The Vertebrate Eye

Question 1

The retina is the area within the eye that detects light and contains two types of photoreceptor cells: rods and cones

Answer 1

Rods - Function in dim light but do not allow colour perception

Cones - Responsible for colour vision and only function in bright light.

Question 2

Retinal

Answer 2

In animals, the light-sensitive molecule retinal is combined with a membrane protein, opsin to form the photoreceptors of the eye

In rod cells, the retinal-opsin complex is called rhodopsin.

Question 3

Rhodopsin and the nerve impulse

Answer 3

Retinal absorbs a photon of light and rhodopsin changes conformation to photoexicted rhodopsin

Photoexcited rhodopsin triggers a cascade of proteins that amplifies the photon signal and generates a nerve impulse.

Question 4

Photoexcited rhodopsin activates a G-protein, called transducin

Answer 4

The photoexcited rhodopsin activates a G-protein called transducin, which activates the enzyme PDE.

A single photoexcited rhodopsin activates hundreds of molecules of G-protein. Each activated G-protein activates one molecule of PDE.

Question 5

PDE catalyses the hydrolysis of a molecule called cyclic GMP

cGMP

Answer 5

Each active PDE molecule breaks down thousands of cGMP molecules per second.

The reduction in cGMP concentration as a result of its hydrolysis affects the function of ion channels in the membrane of rod cells

This results in the closure of ion channels in the membrane of the rod cells, which triggers nerve impulses in neurons in the retina.

Question 6

High degree of amplification in rod cells

Answer 6

A very high degree of amplification results in rod cells being able to respond to low intensities of light