Biology Quiz: Membrane Structures and Functions
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Questions and Answers

What is the diameter and length of the heptamer structure of a-hemolysin?

  • 10 nm in diameter and 6 nm in length
  • 12 nm in diameter and 8 nm in length
  • 6 nm in diameter and 10 nm in length
  • 10 nm in diameter and 10 nm in length (correct)

    • Which of the following statements about gramicidin A is true?

  • It is a type of aquaporin channel.
  • Its N-termini are located outside the bilayer core.
  • It consists of two molecules adopting a helical conformation. (correct)
  • It forms a hydrophilic channel through the membrane.

    • What is the role of aquaporins in eukaryotic cells?

  • They facilitate the rapid transport of water. (correct)
  • They are involved in ion selectivity in membranes.
  • They act as antibiotic agents.
  • They inhibit the movement of water across membranes.

    • What characterizes the structural features of a single aquaporin monomer?

    <p>It is made up of six membrane-spanning helices and two shorter helices with an NPA motif.</p> Signup and view all the answers

    What structural feature of aquaporins provides an electrostatic barrier to H3O+ passage?

    <p>The arrangement of two helical macrodipoles and Arg195</p> Signup and view all the answers

    What structure do lipid molecules typically form when they associate in water?

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

    Which fatty acid is an example of a saturated fatty acid?

    <p>Stearic acid</p> Signup and view all the answers

    What is the primary function of fat storage in animals?

    <p>Energy production</p> Signup and view all the answers

    What is a characteristic of most naturally occurring fatty acids?

    <p>Usually have cis configuration for double bonds</p> Signup and view all the answers

    What is the role of specialized cells in ‘brown fat’?

    <p>Oxidize triacylglycerols for heat production</p> Signup and view all the answers

    What defines the structure of membrane lipids?

    <p>Amphipathic properties</p> Signup and view all the answers

    The major long-term energy storage molecules in many organisms are known as?

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

    What is the effect of adding four Lys to the N-terminus of Lep?

    <p>It results in a mutant Lep with the opposite membrane topology.</p> Signup and view all the answers

    What characterizes membrane rafts?

    <p>Rich in cholesterol and sphingolipids.</p> Signup and view all the answers

    How do proteins interact with membrane rafts?

    <p>Some proteins preferentially interact with rafts while others do not.</p> Signup and view all the answers

    Which factor does NOT affect the rate of nonmediated transport across a membrane?

    <p>Ionic charge of the substance.</p> Signup and view all the answers

    What are the advantages of facilitated transport compared to nonmediated transport?

    <p>It can increase the rate of diffusion significantly.</p> Signup and view all the answers

    What is the primary role of valinomycin in the context of membranes?

    <p>It acts as an ion carrier for potassium ions.</p> Signup and view all the answers

    What is one way substances can circumvent the equalization of concentrations across a membrane?

    <p>Binding of the substance to specific macromolecules.</p> Signup and view all the answers

    What structural characteristic distinguishes membrane rafts from the surrounding membrane?

    <p>Thicker bilayer core.</p> Signup and view all the answers

    What happens when there is a hydrophobic mismatch between a protein and the bilayer?

    <p>The protein or bilayer will undergo conformational changes.</p> Signup and view all the answers

    Which mechanism is NOT a major method of facilitated transport?

    <p>Cation exchange polymers.</p> Signup and view all the answers

    What is the main characteristic of the gel–liquid crystalline phase transition in a synthetic lipid bilayer?

    <p>The transition leads to a more dynamic movement of hydrocarbon chains.</p> Signup and view all the answers

    How does the presence of cholesterol affect the transition temperature (Tm) of a lipid bilayer?

    <p>It broadens the transition but does not change Tm.</p> Signup and view all the answers

    What happens to the lipid composition of the two leaflets of a membrane?

    <p>They usually differ in lipid composition.</p> Signup and view all the answers

    What is indicated by the sharp spike observed in calorimetry when measuring the transition temperature (Tm)?

    <p>The melting of the membrane.</p> Signup and view all the answers

    Which characteristic of lipids generally tends to increase the transition temperature (Tm)?

    <p>Lipids with longer, saturated tails.</p> Signup and view all the answers

    How does a membrane exhibit fluidity based on experimental demonstration involving fluorescent tags?

