Osmosis

Osmosis is the movement of water molecules accross a semi-permeable membrane from a region of highwater concentration to a region of low water concentration.

A solvent is a liquid the dissolved other substance. For example, water is the most common biological solvent. A solute is a substance that has been dissolved. For example, Salt or Sugar dissolved in water are both solutes. A solution is a mixture of the solvent and solute in a solution. For example, Salty Water.

Theory of OsmosisIf pure water is separated from salty water by a semi-permeable membrane, the following will occur:1. Water molecules on both sides of the membrane will move randomly. 2. Some water will cross the membrane in each direction. 3. Because there are more water moelcules in pure water, more molecules will pass from this side to the other side. 4. Solute molecules (salt) cannot cross the membrane. 5. The result is a movement of water from a high water concentration to a low water concentration (i.e. diffusion) accross a semi-permeable membrane.

Osmosis and Animal Cells Animal cells are enclosed by a cell membrane. If an animal cell is in solution that is the same concentration as its cytoplasm, water will move in and out through its cell membrane at the same rate. The volume of the cell will remain the same. Seawater has the same concentration as the cytoplasm of many animals that live in the sear. Also, the cells of most land animals are surrounded by tissue fluid that has the same concentration as the cells. In the same wat, one of the functions of the kidneys is to ensure the plasma, the liquid protion of our blood, as the same concentration as our blood cells. If an animal cell is in a solution that is less concentrated than the cell, it will gain water due to osmosis. The cell will enlarge, may burst and die.

AmoebaAmoeba is a single-celled organism that lives in freshwater pools and ponds. The fresh water is less concentrated than the cell contents of amoeba. This means that amoeba must have a method of controlling osmosis or it will enlarge and burst. The osmoregulation system in amoeba consists of a contractile vacuole. The contractile vacuole works as follows:1. Water enters Amoeba due to osmosis2. Excess water enters the contractile vacuole. 3. The contractile vacuole expands. 4. The contractile vacuole touches the plasma membrane. 5. The contractile vacuole bursts to expel the water. 6. The process is repeated.

Animal Cells in a more concentrated solution If an animal cell is in a solution that is more concentrated than the cell, it will lose water due to osmosis. This will cause the cell to shrivel and possible die. This sometimes happesn to aquatic animals if their environment becomes too salty. It also can happen to red blood cells if they are in a very salty solution.

Osmosis and Plant CellsPlant cells are enclosed by a cell membrane, which in turn is surrounded by a strong cell wall. Cell walls are fully permeable to water, gases and many solutes. The cell wall gives a certain amount of strength to a plant cell.

Plant Cells in a less concentrated solution If a plant cell has a higher concentration of solutes than its surroundings, water moves into the cytoplasm and vacuole of the cell by osmosis. This is how the plant roots abosrob water from the soil. The water enters the vacuole and the plant cell swells slightly, but the cell wall is relatively rigid and prevents the cell from swelling too much. Turgor Pressure is the pressure in the cytoplasm and vacuole against the cell wall of a plant. Turgor pressure gives a plant cell great strength. In this state, the cell is said to be turgid.Plants that do not have wood, such as house plants or lettuce, they get their strength because their cells are fully rigid.

Plant cells is a more concentrated solution If the cytoplasm of a plant cell has a lower concentration of solutes than the surrounding, then the cell is less concentrated than its surroundings. Equally, the surroundings are more concentrated than the cell contents. In this case, water moves out of the cell due to osmosis. The vacuoloe and cytoplasm shrivel and the cell membrane moves away from the cell wall. The less concentrated solution fills the space between the cell membrane and the cell wall. The cell loses turgor pressure. As a result, it is not as strong as it was and it is said to be flaccid (limp). The loss of water from the cytoplasm and the movement of the cell membrane away from the cell wall is called plasmolysis. When all the cells of a plant are plasmolysed the plant is said to wilt. Plasmolysed cells can be restored to normal by placing them in a less concentrated solution. This is called deplasmolysis.

Osmosis in Amoeba

Osmosis in Plant Cell

Osmosis in Red Blood Cell

Osmosis