Translocation of Solutes
In most vascular plants, the body is divisible into three principal organs namely the leaf, stem and root. The leaves are the chief organs of food production and the roots, apart from anchoring the plant, absorb water and mineral nutrients from the soil. There must be an arrangement for transporting food molecules from the site of production in the leaves to other parts of the plant, because all living cells require a source of energy and of chemical building blocks. Similarly mineral ions taken up by the roots must be distributed to the leaves and stem. These functions are performed by the vascular tissues of the plant. There are two types of vascular tissues in plants: the xylem and the phloem. The xylem is concerned primarily with the transport of water and dissolved minerals from the roots to the other parts of the plant. The phloem transports food and certain other solutes from the sites of production (sources) to the sites of utilisation (sinks). In stems and large roots, the xylem is more internally located, whereas the phloem lies closer to the outside of the organ.
The centrally located woody cells called xylem serve to conduct water upward. They are a functional part of the entire plant and consists of several kinds of cells. Their major component is a hard walled elongated cell (tracheids) with open ends at which it joins other cells, thus forming a continuous pipeline. In its functional stage this type of cell is dead, composed only of a hard wall and serving the sole purpose of conducting water upward. On the outside of the xylem and forming the inner bark are the phloem tubes whose function is to transport downward the food manufactured in the leaves. The conducting vessels of phloem are composed of living elongated cells (sieve tubes) which have sieve like openings at both ends (sieve plates). The phloem tubes are formed from a string of these long cells arranged end to end, extending through the entire length of the plant. The xylem and phloem vessels enter the leaves as continuations of the conducting system of the stem. The fine network of the leaf's system of veins and veinlets ensures that the leaf's water supply is as effective as an animal's blood supply with its arteries and veins.
Transport in Xylem
Water enters the plant through roots hairs. As a result of higher concentration of solutes in these cells than in the soil solution, water enters osmotically. At the other end, in the leaves, water evaporates from the moist walls of the mesophyll cells, diffuses through the air spaces of the leaf and finally leaves as water vapour through open stomata. This circulation of water through the xylem vessels also transports the nutrients to all parts of the plant body. There are many physical and chemical factors responsible for the rise of water in xylem vessels. Mineral ions contained in the xylem sap rise passively as the solvent (water) ascends from root to leaf. In this way, the nutritional needs of the shoot are fulfilled. Some of the mineral elements brought to the leaves are subsequently redistributed to other parts of the plant by way of the phloem. The high concentration of salt ions within the leaf cells facilitates entry of water into them through osmosis. Leaf cells also have a high concentration of sugar in their vacuoles.
Transport through Xylem and Phloem
Transport in Phloem
The phloem transports food and certain other solutes from the sites of production (sources) to the sites of utilisation (sinks). The movement of dissolved substances through the phloem is not completely understood. However, it appears that the phloem transport can be explained by a bulk flow of phloem sap, under pressure, by way of sieve-like end walls of the sieve tube elements.
According to mass or passive flow hypothesis two features are important for phloem transport of substances. These are:
- The active (energy driven) transport of sugars and other solutes into the sieve types in sources areas (leaves),
- An active removal of solutes where the sieve tubes enter sinks (roots). These features result in a higher concentration of solutes at the sources end of the sieve tube. This causes water to enter the sieve tube at that end. In turn the entry of water causes greater turgidity (hydrostatic pressure) at the source end so that the entire fluid content of the sieve tube is 'squeezed' toward the sink end of the tube.