    <p>The proteins mix gradually over the fused membrane surface.</p> Signup and view all the answers

    What is a significant effect of increasing cholesterol in the membrane at high C:PL ratios?

    <p>It leads to phase separation forming two distinct lamellar phases.</p> Signup and view all the answers

    What role do glycoproteins serve in the membrane structure?

    <p>They help identify and stabilize cell surfaces.</p> Signup and view all the answers

    What is the typical thickness of a cellular membrane?

    <p>6 nm</p> Signup and view all the answers

    Which type of phospholipid is NOT mentioned in the content?

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

    What is the primary function of bacteriorhodopsin?

    <p>Functions as a light driven proton pump</p> Signup and view all the answers

    What percentage of the total mass of glycophorin A is made up of oligosaccharides?

    <p>60%</p> Signup and view all the answers

    What characteristic describes the transmembrane helix of glycophorin A?

    <p>Highly hydrophobic</p> Signup and view all the answers

    In a translocon, how are transmembrane portions processed during protein insertion?

    <p>They remain embedded in the bilayer</p> Signup and view all the answers

    Which structure facilitates the insertion of integral membrane proteins into the membrane bilayer?

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

    What happens when a hydrophobic segment of a nascent peptide enters a translocon?

    <p>It partitions into the lipid bilayer</p> Signup and view all the answers

    According to the 'inside positive' rule, where do the termini of Lep's transmembrane helices orient?

    <p>Towards the periplasm</p> Signup and view all the answers

    What structural feature is absent in the closed conformation of the SecY complex?

    <p>Hydrophobic core access</p> Signup and view all the answers

    What is a characteristic of the cytosolic C-terminal domain of glycophorin A?

    <p>Highly hydrophilic</p> Signup and view all the answers

    Study Notes

    Biochemistry I (CHM219)

    • Course instructor: Dr. Esra Aydemir

    Lipids, Membranes, and Cellular Transport

    • Lipids are generally insoluble in water

    • Lipids can form structures like micelles, vesicles, and bilayers to become water-soluble

    • Amphipathic lipid molecules have both hydrophilic and hydrophobic regions

    • A simplified representation of an amphipathic lipid molecule shows a polar head group (hydrophilic) and a hydrocarbon tail (hydrophobic)

    The Molecular Structure and Behavior of Lipids

    • Fatty acids are the simplest lipids
    • Their structure consists of a hydrophilic carboxylate group at one end, and a hydrocarbon chain at the other (typically 12-24 carbons)
    • Stearic acid (C18) is an example of a saturated fatty acid (all carbons saturated with hydrogen)
    • Oleic acid (C18) is an example of an unsaturated fatty acid (contains double bonds)
    • Many naturally occurring fatty acids are unsaturated, meaning they have one or more double bonds in their hydrocarbon chains

    Structures of the Ionized Forms of Some Representative Fatty Acids

    • The figures show the ionized forms of stearic and oleic acids
    • Stearate and oleate ions, have a polar head group and a long hydrocarbon tail

    Membrane Lipids

    • Membrane lipids are amphipathic
    • They tend to form monolayers, bilayers, micelles, or vesicles in contact with water
    • Most naturally occurring fatty acids contain an even number of carbon atoms
    • If double bonds (unsaturation) are present, they are usually cis

    Some Biologically Important Fatty Acids

    • Tables of saturated and unsaturated fatty acids display their systematic names, abbreviations, structures and melting points

    Fats or Triacylglycerols

    • Fats, or triacylglycerols, are triesters of fatty acids and glycerol
    • They are major long-term energy storage molecules in many organisms

    Fat Storage in Animals

    • Fat storage in animals serves three distinct functions: Energy production, Heat production, and Insulation

    Lipid Composition of Some Biological Membranes

    • A table shows the percentage of different lipids (e.g., saturated, unsaturated) in various natural fats

    Adipocytes

    • Adipocytes, are animal fat storage cells
    • The designations MFC and VSFC correspond to "mature fat cell" and "very small fat cell" respectively

    Saponification

    • When fats are hydrolyzed with strong bases (e.g., NaOH or KOH, wood ashes historically), soap is produced.
    • This process is called saponification
    • Fatty acids are released as sodium or potassium salts

    Synthetic Detergents

    • Synthetic detergents do not have the same defect as soaps
    • Sodium dodecyl sulfate (SDS) is an example of this class, with salts of dodecyl sulfate being more soluble with divalent cations
    • SDS is widely used in forming micelles around proteins for gel electrophoresis
    • Triton X-100 is another example of synthetic nonionic detergent

    Waxes

    • Waxes are formed by the esterification of fatty acids and long-chain alcohols
    • They are completely water insoluble and often serve as water repellents.

