Passage: Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required. The most widely accepted model for enzyme action is the 'induced fit' model. This suggests that the enzyme's active site is not a rigid shape, but rather a flexible structure that slightly changes its conformation to tightly bind the substrate. This is often compared to a glove stretching to fit a hand. Once the reaction is complete and the product is released, the enzyme returns to its original shape, ready to catalyze another reaction. This flexibility allows enzymes to be highly specific yet efficient, ensuring that only the correct substrates are processed in the complex environment of a living cell.