Why do pistils and stamens mature at different times?

Pistils and stamens mature at different times because this would ensure cross pollination and more variation would be introduced in the offspring. Pistils and stamens also mature at different times for them to avoid the thing called self pollination. The pistil and stamen  are actually the reproductive part of a  male and a female for the plants.

Active transport refers to a procedure, which withdraws substances from a cell against the concentration gradient. During the process, the molecules move from low concentration to a high concentration.  

The mechanisms of active transport need the application of the energy of the cell, generally in the form of ATP. If a substance needs to move within the cell against its concentration gradient, that is, if the substance's concentration within the cell is more than its concentration in the extracellular fluid, then the cell must utilize energy to move the substance.