Chapter 5: Membranes (through Feb. 3)
Chapter 5: Membranes (through Feb. 3) Bio 1510
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This 5 page Class Notes was uploaded by Michelle Notetaker on Sunday February 7, 2016. The Class Notes belongs to Bio 1510 at Wayne State University taught by Dr. Nataliya Turchyn in Summer 2015. Since its upload, it has received 41 views. For similar materials see Basic Life Mechanisms in Biology at Wayne State University.
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Date Created: 02/07/16
Chapter 5: Membranes Lipid Bilayers and Phospholipids o Hydrophobic tails are made up of fatty acids which make it hydrophobic One is a saturated fatty acid (no double bonds) which makes one tail straight The other is a monounsaturated fatty acid since it has 1 double bond which makes the other tail bent o It has a hydrophilic head because of the negatively charged phosphate group o Extracellular fluid and intracellular fluid Extracellular fluid is the fluid outside of the cell Intracellular fluid is cytosol which is the liquid portion of the cytoplasm Hydrophilic ends face fluid o Internal and external membranes are lipid bilayers o Middle section is hydrophobic Makes it hard for water soluble molecules to pass through the membrane Membrane structure o Membrane proteins help pass molecules from one side of the cell membrane to the other o Integral proteins/transmembrane proteins extend completely through the cell membrane They are made up of polar and nonpolar amino acids Non polar amino acids are facing hydrophobic tails Polar amino acids face hydrophilic o Extend to extracellular fluid and cytoplasm o Most of these integral proteins transport ions from one portion of the cell to the other o Peripheral proteins do NOT extend through the membranes They bind to transmembrane proteins and can bind to actin filaments/intermediate filaments o Membrane proteins are found on the outside or inside of the membrane, interacting with the membrane by being connected to transmembrane proteins o Animal cells have cholesterol in their cells Prevents their membranes from becoming too fluid Never find cholesterol in plants o Glycolipid is made in Golgi and attached to membrane proteins (glycoproteins) o Sugars of glycoproteins and glycolipids are found exclusive on the outside of the cell membrane You will NEVER find them facing in the cytoplasm They are involved with cell identity Functions Membrane Proteins o Transporter transport molecules and ions o Enzyme catalyzes chemical reactions by altering reactant (substrate) o Receptor something that receives a signal Ex. Insulin receptor that receives a ligand (insulin) Insulin = hormone Found in our liver, skeletal muscles, and fat cells Convert glucose into glycogen Convert glucose into fatty acids + glycerol = triglyceride (if it is in fat cells) o Glycoproteins are cell surface identity marker o Celltocell adhesion anchoring junction cadherins o Attachment to the cytoskeleton and/or extracellular matrix integrin connects extracellular matrix attaches to actin filaments This influences cell behavior Ways in which membrane proteins associate with the lipid bilayer o Transmembrane extend completely from the membrane Majority of membrane proteins are transmembrane o Phospholipidanchored proteins are attached to either side of the bilayer by covalent attachment to one or more lipid molecules o Peripheral proteins interact with the membrane via noncovalent binding to other membrane proteins o Transmembrane proteins extend across the bilayer as single alpha helix, multiple alpha helices, or beta barrel (rolledup beta sheets) Alpha helix has hydrophilic regions and hydrophobic regions Made up of nonpolar amino acids Made up of polar amino acids that face hydrophilic Single alpha helix it crosses lipid bilayer only once Act as receptors They have extracellular part and intracellular part o Extra cellular part binds with ligand o Intracellular signals to the cell’s interior Alpha helix is a secondary protein structure It forms because of hydrogen bonds o Hydrogen bonds are found inside of the alpha helix Hydrophobic side (R group) face the hydrophobic tails Beta barrels Consists of beta sheets Protein secondary structure and is formed with hydrogen bonds Multiple alpha helices and beta barrels from hydrophilic pores Water fills channels that allow water soluble molecules to pass through hydrophobic