Uptake and transport of water and ions

Plants absorb water from the soil primarily through their root hair cells. These are specialised cells, which are long, thin extensions of root epidermal cells. This structure provides a very large surface area, maximising the plant's ability to absorb water and mineral ions. Water enters the root hair cells from the soil by osmosis. This occurs because the cell sap inside the root hair has a lower (more negative) water potential than the water in the soil. Soil water is a relatively dilute solution, while the cell sap contains dissolved sugars and salts, making it more concentrated. This difference in water potential creates a gradient, causing water molecules to move from the soil (high water potential) into the root hair cell (low water potential) across the partially permeable cell membrane.

While water is absorbed by osmosis, mineral ions (like nitrates, phosphates, and potassium) are absorbed by a different process: active transport. This is because the concentration of these essential ions is often much higher inside the root hair cells than in the surrounding soil water. To move these ions into the root, the plant must transport them against their concentration gradient. This process requires energy, which is supplied by respiration in the mitochondria of the root cells. Specialised carrier proteins in the cell membrane bind to the mineral ions and use energy (in the form of ATP) to transport them into the cell.

Once water enters the root hair cell, it begins its journey up to the leaves. First, it moves across the root cortex, the layer of cells between the root hairs and the central vascular tissue. Water travels from cell to cell across the cortex, primarily by osmosis, following the water potential gradient. Upon reaching the centre of the root, water enters the xylem vessels. The xylem is the plant's main water-conducting tissue. It forms a continuous network of hollow tubes from the roots, up the stem, and into the leaves. This continuous column of water, pulled up by forces from the leaves, is known as the transpiration stream.

The transpiration stream is the movement of water up the xylem from the roots to the leaves. This process is not 'pumped' but 'pulled' from above. As water evaporates from the surfaces of mesophyll cells in the leaves (transpiration), it creates a tension or suction force. This pull is transmitted all the way down the xylem to the roots. This is possible due to two properties of water: cohesion (water molecules are attracted to each other) and adhesion (water molecules are attracted to the xylem walls). Cohesion holds the water molecules together in a continuous, unbroken column, while adhesion helps the water column 'stick' to the sides of the xylem, preventing it from falling back down. Xylem vessels are perfectly adapted for this, being dead, hollow tubes with no cross-walls to impede flow and strengthened with a waterproof polymer called lignin.