The Nobel Prize in Chemistry 2003
The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Chemistry for 2003 “for discoveries concerning channels in cell membranes”, with one half of the prize to
Johns Hopkins University School of Medicine, Baltimore, USA
“for the discovery of water channels”
and one half of the prize to
Howard Hughes Medical Institute, The Rockefeller University, New York, USA
“for structural and mechanistic studies of ion channels”. 
Two Cell Membrane Channels
One Nobel prize "for discoveries concerning channels in cell membranes" unified the work of two men. The following figure represents this unification: "the discovery of water channels" by Peter Agre and "structural and mechanistic studies of ion channels" by Roderick MacKinnon.
Fig 1. The dividing wall between the cell and the outside world – including other cells – is far from being an impervious shell. On the contrary, it is perforated by various channels. Many of these are specially adapted to one specific ion or molecule and do not permit any other type to pass. Here to the left we see a water channel and to the right an ion channel.
Figure 2 shows Peter Agre's experiment on October 9, 1991. In his words: "Although the oocytes expressing the 28 kDa protein and the control oocytes looked the same, when he transferred them from isotonic solution to distilled water, an amazing difference was immediately obvious - all six control oocytes were unaffected whereas all six 28 kDa oocytes immediately exploded like popcorn. Greg ran into my office almost speechless, and we both celebrated with joy. We knew from the first experiment that the 28 kDa protein conferred water permeability and must be the long-sought water channel."
Fig 2. Peter Agre’s experiment with cells containing or lacking aquaporin. The aquaporin is necessary for making the cell absorb water and swell.
A synchrotron is basically a super microscope. Roderick MacKinnon performed his structure research at two facilities: (1) the Cornell High Energy Synchrotron Source (CHESS) of Cornell University and (2) the National Synchrotron Light Source (NSLS) of Brookhaven National Laboratory. He said, "Without exaggeration that most of what is known about the chemistry and structure of ion channels has come from experiments carried out at these SR [synchrotron radiation] centers."
"We had identified the K+ channel signature sequence, but without knowing its structure we never would understand the chemical principles of ion selectivity in K+ channels. I decided at that point to learn X-ray crystallography to someday see a K+ channel."
You can view now the structure of potassium channels with the K+ at the center in .
Finally, http://www.youtube.com/watch?v=XxadMJ9zqpA shows "Water Channels in Cell Membranes" in action.
Note: nobelprize.org is the source for figures and quotes unless designated otherwise.