspended in a double layer of phospholipids Responsible for: selectively exchanging substances Communicating with environment Controlling biochemical reactions Forming connections btwn cells Responsible for: Isolating cells contents
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5.1 How is the Structure of the Cell Membrane Related to its Function?
Crucial functions of the plasma cell membrane: isolates cell’s contents from external environment regulates exchange of essential substances allows communication between cells creates attachments within and btwn cells regulates biochemical reactions extremely thin–10,000 membranes still thinner than txtbk page membranes change in response to their surroundings
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Cell Membrane = Fluid Mosaic Model
named in 1972 by S.J. Singer and G.L. Nicolson proteins and other structures move within the membrane carbohydrate interstitial fluid (outside) protein extra- cellular matrix glycoprotein receptor protein phospholipid bilayer binding site phospholipid transport protein connection protein pore cholesterol recognition protein enzyme cytoskeleton cytosol (fluid inside cell) 5
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Phospholipids are basis of membrane structure
phospholipid bilayer is specifically arranged between an interior (cytosol) and exterior (interstitial fluid) watery layer interstitial fluid–weakly salty liquid (blood w/o its cells & proteins) surrounds outer surfaces of animal cell membranes cytosol – fluid portion of cytoplasm (mostly water) head (hydrophilic) tails (hydrophobic)
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Phospholipids are basis of membrane structure
phospholipid bilayer’s flexibility allows cellular shape changes individual phospholipid molecules not bonded to each other unsaturated fat acids are kinked b/c of their double bond(s) all reasons why membrane is fluid
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Phospholipids structure = selectively permeable
membranes more fluid at high temps (more movement) and less fluid at low temps (less movement) cell membranes of organisms living in low temps tend to be more unsaturated to help maintain fluidity cholesterol stabilizes membranes – makes it less fluid at high temps and less solid at lower temps; becomes more selective
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Phospholipids structure = selectively permeable
water soluble (polar) substances cannot easily cross phospholipid bilayers (i.e. salts, amino acids, sugars) small molecules (i.e. H2O, O2, CO2) can slip in larger lipid soluble (nonpolar) substances (i.e. estrogen, testosterone) can pass through
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Variety of Proteins Form a Mosaic
variety of proteins are embedded within or attached to phospholipid bilayer many have carbohydrates attached to their outer surface (glycoproteins) 5 major categories of membrane proteins enzymes receptor proteins recognition proteins connection proteins transport proteins
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Membrane Proteins 1. Enzymes - proteins that promote chemical reactions that synthesize or break apart molecules (i.e. enzymes that line the small intestine that digest carbs & proteins) 2. Receptor proteins – trigger cellular responses (i.e. hormones that bind, immune response) 3. Recognition proteins – glycoproteins that serve as identification tags on cell surface (i.e. self vs non-self)
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Membrane Proteins 4. Connection proteins: anchor cell membranes
maintain shape by linking membrane to cytoskeleton anchor cell in place within a tissue by linking cell’s cytoskeleton to extracellular matrix outside form connections between adjacent cells 5. Transport proteins: regulate movement of hydrophilic molecules through membrane Channel proteins - form pores (can open and close) Carrier proteins – bind to and carry substances through membrane
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5.2 How Do Substances Move Across Membrane
some substance can move across the membrane by diffusing through phospholipid bilayer through specialized proteins solute: substance that can be dissolved in a solvent (i.e. sugar, salt, etc) solvent: fluid capable of dissolving a solute (i.e. H2O, gasoline, alcohol) Water is a universal solvent
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concentration: defines the amount of solute in a given amount of solvent
gradient: physical difference in temperature, pressure, charge or concentration of a solute in a fluid - btwn 2 adjoining regions of space concentration gradient: differences in solute concentrations across a membrane
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Molecules in Fluids Diffuse in Response to Gradients
