G3P is an intermediate in glycolysis and both an intermediate and a product in the Calvin cycle. In both processes, it is used in substrate-level reactions that propel the system forward, but in the Calvin cycle, it is also a desired end product. The sugar can be used as food for the plant as-is, or it can be rearranged into other sugars and starches for use elsewhere.
Work Step by Step
Figure 9.9 shows glycolysis and figure 10.19 shows the Calvin cycle. Between steps five and six of glycolysis you can see G3P's use as an intermediate. In the Calvin cycle, however, there are two directions G3P can go: some is separated as a product (one for every three CO$_2$ that enter the cycle), and some is used as an intermediate for the generation of RuBP.