Lab 15: Population Genetics - eScience Labs

Lab 6: Diffusion
Concepts to explore:
Diffusion
Rate of diffusion
Direction of diffusion
Concentration gradient
Membrane permeability
Dialysis
Introduction
Molecules are constantly in motion due to the kinetic
energy present in every atom. This energy results in
the net movement of molecules from areas of high
concentration to areas of low concentration, or diffusion
(Figure 1). If uninhibited, this movement will
continue until equilibrium is reached and the molecules
are uniformly distributed.
The rate of diffusion depends on the medium used,
size of the molecule, and polarity of molecule. Because
the medium will not change in a biological system,
the diffusion rate is usually dictated by molecular
characteristics. Small, non‐polar molecules exhibit
a higher rate of diffusion than large, charged
ones.
Figure 1
Diffusion through a semi‐permeable membrane
(lipid bilayer)
The direction of diffusion depends on concentration
gradients, heat and pressure. The concentration gradient is the change of molecular density over a
given area. Temperature and pressure typically remain constant in biological systems, making the concentration
gradient the best indicator of directionality. In general, molecules will move towards areas
of lower concentrations.
A major determinant of diffusion within a biological system is membrane permeability. Cells, as well as
organelles within the cell, are surrounded by selective and differentially permeable membranes. These
membranes control the interaction of the cell and its surrounding environment. Acting as a living gatekeeper,
the membrane allows, slows, or denies access into the cell.
Cellular membranes are composed of two layers of hydrophobic lipids. This lipid bilayer selects for
molecules that can dissolve into the lipid environment and against those that cannot. The ability of a
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