In which type of reaction are complex molecules broken down to release energy?

The correct choice is Catabolic reactions, which are characterized by the breakdown of complex molecules into simpler ones, leading to the release of energy. This process is crucial for living organisms, as it allows for the extraction of energy stored in chemical bonds, which can then be used for various biological functions, such as growth, movement, and maintaining homeostasis.

In catabolic reactions, the degradation of substances, such as carbohydrates, proteins, and fats, involves metabolic pathways that facilitate the conversion of these macromolecules into smaller, energy-rich molecules like ATP (adenosine triphosphate). This ability to harness energy is foundational to cellular metabolism.

In contrast, anabolic reactions involve the synthesis of complex molecules from simpler ones and typically require an input of energy. Exothermic reactions and endothermic reactions classify types of heat exchange during chemical reactions rather than focusing specifically on the breakdown or synthesis of molecules. Exothermic reactions release heat, indicating a release of energy but may not necessarily involve the breakdown of complex molecules. Endothermic reactions absorb heat and require energy input, making them associated with processes that consume energy rather than releasing it.