What is the name of this movement of nutrients? What is the name of the movement of water through a cell membrane? Briefly describe two other processes that cells use to maintain homeostasis.

Substances 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.