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Which statement about light absorption by the two forms of phytochrome is correct?
APr absorbs far red light near 730 nm
BPfr absorbs red light near 667 nm
CPr absorbs red light near 667 nm
DPfr absorbs light at neither wavelength
Answer & Solution
Correct answer: C. Pr absorbs red light near 667 nm
1. Phytochromes are chromoproteins with two photo interconvertible forms, Pr and Pfr.
2. Pr absorbs red light at about 667 nanometres and is immediately converted to Pfr.
3. Pfr absorbs far red light at about 730 nanometres and is quickly converted back to Pr.
4. So the correct pairing is 667 nanometres for Pr and 730 nanometres for Pfr.
5. Swapping the two is the standard trap, since the names Pr and Pfr look interchangeable at a glance.
6. Pfr is the physiologically active form, so exposure to red light yields physiological activity while far red light inhibits it.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 30 "Plant Form and Physiology", section 30.6 Plant Sensory Systems and Responses_
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