Photosynthesis involves two pathways

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Equation 10.2 above summarizes the overall process of photosynthesis, but not the steps by which it happens. Water serves as the electron donor, but there is an intermediary carrier of the H+ and electrons between the oxidation and reduction reactions. The carrier is the coenzyme nicotinamide adenine dinucleotide phosphate (NADP+).

Like glycolysis and the other metabolic pathways that harvest energy in cells, photosynthesis has many steps. These reactions are commonly divided into two main pathways:

  1. The light reactions convert light energy into chemical energy in the form of ATP and the reduced electron carrier NADPH. This molecule is similar to the coenzyme NADH (see Key Concept 9.1) but with an additional phosphate group attached to the sugar of its adenosine. In general, NADPH acts as a reducing agent in photosynthesis and other anabolic reactions.

  2. The light-independent reactions (carbon-fixation reactions) do not use light directly, but instead use ATP, NADPH (made by the light reactions), and CO2 to produce carbohydrate.

The light-independent reactions are sometimes called the dark reactions because they do not directly require light energy. They are also called the carbon-fixation reactions because by these reactions inorganic carbon is assimilated, or “fixed,” into organic compounds. However, in most plants both the light reactions and the light-independent reactions stop in the dark because ATP synthesis and NADP+ reduction require light. So to be accurate, we use the term light-independent reactions. The reactions of both pathways proceed within the chloroplast, but they are compartmentalized, occurring in different parts of that organelle (Focus: Key Figure 10.2).

focus: key figure

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Figure 10.2 An Overview of Photosynthesis Photosynthesis consists of two pathways: the light reactions and the light-independent reactions. These reactions take place in the thylakoids and the stroma of chloroplasts, respectively. See Figure 5.12 for more on the structure of chloroplasts.

Question

Q: In the cell, where does the reduction of CO2 occur and what is the reducing agent?

Reduction occurs in the stroma of the chloroplast and the reducing agent is NADPH.

As we describe these two series of reactions in more detail, you will see that they conform to the principles of biochemistry that we discussed in Chapters 8 and 9: energy transformations, oxidation–reduction, and the stepwise nature of biochemical pathways.