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Ubiquinone in the respiratory chain picks up one H⁺ from the aqueous environment for every electron it accepts, and it can carry either one or two electrons as part of a hydrogen atom (yellow). When reduced ubiquinone donates its electrons to the next carrier in the chain, these protons are released. A long hydrophobic tail confines ubiquinone to the membrane and consists of 6–10 five-carbon isoprene units, the number depending on the organism. The corresponding electron carrier in the photosynthetic membranes of chloroplasts is plastoquinone, which is almost identical in structure. For simplicity, both ubiquinone and plastoquinone are referred to in this chapter as quinone (abbreviated as Q).

Each of these electron carriers in the respiratory chain picks up one H⁺ from the aqueous environment for every electron it accepts, and it can carry either one or two electrons as part of a hydrogen atom (yellow). When it donates its electrons to the next carrier in the chain, these protons are released. In mitochondria the quinone is ubiquinone (coenzyme Q), shown here; the long hydrophobic tail, which confines ubiquinone to the membrane, consists of 6 to 10 five-carbon isoprene units, depending on the organism. The corresponding electron carrier in plants is plastoquinone, which is almost identical. For simplicity, both ubiquinone and plastoquinone will normally be referred to as quinone and abbreviated as Q.

The mobile electron carriers in the chain are plastoquinone (which closely resembles the ubiquinone of mitochondria), plastocyanin (a small copper-containing protein), and ferredoxin (a small protein containing an iron-sulfur center). The b₆-f complex closely resembles the b-c₁ complex of mitochondria and the b-c complex of bacteria (see Figure 14-61): all three complexes accept electrons from quinones and pump H⁺ across the membrane. Note that the H⁺ released by water oxidation and the H⁺ taken up during NADPH formation also contribute to the generation of the electrochemical H⁺ gradient, which drives ATP synthesis by an ATP synthase present in this same membrane (not shown).

The mobile electron carriers in the chain are plastoquinone (which closely resembles the ubiquinone of mitochondria), plastocyanin (a small copper-containing protein), and ferredoxin (a small protein containing an iron-sulfur center). The cytochrome b_6-f complex resembles the b-c₁ complex of mitochondria and the b-c complex of bacteria (see Figure 14-71): all three complexes accept electrons from quinones and pump H⁺ across the membrane. The H⁺ released by water oxidation into the thylakoid space, and the H⁺ consumed during NADPH formation in the stroma, also contribute to the generation of the electrochemical H⁺ gradient. This gradient drives ATP synthesis by an ATP synthase present in this same membrane (not shown here).