Final answer:
Cleaning sample tubes in spectrophotometric experiments is crucial for accurate turbidity measurements of bacterial cultures by eliminating residue that could affect readings, and using a control tube with just broth provides a baseline for comparison.
Step-by-step explanation:
It is important to clean the sample tubes when using a spectrophotometer because any residue or contamination can affect the accuracy of the turbidity measurements of bacterial cultures. This ensures that the light transmission readings taken from the sample tube reflect only the turbidity due to the bacteria in the broth, and not any other factors. Using a control tube with only broth helps in calibrating the spectrophotometer and provides a baseline to compare the turbidity of the actual samples against.