atoms, molecules, & ions are in constant random motion diffusion: net movement of solutes from regions of high concentration to regions of low concentration (down a gradient) gradients cause molecule movement the greater the gradient the faster the rate of diffusion the higher the temperature movement continues until molecules are evenly dispersed (unless disrupted) Hot Water Cold Water
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Dye molecules diffuse into the water; water molecules diffuse into the dye Both dye molecules and water molecules are evenly dispersed A drop of dye is placed in water dye molecules water molecule
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Movement Through Membranes Occurs by Passive or Active Transport
cells w/o gradients are dead so….. membrane proteins must use energy to create and maintain gradients in order to carry out crucial biochemical process selective permeable membrane creates a barrier that helps maintain gradients PASSIVE TRANSPORT diffusion of substances across cell membranes DOWN concentration gradients simple diffusion facilidated diffusion osmosis ACTIVE TRANSPORT requires energy to move substances usually AGAINST concentration gradients endocytosis exocytosis
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Active Transport Passive Transport
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Passive Transport: Simple & Facilitated Diffusion
Simple Diffusion: some molecules diffuse directly through phospholipid bilayers small molecules with no net charge (H2O, O2, CO2) lipid soluble molecules regardless of size (ethyl alcohol, vit A, D, and E, steroid hormones) (interstitial fluid) O2 phospho- lipid bilayer (cytosol) Simple diffusion through the phospholipid bilayer
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channel proteins: form pores through cell membranes
Facilitated Diffusion: transport proteins help larger polar molecules & ions (sugars, K+, Na+, Cl-, Ca 2+) to cross membranes carrier proteins: loosely bind to specific ions/molecules (i.e. sugars & small proteins), change shape & transfer the bound particles across membrane channel proteins: form pores through cell membranes Ion channels very selective (charge/size) channel protein carrier protein Facilitated diffusion through carrier proteins Facilitated diffusion through channel proteins
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small size and positive charge attracts negative side of H2O
aquaporins: specialized water channel proteins for faster diffusion of water (osmosis) small size and positive charge attracts negative side of H2O billions of water molecules can move through an aquaporin in single file every second repels larger molecules and smaller positive ions aquaporin channel (cytosol) water
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Osmosis is the Diffusion of Water
Osmosis: diffusion of water across a selectively permeable membrane in response to concentration gradient, pressure, or temp. H2O moves from regions of high concentration to regions of low concentration across a membrane dissolved substances reduces concentration of free H2O molecules (& purity) in a solution osmotic strength: tendency to attract H2O across a membrane; greater the solute conc., the greater the o.s. Low water concentration High water concentration
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Isotonic, Hypertonic, Hypotonic Solutions
No net flow of water Water flows out; the sac shrinks Water flows in; the sac expands A sac in an isotonic solution A sac in a hypertonic solution A sac in a hypotonic solution iso = same equal concentrations of solute and solvent hyper = more more solute higher osmotic strength H2O moves out of less concentrated sln hypo = less less solute lower osmotic strength H2O moves into more concentrated sln Egg Experiment
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Osmosis Plays an Important Role in Lives of Cells
water uptake by roots of plants absorption of dietary water from intestine reabsorption of water in kidneys organisms that live in fresh water must use energy to counteract osmosis b/c cells are hypertonic to surrounding water Turgor Plasmolysis pressure: when water when water leaves cell flows into a causing cell cell and to shrink inflates the away from cell; pressure cell wall on the cell wall (droopy plants)
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When water is plentiful, it fills the central
central vacuole cytoplasm When water is plentiful, it fills the central vacuole, pushes the cytoplasm against the cell wall, and helps maintain the cell’s shape Water pressure supports the leaves of this impatiens plant Turgor pressure provides support cell wall plasma membrane When water is scarce, the central vacuole shrinks and the cell wall is unsupported Deprived of the support from water, the plant wilts Loss of turgor pressure causes the plant to wilt
