Membranes and Cell Signalling - Lecture 1
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Questions and Answers

What are the two main types of lipid constituents in biomembranes?

Phospholipids and sphingolipids.

What is the main function of membranes?

  • Compartmentalization
  • Signal Recognition
  • Shape Maintenance
  • All of the above (correct)

    • Phospholipids have a _____ head group and _____ tails.

    polar, non-polar

    All lipids found in human plasma have similar structures.

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

    What does the fluid-mosaic model describe?

    <p>The structure and behavior of biomembranes.</p> Signup and view all the answers

    What is the role of receptors in membranes?

    <p>Signal Recognition and Transduction.</p> Signup and view all the answers

    Which technique confirmed the bilayer structure of membranes?

    <p>Freeze-etching</p> Signup and view all the answers

    What are spherocytes?

    <p>Smaller, rounder red blood cells with less surface area.</p> Signup and view all the answers

    What do we mean by integral and peripheral?

    <p>Integral refers to proteins that span the membrane, while peripheral proteins are associated with the membrane surface.</p> Signup and view all the answers

    Proteins in the membrane are completely immobilized.

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

    How do red blood cells (RBCs) maintain their shape?

    <p>Through a network of integral and peripheral membrane proteins anchored to the cytoplasmic face.</p> Signup and view all the answers

    What role does spectrin play in erythrocytes?

    <p>Spectrin provides structural support and helps maintain the flexibility and shape of erythrocytes.</p> Signup and view all the answers

    Which of the following is true about lipid rafts?

    <p>All of the above</p> Signup and view all the answers

    What is one characteristic of cholesterols in membranes?

    <p>They are amphipathic.</p> Signup and view all the answers

    What does FRAP stand for?

    <p>Fluorescence Recovery After Photobleaching</p> Signup and view all the answers

    What are the two types of proteins based on their membrane association?

    <p>Both A and B</p> Signup and view all the answers

    Cholesterol is essential to the process of raft formation.

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

    Match the following terms with their definitions:

    <p>Integral proteins = Proteins that span the membrane. Peripheral proteins = Proteins that are attached to the membrane surface. Lipid raft = Sphingolipid-rich microdomain. FRAP = Technique to measure protein diffusion in membranes.</p> Signup and view all the answers

    What are GPI-linked proteins?

    <p>Glycosylphosphatidylinositol-anchored proteins that are attached to the membrane.</p> Signup and view all the answers

    The process of membrane proteins moving laterally within the bilayer is referred to as ______.

    <p>lateral diffusion</p> Signup and view all the answers

    What is required for transverse diffusion of lipids?

    <p>ATP is often required for the flip-flop movement of lipids.</p> Signup and view all the answers

    What is the role of annexin V in apoptosis?

    <p>Used as a probe to measure apoptosis in vitro and in vivo.</p> Signup and view all the answers

    What are the two hypotheses for maintaining membrane asymmetry?

    <p>A specific mechanism maintains amino phospholipids at the inner leaflet.</p> Signup and view all the answers

    What does an activation of scramblase cause?

    <p>Loss of phospholipid asymmetry.</p> Signup and view all the answers

    Phosphatidylserine (PS) at the outer leaflet indicates a healthy cell.

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

    The process by which phospholipids flip from one leaflet to another in a membrane is called ______.

    <p>flip-flop.</p> Signup and view all the answers

    Which molecules are translocated rapidly to the inner leaflet in ATP-replete erythrocytes?

    <p>Phosphatidylserine (PS)</p> Signup and view all the answers

    What is the conclusion regarding the transport of PS and PE?

    <p>It must be facilitated or active transport.</p> Signup and view all the answers

    What is the role of phospholipase A2 in the outer leaflet?

    <p>Degrades phospholipids.</p> Signup and view all the answers

    How can the activity of ATP-dependent amino-phospholipid translocase be measured?

    <p>By measuring the amount of PS in the outer leaflet.</p> Signup and view all the answers

    What does FRAP stand for?

    <p>Fluorescence recovery after photobleaching.</p> Signup and view all the answers

    A student should use fluorescently-tagged analogues that are more polar than the target phospholipid for better incorporation.

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

    Why is it important to study membrane proteins?

    <p>They constitute more than 30% of cellular proteins and more than half of all FDA approved drugs bind to them.</p> Signup and view all the answers

    What are the two main types of membrane proteins?

    <p>Integral and peripheral proteins.</p> Signup and view all the answers

    Integral membrane proteins are easier to remove than peripheral proteins.

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

    Detergents disrupt the _____ bilayer to solubilize membrane proteins.

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

    What forces hold peripheral proteins to the membrane?

    <p>Noncovalent interactions, including ionic bonds, hydrophobic interactions, hydrogen bonds, and van der Waals forces.</p> Signup and view all the answers

    What is the role of Triton X-100 in the protocol described?

    <p>To solubilize membrane proteins</p> Signup and view all the answers

    What is the purpose of performing hydropathy plots?

    <p>To predict transmembrane regions of proteins.</p> Signup and view all the answers

    Why are membrane proteins challenging to study?

