Biomembranes: Structure and Transport Mechanisms
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Questions and Answers

According to the fluid-mosaic model, the basic membrane structure is primarily made up of _____

phospholipids

Which of the following structures are considered types of lipids in biomembranes? (Select all that apply)

  • Steroids (correct)
  • Nucleotides
  • Phosphoglycerides (correct)
  • Sphingolipids (correct)

    • Cholesterol decreases membrane fluidity at high temperatures.

    True

    What is the primary characteristic of the lipid bilayer?

    <p>It has a hydrophobic core that prevents the diffusion of water-soluble solutes across the membrane.</p> Signup and view all the answers

    What role do glycoproteins play in cellular membranes?

    <p>Cell-cell recognition</p> Signup and view all the answers

    What happens to a cell placed in a hypertonic solution?

    <p>Water flows out of the cell causing it to shrink.</p> Signup and view all the answers

    Match the following types of proteins with their functions:

    <p>Transport Proteins = Provide a hydrophilic channel across the membrane. Receptor Proteins = Bind specific molecules to relay messages. Glycoproteins = Identify cells to the immune system. Enzymatic Proteins = Carry out metabolic reactions.</p> Signup and view all the answers

    What is osmosis?

    <p>The movement of water across a semipermeable membrane from a solution of low solute concentration to one of high solute concentration.</p> Signup and view all the answers

    What do ABC transport proteins enable cells to do?

    <p>Import nutrients against substantial concentration gradients</p> Signup and view all the answers

    Which of the following is NOT an ABC transporter mentioned?

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

    Which proteins span the phospholipid bilayer and are called transmembrane proteins?

    <p>Integral Membrane Proteins</p> Signup and view all the answers

    Cystic fibrosis is caused by dysfunctional Cl- channels that allow Na+ to move out of the cells.

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

    What is the role of P-glycoprotein in cancer cells?

    <p>Pumps drugs out of the cytoplasm</p> Signup and view all the answers

    Cystic fibrosis results in thick mucus in the airways due to a lack of water passing into the ______.

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

    What condition is characterized by ABC transporters present in brain capillary endothelial cells?

    <p>Blood-brain barrier permeability limitations</p> Signup and view all the answers

    Match the following defects with their corresponding genetic diseases:

    <p>Cystic fibrosis = Faulty Cl- transport Tangier disease = Abnormal lipid metabolism Retinal degeneration = Vision loss Anemia = Reduced red blood cells Liver failure = Impaired detoxification</p> Signup and view all the answers

    What is passive transport?

    <p>The movement of molecules across a membrane without the use of energy, often down a concentration gradient.</p> Signup and view all the answers

    Which of the following molecules can readily move by passive diffusion?

    <p>O2</p> Signup and view all the answers

    Facilitated diffusion requires energy.

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

    What do ATP-powered pumps do?

    <p>Transport molecules against a concentration gradient using energy from ATP.</p> Signup and view all the answers

    What is the electrochemical gradient?

    <p>The combined effect of the concentration gradient and electric potential across a membrane.</p> Signup and view all the answers

    Passive transport includes __________ diffusion and facilitated diffusion.

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

    What are uniporters?

    <p>Transporters that move a single type of molecule down its concentration gradient via facilitated diffusion.</p> Signup and view all the answers

    Which class of ATP-powered pumps primarily transports small molecules?

    <p>ABC superfamily</p> Signup and view all the answers

    What does the Na+/K+ ATPase do?

    <p>It maintains the low intracellular Na+ and high intracellular K+ concentrations by transporting three Na+ out and two K+ into the cell.</p> Signup and view all the answers

    V-class pumps transport only H+ ions.

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

    What is the function of proton pump inhibitors?

    <p>They irreversibly block the H+/K+ ATPase system to reduce gastric acid secretion.</p> Signup and view all the answers

    The Ca2+ ATPase is also known as __________.

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

    What is the basic structure of steroids?

    <p>A four-ring hydrocarbon</p> Signup and view all the answers

    How does the presence of unsaturated hydrocarbon chains in phospholipids affect membrane fluidity?

    <p>Increases fluidity at lower temperatures</p> Signup and view all the answers

    What effect does cholesterol have on membrane fluidity at high temperatures?

    <p>Makes the membrane less fluid</p> Signup and view all the answers

    What primarily determines the degree of bilayer fluidity?

    <p>The combination of lipid composition and temperature</p> Signup and view all the answers

    What structural feature does sphingomyelin have?

    <p>An amino alcohol with a long hydrocarbon chain</p> Signup and view all the answers

    What is a characteristic of all membranes related to lipid distribution?

    <p>Asymmetry in lipid composition across the bilayer</p> Signup and view all the answers

    What impact does decreasing temperature have on membrane solidification?

