bound, compared to the plastoquinone at QB, the secondary binding site, which is much more easily removed. QA is only transferred a single electron, so it has to transfer an electron to QB twice before QB is able to pick up two protons from the stroma and be replaced by another plastoquinone molecule. The protonated QB then joins a pool of free plastoquinone molecules in the membrane of the thylakoid. The free plastoquinone molecules eventually transfer electrons to the water-soluble plastocyanin so as to continue the light-dependent reactions. There are additional plastoquinone binding sites within photosystem II (QC and possibly QD), but their function and/or existence have not been fully elucidated. Biosynthesis The p-hydroxyphenylpyruvate is synthesized from tyrosine, while the solanesyl diphosphate is synthesized through the MEP/DOXP pathway. Homogentisate is formed from p-hydroxyphenylpyruvate and is then combined with solanesyl diphosphate through a condensation reaction. The resulting intermediate, 2-methyl-6-solanesyl-1,4-benzoquinol is then methylated to form the final product, plastoquinol-9. This pathway is used in most photosynthetic organisms, like algae and plants. However, cyanobacteria appear to not use homogentisate for synthesizing plastoquinol, possibly resulting in a pathway different from the one shown below. Derivatives Some derivatives that were designed to penetrate mitochondrial cell membranes (SkQ1 (plastoquinonyl-decyl-triphenylphosphonium), SkQR1 (the rhodamine-containing analog of SkQ1), SkQ3) have anti-oxidant and protonophore activity. SkQ1 has been proposed as an anti-aging treatment, with the possible reduction of age-related vision issues due to its antioxidant ability. This antioxidant ability results from both its antioxidant ability to reduce reactive oxygen species (derived from the part of the molecule containing plastoquinonol), which are often formed within mitochondria, as well as its ability to increase ion exchange across membranes (derived from the part of the molecule containing cations that can dissolve within membranes). Specifically, like plastoquinol, SkQ1 has been shown to scavenge superoxides both within cells (in vivo) and outside of cells (in vitro). SkQR1 and SkQ1 have also been proposed as a possible way to treat brain issues like Alzheimer's due to their ability to potentially fix damages caused by amyloid beta. Additionally, SkQR1 has been shown as a way to reduce the issues caused by brain trauma through its antioxidant abilities, which help prevent cell death signals by reducing the amounts of reactive oxygen species coming from mitochondria. References External links Plastoquinones History, absorption spectra, and analogs. Photosynthesis Light reactions 1,4-Benzoquinones Terpenes and terpenoids