Examining Cellular Transport Keith P. Madden 2010 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?