Final answer:
In tube 3 with amylase and cellulose at pH 7.0, no reaction is expected because amylase can't hydrolyze cellulose, and no change should be observed in the buffer.
Step-by-step explanation:
In tube 3 containing amylase, cellulose, and a pH 7.0 buffer, no significant reaction is expected. Amylase catalyzes the breakdown of starch, not cellulose, and its optimal pH is around 6.9, which is close to the pH provided. However, since cellulose's structure differs from starch, amylase will not be able to hydrolyze it, resulting in no color change in the tube, no gas production, or any observable digestion.
For a positive control, typically, the urease test is mentioned where ammonia raises the pH, turning the solution pink. This indicates a urease-positive reaction, but for amylase and cellulose, we expect no change. Moreover, if the pH was significantly different from the optimal (either too high or too low), it could affect amylase activity further, but since pH 7.0 is near optimal, the lack of reaction is primarily due to substrate incompatibility.