a2 biology unit 5- the eye presentation

8/6/2019 A2 Biology Unit 5- The Eye Presentation

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THE HUMAN EYE

Farah, Nisa & Saher



Int roduc t ion

Sense Organs : specialised regio n s composed of a collec t ion of developedrecep t ors

.



Int roduc t ion

W he n ligh t e nt ers the eye, i t passes through :

1 . the cor n ea,2. the n the aqueous humor,

3. le n s a n d vi treous humor.4. The retina (which is the ligh t- se n sin g s t ruc ture of the eye)

W he n ligh t cont ac ts the pho torecep tors (rods and cones),

a series of complex chemical reac t ion s occurs. Thechemical tha t is formed (activated r h odopsin) crea teselec t rical impulses i n the op t ic n erve.



Re t in a

Layer of light sensitive cells and the neurones leadi n g fromthese pho torecep tors to the op t ic n erveTw o types of photoreceptors (also seco n daryex terorecep tors ):

1 ) Rods2) Cones

cont ai n s over a hu n dred millio n ligh t- se n sit ive cellspho torecep tors)

its role is to focus

by perceivi

ng ligh

ta

nd providein forma t ion to the b rai n for processi n g

No te : ex terorecep tors are recep tors tha t de tec t cha n ges i n theex ter n al e n viro n me nt



Rods

spread eve n ly across re t in a excep t a t fovea

provide b lack a n d whi te visio n

used mai n ly for seei n g i n low ligh t int e n sity ( n igh t)

rods co nt ai n rhodopsin (a purple visual pigme nt )they work i n spatial summation where low ligh t int e n sitycould cause accumula t ion of the impulse a n d trigger a n ac t ion po te nt ial)

arra n geme nt of re t in a is ¶ b ack -t o - fro nt· ¶outersegme nt s· are n ex t to choroids wi th n euro n es a t theint erior edge of the eye b all



Rods Arra n geme nt



Rhodopsi n

Re t in al ca n exis t as two isomers :

C is- re t in alTra n s- re t in al

DA RK: Re t in al always exis ts in the cis - re t in al s ta teLIG HT: a pho ton collides wi th the cis - re t in al molecule, i t t ra n sforms i t int o t ra n s- re t in al.This cha n ges the shape of RETINAL causi n g the b on di n g

of opsi n a n d re t in al to s top. He n ce, b reaks Rhodopsi nint o opsi n a n d re t in al.This process is k n ow n as b leac h ing



Rods : Light

Bleachi n g causes the rod cells to b ecome impermea b le to sodiumion s (origi n ally excep t ion ally permea b le)There is n ow less sodium io n s in side the rod cell, the i nt erior is n owmore n ega t ive tha n usual, this is called hy perpolarisation , crea t in gwha t is kn own as the generator potential in the rodMore ligh t mea n s a n in crease i n the amou nt of b leachi n g a n d so a n din crease i n the size of the ge n era tor po te nt ial.If several rods are s t imula ted a t on ce, threshold level is reached ,a n d n euro t ra n smitt ers are released i nt o the sy n apse wi th b ipolarcells ( b ipolar cells co nn ec t ligh t se n sit ive cells (recep tors) wi th se n soryn euro n es)This crea tes a n ac t ion po te nt ial i n the se n sory n euro n e a n d a sig n alis t ra n smitt ed through the op t ical n erve to the b rai n .



Rods : Light

On ce b leachi n g occurs, the rod cannot be stimulated agai n un less rhodopsin is recreated ATP is used to t ra n sform the t ra n s- re t in al to cis - re t in al

a n d joi n it to opsi n t o form rhodopsi n agai nIn dayligh t , the rods are e nt irely b leached a n d soca nn o t respo n d to dim ligh t (as the rhodopsi n n eeds tob e recrea ted o n ce more), this is called lig h t adaptionof the eye,30 mi n utes spe nt in comple te dark n ess allows therhodoposi n t o b e reformed agai n , maki n g i t se n sit ive tovery dim ligh t , n ow the eye is dark adapted



C on es

arou n d 6 millio n con est igh t ly packed toge ther i n fovea

provide sharp vision

in

b

right

light

mitocho n dria

Mem b ra n e li n ed vesiclescont ai n in g the pho tose n sit ivepigme nt iodopsi n

direc t ion of impulse



How do C on es work?

C on es work like rods

however they work usi n g the pigme nt iodopsinIodopsi

ncomes i

n three

types, each o

ne se

nsi

tive

toon e of the primary ligh t colours

It n eeds more ligh t e n ergy to b reak dow n tha n rhodopsi n (therefore is n o t se n sit ive to low ligh t)

The b rai n int erpre ts the n umb er of differe nt typesof co n es as differe nt colours



Summary

Cones Rods

O pera tes b y Iodopsi n O pera tes b y Rhodopsi n

Needs more ligh t e n ergy to opera te Needs less ligh t e n ergy to opera te

3 types 1 type

Lower

se n sit ivityHigher se n sit ivity

C olour Black a n d whi te

Does n·t de tec t mo t ion De tec ts mo t ion

a2 biology unit 5- the eye presentation

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