Absorption by Roots Absorption by Roots: The Processes Involved

Water is absorbed mainly through root hairs, which are slender outgrowths of the single outer layer of cells (the epiblema) just behind the growing tip. They are thin-walled, lack a cuticle, and press closely against the soil particles, so they offer a huge surface area in contact with the film of water around the soil grains. The cell sap inside a root hair contains dissolved salts and sugars, making it more concentrated than the surrounding dilute soil water. Because the cell membrane is semi-permeable, water passes from the dilute soil solution into the more concentrated sap. The root hair does not 'suck'; it simply provides the right concentration difference for osmosis to do the work.

Osmosis is the movement of water molecules from a region of their higher concentration (dilute or hypotonic solution) to a region of their lower concentration (concentrated or hypertonic solution) through a semi-permeable membrane. Once water enters the root hair, that cell becomes more dilute (less concentrated) than the cortex cell next to it. So water now moves from the root hair into the cortex by osmosis, and from cell to cell across the cortex, until it reaches the xylem. As each cell takes in water it becomes turgid (firm and swollen with water), and turgor pressure helps push water on to the next cell. This step-by-step osmotic flow carries water from the soil right into the conducting xylem of the root.

Plants do not only need water; they need mineral ions such as nitrates, phosphates and potassium. Often these are already more concentrated inside the root than in the soil, so they cannot simply diffuse in. The root absorbs them by active transport: the passage of a substance across a living cell membrane from its lower concentration to its higher concentration, using energy released by the cell during respiration (ATP). Because it needs energy, active transport stops if the root cells are killed or starved of oxygen. This is why waterlogged soil, which lacks air, harms mineral uptake. Active transport lets the plant choose and collect exactly the salts it needs, even against the concentration gradient.

Water and dissolved minerals (together called sap) must travel from the root to the topmost leaves, sometimes many metres up, through the xylem. Three forces help. Root pressure is a push that develops in the root xylem as water keeps entering; it can move sap a short way up the stem. Capillarity is the rise of water in the narrow xylem tubes due to surface forces, but only over small heights. The main force is transpiration pull: as water evaporates from the leaves (transpiration), it pulls the continuous column of water upward. The column does not break because water molecules cling together (cohesion) and to the xylem walls (adhesion) - this is the cohesion-tension theory of Dixon and Joly. Together these forces lift sap to the top of even the tallest tree.