    <p>The solidification temperature varies based on lipid composition</p> Signup and view all the answers

    What functions does the hydroxyl group in cholesterol serve?

    <p>Interacts with water, making cholesterol hydrophilic</p> Signup and view all the answers

    What role do phospholipids play in the structure of cellular membranes?

    <p>They form a semi-permeable bilayer that creates a barrier.</p> Signup and view all the answers

    Which components contribute to the identification process of cells?

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

    What characteristic of the lipid bilayer contributes to its stability?

    <p>Hydrophobic and van der Waals interactions between lipid chains</p> Signup and view all the answers

    What is the significance of cholesterol in the cellular membrane?

    <p>It stiffens the membrane, providing structural integrity.</p> Signup and view all the answers

    What defines a membrane as selectively permeable?

    <p>It selectively permits certain molecules while blocking others.</p> Signup and view all the answers

    Which of these describes the arrangement of phospholipids in the bilayer?

    <p>Hydrophilic heads facing outwards and hydrophobic tails facing inwards</p> Signup and view all the answers

    What is the main function of membrane carbohydrates?

    <p>To assist in cell-cell recognition.</p> Signup and view all the answers

    Which structural feature is common to all three classes of lipids in biomembranes?

    <p>Presence of a hydrophilic head group</p> Signup and view all the answers

    What is the main function of protein domains on the extracellular surface of the plasma membrane?

    <p>Bind to signaling proteins and adhesion molecules</p> Signup and view all the answers

    Which type of membrane proteins are classified as lipid-anchored proteins?

    <p>Proteins that are covalently bound to lipid molecules</p> Signup and view all the answers

    What characterizes integral membrane proteins?

    <p>They span the phospholipid bilayer and have hydrophilic domains</p> Signup and view all the answers

    What role do protein domains on the cytosolic face of the plasma membrane primarily play?

    <p>Triggering intracellular signaling pathways</p> Signup and view all the answers

    How do integral membrane proteins differ from peripheral membrane proteins?

    <p>Integral proteins are embedded and span the membrane, while peripheral proteins do not</p> Signup and view all the answers

    What best describes the interactions of integral membrane proteins with the lipid bilayer?

    <p>They have hydrophobic and hydrophilic regions that interact with both the lipid bilayer and the cytosol/extracellular environment</p> Signup and view all the answers

    Why do most transmembrane proteins undergo glycosylation?

    <p>To facilitate interactions with other molecules</p> Signup and view all the answers

    What function do lipid-anchored proteins serve in the cell membrane?

    <p>They facilitate signal transduction and cell interactions</p> Signup and view all the answers

    What is the primary function of F-class proton pumps?

    <p>Synthesis of ATP in mitochondria and chloroplasts</p> Signup and view all the answers

    Which of the following is a characteristic of the ABC superfamily of proteins?

    <p>Includes proteins that are specific for a single substrate</p> Signup and view all the answers

    Which ion is pumped by the SERCA Ca2+ ATPase in skeletal muscle cells?

    <p>Calcium ions (Ca2+)</p> Signup and view all the answers

    What occurs after ATP binds to the Ca2+ ATPase?

    <p>The bound ATP is hydrolyzed to ADP</p> Signup and view all the answers

    What is the initial conformation of the Ca2+ ATPase before calcium ions bind?

    <p>E1 conformation</p> Signup and view all the answers

    What structural components are shared among ABC transport proteins?

    <p>Two cytosolic ATP-binding domains and two transmembrane domains</p> Signup and view all the answers

    What is the role of the sarcoplasmic reticulum in skeletal muscle cells related to Ca2+ ions?

    <p>To store and concentrate Ca2+ ions</p> Signup and view all the answers

    Which substrate can ABC transport proteins be specific for?

    <p>Ions, sugars, and amino acids</p> Signup and view all the answers

    What is the primary role of the Na+/Ca2+ antiporter in cardiac muscle cells?

    <p>To maintain a low calcium concentration in the cytosol</p> Signup and view all the answers

    How does Digitalis affect the Na+/K+ pump?

    <p>It inhibits the phosphorylation of ATPase</p> Signup and view all the answers

    What is the result of inhibiting the Na+/K+ ATPase in cardiac muscle cells?

    <p>Higher intracellular calcium levels</p> Signup and view all the answers

    What role do proton pump inhibitors have in the body?

    <p>They treat acid reflux</p> Signup and view all the answers

    Which of the following correctly describes the action of Ouabain?

    <p>It prevents sodium from binding to Na+/K+ ATPase</p> Signup and view all the answers

    In cardiac muscle cells, how does a decrease in extracellular Na+ affect calcium levels?

    <p>Decreases intracellular calcium concentration</p> Signup and view all the answers

    What is the function of V-class ATPases?

