Oxygen evolution, near membranes

Most proteins in the PS-11 complex are membrane spanning, but the three extrinsic proteins that are involved in oxygen evolution are located on the lumenal side of the thylakoid membrane. Although most proteins in the PS-1 complex are also membrane-spanning, a few are located toward the stromal side of the thylakoid membrane, e.g., the iron-sulfur proteins that contain FeS-A and FeS-B and the Fd-docking protein. On the other hand, some mobile electron-transfer proteins are present on one side or the other of the membrane, with Fd present on the stromal side and near the ferredoxin-NADP" -reductase (FNR) and the FNR-binding protein, and the copper-protein electron carrier, plastocyanin (PCy), present on the lumenal side, close to the PS-1 primary donor P700. 

Impurity-Resistant Membranes. Section 4.8 discusses two fundamental issues related to membranes, the possibility of achieving nearly 100% current efficiency and the development of impurity-resistant membranes. Total rejection of the hydroxyl ion by the membrane is possible if the only anode reaction is the discharge of the chloride ion. However, if oxygen evolution, which is favored thermodynamically, also takes place, the principle of electroneutrality makes it impossible for the membrane to exclude the back-migration of the hydroxyl ion. 

Steroids are derived from the same squalene precursor and have an oxygen-dependent biosynthetic pathway beginning with the formation of the first intermediate, 2,3-oxidosqualene (for details, see Sect. 5.1). Sterols, also known as steroid alcohols, are a subclass of steroids and may be found either as fi ee sterols, acylated, alkylated, sulfated, or linked to a glycoside moiety which can be itself acylated. Sterol biosynthesis is nearly ubiquitous among eukaryotes but almost completely absent in prokaryotes. As a result, the presence of diverse steranes (saturated four-cycle skeleton) in ancient rocks has been considered as evidence for over 2.7 billion years of eukaryotic evolution. Cholesterol is the most well-known sterol found in animal cell membranes (for establishing proper membrane permeability and fluidity) and red blood cells. 

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