Biomembrane Structure and Functions
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What type of proteins are responsible for moving phospholipids from one leaflet to the other in membranes?

  • Passive diffusion proteins
  • ATP-powered transport proteins (correct)
  • Receptor proteins
  • Co-transport proteins

    • Which phospholipid is primarily found in the outer leaflet of the membrane?

  • Phosphatidylcholine (PC) (correct)
  • Phosphatidylinositol (PI)
  • Phosphatidylethanolamine (PE)
  • Phosphatidylserine (PS)

    • What are lipid rafts enriched with?

  • Glycolipids and proteins only
  • Cholesterol and sphingolipids (correct)
  • Phospholipids and cholesterol only
  • Integral proteins and peripheral proteins

    • Which type of membrane protein does not span the membrane?

    <p>Lipid-anchored proteins</p> Signup and view all the answers

    How are transmembrane proteins characterized in terms of their structure?

    <p>Two hydrophilic exterior regions and one hydrophobic interior region</p> Signup and view all the answers

    Which type of proteins are exclusively found in the exoplasmic face of the plasma membrane?

    <p>Glycoproteins and glycolipids</p> Signup and view all the answers

    Which characteristic is most commonly associated with the secondary structure of hydrophobic regions in transmembrane proteins?

    <p>Hydrophobic α-helices</p> Signup and view all the answers

    What role do arginine and lysine residues play in glycoprotein structure?

    <p>They help anchor the protein in the membrane</p> Signup and view all the answers

    What is the primary composition of prokaryotic cell membranes?

    <p>Single plasma membrane containing proteins essential for cell function</p> Signup and view all the answers

    Which function is NOT typically associated with proteins in eukaryotic cell membranes?

    <p>Catalyzing ATP synthesis</p> Signup and view all the answers

    Which term describes the ability of a cell membrane to allow certain substances to pass while blocking others?

    <p>Selective permeability</p> Signup and view all the answers

    What is meant by 'tonicity' in relation to cellular membranes?

    <p>The relative concentration of solutes outside the cell compared to inside</p> Signup and view all the answers

    What structural feature do all biomembranes share?

    <p>Phospholipid bilayers forming leaflets</p> Signup and view all the answers

    What characteristic distinguishes eukaryotic from prokaryotic cell membranes?

    <p>Complex internal membrane system</p> Signup and view all the answers

    How do phospholipids behave in an aqueous environment?

    <p>They spontaneously form bilayers</p> Signup and view all the answers

    What term describes the lateral movement of lipids and proteins within the cell membrane?

    <p>Fluidity</p> Signup and view all the answers

    What is the primary role of desaturase enzymes in fatty acid metabolism?

    <p>They add double bonds to fatty acids.</p> Signup and view all the answers

    What do fatty acid binding proteins (FABPs) primarily interact with?

    <p>Hydrophobic pockets of long fatty acid chains.</p> Signup and view all the answers

    Which enzyme is crucial for converting HMG-CoA to mevalonate in cholesterol synthesis?

    <p>HMG-CoA reductase.</p> Signup and view all the answers

    What is one of the proposed mechanisms for lipid transport between organelles?

    <p>Direct contact between membranes mediated by integral proteins.</p> Signup and view all the answers

    Which statement accurately describes the effect of statins in relation to cholesterol metabolism?

    <p>They inhibit the activity of HMG-CoA reductase.</p> Signup and view all the answers

    What do phosphatidic acid and diacylglycerol have in common in phospholipid synthesis?

    <p>Both contain a glycerol backbone.</p> Signup and view all the answers

    What characterizes atherosclerosis in the arterial walls?

    <p>Formation of plaque due to foam cell accumulation.</p> Signup and view all the answers

    What type of proteins play a role in the movement of fatty acids within cells?

    <p>Cytosolic fatty acid binding proteins.</p> Signup and view all the answers

    What is the primary structural component of a micelle?

    <p>Fatty acyl chains</p> Signup and view all the answers

    What class of membrane lipids is primarily involved in cell membrane fluidity?

    <p>Phosphoglycerides</p> Signup and view all the answers

    Which fatty acid length is NOT commonly associated with phospholipids?

    <p>12 carbons</p> Signup and view all the answers

    What role does cholesterol play in cell membranes?

    <p>Maintains membrane fluidity at temperature extremes</p> Signup and view all the answers

    Which of the following is a characteristic of sphingolipids?

    <p>Have a long hydrocarbon chain</p> Signup and view all the answers

    What type of fatty acid attachment do plasmalogens have?