    <p>Pump protons across lysosomal and vacuolar membranes</p> Signup and view all the answers

    What happens to calcium levels in cardiac muscle when the Na+/K+ ATPase is inhibited?

    <p>Calcium levels increase due to reduced export</p> Signup and view all the answers

    What are sphingolipids derived from and what is their primary structural characteristic?

    <p>Sphingolipids are derived from sphingosine and are characterized by containing a long-chain fatty acid attached to the sphingosine amino group.</p> Signup and view all the answers

    How does the presence of cholesterol influence membrane fluidity at body temperature?

    <p>Cholesterol reduces membrane fluidity at high temperatures by restraining the movement of phospholipids.</p> Signup and view all the answers

    In what way do phospholipids with unsaturated hydrocarbon chains affect membrane fluidity?

    <p>Phospholipids with unsaturated hydrocarbon chains maintain membrane fluidity at lower temperatures.</p> Signup and view all the answers

    What is meant by membrane asymmetry in the context of lipid distribution?

    <p>Membrane asymmetry refers to the unequal distribution of different types of lipids across the bilayer's exoplasmic and cytosolic leaflets.</p> Signup and view all the answers

    What determines the temperature at which a membrane solidifies?

    <p>The temperature at which a membrane solidifies depends on the types of lipids it is composed of.</p> Signup and view all the answers

    Describe the structural feature that characterizes sphingomyelin.

    <p>Sphingomyelin is characterized by having a phosphocholine group attached to the terminal hydroxyl group of sphingosine.</p> Signup and view all the answers

    What factors affect the fluidity of a lipid bilayer?

    <p>Lipid composition, the structure of phospholipid hydrophobic tails, and temperature all affect bilayer fluidity.</p> Signup and view all the answers

    What is the basic structure of steroids and why are they considered amphipathic?

    <p>Steroids have a four-ring hydrocarbon structure and are considered amphipathic because their hydroxyl group can interact with water.</p> Signup and view all the answers

    What are the primary structural components of biomembranes?

    <p>Biomembranes are primarily composed of phospholipids and proteins.</p> Signup and view all the answers

    How does the fluid-mosaic model describe the arrangement of proteins and lipids in biomembranes?

    <p>The fluid-mosaic model describes biomembranes as a flexible structure with various proteins floating in or on the fluid lipid bilayer, resembling a mosaic.</p> Signup and view all the answers

    What is the role of cholesterol in cellular membranes?

    <p>Cholesterol maintains membrane fluidity by preventing the fatty acid chains from packing too closely together.</p> Signup and view all the answers

    Describe how facilitated diffusion differs from passive diffusion.

    <p>Facilitated diffusion requires specific transport proteins to help move substances across the membrane, while passive diffusion does not.</p> Signup and view all the answers

    What defines the selective permeability of a biomembrane?

    <p>Selective permeability is defined by the ability of a membrane to allow certain substances to pass while blocking others based on size, charge, and solubility.</p> Signup and view all the answers

    Which types of molecules have easier access to cross the lipid bilayer, and why?

    <p>Small, nonpolar molecules can easily cross the lipid bilayer due to their ability to dissolve in the lipid environment.</p> Signup and view all the answers

    Explain the significance of the amphipathic nature of phospholipids in membrane structure.

    <p>The amphipathic nature of phospholipids allows them to form a bilayer, with hydrophobic tails facing inward and hydrophilic heads facing outward.</p> Signup and view all the answers

    What is the primary difference in membrane structure between prokaryotic and eukaryotic cells?

    <p>Prokaryotic cells have a plasma membrane without internal compartments, while eukaryotic cells have membranes that partition the cell into organelles.</p> Signup and view all the answers

    What role do cytoskeletal elements play in maintaining cell shape?

    <p>Cytoskeletal elements, like microfilaments, non-covalently bind to membrane proteins, helping to maintain cell shape and stabilize the location of these proteins.</p> Signup and view all the answers

    How does a hypotonic solution affect a cell's volume?

    <p>In a hypotonic solution, water flows into the cell, causing it to swell.</p> Signup and view all the answers

    What occurs when a cell is placed in an isotonic solution?

    <p>In an isotonic solution, there is no net movement of water into or out of the cell, maintaining its volume.</p> Signup and view all the answers

    What happens to a cell placed in a hypertonic solution?

    <p>In a hypertonic solution, water flows out of the cell, causing it to shrink.</p> Signup and view all the answers

    What is the significance of membrane proteins in relation to the extracellular matrix?

    <p>Membrane proteins that bind to extracellular matrix molecules help coordinate extracellular and intracellular changes.</p> Signup and view all the answers

    How do plasma membranes act as a permeability barrier?

    <p>Plasma membranes are selectively permeable, allowing certain molecules to pass while restricting others, maintaining proper ionic composition.</p> Signup and view all the answers

    Describe the movement of water in osmosis.