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Active Transport, Endocytosis, Exocytosis
energy-expending cellular activities are crucial to sustaining life maintaining concentration gradients acquiring food excreting wastes cell to cell communication active transport: membrane proteins use energy to move molecules/ions across a membrane AGAINST their concentration gradient (low concentration to a high concentration)
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Active Transport ATP & specific molecule/ion
ATP & specific molecule/ion Energy comes from breaking high energy bond of 3rd phosphate active transport proteins = pumps (interstitial fluid) Energy from ATP changes the shape of the transport protein and moves the ion across the membrane The transport protein binds both ATP and Ca2 The protein releases the ion and the remnants of ATP (ADP and P) and closes ATP binding site ADP binding site ATP P ATP Ca2 (cytosol)
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Endocytosis Allows Cells to Engulf Particles or Fluids
(interstitial fluid) When cells need materials that are too large to pass through membrane they use… Endocytosis: form of active transport that allows cells to engulf particles or fluids 1. Pinocytosis:“cell drinking” moves liquids into cells vesicle containing interstitial fluid (cytosol) A dimple forms in the plasma membrane, which deepens and surrounds the interstitial fluid The membrane encloses the interstitial fluid, forming a vesicle. Pinocytosis interstitial fluid cytosol TEM of pinocytosis
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2. Receptor-mediated endocytosis: moves specific molecules into cells (packets of protein and cholesterol) Receptor proteins for specific molecules or complexes of molecules are localized at coated pit sites. nutrient molecule (interstitial fluid) receptor The receptor bind the molecules and the membrane dimples inward. coated pit The coated pit region of the membrane encloses the receptor- bound molecules. coated vesicle A vesicle (“coated vesicle”) containing the bound molecules is released into the cytosol. (cytosol) Receptor-mediated endocytosis extracellular particles bound to receptors coated vesicle (interstitial fluid) (cytosol) protein coating coated pit plasma membrane TEM of receptor-mediated endocytosis
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Phagocytosis: “cell eating” moves large particles (or whole
organisms) into cells (interstitial fluid) food particle pseudopods food vacuole (cytosol) The plasma membrane extends pseudopods toward an extracellular particle (for example, food) The ends of the pseudopods fuse, encircling the particle A vesicle called a food vacuole is formed containing the engulfed particle. Phagocytosis An Amoeba engulfs a Paramecium A white blood cell engulfs a disease-causing fungal cell
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Exocytosis Moves Material Out of the Cell
energy is used to dispose of undigested particles of waste or secrete substances (i.e. hormones) into interstitial fluid (interstitial fluid) secreted material plasma membrane plasma membrane vesicle Material is enclosed in a vesicle that fuses with the plasma membrane, allowing its contents to diffuse out (cytosol)
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Exchange of Materials Across Membranes Influences Cell Size and Shape
most cells too small to be seen with naked eye (1-100mm in diameter) WHY? as a spherical cell enlarges, its innermost parts get farther away from the plasma membrane diffusion (already slow) takes too long to supply important processes deep within cell volume increases more rapidly than surface area larger cells (require more nutrients & create more waste) would have a smaller area of membrane for acquiring nutrients and eliminating wastes not enough to exchange/survive
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Case Study many rattlesnake & spider venoms contain phospholipases - break down membrane phospholipids causing cells to rupture and die phospholipases attack membranes of capillary cells causing blood vessels to rupture & release blood into surrounding tissue causes anemia (inadequate # of oxygen-carrying RBC) attack muscle cell membranes antivenin – contains specialized proteins that bind and neutralize snake venom proteins no antivenin for brown recluse bites (treatments only) How does the role of phospholipases in snake venom differ from its role in the snake’s digestive tract?
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Study…. Reread each section; reread your notes Reorganize your notes; re-write; make charts, tables, lists Ch. 5 Vocab Read Summary of Key Concepts (pg. 91) Complete Thinking Through the Concepts 1 – 6 (pg 91-92) Be able to answer Review Questions 1-8 (pg. 92) Concept check questions Use Mastering Biology to help study
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