    <p>Their hydrophobic surface, flexibility, and instability complicate expression, solubilization, purification, and crystallization.</p> Signup and view all the answers

    What class of signaling allows for long-distance communication between cells?

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

    A good signal must be able to travel from site of manufacture to the _____ site.

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

    What does Kd approximate?

    <p>The concentration of a competitor which inhibits binding by 50%.</p> Signup and view all the answers

    What is the Kd for the hepatocyte insulin receptor for insulin?

    <p>1.4 x 10^-10 M</p> Signup and view all the answers

    What is the normal insulin concentration in blood?

    <p>5 x 10^-12 M</p> Signup and view all the answers

    What percentage of receptors will be bound to insulin at a normal concentration of 5 x 10^-12 M?

    <p>Approximately 3.5%</p> Signup and view all the answers

    If the normal circulating insulin concentration was 1.4 x 10^-10 M, what percentage of receptors are bound to insulin?

    <p>50%</p> Signup and view all the answers

    What happens if insulin concentration is increased 5-fold from 1.4 x 10^-10 M?

    <p>The percentage of bound receptors will increase beyond 50%.</p> Signup and view all the answers

    What is the formula for calculating Fractional Occupancy (F.O.)?

    <p>F.O. = [RL]/([R] + [RL])</p> Signup and view all the answers

    What neurotransmitter is released from pre-ganglionic fibers?

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

    What are the principal secretory products of chromaffin cells?

    <p>Catecholamines, adrenaline, and noradrenaline</p> Signup and view all the answers

    How long is the half-life of circulating adrenaline?

    <p>Less than 10 seconds</p> Signup and view all the answers

    Adrenaline has a longer lasting effect in the body compared to its half-life.

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

    What causes adrenaline to have different effects in different tissues?

    <p>Receptor specificity</p> Signup and view all the answers

    What are the three crucial criteria for a good receptor?

    <p>Specificity, binding affinity, and ability to transmit signals</p> Signup and view all the answers

    How does the Kd value relate to receptor-ligand binding?

    <p>Kd is the dissociation constant and measures the affinity of the receptor for its ligand.</p> Signup and view all the answers

    The binding affinity of adrenaline is high due to its low Kd.

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

    What experimental method is used to detect weak binding of a ligand to its receptor?

    <p>Competition assay</p> Signup and view all the answers

    Study Notes

    Introduction to Biomembranes

    • Biomembranes consist of different classes of lipids, influencing their physical properties.
    • Basic structure was determined through various experiments, including freeze-etching and electron microscopy.
    • Original fluid-mosaic model has evolved with new findings about membrane dynamics.

    Functions of Membranes

    • Compartmentation: Enables separation of enzymes, metabolites, and pathways within cells.
    • Specific Transport: Regulates passage of compounds, either passively or actively (e.g., pumps).
    • Maintenance of Electrical Potential: Membranes can create and sustain electrical gradients.
    • Energy Trapping: Some membranes convert light energy into electrical or chemical energy.
    • Signal Recognition/Transduction: Membranes have receptors that interact with hormones and neurotransmitters, generating second messengers.
    • Shape Maintenance: The cytoskeleton helps maintain cell shape and structure through membrane interaction.
    • Endocytosis/Exocytosis: Membrane trafficking is a constant process facilitating material exchange.

    Phospholipids as Main Lipids

    • Phospholipids include hydrophilic head groups and hydrophobic fatty acyl tails, resulting in amphipathic properties.
    • Classes of lipids in biomembranes:
      • Phosphoglycerides: Examples include PC, PE, PS, and PI.
      • Sphingolipids: Derivatives like sphingomyelin with complex structures.
      • Sterols: Cholesterol, an important component affecting membrane fluidity.

    Lipid Composition and Diversity

    • Over 500 lipid species have been identified in human plasma, showcasing diversity.
    • Fatty acids can vary in chain length and saturation (saturated vs. unsaturated).
    • Micelles are rare; natural phosphoglycerides are too bulky for micelle formation.

    Membrane Structure Discovery

    • Robertson's hypothesis in the 1950s linked the appearance of membranes to osmium tetroxide binding.
    • Techniques like freeze-etching revealed bilayer structures and protein distributions within membranes.

    Membrane Proteins and Mobility

    • Protein Distributions: Inner and outer leaflets display different protein densities; proteins play crucial roles in signaling and transport.
    • Phospholipids and proteins are mobile within the bilayer, allowing lateral diffusion (within the same plane); transverse diffusion (flipping) is less common and ATP-dependent.

    Experimental Evidence of Lateral Diffusion

    • Human-mouse heterokaryon experiments showed mobility of proteins was unaffected by ATP depletion or temperature changes, demonstrating lateral diffusion.

    Red Blood Cell Shape and Structure

    • Spherocytes are abnormally shaped red blood cells resulting from defects in cytoskeletal proteins (e.g., spectrin, ankyrin).
    • RBC shape is maintained by a network of integral and peripheral membrane proteins associated with the cytoplasmic face.