    <p>Water moves across a semipermeable membrane from a region of low solute concentration to a region of high solute concentration until concentrations are equal.</p> Signup and view all the answers

    What unique properties do membranes surrounding organelles possess?

    <p>Membranes surrounding organelles contain a specific set of proteins essential for their proper functioning.</p> Signup and view all the answers

    How does digitalis affect intracellular calcium levels in cardiac muscle cells?

    <p>Digitalis increases intracellular calcium levels by inhibiting the Na+/K+ pump, which reduces the Na+ gradient and slows the Na-Ca exchanger.</p> Signup and view all the answers

    What role does the Na+/Ca2+ antiporter play in cardiac muscle cells?

    <p>The Na+/Ca2+ antiporter exports Ca2+ from the cell, helping to maintain low cytosolic calcium levels, which reduces muscle contraction strength.</p> Signup and view all the answers

    Describe how ouabain affects sodium and calcium levels in cardiac muscle cells.

    <p>Ouabain inhibits the Na+/K+ ATPase, leading to increased intracellular sodium and decreased activity of the sodium-calcium exchanger, which results in higher intracellular calcium.</p> Signup and view all the answers

    What is the function of V-class H+ ATPases?

    <p>V-class H+ ATPases transport protons across membranes, helping to acidify the lumen of lysosomes, endosomes, and vacuoles.</p> Signup and view all the answers

    Explain how proton pump inhibitors work in the context of gastric acid secretion.

    <p>Proton pump inhibitors irreversibly block the H+/K+ ATPase system, decreasing gastric acid secretion and promoting ulcer healing.</p> Signup and view all the answers

    What effect does inhibition of the Na+/K+ pump have on cardiac muscle contraction?

    <p>Inhibiting the Na+/K+ pump can increase intracellular calcium, enhancing the strength of cardiac muscle contractions.</p> Signup and view all the answers

    Why are inhibitors of the Na+/K+ ATPase important in treating heart failure?

    <p>They increase the force of heart muscle contractions by disrupting sodium and calcium ion transport.</p> Signup and view all the answers

    How does increased intracellular sodium affect the Na-Ca exchanger in cardiac cells?

    <p>Higher intracellular sodium levels reduce the activity of the Na-Ca exchanger, leading to decreased Ca2+ extrusion from the cell.</p> Signup and view all the answers

    What property of the phospholipid bilayer contributes to its semi-permeable nature?

    <p>The hydrophobic core of the bilayer acts as an impermeable barrier that prevents the diffusion of water-soluble substances.</p> Signup and view all the answers

    How do cholesterol molecules affect the structure of the plasma membrane?

    <p>Cholesterol stiffens the membrane by connecting phospholipids, which helps to maintain membrane integrity.</p> Signup and view all the answers

    What are glycoproteins and what role do they play in cellular membranes?

    <p>Glycoproteins are proteins with attached sugar chains that assist in cell recognition and communication.</p> Signup and view all the answers

    In what way do amphipathic molecules contribute to membrane structure?

    <p>Amphipathic molecules, such as phospholipids, have both hydrophilic heads and hydrophobic tails, forming a bilayer that stabilizes the membrane.</p> Signup and view all the answers

    What role do membrane carbohydrates play in cellular recognition?

    <p>Membrane carbohydrates, attached to glycoproteins and glycolipids, facilitate cell-to-cell recognition and interaction.</p> Signup and view all the answers

    Describe how the fluid mosaic model relates to the arrangement of proteins in the membrane.

    <p>The fluid mosaic model depicts the membrane as a dynamic structure where proteins are embedded and can move within the lipid bilayer.</p> Signup and view all the answers

    What is the significance of the hydrophobic core in the lipid bilayer?

    <p>The hydrophobic core provides a barrier that prevents hydrophilic molecules from freely crossing the membrane.</p> Signup and view all the answers

    How do phosphoglycerides contribute to the structure of the membrane?

    <p>Phosphoglycerides form the main structural framework of the membrane, creating a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.</p> Signup and view all the answers

