Lect 2 S2015 Slides
Slide 9 Fig 1.5
When the light hits the receptor, it will induce a structural change. This will cause some kind of response / change that affects mechanisms (enzyme activity, genes)
Opsins are proteins and usually have some sort of light absorbing molecule.
A lot of aromatics are used to absorb light (chlorophyll, retinal).
Chlorophyll on its own is not green, in pure solution it’s not green. It needs to be complexed in a protein structure for it to be green.
11-cis-Retinal: cis is a double bond. When light hits it momentarily it changes into the trans form where hydrogen lies on either side of it, making it more linear. Therefore it changes structure and consequently the structure of its protein. But almost immediately after it reverts back to its cis form, since the cis for is the favoured state energetically.
If the Trans state became the more favoured state (if the transmission period is lengthened) it will be like a light is continually on (signals sent to the brain), this causes diseases like cancer.
When the light hits the trans retinal, it causes a structural change to the protein enclosing the retinal. This leads to change in activity.
Trans form is more linear consequently, Trans-fats are linear. Normally fats molecules are angled making it difficult to pack, making the fat more liquid, however trans-fat is straight so they can pack making solid fat.
Unicellular eukaryote photosynthesis. The yellow blob is filled with rhodopsin, it picks up light signals. It sends enzymes that affect motility – causes it to move towards the light.
Chloroplast – 2 membranes. Inside are thylakoids that also have a membrane – chlorophyll is on the membrane of the thylakoid. Photosynthetic bacteria contain thylakoids. Colours other than green are absorbed, green is reflected, giving it its green colour.
But as the eye evolved the brain has to evolve at the same time – co-evolution
A primitive eye is a set of photo receptor cells. It can detect light. But not directionality or form. It can’t see something (direct or form), can just tell if light is there or not, can see shadows (e.g. predators).
Evolved to a cup form:
Evolved to a cup form that can determine directionality. The side that is hit by light/lack of light becomes active and nerves send signal to the brain that light is coming from “the top”. Co-evolution: e.g. if a prey evolved, the predator had to evolve at the same time to be able to keep up with the prey or it would die out. This is an argument used against evolution theory: how could two organisms that are so complex co-evolve at the same time. This might have been the driving force.
Evolution is a slow graduate process. The change doesn’t have to be helpful it just needs to gradually change.
It needs selection for adaptation by the environment.
e.g. an eye cup isn’t advantageous on its own. But organisms with the eyecup also evolve their brain to analyse the cups information that would be advantageous. The nerves in the brain to develop to understand the signal coming from the eye-cup.
Part of a neuron that carries the signal
The compound eye can detect colour, direction, form.
The lens helps focus the light
The iris regulates the amount of light entering the eye
The eye evolved gradually (small steps, small selections)
Each change in evolution must confer an advantage.
If you did get aplia there is some videos that can be watched.
Synaptic terminal: meets the other nerves
Chromatic: Can detect different wavelengths of light. Different organisms have the ability to see different kinds of light
In the genetic nature: vision is associated with the X chromosome
Colour blindness would be a mutation in the genes (on DNA) that make the proteins that absorb light. So they don’t work as well