The following are some important structures in human eye:
1. Cornea: As light enters the eye, it first passes through a lubricating tear filmthat coats the cornea. The clear cornea covers the front of the eye and helps to focus incoming light.
2. Iris: The iris is the colored part of the eye. As light conditions change, the iris may dilate to make the pupil bigger or constrict to make the pupil smaller. This allows more or less light into the eye respectively.
3. Lens: After light travels through the pupil, it must pass through the lens. The human lens, much like the lens of a camera is responsible for focusing light. The lens can change its shape to focus on near-by and distant objects.
4. Retina: The retina is a thin, light sensitive tissue lining the back of the eye. It acts much like film in a camera.
Light must be properly focused onto the retina, and the surface of the retina must be flat, smooth, and in good working in order to produce a clear image.
5. Optical Nerve: A bundle of nerve fibers which carry visual information from the eye to the brain.
The retina (as shown in figure1) consists of neural tissue and contains the photoreceptors. Photoreceptors are the cells that detect light waves. Photoreceptors come in two types; rod cells, which detect dim light and cone cells, which detect bright light. At low light levels, the cone cells are useless and we rely on the color blind rod cells to see, and this is called scotopic vision. Our daylight vision, in which we see detailed colored images with great acuity, is called photopic vision, which relies on the cones of the eye exclusively.
These two systems coexist in the human eye, being switched on and off as light levels require. It is almost like having two sets of eyes, one for daylight and one for night.
The retina, therefore, functions in phototransduction. Phototransduction is the converse in of light energy to electric energy.
Figure1. Structure of the eye showing the retina.
The two systems (rod and cone cells) share a similar internal structure but differ in the shape of the outer segment. Figure2 shows the inner segment of both rod and cone cells contain the cell nucleus and various organelles and end at the receptor’s synaptic terminal.
Figure2. Two types of cells in the retina: rods and cones.
The outer segment contains disks that contain molecules that absorb light waves giving the photoreceptors the ability to respond to light. The absorption of light is the first step in phototransduction, and the molecule that absorbs light is a photopigmant.
Outer segment of the rod and cone cell showing Rhodopsin embedded within the membrane of the disk.
There are four different types of photoreceptor contains different photopigments. Three types of photopigments are formed in the cone cells, each respond to a different wavelength. First photopigment in the cone cell responds to the longest wavelength that is sensitive to 560 nm (the red spectrum), second photopigment responds to the middle wavelength that is sensitive to 530 nm (the green spectrum) and the third photopigment in the cone cells responds to the shortest wavelength that is sensitive to 420 nm (the blue spectrum). In this paper we will be investigating the fourth type of photoreceptor that is found in the rod cells of the retina. The photopigment of the rod cell is called Rhodopsin
A single rod cell contains around 108 molecules of rhodopsin, and these are renewed every ten days through out the person’s life.
Rhodopsin embedded within the plasma membrane
Figure3. Absorption of rods and cones pigments.
History Rhodopsin
What is Rhodopsin
Spectroscopy
Studies of Rhodopsin
Lesson plan
References
