Enzyme activity is the rate of enzyme reaction, generally expressed as units of substrate converted (or product formed) per unit time.The activity can be monitored by measuring changes in concentration of substrates or products during the reaction.The enzyme catalase in liver cells catalyses the breakdown of hydrogen peroxide into oxygen and water.The progress of the reaction can be monitored by measuring the volume of oxygen produced.The rate of reaction at a particular time can be determined by calculating the gradient of the tangent at that particular time.In the following curve, the rate is represented by the volume of oxygen produced per minute in cm3/min.