Light reactions, of photosynthesis

Kramer DM, Avenson TJ, Edwards GE (2004) Dynamic flexibility in the light reactions of photosynthesis governed by both electron and proton transfer reactions. Trends Plant... 

FIGURE 19-37 The light reactions of photosynthesis generate energy-rich NADPH and ATP at the... 

The light reactions of photosynthesis are those directly dependent on the absorption of light the resulting photochemistry takes electrons from H20 and drives them through a series of membrane-bound carriers, producing NADPH and ATP. 

In the photosynthetic cells of plants, fatty acid synthesis occurs not in the cytosol but in the chloroplast stroma (Fig. 21-8). This makes sense, given that NADPH is produced in chloroplasts by the light reactions of photosynthesis ... 

Fig. 17.1 Energy level diagram of light reactions of photosynthesis of green plants-...

The Z-scheme describes the oxidation/reduction changes during the light reactions of photosynthesis. The vertical axis in the figure represents the reduction potential of a particular species—the higher the position of a molecular species, the more negative its reduction potential, and the more easily it donates electrons. See Figure 3-3. 

Figure 11.6 Schematic view of photosynthetic centre in chloroplasts (photosystems I and II, cytochrome hi complex) and light reactions of photosynthesis...

In the so-called light reactions of photosynthesis electrons (e-) are donated to the chain from H20 and ultimately accepted by NADP+ to yield NADPH. The difference between the Eo values of the NADPH/NADP+ (-0.32 V) and H20/i 02 (+0.82 V) half reactions (electron acceptor minus electron donor) is — 0.32 0.82 V = —1.14V and accordingly AGo = — rcFAEo = +52.6 kcal mol-1 for the overall reaction ... 

A discussion of the role of LHC phosphorylation and/or cation effects as mechanisms of regulation of energy distribution between the two photosystems in vivo is beyond the scope of this review. However, it seems likely that both mechanisms might cooperate in vivo to achieve a fine regulation of energy distribution to the two light reactions of photosynthesis and therefore an adaptation to the prevailing illumination conditions. 

Oxidative phosphorylation, in Chapter 18, is immediately followed in Chapter 19 by the light reactions of photosynthesis to emphasize the many common chemical features of these pathways. 

The discussion of the light reactions of photosynthesis in Chapter 19 leads naturally into a discussion of the dark reactions—that is, the components of the Calvin cycle—in Chapter 20. This pathway is naturally linked to the pentose phosphate pathway, also covered in Chapter 20, because in both pathways common enzymes interconvert three-, four-, five-, six-, and seven-carbon sugars. 

As animals ourselves, we perhaps easily overlook the ultimate primacy of photosynthesis for our biosphere. Photosynthesis is the source of essentially all the carbon compounds and all the oxygen that makes aerobic metabolism possible. Moreover, as we shall see, there are considerable mechanistic and evolutionary parallels between the light reactions of photosynthesis and steps in oxidative phosphorylation. 

Figure 19.2. The Light Reactions of Photosynthesis. Light is absorbed and the energy is used to drive electrons from water to generate NADPH and to drive protons across a membrane. These protons return through ATP synthase to make ATP.

Many Herbicides Inhibit the Light Reactions of Photosynthesis... 

The proteins that participate in the light reactions of photosynthesis are located in the thylakoid membranes of chloroplasts. The light reactions result in (1) the creation of reducing power for the production of NADPH, (2) the generation of a transmembrane proton gradient for the formation of ATP, and (3) the production of O2. 

Figure 20.14. Light Regulation of the Calvin Cycle. The light reactions of photosynthesis transfer electrons out of the... 

Figure 9 A model of the cyanobacterial carbon concentrating mechanism from Synechococcus PCC 7942, a freshwater species. Solid circles represent transporters located on the plasma membrane and interior to the cell wall. Boxes represent the catalyzing enzymes CA, Carbonic Anhydrase RUBISCO, Ribulose 1-5 bisphosphate Carboxylase Oxygenase. The carboxysome is the site of carbon fixation (dark reactions) and the thylakoid is the site of the light reactions of photosynthesis (after Badger et aL, 2002).

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