Responding to Environmental Stimulus: The Veterbrate Eye Flashcards
The retina
The area within the eye that detects light
Types of photoreceptors cells
Rods and cones
Rods
Function in dim light but do not allow colour perception
Cones
Responsible for colour vision and only function in bright light
Retinal
Retinal is a light sensitive molecule that combines with a membrane protein called opsin to form the photoreceptors in the eye.
In cone cells, different forms of opsin…
Combine with retinal to give different photoreceptors proteins, each with maximal sensitivity to specific wavelengths of light: red, green blue or UV
Retinal-opsin complex
Rhodopsin
When rod cells in the retina are exposed to light
Retinal will absorb a photon of light and rhodopsin changes conformation to photo excited rhodopsin. A cascade of proteins amplifies the signal
Photoexcited rhodopsin
Activates a G-protein, called transducin. This activates an enzyme called PDE
A single photoexcited rhodopsin
Activates hundreds of molecules of G-protein. Each activated G-protein activates one molecule of PDE
PDE catalyses…
The hydrolysis of a molecule called cGMP. Each active PDE molecule will break down thousands of cGMP molecules per second.
The hydrolysis of cGMP causes a reduction in cGMP concentration, this affects….
the function of ion channels in the membrane of rod cells by causing the ion channels to close.
Hyperpolarisation
build up of ions within the cells. This triggers nerve impulses in the neurons in the retina.
Each photon of light activates…
hundreds of G-proteins (transducin) resulting in hundreds of PDE being activated.
PDE breaks down
thousands of molecules of CGMP.
