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Produced by Science & Plants for Schools (SAPS), this activity enables students to investigate the effect of competitive and non-competitive inhibitors on the enzyme beta-galactosidase. The chemical ONPG (o-nitrophenyl beta-D-galactopyranoside) is degraded by the enzyme beta-galactosidase. The product is yellow and so the reaction can be followed using a colorimeter. Enzyme inhibition can be investigated using galactose, which acts as a competitive inhibitor, and iodine solution, which is a non-competitive inhibitor.