Cellular Membranes Overview
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Questions and Answers

What is the primary function of scramblases in cellular membranes?

  • Acting as enzymes to catalyze reactions
  • Facilitating the movement of lipids down their concentration gradients (correct)
  • Serving as receptors for signal transduction
  • Transporting water soluble substances across the membrane

    • Which type of membrane protein is embedded in and anchored to the cell membrane?

  • Peripheral membrane proteins
  • Integral membrane proteins (correct)
  • Channel proteins
  • Transport proteins

    • How do large polar molecules and ions typically permeate lipid bilayers?

  • Through transport proteins (correct)
  • By simple diffusion through the lipid bilayer
  • By active transport mechanisms exclusively
  • By dissolving in lipid bilayers

    • What distinguishes channel proteins from carrier proteins?

    <p>Carrier proteins bind with substances and facilitate their movement</p> Signup and view all the answers

    What describes the overall structure of the cell membrane?

    <p>A 5nm-thick lipid bilayer with associated proteins</p> Signup and view all the answers

    What role does the hydrophobic core of lipid bilayers play in membrane permeability?

    <p>It serves as a barrier to the diffusion of large and/or charged molecules</p> Signup and view all the answers

    Which factor increases the fluidity of the cell membrane?

    <p>Higher number of double bonds in fatty acids</p> Signup and view all the answers

    Which statement correctly describes the distribution of ions inside and outside the cell?

    <p>Ion distribution is influenced by the activity of transport proteins</p> Signup and view all the answers

    What is a function of flippases in the cell membrane?

    <p>To move phospholipids PS and PE from the outer leaflet to the inner leaflet</p> Signup and view all the answers

    What two processes do carrier proteins utilize to transport substances?

    <p>Diffusion and active transport</p> Signup and view all the answers

    Which of the following is true about lipid-soluble substances during membrane transport?

    <p>They dissolve in the lipid bilayer and diffuse through</p> Signup and view all the answers

    Which statement is true about the lipid bilayer of membranes?

    <p>It contains an asymmetrical arrangement of different phospholipids</p> Signup and view all the answers

    What effect does higher temperature have on membrane fluidity?

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

    Which of the following lipids is predominantly found in the outer leaflet of the membrane?

    <p>Sphingomyelin (SM)</p> Signup and view all the answers

    Which statement about cholesterol in membranes is accurate?

    <p>Cholesterol helps maintain membrane fluidity at low temperatures</p> Signup and view all the answers

    What role do floppases play in cellular membranes?

    <p>They utilize ATP to transport PC and SM from inner to outer leaflet</p> Signup and view all the answers

    What is the primary function of cellular membranes?

    <p>To define cell boundaries and act as a barrier</p> Signup and view all the answers

    How does cholesterol affect membrane fluidity?

    <p>It stabilizes the membrane by preventing crystallization</p> Signup and view all the answers

    Which statement correctly describes the asymmetrical distribution of phospholipids in membranes?

    <p>Certain phospholipids are found exclusively on the outer layer</p> Signup and view all the answers

    What mechanisms are employed for the transport of substances across membranes?

    <p>Primary and secondary active transport, along with passive mechanisms</p> Signup and view all the answers

    Which best describes primary active transport?

    <p>Transport using energy from ATP to move substances against their concentration gradient</p> Signup and view all the answers

    What vital role do membrane junctions play?

    <p>They connect adjacent cells, facilitating communication.</p> Signup and view all the answers

    How do transporters maintain the balance between intracellular and extracellular environments?

    <p>By using energy to create concentration gradients</p> Signup and view all the answers

    What effect does temperature have on membrane fluidity?

    <p>Temperature changes can alter the fluidity and permeability of membranes</p> Signup and view all the answers

    What characterizes aquaporins in cell membranes?

    <p>They facilitate rapid water movement.</p> Signup and view all the answers

    How do gated ion channels control ion permeability?

    <p>They open based on electrical and chemical signals.</p> Signup and view all the answers

    What limits the rate of facilitated diffusion?

    <p>Time needed for conformational changes in carrier proteins.</p> Signup and view all the answers

    What is the primary function of the sodium-potassium pump?

    <p>To create a concentration gradient of Na+ and K+ ions.</p> Signup and view all the answers

    Which of the following describes secondary active transport?

    <p>Transport driven by the concentration gradient created by primary active transport.</p> Signup and view all the answers

    What type of channels are highly selective for potassium ions?