    Types of Membrane Proteins

    • Integral Proteins: Span across both layers of the membrane, involved in transport and signaling functions.
    • Peripheral Proteins: Associated with one side of the membrane; often involved in structural support and signaling.### Membrane Protein Mobility in Mice
    • Comparison of lateral mobility of membrane proteins in normal mice and spherocytic mice (lacking spectrin).
    • Sodium dodecyl sulfate polyacrylamide electrophoresis (SDS-PAGE) employed on erythrocyte “ghosts” to analyze protein mobility.
    • Spherocytic red blood cells (RBCs) confirmed via RB2 labeling.
    • Fluorescent labeling techniques used: DTAF (Dichlorotriazinyl aminofluorescein) and Concanavalin A (Con A).
    • Fluorescence recovery after photobleaching (FRAP) utilized to evaluate diffusion rates of labeled proteins in RBCs.

    Lateral vs Transverse Mobility

    • Lateral diffusion in erythrocytes is rapid compared to transverse diffusion.
    • Transverse diffusion for lipids requires significant time (1-2 hours), while lateral movement is much faster.
    • Proteins embedded in membranes typically do not exhibit transverse "flipping."

    Micro-Domains and Bilayer Asymmetry

    • Membrane micro-domains are noted, highlighting non-random distributions of proteins.
    • Spectrin "corrals" influence erythrocyte membrane organization.
    • Lipid asymmetry established between inner and outer leaflets of the bilayer.
    • GPI (Glycophosphatidylinositol)-anchored proteins exhibit unique trafficking and stability characteristics.

    GPI-Linked Proteins

    • GPI-linked proteins demonstrate poor solubility in non-ionic detergents.
    • Breakdown of protein-lipid interactions occurs under harsher conditions.
    • High fractions of sphingolipids found in insoluble fractions of membranes.

    Lipid Raft Model

    • Lipid rafts create "liquid-ordered" regions influenced by sphingolipid and cholesterol composition.
    • Rafts facilitate the retention and trafficking of specific proteins.
    • Stability of rafts enhanced through hydrogen bonding with hydroxyl groups in sphingolipids.

    Discovery Techniques

    • Atomic force microscopy reveals distinct ordering of lipid rafts.
    • Rafts contribute to varied membrane composition between cell types.

    Membrane Dynamics

    • Fluid-mosaic model updated to incorporate stability provided by cytoskeletal interactions.
    • Non-random distribution of membrane proteins leads to functional implications.

    Transport Mechanisms

    • Active transport mechanisms, including flippases, floppases, and scramblases, maintain lipid asymmetry.
    • Apoptosis triggers changes in PL distribution, with phosphatidylserine (PS) exposure indicating cell death.

    Important Concepts in Apoptosis

    • Apoptosis leads to morphological changes and altered lipid asymmetry, including the externalization of PS.
    • Rapid translocation of *PS and *PE to the inner leaflet under ATP-rich conditions exemplifies facilitated transport mechanisms essential for maintaining membrane integrity.### Cell Differentiation and Apoptosis
    • Differentiation of fingers and toes in a human embryo occurs due to apoptosis of cells between digits.
    • Phosphatidylserine (PS) is an important indicator of apoptosis.

    Membrane Proteins Overview

    • Integral and peripheral proteins are distinct types of membrane proteins, with integral proteins spanning the lipid bilayer.
    • Protein asymmetry is absolute, meaning different proteins exist on inner and outer leaflets of membranes.

    Mechanisms of Membrane Asymmetry

    • Maintaining lipid asymmetry requires ATP.
    • Two hypotheses exist regarding phospholipid translocation: slow "flip-flop" rate versus specific mechanisms maintaining inner leaflet phospholipids.
    • Assessing asymmetry can be done through experiments measuring "flip-flop" rates using fluorescent or radiolabeled phospholipids.

    Experimental Techniques

    • Methods include using phospholipid vesicles and incubating erythrocytes with fluorescent phospholipid analogues to measure translocation rates.
    • FRAP (Fluorescence Recovery After Photobleaching) can assess lateral diffusion versus transverse flipping.

    Phosphatidylserine and Experimental Design

    • For ATP-dependent amino-phospholipid translocase activity, choose fluorescently-tagged analogues that closely mimic unmodified phosphatidylserine.
    • Understanding the polarity of analogues can influence incorporation into membranes and affect experimental outcomes.

    Importance of Membrane Proteins

    • Membrane proteins constitute over 30% of cellular proteins and are common targets for FDA-approved drugs, despite challenges in structural determination.
    • RBCs are frequently studied for insights into membrane protein functionality due to their accessible membrane structure.

    Anatomy of Membrane Proteins

    • Integral membrane proteins feature transmembrane domains, disulfide bridges, and oligosaccharides attached in the Golgi and ER.
    • The solubilization process for membrane proteins involves detergents that disrupt the lipid bilayer, forming protein-lipid-detergent complexes.

    Reconstituting Membrane Proteins

    • Reconstituting peripheral proteins requires different methods than those used for integral proteins, emphasizing context-specific techniques to isolate target proteins.

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    Description

    This quiz covers the introduction to biomembranes, focusing on their composition, structure, and physical properties. It highlights the development of the fluid-mosaic model and includes key experiments that shaped our understanding of biomembranes. Ideal for students exploring cell signalling and membrane science.

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