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BIOL 201 Chapter 5 notes

by: Kayla Wisotzkey

BIOL 201 Chapter 5 notes BIOL 201L 004

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Kayla Wisotzkey

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These notes cover Chapter 5; they include notes from the textbook and the lecture.
Introduction to Cell Biology and Genetics
Dr. Sarah Texel
Class Notes
Biology, transport
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This 4 page Class Notes was uploaded by Kayla Wisotzkey on Sunday October 9, 2016. The Class Notes belongs to BIOL 201L 004 at Towson University taught by Dr. Sarah Texel in Fall 2015. Since its upload, it has received 19 views. For similar materials see Introduction to Cell Biology and Genetics in Biology at Towson University.


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Date Created: 10/09/16
Chapter 5  Structure of membranes ­Fluid mosaic model:  a. Integral membrane proteins: embedded in the membrane b. Peripheral proteins: associated with the surface of the membrane ­Cellular membranes are composed of 4 components: 1) Phospholipid bilayer: provides a flexible matrix, is selectively permeable (only lets  certain molecules pass through) 2) Transmembrane proteins: a collection of proteins that float in the lipid bilayer.  They  function in transport and communication across the membrane… many integral proteins  are not fixed in the membrane ­Carriers: transport molecules across the membrane ­Channels: passively transport molecules across the membrane ­Receptors: transmit information into the cell 3) Interior protein network: intracellular proteins that support the membrane and  reinforce its shape.  They control the lateral movement of key membrane proteins,  anchoring them to sites ­Spectrins: determine the shape of the cell ­Clathrins: anchor proteins to certain sites 4) Cell surface markers: glycoproteins (aid in tissue recognition) and glycolipids (aid in  self­recognition)  ­Electron microscopy: allows biologists to closely study the cell membrane ­Ways to prepare a specimen for viewing: 1) Embed the tissue in a matrix and cut it into “epoxy shavings” which are put  into a grid.  Beams of electrons are directed through the grid 2) Freeze­fracturing: the tissue is embedded in a medium and quick frozen with  liquid nitrogen.  The frozen tissue is tapped with a knife and a crack is formed, visibly  revealing the membrane  Phospholipids ­Three classes of lipids: glycerol phospholipids, sphingolipids, and sterols ­Glycerol phospholipids: most diverse, vary in length and composition of their fatty acid  tail ­Sphingolipids: contain saturated hydrogen chains ­sterols: groups of naturally occurring unsaturated steroid alcohols ­Phospholipids spontaneously form bilayers because they are amphipathic (polar heads,  nonpolar tails) ­The nonpolar interior of a lipid bilayer stops the passage of water­soluble  substances through the bilayer ­The phospholipid bilayer is fluid and stable because water’s affinity for hydrogen  bonding never stops ­The hydrogen bonding of water holds the membrane together  Proteins ­6 key classes of membrane proteins: 1) Transport: only certain solutes can enter the cell through channels or carriers  composed of proteins 2) Enzymes: cells use enzymes attached to the membrane to carry out chemical reactions  on the interior surface 3) Cell­surface receptors: surface receptor proteins detect important chemical messages 4) Cell­surface identity markers: identify the cell to other cells; different proteins in each  cell 5) Cell­to­cell adhesion proteins: act by forming temporary or permanent interactions to  another cell 6) Attachments to the cytoskeleton: surface proteins that interact with other cells are  anchored to the cytoskeleton by linking proteins ­Anchoring molecules: modified lipids that have nonpolar regions that insert into the  internal portion of the lipid bilayer and chemical bonding domains that link directly to  proteins; attach some membrane proteins to the membrane  ­Transmembrane domain: a hydrophobic region of a transmembrane protein that anchors  it in the protein ­composed of hydrophobic amino acids usually arranged into ­helices ­pores: nonpolar regions of the membrane with ­pleated sheets that form a polar  environment on the inside of the sheet. This allows molecules to pass through. ­integral proteins: amphipathic, hydrophilic, regions span the protein ­peripheral proteins: found only on one membrane’s side, attached to integral proteins or  lipids  Passive transport: the movement of substances across a cell’s membrane without  expending energy ­concentration gradient: a difference between the concentrations on the inside and outside of the membrane  ­diffusion: the net movement of dissolved molecules from a region where they are more  concentrated to where they are less concentrated ­this will continue until the concentration is the same in all regions… after that,  movement in both directions still occurs, but no net change occurs ­facilitated diffusion: the diffusion of molecules or ions through carrier proteins or ion  channels… no energy needed, but a concentration gradient is needed ­channel proteins: have hydrophilic interiors that provide an aqueous channel  through which polar molecules can pass ­carrier proteins: bind to a specific molecule that cannot cross the membrane and  help it cross the membrane… change shape during the transport process ­The cell membrane is selectively permeable… only some substances can pass through ­Ion channel: has a hydrated interior that spans the membrane; allows ions to pass  through ­3 conditions determine the movement of the ions:  1) The concentrations on both sides of the ions 2) The voltage difference across the membrane and for the gated channels 3) If the gate is open or closed ­osmosis: the net diffusion of water across a membrane towards a higher concentration ­osmotic concentration: the concentration of all solutes in the solution ­hypertonic: a solution with a higher concentration of solutes than the cell; the  water will rush out of the cell and it will shrink ­isotonic: a solution with the same concentration of solutes as the cell; the cell is  healthy and normal ­hypotonic: a solution with a lower concentration of solutes than the cell; the  water will rush into the cell and it will swell and possibly burst  ­aquaporin: a membrane channel that allows water to cross the membrane more easily  than by diffusion  ­osmotic pressure: the force needed to stop osmotic flow ­Maintaining osmotic balance: a. Extrusion: some single­celled eukaryotes use a vacuole to rhythmically contract and pump out water b. Osmotic regulation: many terrestrial animals will circulate a fluid through their  body that bathes the cells in an isotonic solution c. Turgor pressure: the internal pressure in plant cells that presses its cell  membrane against the cell wall, carrying rigidity  Active transport: moving substances up a concentration gradient; requires the expenditure of energy… enables a cell to move substances out of the cytoplasm and into the  extracellular fluid ­protein carriers:  1) uniporters: transports a single type of molecule/ion 2) symporters: transports 2 molecules/ions in the same direction 3) antiporters: transports 2 molecules/ions in different directions ­sodium­potassium pump: ▯ 1) Three Na+ bind to the cytoplasmic side of the protein, causing the protein to change its shape. ▯ 2) In its new shape, the protein binds a molecule of ATP and cleaves it into ADP and  phosphate.  ADP is released, but the phosphate group is covalently linked to the protein. The  protein is now phosphorylated. ▯ 3) The phosphorylation of the protein induces a second shape change in the protein. This  change translocates the three Na+ across the membrane, so they now face the outside. In this new shape, the protein has a low affinity for Na+, and the three bound Na+ break away from the  protein and diffuse into the extracellular fluid. ▯ 4) The new shape has a high affinity for K+, 2 bind to the extracellular side of the protein as soon as it is free of the Na+. ▯ 5)  The binding of the K+ causes another shape change in the protein, this time resulting  in the hydrolysis of the bound phosphate group. 6)  Freed of the phosphate group, the protein reverts to its original shape, exposing the  two K+ to the cytoplasm. This shape has a low affinity for K+, so the two bound K+  dissociate from the protein and diffuse into the interior of the cell. The original shape has  a high affinity for Na+. When these ions bind, they initiate another cycle. ­coupled transport: molecules are moved against their concentration gradient using the  energy stored in the gradient of a different molecule  Bulk transport ­Endocytosis: the uptake of material into cells 1) Phagocytosis: endocytosis of a solid molecule; the plasma membrane folds  inwards around the particle and engulfs it to form a vacuole 2) Pinocytosis: the process of fluid uptake in a cell ­receptor­mediated endocytosis: process by which specific macromolecules are  transported into eukaryotic cells at clathrin­coated pits ­Exocytosis: the discharge of material from vesicles at the cell surface


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