Membrane-Structure-and-Function

• Plasma membrane


  The boundary that separates the living cell from its surroundings
• Plasma membrane


  • Exhibits selective permeability, allowing some substances to cross it more easily than others
• Phospholipids


  The most abundant lipid in the plasma membrane

  
• Phospholipids

  Amphipathic molecules, containing hydrophobic and hydrophilic regions
• Fluid mosaic model


  A membrane is a fluid structure with a "mosaic" of various proteins embedded in it
• Membrane models: Scientific inquiry

  1. Membranes have been chemically analyzed and found to be made of proteins and lipids

  2. Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer

  3. Davson and Danielli proposed a sandwich model

  4. Singer and Nicolson proposed the fluid mosaic model

  5. Freeze-fracture studies supported the fluid mosaic model

  
• Plasma membrane

  • Phospholipids can move within the bilayer
  • Most lipids and some proteins drift laterally
  • Rarely does a molecule flip-flop transversely across the membrane

  
• As temperatures cool


  Membranes switch from a fluid state to a solid state
• Cholesterol

  • At warm temperatures, it restrains movement of phospholipids
  • At cool temperatures, it maintains fluidity by preventing tight packing
• Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions

  
• Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary
• Peripheral proteins


  Bound to the surface of the membrane

  
• Integral proteins


  Penetrate the hydrophobic core of the membrane

  
• Transmembrane proteins


  Integral proteins that span the membrane

  
• Six major functions of membrane proteins

  • Transport
  • Enzymatic activity
  • Signal transduction
  • Cell-cell recognition
  • Intercellular joining
  • Attachment to the cytoskeleton and extracellular matrix (ECM)

  
• Membrane carbohydrates


  • Cells recognize each other by binding to surface molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane
  • Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)
  • Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

  
• Synthesis and sidedness of membranes

  1. Membranes have distinct inside and outside faces

  2. The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus

  
• Plasma membrane


  Selectively permeable, regulating the cell's molecular traffic
• Hydrophobic (nonpolar) molecules


  Can dissolve in the lipid bilayer and pass through the membrane rapidly
• Polar molecules


  Do not cross the membrane easily
• Transport proteins


  • Allow passage of hydrophilic substances across the membrane
  • Some are channel proteins with a hydrophilic channel
  • Others are carrier proteins that bind to molecules and change shape to shuttle them across the membrane

  
• Diffusion


  • The tendency for molecules to spread out evenly into the available space
  • Although each molecule moves randomly, diffusion of a population of molecules may be directional
  • At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other
• Concentration gradient


  The region along which the density of a chemical substance increases or decreases
• Passive transport


  Diffusion of a substance across a membrane with no energy investment
• Osmosis


  The diffusion of water across a selectively permeable membrane
• Tonicity


  • The ability of a surrounding solution to cause a cell to gain or lose water
  • Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement
  • Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water
  • Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water
• Osmosis


  Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides
• Tonicity


  • The ability of a surrounding solution to cause a cell to gain or lose water
• Types of tonicity


  • Isotonic solution
  • Hypertonic solution
  • Hypotonic solution
• Isotonic solution


  Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane
• Hypertonic solution


  Solute concentration is greater than that inside the cell; cell loses water
• Hypotonic solution


  Solute concentration is less than that inside the cell; cell gains water
• Hypertonic or hypotonic environments create osmotic problems for organisms
• Osmoregulation


  The control of solute concentrations and water balance, is a necessary adaptation for life in such environments
• Organism with contractile vacuole

  • Paramecium
• Cell walls help maintain water balance
• A plant cell is in a hypotonic solution


  The cell swells until the wall opposes uptake; the cell is now turgid (firm)
• A plant cell and its surroundings are isotonic


  There is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt
• A plant cell is in a hypertonic environment


  The cell loses water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis
• Facilitated diffusion


  Transport proteins speed the passive movement of molecules across the plasma membrane