🔬Biophysics Unit 6 – Membrane Structure and Dynamics

Key Concepts and Terminology

• Amphipathic molecules have both hydrophilic and hydrophobic regions, allowing them to form lipid bilayers
• Phospholipids are the primary component of cell membranes, consisting of a hydrophilic head and hydrophobic tails
• Integral proteins are embedded within the lipid bilayer, while peripheral proteins are attached to the membrane surface
• Fluid mosaic model describes the dynamic nature of cell membranes, with lipids and proteins moving laterally within the bilayer
• Membrane fluidity refers to the ease with which lipids and proteins can move within the membrane, influenced by factors such as temperature and lipid composition
• Membrane asymmetry describes the different lipid and protein compositions of the inner and outer leaflets of the bilayer
• Membrane curvature plays a role in various cellular processes, such as vesicle formation and membrane fusion

Membrane Composition and Architecture

• Cell membranes are composed primarily of phospholipids, which self-assemble into a bilayer structure due to their amphipathic nature
  • Phospholipids have a glycerol backbone, two fatty acid tails, and a phosphate-containing head group
  • Common phospholipids include phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS)
• Cholesterol is another important component of animal cell membranes, modulating membrane fluidity and permeability
• Membrane proteins make up a significant portion of the membrane and perform various functions, such as transport, signaling, and enzymatic activity
• Glycolipids and glycoproteins are lipids and proteins with attached carbohydrate moieties, respectively, and play roles in cell recognition and adhesion
• The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the dynamic nature of the lipid bilayer with embedded proteins
• Lipid rafts are specialized membrane microdomains enriched in cholesterol and sphingolipids, believed to facilitate protein interactions and signaling

Lipid Bilayer Properties

• The lipid bilayer is a thin, flexible barrier that separates the cell interior from the external environment
• Hydrophobic effect drives the formation of the bilayer, with the nonpolar tails of phospholipids facing each other to minimize contact with water
• Lipid bilayers are selectively permeable, allowing only certain molecules to pass through while restricting others
  • Small, nonpolar molecules (oxygen, carbon dioxide) can diffuse directly through the bilayer
  • Polar and charged molecules (glucose, ions) require specialized transport proteins to cross the membrane
• The thickness of a lipid bilayer is typically around 5 nm, determined by the length of the fatty acid tails
• Membrane fluidity is influenced by factors such as temperature, lipid composition, and the presence of cholesterol
  • Higher temperatures increase membrane fluidity, while lower temperatures decrease it
  • Unsaturated fatty acids (with double bonds) increase fluidity compared to saturated fatty acids
• Membrane asymmetry, with different lipid compositions in the inner and outer leaflets, is maintained by enzymes called flippases and floppases

Membrane Proteins and Their Functions

• Membrane proteins are classified as integral (embedded within the bilayer) or peripheral (attached to the membrane surface)
• Integral proteins can be further categorized as transmembrane (spanning the entire bilayer) or anchored (attached to one leaflet)
• Transmembrane proteins often have hydrophobic amino acid residues that interact with the nonpolar core of the bilayer
• Ion channels are transmembrane proteins that allow the selective passage of specific ions (sodium, potassium, calcium) across the membrane
  • Gated ion channels open or close in response to stimuli such as voltage changes, ligand binding, or mechanical stress
• Transporters are membrane proteins that facilitate the movement of molecules across the membrane, often coupled to energy sources like ATP hydrolysis or ion gradients
  • Examples include the sodium-potassium pump (Na+/K+-ATPase) and the glucose transporter (GLUT)
• Receptor proteins bind specific ligands (hormones, neurotransmitters) and initiate intracellular signaling cascades
  • G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs) are two major classes of receptor proteins
• Enzymes embedded in the membrane catalyze various reactions, such as the synthesis or degradation of lipids and proteins

