Detecting and amplifying an environmental stimulus Advanced Higher Biology 2015

In archaea, bacteriorhodopsin molecules generate potential differences by absorbing light to pump protons across the membrane Bacteriorhodopsin is a protein used by Archaea. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. The resulting proton gradient is subsequently converted into chemical energy

In archaea, bacteriorhodopsin molecules generate potential differences by absorbing light to pump protons across the membrane

In plants, the light absorbed by photosynthetic pigments drives an electron flow that pumps hydrogen ions across the thylakoid membrane of the chloroplast. In both cases the resulting diffusion of hydrogen ions back across the membrane drives ATP synthase.

Electrons leaving photosystem II are transferred transferred through a series of molecules and ultimately end up transferred to a molecule called plastoquinone (PQ). The PQ reacts with two hydrogen ions from the stroma and the two electrons from photosystem II to form PQH 2. 2H + stroma + 2e - + PQ → PQH 2 The PQH 2 diffuses across the thylakoid membrane, passes the two electrons to the next electron carrier and releases the two hydrogen ions into the lumen. PQH 2 → PQ + 2H + lumen + 2e - PQ can then diffuse back across the membrane to repeat the process. The net result of the Q cycle is to move two hydrogen ions from the stroma to the lumen.

The Light Reaction

In animals the light-sensitive molecule retinal is combined with a membrane protein opsin to form the photoreceptors of the eye. A cascade of proteins amplifies the signal. The signal transduction pathway is the mechanism by which the energy of a photon signals a mechanism in the cell that leads to its electrical polarization. This polarization ultimately leads to either the transmittance or inhibition of a neural signal that will be fed to the brain via the optic nerve.

Signalling pathway within eye

Photoreceptors of the eye In rod cells the retinal -opsin complex is called rhodopsin. When stimulated by one photon, a rhodopsin molecule activates hundreds of G - protein molecules, which activate hundreds of molecules of an enzyme. If the enzyme triggers sufficient product formation, a nerve impulse may be generated. A very high degree of amplification results in sensitivities at low light intensities. In cone cells, different forms of opsin combine with retinal to give photoreceptor proteins, each with maximal sensitivity to specific wavelengths (red, green, blue or UV)

Photoreceptor system proteins are found across the three domains.

1.Outline the function of proton pumps and explain how they are coupled with ATP synthase to generate ATP. 2.Describe the role of plastoquinone in the light- dependent reaction of photosynthesis. 3.Outline the role of G proteins as molecular switches within cells. 4.Explain the difference in the photo-sensitivity of rods and cones in terms of the amplification mechanisms of retinal cells.