Transport of Water and Minerals

Root hairs are out-growths of the epidermal layer (outermost layer) of the roots. The cells of the root hairs have a semi-permeable membrane which allows water to enter through osmosis.  If you can recall, osmosis is the movement of particles (usually water) from a higher concentration to a lower concentration across a semi-permeable membrane. Since the root hairs have a semi-permeable membrane, osmosis results in water entering the root hairs if the concentration of water is greater in the growth medium (usually soil) than in the root hairs. This movement continues across the root hairs, into the root cells, and then enters the xylem. The xylem acts as the vessels inside the plant that carries water and minerals from the root hairs to the stem and leaves.

The uptake of minerals by the plant is achieved by active transport.  Active transport is needed because the concentration of some minerals may be greater in the root hairs than in the growth medium. As a result, energy is needed to make minerals travel against their concentration gradients. These minerals, once up taken by the root hairs, travel to the xylem as solutes (dissolved particles).

Now that water and minerals have entered the plant via root hairs, how are they forced up the plant without a pumping mechanism? The answer to that is capillarity. Capillarity may be accurately defined as the movement of a liquid up a narrow tube against the forces of gravity due to inter-molecular forces of attraction. For example, when a narrow tube is placed upright in water, water is seen to force its way up the tubes against gravity. This process is called capillarity and is due to the cohesive nature of water (ability to stick together).

The tubes of the xylem are narrow and hard (xylem cells are made of lignin which makes it hard and woody) causing water to rise through capillarity. Water is continuously pulled up the xylem tubes and carried up and throughout the plant. This process is known as the transpiration pull. There is a continuous loss of water caused by transpiration. As water is lost by transpiration, a vacuum is created which effectively pulls the water up the tubes of the xylem towards the leaves where it is lost.

The Phloem

The phloem does not transport water and minerals, but food. It contains sieve tube elements. Like xylem, the phloem is made of cells that are joined end to end. However, the end walls which have not completely been broken down form sieve plates. These plates have small holes in them and these cells are called sieve tube elements. They have cytoplasm but no nucleus, and there is no lignin in their walls. Next to each sieve tube element is a companion cell which has no nucleus. They are called companion cells because they are believed to supply the sieve tube elements with essential substances.