Membrane Transport Mechanisms

• Passive transport occurs down a concentration gradient without the input of energy, while active transport requires energy to move molecules against a gradient
• Simple diffusion is the passive movement of molecules directly through the lipid bilayer, driven by a concentration gradient
  • Fick's laws of diffusion describe the rate of diffusion based on the concentration gradient and the membrane permeability
• Facilitated diffusion involves the use of transport proteins to move molecules down a concentration gradient without energy input
  • Examples include glucose transport by GLUT proteins and ion movement through channels
• Active transport mechanisms use energy (usually ATP) to move molecules against their concentration gradients
  • Primary active transport directly couples ATP hydrolysis to the movement of molecules (Na+/K+-ATPase)
  • Secondary active transport uses the energy stored in an electrochemical gradient to drive the transport of another molecule (sodium-glucose cotransporter)
• Endocytosis and exocytosis are processes that involve the formation of vesicles to transport large molecules or particles across the membrane
  • Endocytosis includes phagocytosis (cell eating) and pinocytosis (cell drinking)
  • Exocytosis is the fusion of intracellular vesicles with the plasma membrane to release their contents

Membrane Fluidity and Dynamics

• Membrane fluidity refers to the ease with which lipids and proteins can move laterally within the bilayer
• The fluid mosaic model emphasizes the dynamic nature of cell membranes, with lipids and proteins constantly in motion
• Factors affecting membrane fluidity include temperature, lipid composition, and the presence of cholesterol
  • Higher temperatures increase fluidity by increasing the kinetic energy of lipid molecules
  • Unsaturated fatty acids (with double bonds) disrupt tight packing and increase fluidity compared to saturated fatty acids
  • Cholesterol modulates fluidity by interacting with fatty acid tails, increasing fluidity at low temperatures and decreasing it at high temperatures
• Membrane phase transitions occur at specific temperatures, where the bilayer shifts between a gel (solid-like) and a liquid-crystalline (fluid) state
• Lipid rafts are dynamic, ordered microdomains enriched in cholesterol and sphingolipids that can compartmentalize cellular processes
• Membrane curvature and deformation play important roles in processes such as vesicle formation, cell division, and membrane fusion

Experimental Techniques in Membrane Studies

• Electron microscopy (EM) provides high-resolution images of membrane structure and organization
  • Freeze-fracture EM reveals the distribution of integral membrane proteins
  • Cryo-EM allows the visualization of membranes in their native, hydrated state
• Fluorescence microscopy enables the study of membrane dynamics and protein localization using fluorescent probes
  • Fluorescence recovery after photobleaching (FRAP) measures the lateral mobility of membrane components
  • Single-particle tracking (SPT) follows the movement of individual proteins or lipids in the membrane
• Atomic force microscopy (AFM) provides high-resolution topographical images of membrane surfaces and can measure mechanical properties
• Lipid monolayers and bilayers can be studied using Langmuir-Blodgett troughs and supported lipid bilayers, respectively
• Membrane protein structure determination techniques include X-ray crystallography, nuclear magnetic resonance (NMR), and cryo-EM
• Liposomes and giant unilamellar vesicles (GUVs) are artificial membrane systems used to study lipid-lipid and lipid-protein interactions
• Patch-clamp technique allows the measurement of ion channel activity and membrane potential in living cells

Applications and Real-World Relevance

• Understanding membrane structure and function is crucial for developing targeted drug delivery systems and therapies
  • Many drugs target membrane proteins, such as G protein-coupled receptors (GPCRs) and ion channels
  • Liposomes can be used as drug delivery vehicles to encapsulate and transport therapeutic agents to specific tissues or cells
• Membrane research is essential for understanding the pathogenesis of various diseases, such as cystic fibrosis and Alzheimer's disease
  • Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) protein, which disrupts ion transport across epithelial cell membranes
  • Alzheimer's disease is associated with the accumulation of amyloid-beta peptides, which can disrupt membrane integrity and function
• Membrane technology has applications in water purification and desalination, using reverse osmosis and nanofiltration membranes
• Biomimetic membranes, inspired by natural cell membranes, are being developed for applications in sensing, separation, and energy production
  • Artificial ion channels and transporters can be incorporated into biomimetic membranes for selective transport and sensing
  • Membrane-based fuel cells and solar cells aim to harness energy conversion processes similar to those in biological membranes
• Understanding membrane dynamics and organization is crucial for developing artificial cells and synthetic biology applications
  • Bottom-up approaches to creating artificial cells often involve the assembly of lipid bilayers and the incorporation of functional membrane proteins
  • Engineering membrane proteins and pathways can enable the production of novel compounds or the sensing of specific molecules in synthetic biological systems