4.10: Photosynthesis in detail: the captured energy of sunlight is used to make food.

The “synthesis” part of photosynthesis takes place in a series of chemical reactions called the Calvin cycle. All the Calvin cycle reactions occur in the stroma of the leaves’ chloroplasts, outside the thylakoids. Plants carry out these reactions using the energy stored in the ATP and NADPH molecules that are built in the “photo” portion of photosynthesis. This dependency links the light-gathering (“photo”) reactions with the sugar-building (“synthesis”) reactions (FIGURE 4-21).

Figure 4.21: Overview of the “synthesis” reactions of photosynthesis.

If there is any part of photosynthesis that appears magical, it is the Calvin cycle. Just as a magician seems to make a rabbit appear from thin air, the Calvin cycle takes invisible molecules of CO2 from the air and uses them to assemble visible—even edible—molecules of sugar. The processes in the Calvin cycle occur in three steps (FIGURE 4-22).

Figure 4.22: Sugar synthesis. In the Calvin cycle—the “synthesis” portion of photosynthesis—sugars are produced.

Fixation. First, using an enzyme called rubisco, plants pluck carbon from the air, where it occurs in the form of carbon dioxide (which has one carbon), and then attach, or “fix,” it to a visible organic molecule (which has five carbon atoms) within the chloroplast. Not surprisingly, given its role as the critical chemical that enables plants to build food molecules, rubisco is the most abundant protein on earth.

1. Sugar creation. The newly built molecule is chemically modified: a phosphate from ATP is added, and the molecule receives some high-energy electrons from NADPH and is split in two. For every three carbon dioxide molecules added to the Calvin cycle, one sugar product is produced. This product, a three-carbon sugar, is called glyceraldehyde 3-phosphate (G3P).

Some of the G3P molecules are combined to make the six-carbon sugars glucose and fructose. These sugars can be used as fuel by the plant, enabling it to grow. They can also be used as fuel by animals that eat the plant.

2. Regeneration. Not all of the G3P molecules are used to produce sugars. In the third and final phase of the Calvin cycle, some G3P molecules are rearranged to regenerate the original five-carbon molecule in the chloroplast to which the carbon from CO2 is attached. Rearranging G3P to regenerate the starting molecule requires energy from ATP produced in the “photo” reactions of photosynthesis. With this regeneration, the Calvin cycle can continue to fix carbon and produce molecules of G3P.

153

Ultimately, to synthesize one molecule of G3P, the Calvin cycle requires three “turns” of the Calvin cycle and fixation of three atoms of carbon from carbon dioxide to the initial organic molecule; this process consumes nine molecules of ATP and six molecules of NADPH generated in the “photo” reactions of photosynthesis.

TAKE-HOME MESSAGE 4.10

The second part, or “synthesis” part, of photosynthesis is the Calvin cycle, which occurs in the stroma of chloroplasts. During this phase, carbon from CO2 in the atmosphere is attached (fixed) to molecules in the chloroplasts, sugars are built, and molecules are regenerated to be used again in the Calvin cycle. The fixation, building, and regeneration processes consume energy from ATP and NADPH (the products of the “photo” part of photosynthesis).

Describe the three steps in the Calvin cycle.