The selective advantage conferred to organisms that can process the information light contains is illustrated by the existence of opsins. Opsins are a large protein family that functions by processing photons of light in both visual and non-visual phototransduction systems. There are two main types of opsins: type I is found in prokaryotes and fungi; type II in animals. The gene sequence of the two types is similar only to the degree that would be expected by chance - which means that the two types of opsins have evolved separately, an example of convergent evolution.

In biochemistry, crystal structures are used to determine the 3-D structure of molecules. This allows for the identification of key physical and chemical properties, because it allows for models to be hypothesized about which parts of a molecule are active participants in biochemical processes.

This is the crystal structure of the bovine opsin, a well-studied representative of the class:

Opsins are 7-transmembrane-domain proteins. This means they span the lipid bilayer membrane of a cell seven times. They respond to G proteins (discussed in phototransduction), and are therefore called G-protein-coupled-receptors (GPCR).

Opsins have evolved to enhance the physico-chemical properties of chromophores, to contain which they form a pocket. They act as a structural shell composed of around 355 amino acids. The shell provides the necessary environment for the chromophore to absorb a photon of a particular wavelength. This is called ‘spectral tuning’, and it means that differently structured opsins are the basis of color vision.

The first opsin was genetically sequenced in 1982, and since then more than 1000 different opsins have been discovered in organisms ranging from humans to jellyfish.

Of this immense diversity, human DNA contains 9 opsin genes. Four of these serve visual functions, while the rest are non-visual. Here are the main types:

• Rhodopsin - most strongly absorbs green-blue light. These molecules are present in rods and are used in dim illumination.

• Melanopsin - present in intrinsically photosensitive retinal ganglion cells (ipRGCs). These are primarily sensitive to blue light, and play a non-image-forming role in the setting of circadian rhythms. They also play a role in the pupil constriction response of the eye.

• Photopsin (also called cone opsins) - present in cone cells and used in color vision. These can further be subdivided according to the wavelengths of light they preferentially respond to:

i) red opsins (long wavelength sensitive): λ-max of 560 nm.

ii) green opsins (mid wavelength sensitive): λ-max of 530 nm.

iii) blue opsins (short wavelength sensitive): λ-max of 430 nm.

An interesting characteristic to note is that no particular opsin responds to photons of light in a linear fashion. The presence of an opsin only enhances the probability that the molecule of retinal ensconced within it will absorb light of a particular wavelength. This means that a cone opsin responds to a range of wavelengths, rather than to a single one in a deterministic manner.