    Study Notes

    Biomembranes: Structure and Transport Mechanisms

    • Biomembranes are essential for life, compartmentalizing cells and regulating the movement of molecules
    • Prokaryotes have a plasma membrane, but no internal membrane-bound compartments
    • Eukaryotes have internal organelles, each with one or more biomembranes, enabling specialized functions
    • Biomembranes are composed primarily of phospholipids, forming a bilayer structure
    • Phospholipids are amphipathic, with hydrophilic heads and hydrophobic tails.
    • The basic membrane structure is made up of phospholipids like phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol.
    • Biomembranes are fluid mosaics of lipids and proteins, maintaining a stable structure while allowing for movement
    • Membrane fluidity is influenced by lipid composition, temperature, and cholesterol content.
    • Membrane lipids have unequal distribution in the bilayer leaflets, contributing to membrane asymmetry.
    • Biomembranes contain three classes of lipids: phosphoglycerides, sphingolipids, and steroids (cholesterol), all amphipathic
    • Phosphoglycerides are derived from glycerol 3-phosphate with two fatty acyl chains and a polar head group.
    • Sphingolipids are derived from sphingosine, an amino alcohol with a long hydrocarbon chain
    • Cholesterol, a steroid, is amphipathic due to its hydroxyl group interacting with water.
    • Membrane proteins are categorized into three groups based on their interaction with the membrane: integral, lipid-anchored, and peripheral.
    • Integral membrane proteins (transmembrane proteins) span the bilayer and have hydrophilic domains interacting with aqueous solutions. They are often glycosylated with sugar groups.
    • Lipid-anchored membrane proteins are covalently linked to lipids, anchoring them to the membrane without penetrating the bilayer.
    • Peripheral membrane proteins associate with the membrane through interactions with integral proteins or lipid head groups. They reside on either the cytosolic or exoplasmic face.
    • Cytoskeletal filaments, associated with peripheral proteins, contribute to membrane support and communication with the cell interior.
    • Transmembrane proteins can have various secondary structures, including single-pass and multi-pass transmembrane proteins.
    • Covalently attached hydrocarbon chains anchor some proteins to the membrane, contributing to membrane asymmetry.
    • All transmembrane proteins and glycolipids are asymmetrically oriented in the bilayer, reflecting their specific functions and cellular roles.
    • Membrane proteins serve diverse functions: transport, signal transduction, cell-cell recognition, enzymatic activity, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.
    • Transport proteins facilitate the movement of molecules across membranes.
    • Channel proteins provide hydrophilic channels for selective transport of ions and small hydrophilic molecules down their concentration gradients.
    • Aquaporins are channel proteins that facilitate water movement across membranes.
    • Carrier proteins bind specific molecules, change shape, and shuttle them across the membrane.
    • Signal transduction proteins receive extracellular signals, like hormones or neurotransmitters, and initiate intracellular responses.
    • Cell-cell recognition proteins act as ID tags, allowing cells to be recognized by the immune system or other cells.
    • Enzymatic activity of membrane proteins enables metabolic reactions within or at the membrane surface.
    • Intercellular joining proteins connect adjacent cells through junctions like gap junctions or tight junctions.
    • Attachment proteins link the cytoskeleton or extracellular matrix to the membrane, influencing cell shape and communication.
    • Plasma membranes have universal functions: acting as a permeability barrier, maintaining the ionic composition and pH of the cytosol, and providing a unique set of proteins for each organelle.
    • Osmosis is the movement of water across a semipermeable membrane from a region of low solute (high water) concentration to a region of high solute (low water) concentration.
    • Osmosis is driven by the difference in water potential, or the tendency of water to move across the membrane.
    • A hypotonic solution has a lower solute concentration than the cell's interior, causing water to flow into the cell and swell.
    • An isotonic solution has a solute concentration equal to that of the cell's interior, resulting in no net movement of water.
    • A hypertonic solution has a higher solute concentration than the cell's interior, causing water to flow out of the cell and shrink.
    • Osmotic pressure is the hydrostatic pressure required to stop water movement across a membrane separating solutions of different compositions.
    • Transport across membranes can be passive or active.
    • Passive transport occurs without energy expenditure, driven by concentration gradients.
    • Simple diffusion involves the movement of molecules down their concentration gradient across a membrane.
    • Facilitated diffusion involves transport proteins that assist the movement of molecules across the membrane.
    • Active transport requires energy input to move molecules against their concentration gradient, typically using ATP hydrolysis.
    • ATP-powered pumps, or simply pumps, are ATPases that use ATP energy to move ions or small molecules across a membrane.
    • Non-gated ion channels are open most of the time, allowing the passage of specific ions or small molecules down their concentration or electrochemical gradients.
    • An inside-negative electric potential exists across the plasma membrane of all cells, contributing to the movement of ions across the membrane. ### 

    Membrane Potential

    • Generated primarily by movement of potassium ions (K+) from the cytosol through resting potassium channels to the external medium.
    • Non-gated K+ channels are usually open.

    Electrochemical Gradient

    • Influences the movement of charged substances across a membrane.
    • Determined by the combined forces of the concentration gradient and membrane potential.
    • The electrochemical gradient determines the energetically favorable direction of transport.

    Gated Channel Proteins

    • Open only in response to specific chemical or electrical signals.

    Transporters

    • Uniporters: Transport a single type of molecule down its concentration gradient via facilitated diffusion.
      • Glucose and amino acids cross the plasma membrane using uniporters.
    • Co-Transporters: Couple the movement of one type of ion or molecule against its concentration gradient with the movement of one or more different ions down its concentration gradient.
      • Symporters: Movement of the transported molecule and co-transported ion in the same direction.
      • Antiporters: Movement of the transported molecule and co-transported ion in opposite directions.