    Classes of Membrane-Forming Lipids

    • Four major classes of membrane-forming lipids exist:
      • Glycerophospholipids
      • Sphingolipids
      • Glycosphingolipids
      • Glycoglycerolipids

    Phospholipids and Membrane Structure

    • Fatty acids are wedge-shaped and tend to form spherical micelles
    • Phospholipids are more cylindrical and pack together to form a bilayer structure

    Stereochemistry of Glycerophospholipids

    • Glycerol is a prochiral molecule
    • Phosphorylation of one CH₂OH group or the other gives the R- or S- enantiomer of glycerol phosphate
    • The same molecule can be called L-glycerol-3-phosphate or D-glycerol-1-phosphate

    Glycerophospholipid Structure

    • R₃ is a hydrophilic group

    Common Glycerophospholipid Hydrophilic Groups

    Lipid Composition of Some Biological Membranes

    • A table shows the percentage of different lipids (e.g., phosphatidic acid, phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylinositol, phosphatidylserine, sphingomyelin, glycolipids, cholesterol) in various membranes (human erythrocyte plasma membrane, rat liver plasma membrane, pg platelet plasma membrane)

    Sphingolipids

    • Sphingolipids are another class of membrane lipids based on the amino alcohol sphingosine, rather than glycerol
    • They have a long-chain hydrophobic tail, requiring the addition of only one fatty acid
    • Ceramides are a class of sphingolipids linked via an amide bond to the NH2 group

    Sphingomyelin

    • An important example of sphingolipid is sphingomyelin, in which a phosphocholine group is attached to the C-3 hydroxyl group

    Glycosphingolipids

    • Glycosphingolipids result from sphingosine attaching to a sugar
    • Cerebrosides and Gangliosides are examples of glycosphingolipids, important parts of brain cell membranes

    Glycoglycerolipids

    • Another less common class of lipids, particularly in plant and bacterial membranes, are glycoglycerolipids, especially the monogalactosyl diglycerides.

    Cholesterol

    • Cholesterol is a component of many animal membranes, influencing membrane fluidity with its bulky structure

    Fluid Mosaic Model

    • Membranes are a fluid mixture of lipids and proteins.

    • Unsaturated phospholipids increase fluidity.

    • Peripheral membrane proteins are associated with one side of the bilayer

    • Integral membrane proteins are more deeply embedded in the bilayer

    Structure of a Typical Cell Membrane

    • Proteins are embedded in and on the phospholipid bilayer.
    • Some are glycoproteins that carry oligosaccharide chains.
    • The membrane is about 6 nm thick
    • Most membranes are more densely packed with proteins than shown

    Experimental Demonstration of Membrane Fluidity (FRAP)

    • When cells with surface membrane protein marked by fluorescent tags are induced to fuse, the proteins gradually mix over the fused surface

    Protein, Lipid, and Carbohydrate Content of Some Membranes

    • A table shows the percentage by weight of proteins, lipids, and carbohydrates in different membrane types (myelin, human erythrocyte plasma membrane, bovine retinal rod, mitochondria outer membrane, amoeba plasma membrane, sarcoplasmic reticulum, chloroplast lamellae, gram-positive bacteria and mitochondria inner membrane.)

