Examining Cellular Transport

Keith P. Madden2010 Molecular Biology RET

Water transport in cells

• Biological systems are compartmentalized.– Lipid bilayers have a non-polar (“fatty” ) core.– Pores of varying size and charge control passage

of cellular materials.– Water is the ubiquitous solvent in biological

systems.– How does water move from one compartment to

another in cells?– What determines the direction and amount of

flow?

Consider a simple cell…

Lipid Bilayer Structure

Diffusion of free water

• Water molecules are constantly in motion, even at the lowest temperatures.– Brownian motion (Robert Brown, 1827)– Motion is totally chaotic !– Average kinetic energy is 3/2 kT.– The water molecules do not know where they are

supposed to go.– Motion of water from area of lower concentration

to area of higher concentration is statistical.

Example of Brownian Motion

U of Va. – Brownian Motion Applet• Applet• Consider an enclosed volume with a partition (a

porous wall)– One side: high salt concentration (hypertonic)– Other side: no salt (hypotonic)– In each volume element next to the wall, different

numbers of H2O molecules! – Side of lowest salt concentration will have more H2O

in each volume, and will lose them faster!– H2O will appear to flow from low salt to high salt

compartment

Consider a simple cell… again.

Building a model cell

Collect data – H2O height vs. time

Embellish the experiment

• Try different solutes.– Multi-solute systems?

• What about different membranes?– Millipore filters?– Nucleopore filters?– Differing polarity of filter material?