    Glucose Transport (GLUT1)

    • GLUT1 catalyzes the net import of glucose from the extracellular medium into the cell.
    • Glucose concentration is higher extracellularly than intracellularly.

    Na+/H+ Antiporter

    • Exports H+ from cells coupled to the energetically favorable import of Na+.
    • The movement of Na+ into the cell can be coupled to the movement of other molecules against their concentration gradient.

    Transport across the Cell Membrane

    • Passive Transport:
      • Simple Diffusion: Non-polar, hydrophobic molecules diffuse across the membrane down their concentration gradient.
        • Examples: Lipids.
      • Facilitated Transport: Polar, hydrophilic molecules diffuse across the membrane through a protein channel down their concentration gradient.
    • Active Transport:
      • Diffusion against the concentration gradient.
      • Uses a protein pump.
      • Requires ATP.

    Active Transport Mechanisms

    • ATPases: Proteins that hydrolyze ATP to ADP and Pi to power transport processes.
    • Transmembrane Proteins: ATP-powered pumps have one or more binding sites for ATP located on the cytosolic face of the membrane.

    Classes of ATP-Powered Pumps

    • Four Classes:
      • P-Class: Transport ions only.
        • Examples: Na+/K+ ATPase, Ca2+ ATPases.
      • V-Class: Transport protons (H+) only.
        • Function in acidifying lysosomes, endosomes, and plant vacuoles.
      • F-Class: Transport protons (H+) only.
        • Function in powering ATP synthesis in mitochondria and chloroplasts.
        • Commonly called ATP Synthases.
      • ABC Superfamily: Primarily transport small molecules.
        • Includes various transport proteins in bacteria and humans.

    P-Class Pumps

    • Na+/K+ ATPase: Maintains low cytosolic Na+ and high cytosolic K+ concentrations in animal cells.
    • Ca2+ ATPases: Pump Ca2+ ions out of the cytosol, into the external medium, or into the endoplasmic reticulum (ER).
    • H+/K+ ATPase: Found in acid-secreting cells of the stomach.

    V-Class & F-Class Pumps

    • V-Class: Function in acidifying organelles by pumping protons from the cytosolic to the exoplasmic face of the membrane.
    • F-Class: Found in bacterial plasma membranes, mitochondria, and chloroplasts.
    • ATP Synthases: Function in ATP synthesis by moving protons down their concentration gradient.

    ABC Superfamily

    • Structure: Two transmembrane (T) domains and two cytosolic ATP-binding (A) domains.
    • Function: Transport a variety of substances, including ions, sugars, amino acids, phospholipids, peptides, and proteins.
    • Examples: CFTR (Cystic Fibrosis Transmembrane Conductance Regulator), TAP (Transporter associated with Antigen Processing), MDR proteins (Multi-Drug Resistance).

    Ca2+ ATPase Transporter (SERCA)

    • Found in the sarco(endo)plasmic reticulum (SR) of skeletal muscle cells.
    • Pumps Ca2+ from the cytosol into the lumen of the SR.
    • Involved in muscle relaxation.

    Mechanism of Action of Ca2+ ATPase

    • Step 1: Ca2+ binds to high-affinity binding sites on the cytosolic side.
    • Step 2: ATP binds to a site on the cytosolic surface.
    • Step 3: ATP is hydrolyzed to ADP, and the phosphate is transferred to a specific aspartate residue.
    • Step 4: Protein undergoes a conformational change, generating E2, and Ca2+ binding sites become accessible to the SR lumen.
    • Step 5: Ca2+ ions dissociate and enter the SR lumen.
    • Step 6: Aspartyl phosphate bond is hydrolyzed, and the protein returns to the E1 conformation, ready to transport more Ca2+.

    P-Class Ion Pumps

    • All P-class pumps are phosphorylated on a conserved aspartate residue during transport.
    • Examples: Na+/K+ ATPase, H+/K+ ATPase, Ca2+ ATPase.

    Na+/K+ ATPase Transporter

    • Maintains low cytosolic Na+ and high cytosolic K+ concentrations.
    • Tetrameric protein composed of two alpha (α) and two beta (β) subunits.
    • The α subunit is similar to the muscle SR Ca2+ ATPase.

    Function of Na+/K+ ATPase

    • Moves three Na+ ions out of the cell and two K+ ions into the cell per ATP molecule hydrolyzed.
    • Creates and maintains ion gradients across the cell membrane.

    Mechanism of Action of Na+/K+ ATPase

    • E1 Conformation: High-affinity Na+ sites and low-affinity K+ sites are accessible on the cytosolic face.
    • E2 Conformation: Na+ sites become accessible on the exoplasmic face, K+ binding sites become accessible on the exoplasmic face.