    The Transition Temperature in a Synthetic Lipid Bilayer

    • The transition temperature (Tm) depends on lipid composition
    • Lipids with longer, saturated tails tend to increase Tm
    • Those with more cis double bonds and/or shorter tails reduce Tm

    Effects of Cholesterol on Synthetic Membrane Structure

    • Cholesterol's influence on membrane structure in different C:PL ratios is described (membrane width and phase separation as membrane “saturates” with cholesterol)

    Composition of Membrane Leaflets

    • The two leaflets (outer and inner membrane layers) of a membrane typically differ in lipid composition

    Integral Membrane Proteins

    Bacteriorhodopsin

    • Acts as a light-driven proton pump in certain bacteria
    • Contains seven helices that holds a retinal pigment

    Glycophorin A

    • The first integral membrane protein to be sequenced
    • Its external (N-terminal) domain carries 15 O-linked and one N-linked oligosaccharides; ~60% of the protein mass
    • The single transmembrane helix is highly hydrophobic
    • The cytosolic C-terminal domain is quite hydrophilic

    Co-translational Insertion and Folding of Transmembrane Helices

    Crystal Structures of Secy Complex Conformational Changes

    The “Translocon” Function

    • Acts as a protein channel for the insertion of integral membrane proteins into the membrane bilayer

    The “Inside Positive” Rule

    • Wild-type leader peptidase (Lep) orientation of its two transmembrane helices
    • Addition of four Lys to the N-terminus yield a mutant Lep that has the opposite membrane topology

    Membrane Rafts

    • Rich in cholesterol, sphingolipids, and glycoproteins linked to glycosylphosphatidylinositol
    • The bilayer is thicker in rafts than other regions of the cell membrane
    • Proteins coalesce, forming nanometer-sized dynamic domains, stabilized by actin fibers
    • Rafts can associate to form larger structures.

    Transport Across Membranes

    • Adaptation to hydrophobic mismatch in a membrane
    • If the thickness of the bilayer core and the hydrophobic surface area do not match, either the protein will change conformation or the bilayer's composition will change

    Permeability Coefficients

    • A table shows permeability coefficients for ions (K+, Na+, Cl−) and molecules (glucose, water) across both synthetic and biological membranes

    Facilitated Transport

    • Three major mechanisms for facilitated transport exist.
      • Protein pores
      • Carrier molecules
      • Permeases

    Valinomycin

    • Acts as an ion carrier
    • Cyclic peptide with hydrophobic outside
    • Oxygens complexes a K+ ion surrounded by CH3 groups

    Channel-Forming Hemolysin from S. aureus

    • A heptamer with a β-barrel stem that penetrates the cell membrane

    Gramicidin A

    • Acts as an ion pore formed by two molecules that adopt a helical conformation
    • Hydrophobic side chains contact lipid
    • N-terminus in cytosol, C-terminus extracellular
    • Inside of helix forms hydrophilic pore

    Aquaporins

    • Water-specific channels that rapidly transport water
    • Function as tetramers of identical monomers, each with six membrane-spanning helices and two shorter helices (NPA motif)

    Ion Selectivity

    Selective Binding of Na⁺ and K⁺ in Ion Channels

    • Two binding sites make up the selectivity filter of LeuT
    • Four bind in the KcsA channel

    Voltage-Gated Channels and Action Potentials

    • The channel portion of the voltage-gated channel is homologous to the KcsA channel
    • The arginine- and lysine-rich S4 helices are highlighted in blue
    • Depth of these helices changes as a function of membrane potential

    Action Potential

    • The membrane first becomes permeable to Na+, followed by a decrease in Na+ permeability and an outward flow of K+
    • Changes in membrane potential accompany these permeability changes, increasing to a positive value before decreasing to undershoot the resting potential

    Transmission of the Action Potential

    Neurotoxins

    Techniques for Studying Membranes

    • Freeze fracture technique
    • Membrane isolation

    Preparation of Vesicles and Bilayers

    • Isolating and preparing membrane fragments

    Reconstitution of the Ca²⁺ Pump

    • Combining phospholipids with detergent-solubilized Ca²⁺ pump protein to form vesicles
    • Ca²⁺ accumulates in vesicles

    Preparation and Resealing of Erythrocyte Ghosts

    • Isolating and preparing erythrocyte ghosts to study membrane function

    Fluorescence Recovery After Photobleaching (FRAP)

    • An experimental method to study membrane dynamics

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    Biochemistry I CHM219 Lecture Notes Fall 2021 PDF

    Description

    Test your knowledge on key biological concepts including the structures of membranes, the function of aquaporins, and the characteristics of fatty acids. This quiz covers various aspects from cellular structures to energy storage in animals. Perfect for students studying cell biology or biochemistry.

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