    <p>One ether and one ester linkage</p> Signup and view all the answers

    Which lipid is the most abundant in mammalian plasma membranes?

    <p>Cholesterol</p> Signup and view all the answers

    How does the presence of sphingolipids affect membrane fluidity?

    <p>Decreases membrane fluidity</p> Signup and view all the answers

    What is the significance of uneven lipid distribution in cellular membranes?

    <p>Influences signaling pathways in the cell</p> Signup and view all the answers

    Which of the following statements about membrane proteins is true?

    <p>All transmembrane proteins are also integral membrane proteins.</p> Signup and view all the answers

    Which components of biological membrane are considered amphipathic?

    <p>Glycolipids and membrane steroids</p> Signup and view all the answers

    Which of the following best describes phosphoglycerides?

    <p>Have diverse head group attachments</p> Signup and view all the answers

    Which of the following about membrane lipids is accurate?

    <p>Flippases are responsible for lipid transfer across the membrane.</p> Signup and view all the answers

    In which leaflet of the plasma membrane are glycolipids primarily found?

    <p>The outer leaflet of the plasma membrane</p> Signup and view all the answers

    What can be said about lipid movement in biological membranes?

    <p>Both integral and peripheral proteins affect lipid movement.</p> Signup and view all the answers

    What determines the blood type in individuals?

    <p>The presence or absence of glycosyltransferases</p> Signup and view all the answers

    Which fatty acids are primarily synthesized in the cytosol from Acetyl-CoA?

    <p>14 and 16 Carbon fatty acids</p> Signup and view all the answers

    Which statement best describes the role of detergents in membrane biology?

    <p>Detergents can both denature and solubilize proteins</p> Signup and view all the answers

    From where are many lipids synthesized in the cell?

    <p>Smooth ER</p> Signup and view all the answers

    What are the characteristics of amphipathic molecules like detergents?

    <p>They have both hydrophilic and hydrophobic parts</p> Signup and view all the answers

    What role do oligosaccharide chains play in determining blood types?

    <p>They identify blood types by varying sugar compositions</p> Signup and view all the answers

    What is the final location for the last steps of membrane lipid synthesis?

    <p>Destination membranes</p> Signup and view all the answers

    Which of the following is a function of glycosyltransferases?

    <p>To add sugars to O Antigen</p> Signup and view all the answers

    Which of the following blood type percentages is the highest among Caucasians in the United States?

    <p>O+</p> Signup and view all the answers

    How are fatty acids generated from stored triacylglycerol in adipocytes?

    <p>By enzymatic hydrolysis</p> Signup and view all the answers

    Study Notes

    Biomembrane Structure

    • Biomembranes are integral components of all cells
    • Prokaryotic cells have a single plasma membrane, usually without internal membrane compartments
    • Prokaryotic cell membranes contain hundreds of proteins crucial for cell function (e.g., catalyzing ATP synthesis, initiating DNA replication, transporting ions, molecules)
    • Proteins can also be involved in cell signaling
    • Eukaryotic plasma membranes are studded with proteins with various functions
    • Membrane transport proteins facilitate ion, sugar, amino acid, and vitamin passage

    Membrane Functions

    • Membranes contribute to the shape of cells and organelles
    • Membranes possess stability due to hydrophobic and van der Waals interactions between lipid chains
    • Membranes have selective permeability (tonicity) including isotonic, hypertonic and hypotonic conditions

    Lipid Composition

    • Biomembranes share common phospholipid features
    • Phospholipids spontaneously form bilayers
    • The bilayer is composed of two leaflets
    • Lipids are amphipathic

    Membrane Protein Distribution

    • In membranes, proteins can be peripheral, integral or lipid-anchored
    • Transmembrane proteins span the membrane, typically with hydrophobic α-helices interacting with fatty acid tails
    • Integral membrane proteins have multiple portions embedded within the membrane
    • Peripheral proteins are loosely associated with the membrane surface
    • Cell types and subcellular locations affect membrane protein amounts and types

    Membrane Proteins

    • Porins: a class of trimeric transmembrane proteins that form barrel-like structures with central pores, allowing molecules to traverse the membrane
    • Aquaporins: transmembrane proteins that transport water and other hydrophilic molecules

    Lipid Distribution

    • Membrane lipids are asymmetrically distributed between exoplasmic and cytoplasmic leaflets
    • Cholesterol is generally evenly distributed
    • Mechanisms like flippases facilitate asymmetric lipid distribution and lipid transport between leaflets to maintain asymmetries
    • Lipid composition influences membrane physical properties (e.g., fluidity, membrane thickness, curvature)

