lecture 3: diffusion, osmosis and toxicity

nanoscale pores

responsible for transporting molecules in and out of a cell membrane

diffusion

essential biological process

• transports O₂ and CO₂ into/out of the bloodstream, ions across cell membranes etc.

• Occurs due to random thermal motion of molecules, similar to motion in a gas

  • Higher temperature (higher thermal motion) leads to faster diffusion

cause of diffusion

• Due to random thermal motion; molecules are bouncing into something

• Net motion; characterised by a root mean square distance

  • Motion of the di-molecule can be characterised by this

• Occurs from a region of high concentration towards a region of lower concentration

diffusion equation; fick’s law

provides the rate of diffusion

• the greater the difference in concentration per unit distance, the greater the flow rate (i.e. the rate of diffusion J)

alveolar sac structure

• Lung contains about 600 million alveoli (SA - 70m²)

• Alveolar membrane between air and blood is thin, typically 0.5 μm

• Roughly spherical in shape

• Oxygen molecules diffuse across membrane rapidly approx. 0.25 s

• Oxygen concentration in blood leaving lungs is almost equal to the concentration in alveolus

diffusion example; oxygen in blood

osmosis

osmosis is the movement of water molecules from a solution with a high concentration of water molecules to a solution with a lower concentration of water molecules, through a cell's semi permeable membrane

osmotic concentration/osmolarity

• Dissolve sugar or salt (solute) in water (solvent)

• The osmotic concentration, or osmolarity, is;

  • The number of moles of solute particles, or osmoles, per litre of solution (osmol/L)

• Osmole (or Osm or osmol) is a non-SI unit defining;

  • The number of moles of particles that contribute to the osmotic pressure

  • E.g. NaCl dissociates into Na+ and Cl= ions, so 1 mole/L of NaCl gives 2 osmol/L of solute particles

  • Glucose does not dissociate, so 1 mole/L of glucose gives 1 osmol/L of glucose

osmosis via a semi permeable membrane

spontaneous movement driven by diffusion

semi permeable membrane

• Barrier that blocks some molecules, but lets others through

• Typically lets through polar molecules (like water), but blocks bigger polar or charged molecules, like proteins or ions

• Common feature in many biological membranes

osmosis and equalising of water pressure

hemodialysis

tonicity

the effect a solution has on the volume of a cell due to osmosis, specifically the relative concentrations of solutes that cannot freely cross the cell membrane, leading to water movement and cell swelling or shrinking

• a measure of the osmotic pressure gradient