Final answer:
The factor that should be kept constant during the biological investigation, apart from temperature and pH, is the type of algal cells used. This ensures that the results are not influenced by variations in the biological material being studied.
Step-by-step explanation:
The factor that should be kept constant during the investigation apart from temperature and pH is the type of algal cells used. This is because different types of algal cells may respond differently to the experimental conditions, thus affecting the results of the study. Therefore, to ensure accuracy and reliability in the investigation, the same type of algal cells must be consistently used throughout.
When conducting scientific experiments, it's imperative to control variables to obtain trustworthy results. Controlled variables are the elements of the experiment that are kept constant to prevent them from influencing the outcome. This allows for a clear understanding of the effects of the experimental variables being tested.
For instance, in an investigation focused on enzyme activity like AP Biology Investigation 13, besides the main factors being studied like temperature and pH, it is crucial to control other potential influences such as the amount of water, the amount of plant food, and the type of soil used. Failing to control these could lead to individual plant variation, changes in soil temperature due to different colors of containers, water loss due to evaporation from the soil, or the effect of insects, all of which could skew the results.