Plant cells behave differently from animal cells in osmosis because of their structure:
• a strong, fully permeable cellulose cell wall, which resists expansion and stops the cell bursting;
• a cell surface membrane, which is partially permeable;
• a large, permanent central vacuole that holds most of the cell's water;
• the vacuole is surrounded by its own membrane, the tonoplast, and contains cell sap, a solution of sugars, ions and other solutes.The solutes in the cell sap and cytoplasm give the cell a negative water potential. As water enters, the cell contents push outwards against the wall, and the wall pushes back. This pressure raises the cell's water potential. So the water potential of a plant cell depends on both its solute concentration and the pressure from the wall (these two parts are covered separately in the next lessons).Water moves between neighbouring plant cells by osmosis, from a cell with a higher water potential to one with a lower water potential. In a tissue, water flows down a water potential gradient.Worked example: three cells in a row have water potentials A = −300 kPa, B = −500 kPa, C = −700 kPa. Water moves from A to B and from B to C, because each step is to a lower (more negative) water potential.This is how water crosses a root. Soil water usually has a high water potential, close to 0 kPa. A root hair cell must have a water potential lower than the soil water for water to enter it, and the cells further in have lower water potentials still.
• a strong, fully permeable cellulose cell wall, which resists expansion and stops the cell bursting;
• a cell surface membrane, which is partially permeable;
• a large, permanent central vacuole that holds most of the cell's water;
• the vacuole is surrounded by its own membrane, the tonoplast, and contains cell sap, a solution of sugars, ions and other solutes.The solutes in the cell sap and cytoplasm give the cell a negative water potential. As water enters, the cell contents push outwards against the wall, and the wall pushes back. This pressure raises the cell's water potential. So the water potential of a plant cell depends on both its solute concentration and the pressure from the wall (these two parts are covered separately in the next lessons).Water moves between neighbouring plant cells by osmosis, from a cell with a higher water potential to one with a lower water potential. In a tissue, water flows down a water potential gradient.Worked example: three cells in a row have water potentials A = −300 kPa, B = −500 kPa, C = −700 kPa. Water moves from A to B and from B to C, because each step is to a lower (more negative) water potential.This is how water crosses a root. Soil water usually has a high water potential, close to 0 kPa. A root hair cell must have a water potential lower than the soil water for water to enter it, and the cells further in have lower water potentials still.