 Substances can move in and out of living cells by the process of osmosis. Osmosis is the diffusion of a substance through a semi-permeable membrane. When a substance moves through a membrane by osmosis, it moves from a more dilute solution on one side of the membrane to a more concentrated solution on the other side of the membrane. This type of movement is passive because external energy is not needed to make it happen. Osmosis occurs because the molecules of any given substance seek to distribute themselves equally on both sides of the membrane barrier. In this beaker, you can see a type of green algae called nitella floating in water. When we move the algae from the beaker of water into a beaker that contains a solution of sugar dissolved in water, we trigger osmosis. The osmotic potential of the sugar solution is more negative than the osmotic potential in the cells, so water moves out of the cells into the sugar solution. This causes the turgor pressure of the nitella cells to drop and the algae becomes limp. When we move the nitella back into the beaker with just water, the osmotic potential in the cells is more negative than the water because there is a greater solute concentration inside the cells than in the water. This causes the water molecules to move by osmosis through the nitella cell membranes into the cells and the turgor pressure increases. The root cells of plants also take up water by osmosis. The membranes of the outer layers of root cells are like a semi-permeable barrier that allows water molecules to pass through. Usually, the concentration of solutes inside the root cells is higher than on the outside, so water moves into the cells. Most plant roots have root hairs to increase the surface area of root cell membrane and maximize water and nutrient uptake. Root cells have transport proteins in their membranes that help them to take up water and nutrients from soil. Osmosis also enables the creation of pressure. If you fill a beaker with water, separate the beaker with a suitable semi-permeable barrier and dissolve salt into one of the compartments. The water level in the compartment with salt rises. This happens because osmotic pressure is greater than the pressure exerted on the surface of the water by the atmosphere. If you seal the container to prevent the water level from rising, osmotic pressure causes the membrane to swell in the direction of the side containing pure water. Osmosis is important for regulating the internal environment of a cell or organism because it can be used by the organism to control water and solute movement. In all living organisms, the movement of many key nutrients and water into and out of or between cells and tissues and the regulation of pressure can occur because of osmosis.