    Inhibiting Na+/K+ ATPase

    • Ouabain: Somali toxin that inhibits ATPase activity by binding to the exoplasmic domain.
    • Digitalis: Mixture of cardiotonic steroids that inhibit dephosphorylation of the phosphorylated form of ATPase.

    Na+/Glucose Symporter

    • Transports both Na+ and glucose into the cell.
    • Driven by the electrochemical gradient of Na+ established by the Na+/K+ ATPase.

    H+ ATPases (V-Class)

    • Function in acidifying lysosomes, endosomes, and plant vacuoles by pumping protons from the cytosol to the exoplasmic face of the membrane.

    ABC Transporters

    • Structure: Two transmembrane (T) domains and two cytosolic ATP-binding (A) domains.
    • Function: Transport various substances across the membrane.
    • Examples: CFTR (Cystic Fibrosis Transmembrane Conductance Regulator), TAP (Transporter associated with Antigen Processing), MDR proteins (Multi-Drug Resistance).

    CFTR (Cystic Fibrosis Transmembrane Conductance Regulator)

    • Defective Cl- channels in cystic fibrosis prevent Cl- and Na+ from exiting the cells lining the airways.
    • Mucus becomes thick and clogs the airways.

    MDR Proteins (Multi-Drug Resistance)

    • Pump drugs and toxins out of eukaryotic cells, increasing drug resistance.
    • P-glycoprotein (P-gp) is an example of an MDR protein.
    • Overexpression of MDR proteins in cancer cells contributes to multi-drug resistance, making cancer cells more difficult to treat.

    Blood-Brain Barrier

    • ABC transporters in brain capillary endothelial cells pump drugs back into the blood, limiting drug delivery to the brain.

    Defects in ABC Transporters

    • Can lead to various genetic diseases, including cystic fibrosis, Tangier disease, retinal degeneration, and anemia.

    Cellular Membranes

    • Cellular membranes are fluid mosaics of lipids and proteins
    • The fluid mosaic model describes the cell membrane as a semi-permeable double layer of phospholipids with embedded proteins
    • Cell membranes are responsible for maintaining a steady internal environment for cells
    • They facilitate communication between cells through signal transduction
    • They help cells to identify each other

    Components of the Plasma membrane

    • Phospholipids are amphipathic, composed of a hydrophilic head and a hydrophobic tail
    • The phospholipid bilayer acts as a barrier to water-soluble solutes due to its hydrophobic core
    • Cholesterol helps to stiffen the membrane by connecting phospholipids
    • Glycolipids act as signaling molecules
    • Glycoproteins, with attached sugar chains, are crucial for cell-cell recognition and immune system function

    Lipid Composition and Structural Organisation

    • Phospholipids spontaneously form sheet-like bilayers within cells
    • The hydrocarbon chains of phospholipids in each layer create a hydrophobic core

    Properties of the Lipid Bilayer

    • The hydrophobic core prevents the diffusion of water-soluble solutes
    • The bilayer structure is stable due to hydrophobic and van der Waals interactions

    Types of Lipids in Biomembranes

    • Biomembranes are composed of phosphoglycerides, sphingolipids, and steroids (cholesterol)
    • All three types of lipids are amphipathic molecules with a polar head and a hydrophobic tail

    Phosphoglycerides

    • Derivatives of glycerol 3-phosphate
    • Contain a hydrophobic tail composed of two fatty acyl chains esterified to glycerol phosphate and a polar head group attached to the phosphate group

    Sphingolipids

    • Derived from sphingosine, an amino alcohol with a long hydrocarbon chain
    • Contain a long-chain fatty acid attached to the sphingosine amino group
    • Sphingomyelin, the most abundant sphingolipid, has phosphocholine attached to the terminal hydroxyl group of sphingosine

    Steroids: Cholesterol

    • The basic structure of steroids is a four-ring hydrocarbon
    • Cholesterol is amphipathic, with a hydroxyl group that can interact with water

    Factors Affecting Membrane Fluidity

    • The fluidity of the membrane is influenced by lipid composition, structure of the phospholipid hydrophobic tails, and temperature
    • Membranes with unsaturated hydrocarbon chains remain fluid to a lower temperature
    • Cholesterol makes the membrane less fluid at higher temperatures, restraining phospholipid movement

    Asymmetry in Lipid Composition

    • Biomembranes exhibit asymmetry in lipid composition across the bilayer
    • While most phospholipids are present in both membrane leaflets, they are more abundant in one leaflet than the other

    Membrane Proteins

    • Membrane proteins are the mosaic part of the fluid mosaic model
    • They can be located within the phospholipid bilayer or at its surface
    • The amount of protein associated with biomembranes varies depending on cell type and subcellular location

