Advanced Placement (AP) Biology Practice Exam

Enzyme activity in biological reactions is primarily defined by the speed at which an enzyme catalyzes a reaction. This reflects how effectively an enzyme converts substrates into products within a specific timeframe, which is a fundamental aspect of why enzymes are important in biological systems. Catalytic efficiency can be influenced by various factors such as enzyme concentration, substrate concentration, temperature, and pH, but the essence of activity is focused on the rate of reaction that the enzyme can achieve.

For instance, the turnover number, which is a measure of an enzyme's efficiency, indicates how many substrate molecules one enzyme molecule can convert per unit of time. Therefore, option B accurately captures the core concept of enzyme activity by emphasizing the importance of reaction speed in the overall biochemical process. This understanding is crucial for studying metabolic pathways and the regulation of enzyme functions in living organisms.