Page 93 - Design in Nature

Communication and Target Location Systems 91

a smaller chemical molecule (GTP) is called "GTP-transducinrhodopsin".
The new GTP-transducinrhodopsin complex can now very quickly
bind to another protein resident in the cell called "phosphodiesterase". This
enables the phosphodiesterase protein to cut yet another molecule resident

in the cell, called cGMP. Since this process takes place in the millions of
proteins in the cell, the cGMP concentration is suddenly reduced.
How does all this help with sight? The last element of this chain
reaction supplies the answer. The fall in the cGMP amount affects the ion
channels in the cell. The so-called ion channel is a structure composed of

proteins that regulate the number of sodium ions within the cell. Under
normal conditions, the ion channel allows sodium ions to flow into the cell,
while another molecule disposes of the excess ions to maintain a balance.
When the number of cGMP molecules falls, so does the number of sodium
ions. This leads to an imbalance of charge across the membrane, which
stimulates the nerve cells connected to these cells, forming what we refer to
as an "electrical impulse". Nerves carry the impulses to the brain and

"seeing" happens there.
In brief, a single photon hits a single cell and, through a series of chain
reactions, the cell produces an electrical impulse. This stimulus is modulated
by the energy of the photon, that is, the brightness of light. Another
fascinating fact is that all of the processes described so far happen in no more
than one thousandth of a second. Other specialised proteins within the cells

convert elements such as 11-cis-retinal, rhodopsin and transducin back to
their original states. The eye is under a constant shower of photons, and the
chain reactions within the eye's sensitive cells enable it to percieve each one
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of these photons.
The process of sight is actually a great deal more complicated than the
outline presented here would indicate. However, even this brief overview is

sufficient to demonstrate the extraordinary nature of the system. There is
such a complicated, finely calculated design inside the eye that chemical
reactions in the eye resemble the domino shows in the Guinness Book of