    <p>Potassium channels.</p> Signup and view all the answers

    What is a key characteristic of non-gated ion channels?

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

    During the action of the sodium-glucose co-transporter, what happens as Na+ ions move down their concentration gradient?

    <p>They transport glucose against its concentration gradient.</p> Signup and view all the answers

    Which of the following is not a type of gated ion channel?

    <p>Carrier-mediated channels.</p> Signup and view all the answers

    What defines primary active transport?

    <p>It directly utilizes ATP for energy.</p> Signup and view all the answers

    What is the primary role of the Na+-Ca2+ counter-transport system?

    <p>To aid in the relaxation of muscle cells by removing Ca2+</p> Signup and view all the answers

    How does ouabain affect the Na+/K+ ATPase pump?

    <p>It partially inhibits the pump's activity</p> Signup and view all the answers

    Which type of transport does not require metabolic energy?

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

    What characterizes secondary active counter-transport?

    <p>Utilizes ion gradients created by primary active transport</p> Signup and view all the answers

    Which statement best describes facilitated diffusion?

    <p>It is a passive process involving carrier proteins</p> Signup and view all the answers

    What is a key difference between primary active transport and secondary active transport?

    <p>Primary active transport uses direct energy while secondary uses indirect energy</p> Signup and view all the answers

    Which of the following transport mechanisms depends on a sodium gradient?

    <p>Secondary active transport</p> Signup and view all the answers

    What is the significance of sodium's role in cellular transport mechanisms?

    <p>Sodium gradients are crucial for both cotransport and counter-transport</p> Signup and view all the answers

    Study Notes

    Cellular Membranes

    • Defines the cell boundaries
    • Serves as a barrier between cytoplasm and external environment
    • Regulates the movement of substances into and out of the cell
    • Contributes to intra- and inter-cellular communication
    • Forms membrane junctions linking adjacent cells together

    Structure & composition

    • Membrane is a 5nm-thick lipid bilayer with associated proteins
    • Proteins can be integrated into the bilayer or attached to the inner or outer surface of the membrane
    • It follows the fluid-mosaic model

    Membrane Fluidity

    • Molecules within the membrane can move and change places
    • It depends on lipid composition, temperature, and cholesterol
    • Factors that increase fluidity include: short tails of phospholipids, more unsaturated bonds, higher temperature, and low cholesterol levels

    Membrane Asymmetry

    • The membrane is asymmetric
    • The outer leaflet is rich in sphingomyelin and phosphatidylcholine
    • The inner leaflet is rich in phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol

    Role of enzymes in lipid asymmetry

    • Flippases: Utilize ATP to move PS and PE from the outer leaflet to the inner leaflet
    • Floppases: Utilize ATP to move PC and SM from the inner leaflet to the outer leaflet
    • Scramblases: Non-specific ATP-independent enzymes that facilitate the movement of lipids down their concentration gradients

    Membrane Proteins

    • Integral membrane proteins: Embedded in, and anchored to the cell membrane
    • Peripheral membrane proteins: Adhere to integral proteins on either side of the membrane
    • Serve as receptors, adhesion molecules, transporters, and enzymes

    Permeation of Lipid Bilayers

    • Gasses and small molecules permeate lipid bilayers
    • Hydrophobic core of lipid bilayers acts as a barrier to the diffusion of large and/or charged molecules
    • Membrane permeation of large polar molecules and ions is mediated by transport proteins

    Principles of Membrane Transport

    • Ion concentrations differ inside and outside the cell
    • Distribution of ions inside and outside the cell is controlled by:
      • activity of transport proteins
      • permeability characteristics of the lipid bilayer

    Transport Pathways

    • Channel proteins: have open spaces and allow movement of molecules based on charge or size
    • Carrier proteins: bind with the substance and carry it to the other side
    • Function via two processes:
      • Diffusion
      • Active transport

    Simple Diffusion

    • Lipid-soluble substances dissolve in the lipid bilayer and diffuse through, like oxygen
    • Lipid-insoluble molecules pass through channels, like water
    • Transport is selectively permeable
    • Flow is down the concentration gradient

    Ion channels

    • Integral, membrane-spanning proteins
    • Can be gated or non-gated
    • Selectivity is based on:
      • Size of the channel
      • Charge of the lining
      • Negatively lined channels allow cations, not anions
      • Positively lined channels allow anions, not cations

