Ubiquinone has an absorbance band at 275 nm. This band bleaches when the quinone
ID: 222464 • Letter: U
Question
Ubiquinone has an absorbance band at 275 nm. This band bleaches when the quinone is reduced to either the semiquinone or the dihydroquinone. The semiquinone has an absorbance band at 450 nm, but neither the quinone nor the dihydroquinone absorbs at this wavelength. A suspension of purified bacterial reaction centers was supplemented with extra ubiquinone and reduced cyctochrome c and was then illuminated with a series of short flashes of light. The absorbance at 275 nm decreased on the odd-numbered flashes as shown on the first curve. The absorbance at 450 nm increased on the odd numbered flashes but returned to the original level on the even-numbered flashes as shown in the second curve.
Why is it necessary to have reduced cytochrome c present in order to see these effects? Make sure to explain how you arrived at your answer.
2 3 4 Flash number MmeExplanation / Answer
In this experiment the role of reduced cytochrome is to act as an electron donor and it donates its electron to Ubiquinone converting it into semiquinone and this molecule is then detected at 450nm.
Upon illumination reduced cytochrome gets oxidized and donates the electron to Ubiquinone that is why upon fisrt illumination you see an increase in the absorption at 450nm ( semiubiquinone formed) now upon second illumination this reduced ubiquinone (Semiubiquinone) is photooxidized and donates its electrons back to cytochrome returning it into its original reduced state. Again upon third illumination reduced cytochrome c gets photooxidised donating its electron to oxidized ubiquinone converting it back to semiubiquinone. hence you see an increase in absorption at odd numbered flashes.
This kind of experiment has been used to study and demonstrate cyclic nature of electron flow in photosynthetic bacterias as opposed to non cyclic electron flow in plants.
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