Chapter 7 - Membrane Structure and Function

Plasma Membrane

  • 1895 (Charles Overton) - discovered membrane made of lipids
  • 1910 - membrane made of lipids and proteins
  • 1917 (Langmuir) - made artificial membrane
  • 1925 (Gorter and Grendel) - 2 layers of phospholipids in membrane
  • 1935 (Davson and Danielli) - sandwich model
    • 1960 - accepted model
    • Challenge #1: not all membranes are identical
    • Challenge #2: placement of proteins on surface
  • 1972 (Singer and Nicolson) - membranes can change molecules and proteins are inside membrane

Fluidity

  • Amphipathic: hydrophilic and hydrophobic
  • Held by hydrophobic bonds
  • Lateral movement of lipids and proteins
  • Temperature: remains fluid at low temp until threshold solidifies; due to type of bonds in lipids (2x, remains fluid)
  • Cholesterol: membrane less fluid, but also hinders solidification
  • Solidify: change protein conformation and characteristics of membrane components

Water Balance in Cells - Osmosis

  • Osmosis: diffusion of water through a selectively permeable membrane
  • Animal - lysis or shrivel
  • Plant - live best in hypotonic
  • Know the terms:
    • Hypotonic
    • Isotonic
    • Hypertonic

Passive Transport

  • Diffusion/Osmosis
  • Facilitated diffusion
    • Molecule can’t flow through membrane on its own
    • Specificity and Saturation
    • Channel Protein (corridor)
    • Gated channel
    • Aquaporin
    • Carrier Protein
    • Cotransport: symport and antiport

Traffic Across Membranes

  • Selectively permeable
  • Hydrophobic interior
    • Hydrocarbons, gases dissolve freely
  • Transport proteins - facilitated diffusion
    • Carrier - physically hold molecule
    • Channel - tunnels through membrane

Active Transport

  • Go against concentration gradient
  • Required energy input (usually ATP)
  • Ex: sodium/potassium pump
    • Na+ = high concentration on outside
    • K+ = high concentration inside cell
    • Pump will maintain this using ATP
  • Cotransport - “piggy back”

Transport Vesicles

  • Exocytosis: secretion; from Golgi to plasma membrane
  • Endocytosis:
    • Phagocytosis - large molecules taken in (specialized cells only; like a cell eating)
    • Pinocytosis - small molecules from extracellular fluid; nonspecific “gulps” (think opening mouth underwater; done automatically, no control over what goes in)
    • Receptor-Mediated Endocytosis - receptors for certain ligands are clustered in certain regions of the cell (whatever gets bound to the receptors gets brought in)
  • Rejuvenates membrane