ENDODERMIS AND CASPARIAN STRIP

Some of the root epidermal cells, situated near the growing root tips, have projections called root hairs (unicellular hairs) which increase the surface area of the root for water absorption, though other epidermal cells may also absorb water. The diagram below shows two possible routes of this water absorption. The bottom arrow (blue) shows water moving through plasmodesmata in the symplast and is the symplastic pathway. Note that to reach the symplast the water first had to cross the cell membrane of the root hair cell. (See plant cell structure). The top arrow (red) shows water being absorbed through the apoplast (apoplastic pathway) traveling through the sponge-like meshes of the cell walls by capillarity, until it reaches a cylinder of cells called the endodermis. (The apoplast pathway is probably faster than the symplast pathway). These cells have a hoop of water-proof material in their cell walls (suberin) forming the Casparian strip. At this point the apolastic pathway is blocked and the water is forced to detour along the dashed part of the arrow, which leads into the symplast. At this point the water was forced to cross the cell membrane of the endodermis cell. This is the key point - whichever route the water takes to reach the xylem, it has to cross at least one cell-surface membrane.

These cell-surface membranes are able to control water entry and so the root can control water and mineral uptake at these points. Protein pumps in these membranes actively pump mineral salts, obtained from the soil water, into their cytoplasm, using cellular energy in the form of ATP (adenosine trisphosphate). This loading with salts creates an osmotic gradient (the water potential inside the cell is lowered - see below for an explanation of water potential) which then drives water into the cell, across the membrane (as water moves from a region of low solute gradient, or high water potential, to a region of high solute concentration, or low water potential). Thus, by using energy to accumulate the dilute and much needed mineral slats from the soil, the cells are essentially pumping water into the root and into the xylem! The xylem will then carry this water and these minerals (the minerals are essential nutrients for the plant) in the transpiration stream. This pumping creates a positive pressure in the root xylem, helping to push water up the stem - the so-called root pressure. This root pressure may force water up by a metre or so and may be sufficient tod rive water transport in small plants, however, in tall trees transpiration is by far the main driving force for xylem sap movement.