In which type of reaction does energy input lead to the synthesis of larger molecules?

The correct choice is the one that identifies anabolic reactions as those that require energy input to combine smaller molecules into larger, more complex ones. Anabolic reactions are essential in the growth, repair, and maintenance of cells and tissues in living organisms. This process often involves the synthesis of macromolecules like proteins, nucleic acids, and polysaccharides from their building blocks, such as amino acids, nucleotides, and monosaccharides, respectively.

Energy is utilized in these reactions, typically in the form of ATP (adenosine triphosphate) or other high-energy molecules, to drive the formation of chemical bonds, thus increasing the molecular complexity. Through pathways like photosynthesis in plants or protein synthesis in cells, anabolic processes build up the necessary components of life, demonstrating how energy input is converted into stored chemical energy.

In contrast, catabolic reactions break down larger molecules into smaller units, releasing energy, while exergonic reactions are those that occur spontaneously and also release energy. Spontaneous reactions do not require energy input, highlighting the distinct nature of anabolic processes in requiring energy for the synthesis of larger molecules.