cell membrane

• Membranes define the boundaries of a cell and its internal compartments, playing various roles in the life of a cell
• Functions of membranes:

  • Define boundaries of a cell and organelles, acting as permeability barriers
  • Serve as sites for biological functions such as electron transport
  • Possess transport proteins that regulate the movement of substances into and out of the cell and organelles
  • Contain protein molecules that act as receptors to detect external signals
  • Provide mechanisms for cell-to-cell contact, adhesion, and communication
• Membranes separate the interior and exterior of cells and organelles, acting as effective permeability barriers due to their hydrophobic interior
• Membranes are associated with specific functions because the molecules responsible for the functions are embedded in or localized on membranes
• Membrane proteins carry out and regulate the transport of substances across the membrane, allowing cells and organelles to take up nutrients, ions, gases, water, and other substances
• Membrane proteins detect and transmit electrical and chemical signals, allowing cells to receive information from their environment and trigger chemical events that lead to changes in cell function
• Cell-to-cell contacts, critical in animal development, are often mediated by cadherins which promote adhesion between similar types of cells in a tissue
• Types of junctions between cells in animal tissues:

  • Adhesive junctions hold cells together
  • Tight junctions form seals that block the passage of fluids between cells
  • Gap junctions allow for communication between adjacent animal cells
  • In plants, plasmodesmata perform a similar function
• The fluid mosaic model envisions a membrane as two fluid layers of lipids with proteins within and on the layers
• Membrane proteins are embedded in the lipid bilayer due to their hydrophobic regions, while peripheral proteins are hydrophilic and located on the surface of the bilayer
• Lipids in the bilayer are in constant motion, allowing for fluidity, while proteins can move laterally within the membrane
• Most integral membrane proteins have one or more hydrophobic segments that span the lipid bilayer, anchoring the protein to the membrane
• Membranes are not homogenous but freely mixing, ordered through dynamic microdomains called lipid rafts, and most cellular processes involving membranes depend on specific lipid-protein complexes
• Membrane lipids are important components of the fluid mosaic model, with several major classes including phospholipids, glycolipids, and sterols
• Most eukaryotic cell membranes contain sterols, with cholesterol being the main sterol in animal cell membranes
• Plant cell membranes contain small amounts of phytosterols, while fungal cell membranes contain ergosterol, similar to cholesterol
• Thin-Layer Chromatography (TLC) is used for lipid analysis:

  • Lipids are isolated, separated, and studied using nonpolar solvents like acetone and chloroform
  • TLC separates different lipids based on their relative polarities
  • A glass plate coated with silicic acid is used, and lipids are spotted near the bottom of the plate at the origin
• Principle of Separation of Lipids by TLC:

  • A nonpolar organic solvent moves up the plate by capillary action, carrying different lipids to varying degrees
  • Nonpolar lipids move readily with the solvent, near the solvent front
  • Polar lipids interact variably with the silicic acid, and their movement is slowed proportionately
• Fatty acids are essential to membrane structure and function:

  • Found in all membrane lipids except sterols
  • Long hydrocarbon tails provide a barrier to diffusion of polar solutes
  • Membrane fatty acids range between 12-20 carbons long, optimal for bilayer formation
• Fatty acids vary in degree of saturation:

  • Palmitate and Stearate are common saturated fatty acids
  • Oleate and linoleate are unsaturated fatty acids
  • Saturated fatty acids pack tightly, while unsaturated fatty acids are more fluid
• Membrane asymmetry:

  • Most glycolipids in animal cell membranes are in the outer layer
  • Established during membrane synthesis and tends to be maintained
  • Lipids move freely within their monolayer through lateral diffusion
• Transverse diffusion:

  • Rare phospholipid flip-flop occurs in natural membranes
  • Some membranes have proteins that catalyze flip-flop, called phospholipid translocators or flippases
• The lipid bilayer is fluid:

  • Permits movement of lipids and proteins
  • Lipids can move rapidly within the monolayer
  • Lateral diffusion can be demonstrated using Fluorescence Recovery After Photobleaching (FRAP)
• Measuring membrane fluidity:

  • Membrane fluidity decreases with temperature
  • Each lipid bilayer has a characteristic transition temperature (Tm)
  • Tm is measured using differential scanning calorimetry
• Effects of fatty acid composition on membrane fluidity:

  • Length and saturation of fatty acids affect fluidity
  • Saturated fatty acids have higher Tms, unsaturated fatty acids have lower Tms
• Effects of sterols on membrane fluidity:

  • Sterols like cholesterol influence fluidity and Tm
  • Cholesterol decreases fluidity but prevents tight packing of hydrocarbon chains
• Regulation of membrane fluidity:

  • Organisms regulate fluidity by varying lipid composition
  • Important for poikilotherms that cannot regulate body temperature
• Lipid rafts in membranes:

  • Localized regions involved in cell signaling
  • Dynamic regions with changing compositions
  • Thought to play roles in detecting and responding to extracellular signals