: The Light Reactions 193

complex. In their reduced form, the electron carriers that (A) Cl function in the acceptor region of PSI are all extremely strong Cl reducing agents. These reduced species are very unstable and H CH N+ N+ CH thus difficult to identify. Evidence indicates that one of these N 3 3 early acceptors is a chlorophyll molecule, and another is a Cl Cl quinone species, phylloquinone, also known as vitamin K . C 1 O N(CH3)2 Additional electron acceptors include a series of three (methyl viologen) membrane-associated iron–sulfur proteins, also known DCMU (diuron) (3,4-dichlorophenyldimethylurea) as Fe–S centers: FeSX, FeSA, and FeSB (see Figure 7.27). Fe–S center X is part of the P700-binding protein; centers (B) A and B reside on an 8-kDa protein that is part of the PSI P700* Paraquat reaction center complex. Electrons are transferred through centers A and B to ferredoxin (Fd), a small, water-soluble P680* DCMU NADP+ iron–sulfur protein (see Figures 7.19 and 7.27). The mem- + brane-associated flavoprotein ferredoxin–NADP reduc- PQA tase (FNR) reduces NADP+ to NADPH, thus completing PQ the sequence of noncyclic electron transport that begins B NADPH with the oxidation of water. H2O P700 In addition to the reduction of NADP+, reduced fer- redoxin produced by PSI has several other functions in O2 P680 the chloroplast, such as the supply of reductants to reduce nitrate and the regulation of some of the carbon-fixation enzymes (see Chapter 8). Figure 7.28 Chemical structure and mechanism of action of two important . (A) Chemical structure of Cyclic electron flow generates ATP but no NADPH dichlorophenyldimethylurea (DCMU) and methyl viologen Some of the cytochrome b f complexes are found in the (paraquat), two herbicides that block photosynthetic elec- 6 tron flow. DCMU is also known as diuron. (B) Sites of action stroma region of the membrane, where PSI is located. of the two herbicides. DCMU blocks electron flow at the Under certain conditions, cyclic electron flow is known acceptors of PSII by competing for the bind- to occur from the reducing side of PSI via plastohydro- ing site of plastoquinone. Paraquat acts by accepting elec- quinone and the b6 f complex and back to P700. This cyclic trons from the early acceptors of PSI. electron flow is coupled to proton pumping into the lumen, which can be used for ATP synthesis but does not oxidize water or reduce NADP+ (see Figure 7.16B). Cyclic elec- Proton Transport and ATP Synthesis tron flow is especially important as an ATP source in the in the Chloroplast bundle sheath chloroplasts of some plants that carry out

C4 carbon fixation (see Chapter 8). The molecular mecha- In the preceding sections we learned how captured light nism of cyclic electron flow is not well understood. Some energy is used to reduce NADP+ to NADPH. Another frac- proteins involved in regulating the process are just being tion of the captured light energy is used for light-depen- discovered, and this remains an active area of research. Plantdent PhysiologyATP synthesis, 6/E Taiz/Zeiger which is known as photophosphor- Sinauerylation Associates. This process was discovered by Daniel Arnon Some herbicides block photosynthetic Moralesand his Studio coworkers in the 1950s. Under normal cellular TZ6E_07.28 Date 04-17-14 electron flow conditions, requires electron flow, The use of herbicides to kill unwanted plants is widespread although under some conditions electron flow and pho- in modern agriculture. Many different classes of herbi- tophosphorylation can take place independently of each cides have been developed. Some act by blocking amino other. Electron flow without accompanying phosphoryla- acid, carotenoid, or lipid biosynthesis or by disrupting tion is said to be uncoupled. cell division. Other herbicides, such as dichlorophenyldi- It is now widely accepted that photophosphorylation methylurea (DCMU, also known as diuron) and paraquat, works via the chemiosmotic mechanism. This mecha- block photosynthetic electron flow (Figure 7.28). nism was first proposed in the 1960s by Peter Mitchell. DCMU blocks electron flow at the quinone acceptors The same general mechanism drives phosphorylation of PSII, by competing for the binding site of plastoqui- during aerobic respiration in bacteria and mitochondria none that is normally occupied by PQB. Paraquat accepts (see Chapter 12), as well as the transfer of many ions and electrons from the early acceptors of PSI and then reacts metabolites across membranes (see Chapter 6). Chemios- – with oxygen to form superoxide, O2 , a species that is very mosis appears to be a unifying aspect of membrane pro- damaging to chloroplast components. cesses in all forms of life.

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