Water Transport
The movement of plants from water to land has necessitated the development of
internal mechanisms to supply all the parts of the plant with water. As
discussed in Plant Classification, Vasular Tissues
,
tracheophytes (including
virtually all
terrestrial plants except for mosses and liverworts), have developed complex
vascular systems that move nutrients and water throughout the plant body
through "tubes" of conductive cells. The vascular tissues of these plants
are called xylem and phloem. The xylem of vascular plants consists of
dead cells placed end to end that form tunnels through which water and minerals
move upward from the roots (where they are taken in) to the rest of the plant.
Phloem, which is made up of living cells, carries the products of
photosynthesis (organic nutrients) from the leaves to the other parts. The
vascular system is continuous throughout the whole plant, even though the xylem
and phloem are often arranged differently in the root than they are in the
shoot.
The major mechanism by which water (along with dissolved materials) is carried
upward through the xylem is called TATC
(Transpiration-Adhesion-Tension-Cohesion). It should be noted that TATC,
while supported by most scientists, is speculated but not proven to be at work
in very tall trees. In this theory,
transpiration, the evaporation of water from the leaf, is theorized to create a
pressure differential that pulls fluids (held together by cohesion) up from the
roots.
Water transport also occurs at the cellular level, as individual cells absorb
and release water, and pass it along to neighboring cells. Water enters and
leaves cells through osmosis, the passive diffusion of water across a
membrane. In plants, water always moves from an area of higher water
potential to an area of lower water potential. Water potential results from
the differences in osmotic concentration (the concentration of solute in the
water) as well as differences in water pressure (caused by the presence of rigid
cell walls) between two regions. The relationship between the amount of
dissolves solute and water potential is inverse: where there is a lot of
dissolved solute the water potential is low.
Most of the water that a plant takes in enters through the root hairs. The
water diffuses easily (and osmotically) into the root hairs because the
concentration of dissolved materials in the plant's cellular cytoplasm is high.
As discussed in Plant Classification, Root
Hairs,
there are two pathways through which water travels from the outside of the root
to the core, where it is picked up by the xylem. The first of these pathways is
the symplast, in which water moves across the root hair membrane and through
the cells themselves, via channels that connect their contents. An alternate
route for water is the apoplast, in which water travels along cell walls and
through intercellular spaces to reach the core of the root. Once in the xylem,
the water can be carried by TATC to all the other parts of the plant.
Overall, water is transported in the plant through the combined efforts of
individual cells and the conductive tissues of the vascular system. Water from
the soil enters the root hairs by moving along a water potential gradient and
into the xylem through either the apoplast or symplast pathway. It is carried
upward through the xylem by transpiration, and then passed into the leaves along
another water potential gradient. In the leaf, some water is lost through
evaporation from the
stomata and the
remaining fluid moves along a water potential gradient from the xylem into the
phloem, where it is distributed along with the organic nutrients produced by
photosynthesis throughout the plant.
