Advanced Placement (AP) Biology Practice Exam

The second law of thermodynamics states that in any energy transfer or transformation, the total entropy of an isolated system can never decrease over time. In this context, entropy is a measure of disorder or randomness in a system. As energy transformations occur, they tend to spread energy out more evenly, which leads to an increase in the total entropy of the system.

This principle explains why processes are not 100% efficient; some energy is always lost as unusable heat, contributing to a higher level of disorder in the system. For example, when fuel is burned in an engine, some energy is transformed into useful work, but a significant portion is released as heat, which increases the overall entropy.

This concept has profound implications for understanding biological processes, ecological interactions, and the direction of natural processes, as living systems must constantly import energy to maintain order against the backdrop of the natural tendency toward increased entropy.