A reversible reaction in a closed system, such as a capped flask, reaches equilibrium and cannot carry out any work in the equilibrium state. In contrast, living cells are open systems that exchange energy and matter with their surroundings. This exchange helps them to avoid an equilibrium state and allows biochemical reactions to proceed to meet the cell's needs. Instead, cells maintain a steady state where concentrations of reactants and products remain relatively constant over time. The controlled amounts of these metabolites at non-equilibrium concentrations are necessary for cell survival and function. A cell maintains non-equilibrium through a continuous supply of reactants and rapid removal of products. The products are either transferred out of the system or act as reactants for another reaction. In a steady state, the cell senses changes in concentrations, and metabolic and signaling pathways react to counteract the change. For example, during fasting, glycogen is broken down into glucose molecules to maintain a regular glucose supply to cells.