The top of the figure above shows the carbon fixation phase of the Calvin cycle. The five-carbon compound, ribulose bisphosphate (RuBP), is joined with one molecule of carbon dioxide to produce a highly unstable 6-carbon compound. The six-carbon compound that results immediately splits into two molecules, each with three carbons. The enzyme rubisco (which is one of the most abundant proteins on Earth), catalyzes this reaction.
The carbon dioxide used in the Calvin cycle can get to the chloroplast in several different ways. In algae it simply diffuses through membranes from the surrounding water. In plants the carbon dioxide comes in through pores (stomata) in the leaves. These same pores are where oxygen, produced in the light reactions, escapes into the atmosphere.
In the first phase of the Calvin cycle, carbon dioxide is fixed into a 6-carbon molecule, which splits into two, 3-carbon molecules. In the second phase (shown in the figure below), the 3-carbon molecules are reduced to glyceraldehyde 3-phosphate (G3P) , a 3-carbon molecule. Remember, reduction means that electrons are added to the molecule. NADPH, produced during the light-dependent reactions, provides the high-energy electrons for this process. ATP is also used in this anabolic reaction. Thus, the Calvin cycle is energetically tied to the light-dependent reactions of photosynthesis.
At the end of the reduction phase, some of the G3P leaves the cycle to become sugar, however, most of it is used to regenerate RuBP. Notice that glucose is not produced by the Calvin cycle; rather it is the sugar G3P. This G3P can then be used by the cell as an immediate energy source (to be metabolized in cellular respiration) or used to build glucose molecules that will be used by the plant to build cell walls, starches, and other polysaccharides.
As seen in the images below, the Krebs cycle and the Calvin cycle have some general similarities. Remember, the Krebs cycle regenerates oxaloacetate at the end of one cycle to begin the next. In much the same way, the Calvin cycle regenerates RuBP to begin the next cycle. For every three carbon dioxide molecules that are fixed, three molecules of RuBP were needed. Thus, at the end of the cycle there must be three molecules of RuBP or the cycle would get out of balance.
The three molecules of RuBP that began the cycle each had a total of five carbons multiplied by three molecules, or 15 atoms of carbon. Three molecules of carbon were then fixed for a surplus of three carbons in the cycle. Those three carbons are expelled from the cycle as one molecule of G3P. The remaining 15 carbons are still in the form of G3P. Therefore, they must be converted back to RuBP to start the process again. More ATP, as well as many steps involving enzymes, are necessary for this regeneration.