Final answer:
In cells, enzymes are the macromolecules that specifically catalyze reactions by lowering activation energy, thus determining which cellular reactions occur and their rates. Enzymes are highly specific, enabling stereoselective reactions and adaptation to changing environmental conditions through evolution.
Step-by-step explanation:
Any molecule can react in several ways, but enzymes in the cell cause specific transformations to be favoured. Enzymes are macromolecules, most often proteins, that act as catalysts, accelerating chemical reactions by lowering activation energy barriers. They are highly specific for the reactions they catalyze, attributed to the unique three-dimensional shapes of their active sites. When a substrate binds to the enzyme's active site, the enzyme changes shape slightly to accommodate the substrate, a process known as the induced fit model of interaction.
Such specificity ensures that each chemical reaction within a cell occurs efficiently and under controlled conditions. Factors like pH, temperature, salt concentration, and the presence of cofactors or coenzymes partly control enzyme activity. The importance of enzymes is also evident in anabolic reactions, where they utilize ATP to build larger molecules from smaller ones, and in oxidation-reduction reactions, which are vital for the production of ATP. Enzymes can also facilitate stereoselective reactions, which produce one stereoisomer preferentially, playing a crucial role in the correct functioning of biological systems.
Evolution has shaped these biochemical pathways. When environmental conditions change, natural selection can favor cells that have evolved novel enzyme activities that allow them to survive under the new conditions, such as synthesizing a vital molecule that is scarce in the environment. This dynamic interplay between enzymes and the cell's internal and external environments underlies the complexity and specificity of cellular reactions.