Cell Membranes

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  • CELL MEMBRANES

Composition

  • Cell membranes are partially permeable
  • They act as gates controlling what goes in and out of a cell 
  • Bilayer of phospholipid
    • PHOSPHOLIPID: 1 x Phosphate group, 2 x Fatty acids
    • Hydrophilic head, hydrophobic tail 

(http://healthspotlight.com/phospholipid1.gif)

(http://kentsimmons.uwinnipeg.ca/cm1504/Image127.gif)

FLUID MOSAIC

- Cholesterol stability
- Intrinsic Proteins (crossing both layers)
- Extrinsic Proteins (crossing one layer)
- Extremely flexible

Function

  • Barrier (partially permeable)
  • structural support of the cell
  • secreting chemicals
  • cell to cell recognition
  • takeup of nutrients

Passing substances

  • SMALL UNCHARGED MOLECULES
    • oxygen & c02
    • glycerol
  • LARGE &/OR  CHARGED MOLECULES
    • ions
    • polar molecules
    • glucose

TRANSPORT

Diffusion 

  •  Passive
  • High ---> low concentration
  • Net movement until equilibrium
  • Rate affected by
    • Concentration gradient
    • Travelling distance
    • Surface area of membrane
    • Membrane thickness
    • Temperature

Facilitated Diffusion

  • Using channel proteins & carrier proteins
  • Channel Proteins
    • Acts as a channel
    • Ion specific (only lets one particular type of ion through)
    • Opens and closes
  • Carrier Proteins
    • Allows diffusion of larger, polar molecules incl. sugar & amino acids
    • Molecule attaches to carrier protein @ binding site
    • Causes carrier protein to change its shape
    • Molecule realeased on other side and into or out of cell
  • Both carrier & channel proteins push molecules against the natural concentration gradient and therefore require ATP (from respiration)

Osmosis

  • concerns the movement of WATER MOLECULES ONLY
  • 'The net movement of water from a region where it is highly concentrated to a region where it is lower through a partially permeable membrane'
  • ----> water moves from low solute concentration ---> high solute concentration
  • Water potential: The…

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