This applet is a simulation of what goes on inside of your eyes when light (in the form
of photons) enters. The photons hit Rhodopsin molecules located on disks located in rods located
in the eye cells. This activates the Rhodopsin. The active Rhodopsin then moves around the disk
and activates any Transducin molecules it runs into. After a period of time, the Rhodopsin
looses its activity and must be 'recharged' through a different process before being used again. The
period of time Rhodopsin is active is known as its lifetime. The activated Transducin then moves
around and activates any Phosphodiesterase molecules it collides with. The Transducin, like the
Rhodopsin, looses its activity at the end of its lifetime. After being active, the Phosphodiesterase
must be recharged before becoming active again. The activated Phosphodiesterase is the agent which
sends the signal to the brain. The brain then interprets the signals and produces the image you see. Active
Phosphodiesterase also reverts to 'rechargeable' form of Phosphodiesterase after a
period of time.
Note: In the applet Phosphodiesterase is reffered to as PDE and Transducin is reffered to as GDP.
Directions:
To start the reaction press the 'Start' button. The text on the start button will change to
'Stop' and the reaction will start. To stop the reaction hit the 'Stop' button. The text will
change back to 'Start' and the reaction will stop.
To reset the reaction or to implement any changes made by the edit buttons hit the 'Reset'
button. This is the only way to implement any edited options. To edit any of the editable
properties of the reaction or its animation, click on the respective edit button and then
change the items in the text boxes to what you would like (the times are in milliseconds,
the sizes are in nanometers and the original speeds are located in the table below). Be sure that
the numbers you enter in the text boxes are integers. Only the sizes of molecules and the number of
milliseconds per frame options can be given non-integer values. Keep all values greater than zero.
Click outside of the box after you are done editing. Click the reset button to implement your changes.
You may change as many things as you like before you reset it, but you must be sure to
reset the applet or you will notice no change.
Original Speeds
Active Rhodopsin
989.95 nm/s
Transducin
1697.06
Active Transducin
2121.32
Phosphodiesterase
1131.37
Active Phosphodiesterase
707.11
Molecules that need recharging
1414.21
These values are actually (in every case except the Active Phosphodiesterase and used molecules)
coefficients of
lateral diffusion (in micrometers squared per second). They should provide an accurate ratio
of speeds between the forms of molecules. The values come from:
A Quantitative Account Of the Activation Steps Involved in
Phototransduction in Amphibian Photoreceptors by: T.D. Lamb and E.N. Pugh Jr.
The Physiological Laboratory, University of Cambridge
Downing Street
Cambridge CB2 3EG
Psychology Department University of Pennsylvania
3815 Walnut Street
Philadelphia PA 19104-6196, USA
Bugs
The editing windows do not close when you leave the web page. Please do not worry about this, they
will close when you exit your browser. Just minimize them or place them in the background and ignore them.
They will disappear when you exit your browser. Most computers will not be able to handle numbers
of molecules that exceed 5000. the higher the number the more time you are going to have to wait for
the applet to reset and to change frames. There is a point beyond witch the applet will be setting up
indefinitely. So keep the numbers small. The reaction is collision based. That means that when two
molecules hit each other a reaction should happen. Sometimes this does not work. It could be because
the speeds are too great for the animation. Try decreasing the speed or decreasing the number of
milliseconds per frame.