Biology Unit 1.4 Water Potential

• All cells are surrounded by a cell membrane which is partially permeable
• Water can move in and out of cells by osmosis
• The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)
• If a cell is placed in a solution with a lower solute concentration than the cytoplasm of the cell, then there will be a net movement of water into the cell by osmosis
• If the solution outside the cell has a higher solute concentration  than the cytoplasm of the cell, then there will be a net movement of water out of the cell, these solutions are said to be hypertonic
• If the solute concentration is the same on both sides of the cell membrane, there will be no net movement of water into or out of the cell by osmosis
• The direction of the net movement of water will depend on whether a cell is placed in a hypertonic or hypotonic solution
• In a hypertonic solution there will be a net movement of water out of the cell, as the cytoplasm is more dilute than the outside solution
• In a hypotonic solution there will be a net movement of water into the cell because now the outside solution is more dilute than the cytoplasm
• In an isotonic solution, the movement of water into the cell will be balanced out by the movement of water out of the cell
• Animal cells lose and gain water as a result of osmosis
• If an animal cell is placed into a hypertonic solution , it will lose water by osmosis and become crenated 
• If an animal cell is placed into a hypotonic solution, it will gain water by osmosis and, as it has no cell wall to create turgor pressure, and will continue to do so until the cell membrane is stretched too far and it bursts
• Multicellular organisms must therefore maintain isotonic tissue fluid around their cells to prevent these harmful changes from happening
• If a plant cell is placed in a hypotonic solution, water will enter the plant cell through its partially permeable cell surface membrane by osmosis, as the solution has a lower solute concentration than the plant cell
• As water enters the vacuole of the plant cell, the volume of the plant cell increases
• The pressure created by the cell wall also stops too much water entering and this also helps to prevent the cell from bursting
• When a plant cell is fully inflated with water and has become rigid and firm, it is described as fully turgid
• This turgidity is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant 
• If a plant cell is placed in a more concentrated solution, water will leave the plant cell through its partially permeable cell surface membrane by osmosis
• As water leaves the vacuole of the plant cell, the volume of the plant cell decreases
• A plant cell that loses enough water becomes plasmolysed - the plasma membrane pulls away from the cell wall