In an experiment, you measure the concentration of a polar molecule inside and outside a cell. You find that the concentration is high and gradually increasing inside the cell. You also measure the ATP concentration inside the cell and find that it is dropping. What is your best hypothesis for the process you are observing?

a) facilitated diffusion
b) passive transport
c) simple diffusion
d) active transport
e) endocytosis

The correct answer is: d).

Explanation:

Active transport can be defined as the process by which molecules are transported from a region where the concentration of the molecules is low to a region where the concentration of molecule is high across a selectively permeable membrane.Here, the molecules are transported up or opposite to their concentration gradient and so the process requires the expenditure of energy in the form of ATP or Adenosine Triphosphate.In the given question, it is said that the concentration of a polar molecule is high inside the cell initially. But still with time the concentration of the polar molecule continues to increase inside the cell. This is accompanied by the expenditure of energy in the form of ATP as concentration of ATP inside the cell is found to drop gradually.Hence, the given hypothesis describes the process of an Active Transport.