Page 92 - Design in Nature

90 DESIGN IN NATURE

The Chemistry of Seeing
When photons hit the cells of the retina, they activate a chain reaction,
rather like a domino effect. The first of these domino pieces is a molecule
called "11-cis-retinal" that is sensitive to photons. When struck by a photon,
this molecule changes shape, which in turn changes the shape of a protein
called "rhodopsin" to which it is tightly bound. Rhodopsin then takes a form

that enables it to stick to another resident protein in the cell called
"transducin".
Prior to reacting with rhodopsin, tranducin is bound to another
molecule called GDP. When it connects with rhodopsin, transducin releases
the GDP molecule and is linked to a new molecule called GTP. That is why
the complex consisting of the two proteins (rhodopsin and transducin) and

RHODOPSIN

Light

RHODOPSIN

The first step in seeing is a small change created by light in
the structure of a minute molecule called 11-cis-retinal that
causes a change in a larger protein called rhodopsin to
which it is attached.