Photo-chemistry of Vision

Photo-chemistry of Vision

1. PHOTOCHEMISTRY OF VISION By- Manish Sahu M.Sc. Chemistry (Final) Sp.- Physical Chemistry

2. CONTENTS Introduction Rods and cones Photochemistry of vision Visual pigments Absorption spectrum of visual pigments Structure of Rhodopsin Rhodopsin- - Retinal visual cycle Photo transduction in Rods Photo receptors and their activation Conclusion References

3. INTRODUCTION:- Photochemistry of vision basically means detection of light which strikes the eyes and brains that receives a signal. The eye of a vertebrate consists of Lens an Retina. Retina is made up of two types of light sensitive cells:- i)Rods ii) Cones. Eyes of humans are made up of photosensitive compound i.e. PROTEINS. This proteins are made up of two types :- i) Opsin. ii) Retinal.

4. RODS AND CONES:- Rods :- Rods are sensitive therefore functions at low intensity . Responsible for vision in dim light. Cone :- Comparatively low sensitivity. Responsible for colour selectors Both of them present in Retina acts as the receptors of an Eye.

5. PHOTOCHEMISTRY OF VISION i.Photopic vision :- Daylight vision due to cones. Mainly for colour vision ii. Scotopic vision :- Dim lights vision due to rods. Mesopic vision :- Full moonlight vision. Both rods and cones .

6. VISUAL PIGMENTS The photosensitive pigment in the rod is known as Rhodopsin or visual purple. 3 cones pigments :- Erythrolable – maximum sensitivity for RED. Cholorable – maximum sensitivity for GREEN. Cyanolable – maximum sensitivity for BLUE .

7. ABSORPTION SPECTRUM OF VISUAL PIGMENTS . Rhodospin absorbs light most of the visible spectrum shows absorption at 500nm. 3 types of cones which contain photo pigment that absorbs light at particular wave length. RED CONES GREEN CONES BLUE CONES

8. STRUCTURE OF RHODOPSIN Rhodopsin also known as visual purple pigment i.e. photo sensitive pigment in the rods. Its opsin (protein) ,scotopsin and 11-cis-retinal derivative of vitamin A. Molecular weight of a human Rhodopsin is 41,000 and it consists of 348 amino acids. Location:- present in the membrane of rod disks. It is very sensitive light receptors.

9. RHODOPSIN – RETINAL VISUAL CYCLE REGENERATION OF RHODOPSIN….. All the trans retinol is enzymatically isomerized to all 11-cis-retinol then enzymatically oxidized to11-cis retinal . The 11-cis-retinal then spontaneously combines with opsin reforming rhodopsin. Conversion of the retinol to retinal is dependent on the pigment epithelium. Deficiency of vitamin A Prevents regeneration of Rhodopsin, for this reason lack of vitamin A leads of Night blindness.

10. PHOTOTRANSDUCTION IN RODS… Photo transduction is a process by which light is converted into an electrical signals in the rod cells and cone cells. PHOTOTRNSDUCTION PROCESS..

11. PHOTORECEPTORS Photoreceptors are the cells in the retina that respond to light. Activation of Rods by Light.. DARK CONDITIONS.. In dark conditions, cGMP levels are high and keep cGMP gated sodium channels open allowing a steady inward current called dark current. This dark current keeps the cell depolarized at about -40mv. LIGHT CONDITIONS.. In photopic conditions photoreceptors hyperpolarized to potential of -60mv. Light closes cGMP gated sodium channels reducing influx of sodium ions and lead to hyperpolarization .

12. Conclusion Photo isomerization reaction occurs as the first step in the chemistry of vision. Photo isomerization is a molecular behaviour in which structural change between isomers is caused by photoexcitation.The cis-transisomerization of rhodopsin is the primary reaction in vision .

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