Diffusion and Osmosis
Bio 21L
Spring 2003
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.
Diffusion
• Net movement of molecules from an area of
high concentration to one of lower
concentration
• Fick’s First law of Diffusion:
Rate of diffusion is directly
proportional to concentration
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 @higher osmotic pressure than
the reference solution
• Hypertonic solutions: are @ higher osmotic
pressure than the reference solution. Exosmosis
• Hypotonic: are @ lower pressure than reference.
Endosmosis
• Isotonic: are @ the same osmotic pressure as
reference; e.g. 0.9% NaCl Sol; 5% sucrose sol
with human blood cells
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
Water Potential ۴ & Uptake of
Water by Plants
• Forces of adhesion & cohesion
• Evaporation & transpiration at leaf tip
• Generation of water potential
• Endosmosis into root cells
• Water uptake
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!
Cool! Let’s get started!