Unit 1-Detecting Light Flashcards
What does bacteriorhodopsin do in Archaea?
These transmembrane proteins have a prosthetic group called retinal which absorbs energy from photons. The energy absorbed by the retinal is used to pump protons through the bacteriorhodopsin, so that a protein gradient is generated across the membrane. The hydrogen ions then diffuse back through the membrane through ATP synthase to generate ATP from ADP and phosphate ions.
How to plants generate a proton gradient to create energy from light?
The energy from the photons is absorbed by photosynthetic pigments in the chloroplast. The absorbed energy drives a flow of electrons along an electron transport chain and this flow of electrons is used to pump protons across the thylakoid membrane into the stroma. As with the archaea the hydrogen ions then diffuse back through the membrane through ATP synthase to generate ATP.
Describe the structure of rhodopsin
It contains the protein opsin covalently bonded to prosthetic group retinal. Opsins are G-protein coupled receptors and have seven transmembrane alpha helices.
How is a nerve impulse created by animals in the eye when a photon of light is detected?
1) When a photon of light is absorbed by a retinal molecule it causes the retinal to change shape, which in turn causes a conformational change in the rhodopsin.
2) The change in the rhodopsin activates hundreds of G-proteins, each of which goes on to activate hundreds of molecules of an enzyme.
3) Each activated enzyme catalyses the breakdown of cGMP from many ligand-gated Na+ channels so the channels close and the inward leakage of sodium ions stops.
4) If there is sufficient build-up of Na+ then the membrane will become hyperpolarized and a nerve impulse will be generated
What do cone cells in the eye contain which allow colour vision?
photopsins
What are the differences between rod and cone cells?
Rod cells contain the photon receptor rhodopsin and cone cells contain photopsin. Rod cells are adapted to a low level of light and cone cells need bright conditions. Up to 100 rod cells feed into one ganglion whereas each cone cell is linked to a separate ganglion. Rod cells only register greyscale whereas cone cells register colour. Rod cells are located at the peripheral edge of the retina and cone cells in the centre of the retina.
Why are rod cells good at functioning in low light levels?
They can absorb light at a wide range of wavelengths.
What are the names given to proteins which react to specific wavelengths of light?
Photoreceptors
How do rod cells have a greater sensitivity at low light intensities than cone cells?
They have a higher degree of amplification
What is the additional wavelength range to which birds are sensitive to?
Ultraviolet
