Which law of thermodynamics states that energy cannot be created or destroyed?

The First Law of Thermodynamics establishes that energy cannot be created or destroyed; it can only be transformed from one form to another. This principle underlies many biological processes, as organisms convert energy from food into usable forms for growth, maintenance, and reproduction. For instance, when a plant performs photosynthesis, it transforms light energy from the sun into chemical energy stored in glucose.

This law is foundational in science because it implies that the total energy in a closed system is constant, which is crucial for understanding energy transfer in biological systems and the environment. By recognizing that energy changes forms rather than being lost or gained, we can better understand metabolic processes and energy conservation in ecosystems.

The other laws of thermodynamics focus on different aspects of energy and entropy, such as the tendency of systems to become more disordered over time or the absolute zero point of temperature. However, it's the First Law that explicitly addresses the constancy of energy in any process.