Let's start with the plasma membrane, the outermost protective covering of a cell. The plasma membrane of a cell lets certain molecules travel across it but can regulate the movement of some molecules. These membranes that control the travel of certain molecules like sugar, but do not control the movement of other molecules like water, are called selectively permeable membranes. The reason that water travels across a selectively permeable membrane is based on molecules called salutes. What is a solute? A solute is a substance that is dissolved in another substance. For example, when sugar is dissolved in water, it is known as a solute. The amount of solute, in our case sugar, on either side of a membrane will decide how water moves across a membrane. The difference in the amount of a type of a molecule across a membrane is called a concentration gradient. Say that we have more sugar on one side of a membrane than the other. That's a concentration gradient. And that's what we're interested in. The concentration gradient of a solute is what determines whether or not water will travel across a membrane. The measurements of a concentration gradient is called tonicity. There are three main types of tonicity that can occur with a concentration gradient. In an isotonic solution, the concentration of a solute is the same on both sides of a membrane. So there is no movement of water across the membrane. For a red blood cell, an isotonic solution is a state of equilibrium. The red blood cell will appear as a dimpled hockey puck shaped called a biconcave disc. In a hypertonic solution, there is a higher concentration of solutes outside of a cell than inside a cell. Because there is a higher concentration of sugar outside of the cell than inside, the water molecules move from the side of the membrane with the lower concentration of sugar to the side of the cell with the higher concentration of sugar. In the case of a red blood cell, water molecules leaving a cell that is in a hypertonic solution causes the red blood cell to shrivel up like a raisin. With a hypotonic solution, water molecules enter the cell and in the case of a red blood cell cause it to swell up. The process of osmosis doesn't require any energy in the form of ATP to occur and is known as a form of passive transport. Osmosis is important in the human body for the uptake of water that is used for vital cell processes.