Structural adaptations of root hair cells
The root hair cell has a long, thin hair-like extension that projects into the soil between soil particles. This extension gives the cell a very large surface area relative to its volume, increasing the rate of absorption.
Root hair cells also contain many mitochondria to supply energy from respiration for active transport of mineral ions. The thin cell wall and partially permeable cell membrane allow water to enter freely by osmosis. The large central vacuole contains cell sap with a relatively low water potential, maintaining the osmotic gradient.
Water uptake by osmosis
Soil water is a dilute solution with a higher water potential than the root hair cell sap. Water moves into the root hair cell by osmosis through the partially permeable cell membrane, from higher water potential outside to lower water potential inside.
Once inside the root hair cell, water continues to move across the root cortex from cell to cell by osmosis, always toward the next cell with a lower water potential, until it reaches the xylem in the centre of the root.
Mineral ion uptake by active transport
Mineral ions such as nitrate, magnesium and potassium are often at a lower concentration in the soil than inside the root hair cell. Diffusion cannot move them against this concentration gradient. Instead, the cell uses active transport: carrier proteins in the membrane move ions into the cell using energy from respiration.
This is why root hair cells need many mitochondria. If respiration is inhibited (for example by waterlogging reducing oxygen supply), active transport slows and the plant may show mineral deficiency symptoms even if minerals are present in the soil.