Certain reactions in the Monod Cell consume or produce energy. Unlike in a biological cell, where energy is carried by dedicated molecules, the energy in the Monod Cell is maintained by as a simple tally for each component. When a reaction consumes energy, the tally is lowered, while when it is produced, the tally is increased.
To produce energy, a cell must successfully metabolize absorbed substrates, as will be shown later. Through the operation of proteins, ligands are gradually broken down. When tagged with a particular indicator by proteins, the sequences are recognized by the containing compartment's basic functions and categorized as either poisonous or energetic. A poisonous base sequence lowers the energy tally proportionally to its length, while an energetic one increases the tally.
Energy-consuming reactions can not take place within a cell with no energy left. Furthermore, such a cell will eventually die. Note that the energy of a cell is the sum of the energy of each compartment within the cell.
Energy within a biological cell serves to power certain reactions. In a Monod Cell, power is not an issue, of course. Also in a biological cell, energy is also used to give direction to certain reactions by making them irreversible. While irreversibility may be built into proteins without recourse to energy, a limited supply of energy helps drive the evolution of Monod Cell genomes away from wasteful cells which include lots of reversible operations - this will be explored in the next Monod book.