Final answer:
ATP (adenosine triphosphate) is the molecule that is most readily used by cells to perform work, as it is the primary currency of energy within the cell, converted from energy in glucose via catabolic reactions.
Step-by-step explanation:
Energy Conversion in Cells
The molecule that cells use as a direct source of energy is adenosine triphosphate (ATP). When energy is needed to perform work within cells, ATP is converted to ADP (adenosine diphosphate) and a phosphate group, releasing energy. ATP is generated through the process of cellular respiration, whereby glucose is broken down. Such catabolic reactions release the molecular energy stored in glucose to produce ATP, which can be used for various cellular functions.
In terms of the question provided, the molecule formed from the conversion of energy in glucose that is most readily used to do work in cells is ATP. Substances like NADPH and FADH2, while important in other aspects of metabolism, specifically in the transport of electrons during cellular respiration and photosynthesis, are not the direct currency for energy within the cell like ATP is.