Cell Membranes

• 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 

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…