region of lipid bilayer A Segment of Alpha Helix Crossing Lipid Bilayer o Only crosses lipid bilayer once Multiple Alpha Helices and Beta Barrels Form Hydrophilic Pores o Hydrophilic pores are waterfilled channels that allow water soluble molecules to pass through hydrophobic region of lipid bilayer How Do Molecules Move Across the Cell Membrane? o Passive transport – movement of molecules down their concentration gradients; no energy is needed (molecules from a region of higher to lower concentration) Simple diffusion Allows molecules to cross the membrane without the aid of membrane proteins Gases (oxygen, carbon dioxide, nitric oxide), water, and nonpolar molecules (such as steroid hormones) Molecules are going to keep moving until it reaches equilibrium No membrane proteins are needed to pass through membrane Ex. Adding creamer, spraying perfume, and spreading smoke from a cigarette Facilitated diffusion Allows molecules to cross membrane with the help of membrane proteins (channels or carriers) Channels integral proteins that create a hydrophilic pore through which specific ions diffuse o Ex. Sodium ion channel Only allows sodium ions to pass through the channel Diffusion is facilitated through membrane proteins Carriers integral proteins that change their shape to allow polar molecules to pass through the membrane o Polar molecules can be sugars and some ions Osmosis Diffusion of water down its concentration gradient Only water can use osmosis to enter and leave our cells o Water can enter because of its small size Aquaporins are specialized water channel proteins that facilitate the flow of water through the membrane Water always moves from the region where concentration is high and concentration of solutes is low to the region where its concentration is low and concentration of solutes is high o Water is a solvent and wants to dissolve those solutes therefore water wants to go to where there are lots of solutes in order to dissolve them What Controls Osmosis? o Isotonic solution has the same [solutes] as the cell Iso = the same Tonic = concentration of solutes Cell volume does not change Hypotonic has much lower [solutes] than the cell smell and may burst or lyse Pure water is hypotonic “hypo” means below Hypertonic = “above” has much higher [solutes] than the cell Shrink Salt water o In plant cells… Hypertonic solutions cause the plasma membrane pulls away from cell wall = plasmolysis Isotonic solutions make the cells flaccid Hypotonic solution makes the cells turgid which is normal Active transport o Movement of molecules up their concentration gradients with the help of energy o Molecules move from regions of low to higher concentration and requires energy IF IT’S ACTIVE TRANSPORT o Sodium potassium pump [Na+] outside > [Na+] inside [K+] outside < [K+] inside Sodium potassium pump uses ATP to move 3 sodium ions out of the cell and 2 potassium ions into the cell against the concentration gradient ATP – driven pump uses ATP DIRECTLY Coupled Transport: Cotransport o Na+/glucose transporter uses Na+ gradient established by Na+/K+ pump as a source of energy to move both Na+ and glucose into the cell o Uses ATP INDIRECTLY o The movement of glucose up its concentration gradient is coupled to the movement of sodium ion down its concentration gradient o Cotransport = symport when 2 molecules to the same side of the cell Coupled Transport: Countertransport o Antiport when 2 molecules move to the opposite sides of the cell membrane Endocytosis o An energyusing process by which cells import substances from the external medium A form of active transport Phagocytosis = ingestion of large particles, such as microorganisms and cell debris Used by protists and our white blood cells o Amoebas use it to engulf their prey o Plasm membrane extends outward forming a vesicle Vesicle = food vesicle delivers it to lysosomes that will break down the bacteria Endocytosis o Pinocytosis = uptake of fluid and small molecules o Dissolve solve molecules o “cellular drinking” o Invaginate = bends inward forming a vesicle o Performed by all cells in our body o Receptormediated endocytosis = requires the receptor to bring specific molecules in the cell LDL = Bad cholesterol Lysosomes Coated vesicle Clathrin = protein Exocytosis (secretion o An energyusing process by which cells export substances to the extracellular environment
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