chapter 6

• Biological Membranes


  Lipid–Protein Bilayers
• Cell Membrane


  • Important in cell adhesion & recognition
• Substances can cross membranes


  1. Passive processes

  2. Active transport across membranes requires energy
• Large molecules enter and leave a cell

  Through vesicles
• Phospholipids


  • Amphipathic
  • Hydrophilic head group
  • Hydrophobic tails
• Fluid-Mosaic Model of Biological Membranes


  • Membranes exhibit properties that resemble a fluid
  • Membrane is a mosaic of lipid, protein, and other molecules
  • Phospholipids form a bilayer, which is like a "lake" in which a variety of proteins "float"
• Diffusion


  • The physical process in which particles spread out from regions of high concentration to regions of low concentration through space over time due to random independent motions and collisions
  • Particles eventually reach equilibrium in which their concentration becomes constant
• Factors influencing rate of diffusion


  • Particle size
  • Temperature & density of the solution
  • Magnitude of the concentration gradient
• Diffusion only occurs when very small particles are part of a physical phase that allows particles to move around independently
• Diffusion in gas phase


  Molecules lack significant intermolecular forces and move ~100% independently
• Diffusion in liquid phase


  Molecules have significant intermolecular forces so motion is somewhat dependent on neighboring molecules
• Diffusion in solid phase


  Molecules are unable to move at all relative to other molecules
• Factors affecting membrane fluidity


  • Length of fatty acid tails
  • Presence of double bonds in fatty acid tails
  • Presence of cholesterol
  • Temperature
• Integral membrane proteins


  • One or more regions physically embedded in the hydrophobic region of the phospholipid bilayer
  • Transmembrane proteins span the entire membrane with one or more transmembrane domains
• Peripheral membrane proteins


  Found on one side of the membrane, not integrated in the membrane
• Membrane proteins can move rapidly throughout the membrane
• Cell adhesion & cell recognition

  • Depend on intercellular connections between cell membranes of adjacent cells
  • Cell recognition: one cell specifically recognizes and binds to another cell
  • Cell adhesion: connection between two cells is strengthened
  • Recognition & adhesion molecules are specific and depend on proteins and carbohydrates in cell membrane
• Homotypic binding


  Cell-cell adhesion in tissues, the same molecule sticks out from both cells and bind to each other
• Heterotypic binding


  Adhesion between different types of cells, the cells have different proteins with an affinity for one another
• Types of cell junctions


  • Tight junctions
  • Desmosomes
  • Gap junctions
• Integrins


  • Transmembrane proteins with binding sites for both extracellular matrix and cytoskeleton (actin)
  • Integrin binding is non-covalent & reversible, allowing cell movement within a tissue
• Concentration gradient


  Solute concentration higher on one side of a membrane than the other
• Passive transport


  Movement of molecules through a membrane with no energy required, down their concentration gradient
• Types of passive transport


  • Simple diffusion
  • Facilitated diffusion
  • Osmosis
• Simple diffusion


  Movement of a molecule from high to low concentration without the help of a transport protein
• Facilitated diffusion


  Movement of a molecule from high to low concentration with the help of a transport protein
• Osmosis


  Diffusion of water across a semipermeable membrane from an area with more free water to an area with less free water
• Tonicity


  Ability of a solution to make a cell gain/lose water
• Aquaporins


  Special channels that allow water to cross membranes, while excluding ions
• Active transport


  Movement of a solute across a membrane against its gradient, from lower to higher concentration, requiring energy input
• Types of active transport


  • Calcium-Binding Protein
  • Na+-Amino Acid Symporter
  • Na+-K+ Pump
• Sodium-Potassium Pump


  Primary active transport mechanism
• Secondary active transport


  No direct use of ATP
• Exocytosis/Endocytosis


  1. Transport larger molecules such as proteins and polysaccharides, and even very large particles

  2. Endocytosis: Material brought into the cell

  3. Exocytosis: Material secreted outside of cell
• Types of endocytosis


  • Phagocytosis ("cell-eating")
  • Pinocytosis ("cell-drinking")
  • Receptor-Mediated Endocytosis