    Gated Ion Channels

    • Controls ion permeability
    • When the gate is open, ions flow through by passive diffusion
    • Voltage gated respond to changes in electrical potentials
    • Chemical gated respond to signaling molecules, like cAMP or IP3
    • Ligand-gated respond to hormones or neurotransmitters, like ACh

    Facilitated Diffusion

    • “Carrier-mediated diffusion”
    • Occurs down the concentration gradient
    • Requires membrane carrier
    • Molecule enters the pore within the protein, binds to the receptor
    • Protein undergoes a conformational change
    • Pore opens to the opposite end, molecule is released
    • Examples include: glucose (GLUT) and amino acids

    Limiting Rate of Facilitated Diffusion

    • Rate of transport is limited by the time of the conformational changes
    • Rate of facilitated diffusion reaches a maximum, Vmax

    Active Transport

    • Maintains ion concentration
    • Goes "uphill" against the concentration gradient
    • Two types:
      • Primary Active Transport: Energy derived from ATP
      • Secondary Active Transport: Energy is derived from energy created originally by primary active transport
    • Depends on carrier proteins

    Sodium-Potassium Pump (Primary)

    • 3 Na+ ions are exported out of the cell
    • 2 K+ ions are imported into the cell
    • 1 ATP molecule is consumed
    • Binding of ions stimulates phosphorylation of the pump
    • Works against the concentration gradient
    • Maintains low Na+ and high K+ inside the cell
    • Critical for cell function

    Secondary Active Transport

    • When ions are transported by primary active transport, a large concentration gradient forms
    • This gradient is a storehouse of energy
    • Diffusion energy of the ion can pull other substances along with it
    • Requires carrier proteins

    Sodium-Glucose Co-Transporter

    • As Na+ moves down its concentration gradient, it carries along glucose
    • Sodium-dependent glucose transporters (SGLT) are found in the epithelial cells lining the small intestine and the proximal tubule of the nephron

    Sodium Counter-Transporter (Antiport)

    • Counter-transport: transport in the opposite direction of the primary ion
    • Na+-Ca2+ counter-transport occurs through almost all cell membranes
    • Na+-H+ counter-transport occurs in several tissues, like the kidneys
    • Na+ into the tubular cell
    • H+ into the tubular lumen
    • Can transport a large number of H+ at once
    • Key to maintaining body fluids

    Transporters Comparison

    Type of Transport Active or Passive Carrier-Mediated Uses Metabolic Energy Dependent on Na+ Gradient
    Simple diffusion Passive No No No
    Facilitated diffusion Passive Yes No No
    Primary Active transport Active Yes Yes, direct No
    Secondary Active Transport (Cotransport) Active Yes Yes, indirect Yes
    Secondary Active Transport (Counter-transport) Active Yes Yes, indirect Yes

    Review: Na+/K+ ATPase

    • Pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell, using ATP

    Review: Na+/K+ ATPase

    • If an ATPase-inhibitor (ouabain, glycosides) is used, the pump will be inhibited and the ion gradients will be disrupted

    Clinical Relevance

    • Ouabain and Digitalis are given to patients with heart disease to make the heart pump more strongly
    • These drugs work by partially inhibiting the Na+-K+ pump in the heart
    • This inhibition leads to an increase in intracellular Na+
    • The increase in Na triggers the Na+-Ca2+ antiporter to import Ca2+, increasing intracellular Ca2+
    • The increase in Ca2+ leads to a stronger heart contraction

    Review Questions (1st order)

    • Membrane fluidity depends on: lipid composition, temperature, and cholesterol
    • Cholesterol maintains fluidity of the membrane
    • Flippases move phospholipids from the outer to the inner leaflet, Floppases move from the inner to the outer leaflet, and Scramblases move lipids in both directions
    • Facilitated diffusion uses carrier molecules
    • Ion channels can be gated by voltage, chemicals, or ligands
    • The time required for conformational changes limits the rate of diffusion
    • Primary active transport uses energy derived from ATP, while secondary active transport uses the energy from a pre-established gradient

    2nd Order statements/questions

    • Cholesterol plays a regulatory function in antibiotic resistance
    • Consider how this is possible by considering the function of cholesterol in membrane fluidity and its interaction with antibiotics

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    Membranes and Membrane Transport Lecture Notes PDF

    Description

    Explore the key concepts and structures of cellular membranes. This quiz covers membrane composition, fluidity, and asymmetry, providing insights into how membranes serve as barriers and facilitate communication between cells. Test your understanding of the fluid-mosaic model and the factors influencing membrane dynamics.

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