    Lipid Rafts

    • Regions of cell membranes with enriched specific lipids (cholesterol, sphingolipids) and proteins
    • They may promote specific functions
    • Decreased cholesterol content in membranes can disrupt lipid rafts

    Membrane Biogenesis

    • New bio-membranes are formed by expanding existing ones
    • Lipids are synthesized mainly on smooth endoplasmic reticulum membranes
    • Steps of lipid synthesis complete on destination membranes
    • Lipid distribution needs to occur after synthesis to appropriate leaflets and organellar membranes

    Fatty Acid Synthesis

    • Fatty acids can be synthesized from other organic molecules or Acetyl CoA
    • They are primarily synthesized in the cytoplasm
    • Elongation to longer chains may occur in the ER membrane
    • Some fatty acids have double bonds, created by desaturase enzymes

    Intracellular Fatty Acid Movement

    • Fatty acids are transported within cells using cytosolic protein carriers known as fatty acid binding proteins (FABPs)
    • FABP upregulation/downregulation is responsive to cellular demands

    Phospholipid Synthesis

    • Fatty acyl CoA and glycerol phosphate are synthesized into phosphatidic acid
    • Then the molecule is produced into diacylglycerol
    • Lastly phosphocholine is transferred to diacylglycerol to form the phospholipid
    • Flipases transfer lipids between cytosolic and exoplasmic leaflets

    Cholesterol Synthesis

    • Acetyl CoA and acetoacetyl CoA form HMG-CoA, which converts to mevalonate, a 6-carbon molecule
    • HMG-CoA reductase converts HMG-CoA to mevalonate
    • Mevalonate, IPP and farnesyl pyrophosphate are intermediates in cholesterol biosynthesis
    • Squalene is converted to cholesterol on the SER

    Atherosclerosis

    • Atherosclerosis is the hardening of arteries due to lumen blockage
    • It arises from damage to the endothelial cells
    • Monocytes enter and are converted into macrophages which then take up LDL cholesterol
    • Excess LDL forms foam cells, eventually accumulating to form plaques that obstruct arteries

    Transport of Lipids between Organelles

    • Cholesterol and phospholipids are often transported via Golgi-dependent and Golgi-independent processes
    • There is also transport via vesicles; direct interaction between organellar membranes mediated by integral proteins
    • Lipid transfer proteins can mediate transport

    Membrane Proteins

    • Membrane proteins are diverse in kind and amount dependent on the cell and subcellular location
    • Proteins interact with membranes in 3 ways: peripheral, integral and lipid-anchored proteins

    Typical Single-pass Transmembrane Protein

    • Glycophorin A (an integral protein in RBC membranes) has a hydrophobic alpha-helix portion and carbohydrate chains
    • The protein is usually a dimer

    Multipass Membrane Protein

    • Bacteriorhodopsin (a transmembrane protein) crosses the membrane multiple times with hydrophobic alpha-helices
    • Other multipass membrane proteins are also integral to the membrane

    Porins

    • Porins are a type of transmembrane protein that forms channels
    • The channels are made up of Beta pleated sheets
    • Examples include mitochondrial porin and E.coli porin (OmpX)

    Aquaporins

    • Aquaporins are tetrameric transmembrane proteins
    • They transport water and hydrophilic molecules

    Anchored Membrane Proteins

    • Anchored membrane proteins are covalently linked to lipids by hydrocarbon chains
    • These chains provide direct connections to the membrane

    Human ABO Blood Group Antigens

    • Antigens are oligosaccharide chains covalently attached to glycoproteins or glycolipids
    • Differences in these chains determine blood type (A, B, O, AB)

    Q1. Membrane Protein Characteristics

    • A: Incorrect
    • B: Correct
    • C: Correct
    • D: Incorrect

    Q2. Amphipathic Membrane Components

    • a: Partially correct. Integral membrane proteins have hydrophobic and hydrophilic regions, making them amphipathic.
    • b: Correct
    • c: Correct
    • d: Correct

    Q3. Properties of Membrane Lipids

    • a: Incorrect. Membrane lipids have hydrophobic and hydrophilic regions, making them amphipathic.
    • b: Correct. Flippases catalyze the transfer of lipids across the membrane.
    • c: Incorrect. While lipids can move, it's not spontaneous in all cases. Flippases and other mechanisms are required, in most cases, for movement to occur.
    • d: Correct. Different lipid compositions are found in the different membrane leaflets.

    Q4. Glycolipid Location in Plasma Membrane

    • c: Correct. Glycolipids are predominantly found in the outer leaflet of the plasma membrane

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