1. Communication Topic 7: Visual Pigments
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis

2. DOT Point(s) outline the role of rhodopsin in rods
identify that there are three types of cones, each containing a separate pigment sensitive to either blue, red or green light

3. Rhodopsin in Rods
All rods have only one type of pigment, rhodopsin. They are not sensitive to different colours.
Rhodopsin is a broad spectrum pigment. Its peak sensitivity is in the 500nm wavelength region of the visible spectrum, but rods do not allow us to perceive any colour.
Vision involving rhodopsin in rods allows us to see in shades of black, white and grey.

4. Other Opsins in Cones
Cones contain one of three types of iodopsin pigments and are most sensitive to light in one of three wavelengths. These pigments result in cone cells being sensitive to:
The short wavelengths of blue light (peak 455nm)
The medium wavelengths of green light (peak 530nm)
The long wavelengths of red light (peak 625nm)

5. Other Opsins in Cones
However, the sensitivity of a particular cone cell allows it to detect light to some extent on either side of these peak sensitivities, giving it an overlap in some of the colours detected. ‘Red’ cones are actually more sensitive to yellow light ( nm) than to red light, but they respond to red light before any of the others do, therefore behave as red receptors.

6. Other Opsins in Cones
Light of a particular wavelength may stimulate more than one cone which allows us to see a variety of colours.
By comparing the rate at which various receptors respond, as well as the overlap in colours detected, the brain is able to interpret these signals as intermediate colours..

7. Rhodopsin in Detail
The visual pigment rhodopsin, present in rods, consists of a protein molecule, opsin, combined with a simple, light absorbing part called retinal.
Retinal is a derivative of vitamin A (carotene). If this vitamin is lacking, vision is affected and a condition known as ‘knight blindness’ may result.

8. Rhodopsin in Detail
The main function of the photochemical pigments rhodopsin is to absorb light.
When light strikes the rhodopsin pigment the light energy is absorbed and rhodopsin changes from its resting state to an excited state. This change is due to the activation of the retinal part of rhodopsin.

9. Rhodopsins
This causes rhodopsin to split into its protein opsin part and a free retinal part.
The split rhodopsin pigment is said to be ‘bleached’, but this change is temporary.
A series of biochemical steps follows, whereby the activated pigment causes a change in electrical charges of the membrane of the cone (photoreceptor).

10. Rhodopsins
This is the start of an electrical impulse that moves along the receptor to the brain by first triggering the release of a chemical substance known as a neurotransmitter. The neurotransmitter then stimulates a bipolar cell, generating an impulse in this cell.

11. Rhodopsins
The signal is termed electrochemical because it involves both an electrical change in membrane and a chemical release of a neurotransmitter. The bipolar cell transmits the electrochemical signal to the ganglion cells which in turn carry the signal to the brain.

12. Rhodopsins
Rhodopsin, which was temporarily bleached or broken down in the presence of light, is then regenerated so that is can be reused.
Retinal and opsin recombine with the help of enzymes. This allows a new image to be received. The presence of vitamin A is essential for these steps to occur.

13. Activity
-Students to complete DOT Point 4.6 Photoreceptor cells in mammals, insects and another animal
Before we go to the library, flip your page over and create this table on the back. This is how I would like you to present your information.
Mammal
Insect
Other
Diagram of photoreceptor (eyeball)
Type of photoreceptor
Eg: Single lens eye
Compound eye
Type of image
Image on retina is inverted
Mosaic image
Similarities in photoreceptors
Detects light
Differences