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Osmosis Plan An introduction to osmosis. The movement of water molecules from a high concentration of water molecules to a lower concentration of water molecules through a partially permeable membrane is called Osmosis. Under normal conditions, the concentration of water molecules and salts is the same in both cells and plasma. After sweating, water and salts are lost from the plasma and there is a lower concentration of water in the plasma than inside the cells. Water molecules therefore diffuse out from the cells through the cell membrane. We call a membrane like this partially permeable. Water molecules are small, so they pass through the membrane much more easily than salts. Diagram: There are several different types of cells in xylem but the one that transports most water and minerals to the leaves are called xylem vessels. These are formed from dead xylem cells whose end walls have broken down. These form continuous tubes. The xylem vessels act like pipes in transporting water up the plant. Wood is made up of xylem cells; most xylem cells have substances in their walls which strengthen the cells. This is why wood is a strong material. Osmosis in plants The root hairs together increase the surface area of th