    Protein Domains: Exoplasmic

    • Proteins on the extracellular surface of the plasma membrane bind to external signaling proteins, ions, small metabolites, and adhesion molecules

    Protein Domains: Cytosolic

    • Cytosolic domains of the plasma membrane have a variety of functions including anchoring cytoskeletal proteins and triggering intracellular signaling pathways

    Types of Membrane Proteins

    • Membrane proteins can be classified into three categories: integral, lipid-anchored, and peripheral
    • This classification is based on the nature of their interaction with the membrane

    Integral Membrane Proteins

    • Also known as transmembrane proteins
    • Span the phospholipid bilayer and have a hydrophilic exterior that interacts with aqueous solutions on both sides of the membrane

    Lipid-anchored Membrane Proteins

    • Bound covalently to one or more lipid molecules

    ATP-Powered Pumps: F-Class - Mitochondria

    • F-class proton pumps, known as ATP synthases, are highly important in ATP synthesis in mitochondria

    ATP-Powered Pumps: F-Class - Chloroplast

    • F-class proton pumps play a crucial role in ATP synthesis in chloroplasts

    ATP-Powered Pumps: ABC-Superfamily

    • The ATP-binding cassette (ABC) superfamily is a diverse group of transport proteins found in various organisms
    • They have a characteristic structure with two transmembrane domains and two ATP-binding domains
    • Each ABC protein is specific for a single substrate or group of related substrates

    Ca ATPase Transporter: SERCA

    • The ER Ca2+ pump is called SERCA: Sarco(Endo)plasmic Reticulum Ca2+ ATPase

    Role of Ca2+ ATPase in Muscle Contraction

    • In skeletal muscle cells, Ca2+ ions are stored in the sarcoplasmic reticulum (SR)
    • Release of Ca2+ from the SR lumen into the cytosol causes muscle contraction
    • The Ca2+ ATPase pumps Ca2+ from the cytosol into the SR lumen, inducing muscle relaxation

    Mechanism of Action of the Ca2+ ATPase

    • The Ca2+ ATPase undergoes a conformational change, binding to Ca2+ ions on the cytosolic side in the E1 conformation
    • ATP binds to a site on the cytosolic surface and is hydrolyzed to ADP, transferring a phosphate group to a specific aspartate residue in the protein (E1 ~ P)

    Glucose Transporters

    • Glucose transporters facilitate the movement of glucose across cell membranes
    • They are involved in transporting glucose from the blood into cells
    • Glucose is transported by facilitated diffusion, driven by the concentration gradient
    • GLUT4 is the primary glucose transporter in muscle and adipose tissue

    Sodium-Potassium Pump (Na+/K+ ATPase)

    • Essential for maintaining cell volume, nerve impulse transmission, and muscle contraction
    • Active transport system that pumps three sodium ions out of the cell for every two potassium ions brought into the cell
    • Requires ATP for energy
    • Maintains a higher concentration of potassium ions inside the cell and a higher concentration of sodium ions outside the cell

    Mechanism of Action of the Na+/K+ ATPase

    • The protein undergoes conformational changes, with a high affinity for potassium ions on the extracellular side and high affinity for sodium ions on the cytosolic side
    • ATP hydrolysis provides energy for the conformational changes and ion transport

    Inhibitors of the Na+/K+ Pump

    • Digitalis, a cardiotonic steroid, inhibits the dephosphorylation of the Na+/K+ ATPase, leading to increased intracellular sodium concentration
    • Ouabain, a Somali toxin, binds to the extracellular form of the Na+/K+ ATPase, preventing conformational changes and ion transport

    Na+-Linked Antiporter

    • In cardiac muscle cells, the Na+/Ca2+ antiporter plays a crucial role in maintaining low cytosol Ca2+
    • It pumps one calcium ion out of the cell for every three sodium ions brought into the cell
    • Inhibitors of the Na+/K+ pump are used in the treatment of congestive heart failure due to their effect on the Na+/Ca2+ antiporter

    H+ ATPases Transporter: V-Class

    • V-class proton pumps are found in the membranes of lysosomes, endosomes, and plant vacuoles
    • They transport only H+ ions and are responsible for acidifying the lumen of these organelles

    Inhibitors of H+ ATPases Pump

    • Proton pump inhibitors block the H+/K+ ATPase system in the stomach, promoting healing of ulcers.

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    Biomembranes: Structure and Transport Mechanisms PDF

    Description

    Explore the fascinating world of biomembranes, which are vital for life due to their role in cell compartmentalization and molecule transport. This quiz covers the differences between prokaryotic and eukaryotic membranes, the composition of phospholipids, and the fluid mosaic model. Test your knowledge on how lipid composition and temperature influence membrane fluidity and functionality.

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