Diffusion and Osmosis

1
Diffusion and Osmosis

• Bio 21L
• Spring 2003

2
Molecular Motion

• Molecules posses kinetic energy.
• Random motion
• Rotational, translation vibration movements
• Brownian Movement random, zigzag movement,
  arising due to collisions of molecules with
  particles.
• BM is typical of colloids.

3
Diffusion

• Net movement of molecules from an area of high
  concentration to one of lower concentration
• Ficks First law of Diffusion
• Rate of diffusion is directly
  proportional to concentration

4
Osmosis

• Diffusion of molecules across a semi-permeable
  membrane from a place of higher concentration to
  one of lower concentration until the
  concentration on both sides is equal
• Osmolarity Concentration of osmotically active
  particles
• Tonicity The osmotic pressure/tension of a
  solution wrt blood
• Hypertonic solutions are _at_higher osmotic
  pressure than the reference solution

5

• Hypertonic solutions are _at_ higher osmotic
  pressure than the reference solution. Exosmosis
• Hypotonic are _at_ lower pressure than reference.
  Endosmosis
• Isotonic are _at_ the same osmotic pressure as
  reference e.g. 0.9 NaCl Sol 5 sucrose sol
  with human blood cells

6
Dialysis

• Separation of molecules based on size, using a
  semi-permeable membrane.
• Semi-permeable membranes natural-cell membranes
  synthetic- nitrocellulose memb.
• Factors affecting rate Pore size of memb Size
  of molecule polarity of molecules tonicity of
  solution outside dialysis bag

7
Water Potential ? Uptake of Water by Plants

• Forces of adhesion cohesion
• Evaporation transpiration at leaf tip
• Generation of water potential
• Endosmosis into root cells
• Water uptake

8
Haemeolysis Crenation Ghost Studies

• Loss of water by cells- cell shrinkage or
  crenation
• In RBCs-leaves just membranes-used extensively to
  study memb structure
• In plant cells- cell shrinkage- allows plant cell
  memb to be seen
• Plasmolysis-rupturing plant cells by dunking in
  hypotonic solution.
• Turgid cell- plant cells that maintain a good
  osmolarity tonicity- appear round happy!

9

• Cool! Lets get started!