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 * 11. Describe the differences in distribution, structure and function of the photoreceptor cells in the human eye**

There are two types of photoreceptor cells, rod cells and cone cells. Rods are sensitive to light and do not cause colour vision, however cones are less light-sensitive and cause colour vision. There are more rods in the retina, however there are more cones in the fovea area. There are 125 million rods and 6 million cones abundant in the human eye. Rods and cones are long shaped cells consisting of a synaptic terminal, a cell body and an outer terminal containing discs. In the rods, the visual pigment rhodopsin is located in the discs, and the visual pigments photopsin are found in the cones that are part of the discs. Rods are more suitable for night vision and cones are more appropriate for day vision, when the pupil is contracted. Rods are good for detecting movement but sensitive to low light levels.They are usually more exposed when the pupil is dilated. Rods have only one type of rhodopsin and in the visible spectrum, is sensitive to lower wavelengths. Cones are in 3 forms each having photopsins. One absorbs blue light, the other green light and the third red light. The wavelengths in which they absorb the light greatly are different to each other. These absorb all the wavelengths of visible light.



This image represents the different structure of the rods and cones while working at the back of the eye. It also provides a visual dynamic to where they are situated and process the light that is coming into the human eye.


 * 12. Outline the role of rhodopsin in rods

Rhodopsin is a light absorbing pigment found in the rods of the human eye. It is composed of Vitamin A called retinal bonded to a protein called opsin. Retinal, part of the rhodopsin molecule is responsible for the initial absorption of light. Different opsin molecules affect the light absorbing and the ability of the retinal. Rhodopsin is sensitive to lower wavelenghts in the visible spectrum(blue-green). Rhodopsin is more active during duller light or darkness and is sensitive to blue-green light.

13. Identify that there are three types of cones, each containing a separate pigment sensitive to either blue, red or green light.

There are three types of cones found in the human eye which are all sensitive to different wavelengths of the visible spectrum. This is due to the type of opsin molecule which best absorbs that certain colour. The opsin molecule and its absorbing wavelength are as: blue region with a maximum of 420 nanometres, green region with a maximum of 530 nanometres and the red region with a maximum of 560 nanometres. These cones are responsible for all the colours of the spectrum which we see.

14. Explain that colour blindness in humans results from the lack of one or more of the colour-sensitive pigments in the cones

Colour blindness occurs when individuals are unable to distinguish certain colours. Because it is caused by a sex-linked genetic deficiency it is more common in males than females. There are three types of colour blindness. In individuals with red-green colour blindness, either the red cones or the green cones are absent. Their remaining two types of cones enable these people to see all the other colours, but cannot distinguish between red and green or cannot see red or green properly. Therefore a lack of one or more colour-sensitive pigments in the cones causes colour blindness.

15. Explain why sound is a useful and versatile form of communication**

Sound is a very useful and versatile form of communication as it can reverberate around corners and can travel long distances. It can vary in the nature of its transmission (i.e. pitch) and is thus versatile in communicating different information. It can be more effective than visual aids and is a very effective means of communicating for animals. It can travel through solid objects and can operate 24 hours a day. There are no complex structures involved with sound and it travels well in water. Sound can be used for sending and detecting messages and is the basis of language upon which most of our civilisation depends. Sound also alerts to stimuli/dangers and the detection of sound occurs from differing directions.