Eye Spy

Every day we awaken to the world around us, opening our eyes to see what the day may bring. But have you ever thought about how we’re able to see all that the world around us has to offer. How is it that the light reflecting off of objects is translated in our brains to produce the images we see in our head?

Although it may seem rather complex, vision can be broken down into several major steps. First, light enter the pupil, is focused by the lens, and then hits the retina, a surface covered in light-sensitive detectors. These detectors, known as rods and cones, convert the light into electrical impulses which are then transmitted to the brain via brain nerves. It is from this array of impulses that the brain then creates a picture, enabling us to see.

Isomerization of Retinal

The chemistry of vision lies in the photoreceptor cells lining the retina. Within the retina, there are 7 million cones cells, which provide color information and sharpness, while there are 120 million rods cells which are responsible for detecting white light and providing most of our night vision. Just under the surface of rod cells lie pigment discs which contain proteins bound to the molecule 11-cis-retinal. Upon absorption of light, 11-cis-retinal isomerizes into all-trans-retinal. 

Courtesy of chemistry.wustl.edu

When retinal changes shape, it causes a change in the shape of the protein it sits within, called opsin. Together, 11-cis-retinal and opsin are known as rhodopsin.  Upon isomerization to all-trans-retinal, the complex is known as bathorhodopsin. In order to fit properly in the protein, the retinal molecule has to twist into an unfavorable shape. Due to this instability, bathorhodopsin rapidly changes its shape and expels the retinal molecule from it. 

Just before the molecule leaves the protein (now called metarhodopsin II), the protein complexes with another protein, transducing, which then activates the enzyme photodiesterase. Photodiesterase leads to the hydrolysis of cyclic GMP, a molecule required to open Na⁺ channels in the cells membrane. With closed Na⁺ channels, the cell develops a difference in charge across it’s cell membrane, producing an electrical signal, which can then be sent to the brain and create vision!

Courtesy of RSC Publishing

In cone cells, the process is mostly the same. There are three different cone cells within the body, responsible for perceiving red, blue, and green. Each of these different cells contains a different protein bound to 11-cis-retinal. Given the number and type of cone cells activated by incoming light, the brain is able to decipher what images we are looking at. 

Just think, all of this is happening millions of times every second! The body is certainly an amazing thing!

Cool Videos about the Chemistry of Vision:

1. http://www.youtube.com/watch?v=r6v21W8zRIw
2. http://www.youtube.com/watch?v=Fm45A4yjmvo

References:

1. http://chemwiki.ucdavis.edu/Biological_Chemistry/Photoreceptors/Chemistry_of_Vision
2. http://www.chemistry.wustl.edu/~edudev/LabTutorials/Vision/Vision.html