A prokaryotic cell is being looked at under the microscope and a scientist is watching molecules such as sodium enter and exit the cell. What part of the prokaryotic cell is under the microscope?

Cell membrane

Explanation:

In the cell membrane, there is the process of transport of substances, with energy expenditure, through the mediation by specific carrier proteins.

This usually occurs from the lowest to the highest concentration medium in the cell (against the concentration gradient). This process is called active transport.

Some examples of substances that can be actively transported across the membrane are: Iron, potassium, iron, hydrogen, calcium and sodium ions (as described in the question), as well as some types of amino acids and sugars.