Vision and eyes Flashcards
how do you calculate the energy per photon?
E= hv = hc/WL h= Planck's constant v = freq c/WL = freq
what is the average natural irradiance at these levels? sunlight sunrise/set twilight full moon starlight
sunlight 10^20 sunrise/set 10^18 twilight 10^16 full moon 10^14 starlight 10^12
what are two outcomes of a molecule absorbing a photon?
- high energy - vibration, electrons jump to higher energy levels. this can result in fluorescence, change of shape, chemical reaction, or break apart molecules. (night is essential for bodies molecules to replenish)
- low energy - Electron is stripped off and the molecule is ionised.
what is the basis of 3 types of cones in the retina?
long wave pass/red filters
short wave pass/blue filters
intermediate - green filters
what cells are in the retina and in what order?
central - ganglion cells acramine cells bipolar cells horizontal cells cone and rods - flat discs contain the photoreceptors, containing rhodopsin
what causes rhodopsin to convert into what?
Cis isomer under light converted into a trans isomer. Enzyme used to regenerate cis, this is a costly process. regeneration involves pigment migration and metabolic energy in the night to restore eyes in a fresh state.
how is luminance and different colours measured in the human eye?
luminance - overall light level = R+B+G
colour eg green: R+B-G
what are 2 steps in the process of vision?
- Trichromatic stage
trichromatic cone cells respond positively to one of 3 freqs of photons arriving at cell surface. - Opponent process stage
Opponent cells are near cone cells and detect 3 colour channels. Cg - excited by R, B, inhibited by green etc.
What is photon univariance?
photons have the same effect in transduction, regardless of the WL.
therefore, sensitivity of a photoreceptor is determined by its absorbance.
Describe the structure of rhodopsin
Made of 7 Trans-membrane alpha helices, with mainly non polar AAs so can float in the cell membrane easily, allows for high density in stacking.
retinal attachment site on 7th helix.
are opsins common in the animal kingdom?
hundreds of vertebrate opsins have been sequenced and has helped in understanding vertebrate vision and its evolution. opsins are highly conserved and probably homologous throughout the animal kingdom,
Common molecular ancestor.
describe the structure of a rod cell
scotopic vision
inner segment contains cell organelles and specialist features of neurones.
outer segment contains stacks of discs of lipid membrane - large SA and volume of visual pigments embedded in the membranes. 50% of outer segment is filled w rhodopsin.
Remnant cilium joins inner and outer segment = rods and cones evolved from an ancestral ciliated cell type.
what are some physiological differences between rods and cones?
rods - more sensitive, longer time constants for internal physiological processes (work slower), in most vertebrates they work at low light levels.
cones - opposite.
what is a pigment made of?
opsin (7 thrans membrane helice structure) + chromophore (aldehyde of vit A. In rhodopsin it is called retinal {A1}).
what is one cause of blindness?
vit A deficiency, as retinal cannot be synthesised (retinal,A1, is the chromophore in rods, alternatively in very few terrestrial animals, it is A2, 3,4,di-dehydroretinal).
which animals use 34 di-dehydroretinal in their chromophore?
in very fewterrestrial, but common in aquatic FW, mainly fish.
opsins based on 3,4di-dehydroretinal are called porphyropsins.
describe the vertebrate photoreceptor response to a flash of light
arrival of light is encoded by hyperpolarisation.
transduction leads to the a change in the photoreceptor potential caused by ion movement, causing release of NT.
- amount of change in membrane potential is related to the brightness of the light (graded response) – how many
photons fall on a photoreceptor and are absorbed in a period of time.
- the transduction cascade is an amplifier, as one photon can isomerise one rhodopsin, which then can activate around 700 molecules of transducin.
What causes light adaptation
transduction system is being regulated.
in the dark - more amplification of the light signal. down regulated in brighter conditions. Involves calcium.
How many eye types are there?
10
Vertebrates have 2
inverts have all 10
When did eyes evolve and what pushed this?
530mya earliest fossils with large eyes
Cambrian explosion - 5m animals w complex eyes appeared. increase in oxygen allowed transition from crawling on sea floor to swimming.
describe an early compound eye
Trilobite - hexagonal design
lenses made of calcite, not protein as insect eyes are.
v large eyes, anomalocaris had 15000 facets.
where did camera eyes first evolve?
in jawless fish
Haikouichthys are earliest fossil evidence.
How fast did eyes evolve?
Nilsson and Pegler: very fast. suggested 40,000 generations for an eye like ours to evolve from just a light sensitive patch.
Do all animals require good vision?
beh function determines why different eye types have evolved.
visually guided beh needs acute vision whereas others work with low resolution vision.
the world is blurry to the majority of animals.
