Define an enzyme.

An enzyme is defined as a protein that accelerates biochemical reactions. Enzymes act as catalysts, which means they speed up the rate of chemical reactions without being consumed in the process. They lower the activation energy required for a reaction to occur, enabling the reaction to proceed more quickly than it would without the enzyme.

Enzymes are highly specific in their function, typically catalyzing only one type of reaction or acting on a specific substrate. They have an active site that allows substrates to bind, facilitating the conversion of reactants into products. This catalytic action is essential for numerous biological processes, including digestion, metabolism, and DNA replication, making enzymes crucial for life.

In contrast, the other options describe different biological molecules: lipids serve primarily for energy storage and structural functions, carbohydrates provide support in cell structures like plant cell walls, and hemoglobin is an example of a molecule that transports oxygen but is not classified as an enzyme. Thus, the definition of an enzyme as a protein that accelerates biochemical reactions is accurate and reflects its fundamental role in biological systems.