What is a substance required by a cell that the cell synthesizes and therefore cannot obtain from the environment?

The correct answer is the metabolite. A metabolite is a substance produced within living cells during metabolic processes, and it plays a crucial role in cellular functions. These compounds are essential for various biochemical reactions and can include intermediates in metabolic pathways, such as amino acids, nucleotides, and lipids, which cells must produce themselves because they cannot acquire them directly from their external environment.

In contrast, vitamins, hormones, and enzymes have different natures concerning cellular synthesis and acquisition. Vitamins are organic compounds that organisms typically acquire from their diets because they cannot synthesize them in sufficient quantities. Hormones are biochemical messengers produced by glands and released into circulation to regulate physiological processes, often synthesized in response to specific stimuli rather than being a constant requirement. Enzymes are proteins that act as catalysts to accelerate biochemical reactions; while they are synthesized by the cell, it's the substrates that they act upon that are often obtained from the environment. Therefore, the best term to describe a substance that a cell requires and synthesizes internally, without relying on external sources, is a metabolite.