alleryStart learning →Why must a second photon be absorbed at photosystem I?
AThe electrons were destroyed at the first stop
BThe electrons need to be split into halves
CThe electrons must be returned to water
DThe electrons lost energy on the way there
Answer & Solution
Correct answer: D. The electrons lost energy on the way there
1. Electrons give up energy at each transfer between photosystem II and photosystem I.
2. Because they have lost energy prior to arrival, they must be re-energised by photosystem I.
3. Another photon is therefore absorbed by the photosystem I antenna.
4. That energy is relayed to the photosystem I reaction centre, called P700.
5. The electrons are never destroyed, only lowered in energy, so a replacement supply is not needed here.
6. Returning electrons to water would undo the splitting that started the whole chain.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 8 "Photosynthesis", section 8.2 The Light-Dependent Reactions of Photosynthesis_
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