Membrane Structure & Transport

    Cards (28)

    • Phospholipid bilayer
      The basic structure of the cell membrane, formed by two layers of phospholipids with hydrophilic heads facing outwards and hydrophobic tails facing inwards
    • Phospholipids
      • Amphipathic - have a hydrophilic phosphate head and hydrophobic hydrocarbon tails
      • Form a phospholipid monolayer when in water, with heads facing out and tails facing in
      • Form a phospholipid bilayer when at sufficient concentration, with two layers of phospholipids
    • Micelle
      A spherical structure formed when phospholipids are mixed with water, with the hydrophilic heads facing outwards and the hydrophobic tails facing inwards
    • Phospholipid bilayer
      • Composed of two layers of phospholipids with hydrophobic tails facing inwards and hydrophilic heads facing outwards
      • Allows for some fluidity due to weak hydrophobic interactions between the hydrocarbon tails
      • Acts as a barrier to most water-soluble substances due to the non-polar fatty acid tails
    • Cholesterol
      A lipid that is a component of animal cell membranes, positioned between phospholipids with its hydrophobic region interacting with the phospholipid tails and its hydrophilic hydroxyl group interacting with the phosphate heads
    • Role of cholesterol in mammalian membranes
      • Stabilizes the membrane at higher temperatures by preventing the membrane from becoming too fluid
      • Increases fluidity of the membrane at colder temperatures to prevent it from becoming too rigid
      • Reduces the permeability of the membrane to hydrophilic ions like sodium and hydrogen
      • Increases the mechanical strength and stability of the membrane
    • Integral membrane proteins
      Partially hydrophobic proteins embedded in the phospholipid bilayer, either in one or both layers
    • Peripheral membrane proteins
      Hydrophilic proteins temporarily attached to the surface of integral proteins or connected to the membrane via a hydrocarbon chain
    • Functions of membrane proteins
      • Transport
      • Receptors
      • Cell adhesion
      • Cell-to-cell recognition
      • Immobilized enzymes
    • History of fluid mosaic model
      1. Gorter and Grendel model (1920s) - phospholipids arranged in a bilayer
      2. Davson and Danielli model (1930s) - proteins arranged in layers above and below the phospholipid bilayer
      3. Singer and Nicolson fluid mosaic model (1970s) - proteins are both peripheral and integral, dispersed throughout the fluid membrane
    • Simple diffusion
      The net movement of particles from a region of higher concentration to a region of lower concentration, driven by the random motion of the particles
    • Simple diffusion
      A type of transportation that involves particles passing between phospholipids in the plasma membrane
    • Simple diffusion
      The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration
    • Molecules or ions move down a concentration gradient during simple diffusion
    • As a result of diffusion, molecules or ions tend to reach an equilibrium (given sufficient time), where they are evenly spread within a given volume of space
    • Factors affecting the rate of diffusion across a membrane
      • Steepness of the concentration gradient
      • Temperature
      • Surface area
      • Properties of the molecules or ions
    • Large molecules
      Diffuse more slowly as they require more energy to move
    • Uncharged molecules (e.g. oxygen)

      Diffuse faster as they move directly across the phospholipid bilayer
    • Non-polar molecules
      Diffuse more quickly as they are soluble in the non-polar phospholipid bilayer
    • Polar molecules
      Smaller polar molecules (e.g. urea) can diffuse at low rates, although larger polar molecules cannot easily pass through the hydrophobic part of the membrane
    • Facilitated diffusion
      The movement of substances that cannot diffuse through the phospholipid bilayer of cell membranes, with the help of certain proteins
    • Substances that require facilitated diffusion
      • Large polar molecules such as glucose and amino acids
      • Ions such as sodium ions (Na+) and chloride ions (Cl-)
    • Channel proteins
      Water-filled pores that allow charged substances (e.g. ions) to diffuse through the cell membrane
    • Carrier proteins
      Proteins that can switch between two shapes to allow the movement of molecules or ions across the membrane
    • During facilitated diffusion, the net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)
    • Osmosis
      The diffusion of water molecules from a dilute solution to a more concentrated solution across a partially permeable membrane
    • Osmolarity
      The solute concentration of a solution; a solution with high osmolarity has a high solute concentration and a solution with low osmolarity has a low solute concentration
    • Water will move from a solution of low osmolarity to a solution of high osmolarity across a partially permeable membrane
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