Summarizing Molecular Transport Phenomena

Summary

• Diffusion is the movement of substances due to random thermal molecular motion.
• The average distance \({x}_{\text{rms}}\) a molecule travels by diffusion in a given amount of time is given by

  \({x}_{\text{rms}}=\sqrt{2D\text{t}},\)

  where \(D\) is the diffusion constant, representative values of which are found in this table.

• Osmosis is the transport of water through a semipermeable membrane from a region of high concentration to a region of low concentration.
• Dialysis is the transport of any other molecule through a semipermeable membrane due to its concentration difference.
• Both processes can be reversed by back pressure.
• Active transport is a process in which a living membrane expends energy to move substances across it.

Glossary

diffusion

the movement of substances due to random thermal molecular motion

semipermeable

a type of membrane that allows only certain small molecules to pass through

osmosis

the transport of water through a semipermeable membrane from a region of high concentration to one of low concentration

dialysis

the transport of any molecule other than water through a semipermeable membrane from a region of high concentration to one of low concentration

relative osmotic pressure

the back pressure which stops the osmotic process if neither solution is pure water

osmotic pressure

the back pressure which stops the osmotic process if one solution is pure water

reverse osmosis

the process that occurs when back pressure is sufficient to reverse the normal direction of osmosis through membranes

reverse dialysis

the process that occurs when back pressure is sufficient to reverse the normal direction of dialysis through membranes

active transport

the process in which a living membrane expends energy to move substances across