Final answer:
When the temperature of a system is decreased to 0° Celsius, enzyme activity decreased due to enzymes becoming mostly inactive at low temperatures. This drop in activity results from the loss of the enzyme's three-dimensional structure essential for its function. Therefore, the correct answer is (b) Decreased.
Step-by-step explanation:
The question pertains to the effect of temperature changes on enzyme activity. When the temperature of the second test tube was decreased to 0° Celsius, enzyme activity decreased. This is based on the general principle that enzymes, which catalyze biochemical reactions in living organisms, perform optimally at certain temperatures. Human enzymes, for instance, have their optimum temperatures between 37° C and 40° C. Below these temperatures, enzyme activity generally diminishes. It is known that at temperatures around 0° C to 4° C, enzymes are mostly inactive. The cold temperature can cause enzymes to lose their three-dimensional structure and, consequently, their functionality.
Moreover, enzymes become inactive at very high temperatures because the heat causes them to denature, losing their specific structure that is crucial for their catalytic function. During enzyme-catalyzed reactions, the reaction rate increases with the increase in substrate concentration, but this rate eventually levels off. This is different from non-enzyme-catalyzed reactions where reaction rates continue to increase with reactant concentration.