There are a number of similarities, and some differences, between the Q cycle in the mitochondrial electron transport system and the PQ cycle found in the chloroplast electron transport system. Sort the items into the appropriate locations, depending on whether they are unique to the PQ cycle, unique to the Q cycle, or common to both. The cycle converts a two-electron transfer process into two one-electron transfer processes. Two electrons are transferred from the reduced carrier to cytochrome c. Addition of electrons one at a time generates a semiquinone intermediate. Used to translocate protons across a membrane. Two electrons are transferred from the reduced carrier to plastocyanin.

1. unique to the PQ cycle:

Two electrons are transferred from the reduced carrier to plastocyanin

2. unique to the Q cycle:

Two electrons are transferred from the reduced carrier to cytochrome c

3. common in both PQ cycle and Q cycle:

- The cycle converts a two-electron transfer process into two one-electron transfer processes

- Addition of electrons one at a time generates a semiquinone intermediate

- Used to translocate protons across a membrane

Explanation:

The Q cycle (Q due to quinol) refers to the cycle by which electrons are transferred from ubiquinol to cytochrome c. This cycle is associated with the sequential mechanism of oxidation-reduction between ubiquinol and ubiquinone, also known as Coenzyme Q10 (CoQ10, used as electron carrier), in order to move protons across the inner mitochondrial membrane. In the first phase of the cycle, ubiquinol attaches onto the cytochrome c oxidoreductase, also called complex III (because this complex is the third complex in the electron transport chain), in order to transfer two electrons to the complex. On the other hand, the PQ cycle (PQ due to plastoquinone) refers to the cycle where plastoquinone transports electrons during light-dependent photosynthetic reactions. In the PQ cycle, the plastoquinone serves as an electron carrier through the membrane of the thylakoid. In the first step of this cycle, plastoquinone obtains two electrons from photosystem II and absorbs two protons from the chloroplast stroma, resulting in plastoquinol, which subsequently transfers electrons to plastocyanin in a process coupled with the generation of a proton gradient between the lumen of thylakoid discs and stroma.

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