Final answer:
Independent variables are controlled by the experimenter to influence the measured outcome, known as the dependent variable. In a bacteria growth experiment, the type of gel is the independent variable, while the number of bacteria after 24 hours is the dependent variable.
Step-by-step explanation:
The main difference between independent and dependent variables is that the independent variable is manipulated or controlled by the experimenter to observe its effect on the dependent variable, which is the outcome that is measured. In an experimental study, the independent variable is what is altered or the condition that is changed, and it's the 'driver' of the experiment. The dependent variable, on the other hand, is the 'receiving' variable that responds to the change created by the independent variable.
In the given scenario where a scientist is investigating if bacteria grow faster on gel A or gel B, the independent variable is the type of gel used (gel A or gel B) since it is the condition that the scientist changes. The dependent variable is the number of bacteria present after 24 hours because that is the result being measured and is expected to be influenced by which type of gel is used.