ubstances that must be able to enter a cell are water, oxygen, and
nutrients. On TV medical dramas, you'll often see technicians
administering a saline drip to a patient who has lost blood or is
dehydrated. This saline solution maintains a specific concentration of
dissolved substances in the blood and body fluids. When dehydrated, our
concentration of dissolved substances increases, upsetting the balance
in our cells. Water can freely move in and out of cells to maintain the
same water pressure on both sides of the plasma membrane. Oxygen is
important because cells are undergoing cellular respiration. A cell
takes chemical bond energy and converts it to a form of energy that it
can use--a molecule of ATP. ATP contains small amounts of energy
appropriate to powering cellular processes. This process of energy
conversion requires oxygen (we will discuss this in more detail in
Lesson 4). For aerobic cellular respiration to occur inside this cell,
oxygen must move through the plasma membrane. Thats the answer
