Photoreceptors | Bright Minds Mentori

Retina photoreceptors

Retina

Area of the eye that detects light via rods & cone photoreceptor cells.

Structure of Photoreceptors

1. membrane protein opsin 2. light sensitive molecule retinal

Only in rod cells the retinal opsin structure is called rhodopsin

Types of Photoreceptors

1. Rods

Function in dim light.

Adaptation

A very high degree of amplication

2. Cones

Enable colour detection

Adaptation

Different forms of opsin allows maximal sensitivity to red, green, blue

wavelengths & birds also have UV light sensitive membrane opsin.

Rod Mechanism

Stages of Hydrophobic signalling

1. Retinal absorbs a photon of light causing a conformational change in rhodopsin forming a single photoexcited./activated rhodopsin.

2. A cascade of proteins amplifies the signal.

3. Photorexcited rhodopsin activates hundreds of G protein transducin

per second.

4. Hundreds of transducin bind to and activate hundreds of PDE

(1 to 1 ratio of activation).

5. Hundreds of PDE catalyses the hydrolysis of thousands of cyclic GMP

6. This causes cyclic GMP to come off the ion channel to close the channel.

7. This depolarisation triggers a nerve impulse from a single photon of light.

Tissue Specific Response

A signals can only be detected if a cell has the specific receptor

-cell surface (hydrophilic signalling)

-cytosol/nucleus (hydrophobic signalling)

The same signal may have different effects on different target cell

types due to different intracellular signalling molecules/pathways.