The document summarizes water transport through plants. It describes how water is absorbed through root hairs into the xylem and transported upwards. Water moves from root hair cells to the cortex and endodermis by osmosis. In the stem, water is transported through xylem vessels by tension from the leaves. Water then diffuses out of xylem cells in the leaves down a concentration gradient to be used in photosynthesis.
2. To start with the Roots
Most of the water absorption is carried out by the younger part
of the roots. Just behind the growing tip of a young root,is a
region made up of hundreds of projections of the epidermal
tissue, the root hairs.
Root hairs can be seen clearly in newly germinated seeds, such as
the radish shown here in the photograph. The root hairs are
short lived being constantly replaced as new growth takes its
place.
The narrow walled hairs greatly increase the surface area over
which water absorption can take place faster. Water in the soil
spaces is taken into the root hairs by the process of
osmosis, there being a higher water concentration
outside than within the root hair cells.
3. Further into the roots
Once inside the root hair, the water needs to be transferred to
the xylem, the vascular tissue, (also known as the vascular
bundles) involved in water transport through the plant. Vascular
tissue is a complex conducting tissue, formed by more than one
cell type. The primary components of vascular tissue are
the xylem and phloem. In the roots, the vascular tissue is found
in the central column.
There is a water concentration gradient across the root which
means that water which has entered the root hair cell continues
to move across the cortex to the endodermis by osmosis. The
route taken by much of the water is through the cellulose cell
walls, the rest of the water either passes through the cytoplasm
of the cells or via the cell vacuoles.
5. Movement through the Stem
The xylem vessels form continuous pipes from the roots to the
leaves. Since the xylem vessels are dead, open tubes, no osmosis
can occur within them. This causes low pressure in the leaves, so
water is sucked up the stem to replace the lost water. The
column of water in the xylem vessels is therefore
under tension (a stretching force).
The very strong lignin walls of the xylem vessels stops them
collapsing under the suction pressure.
7. Water transport through leaves
In broad leaved plants, the leaves are large, thin, flat
structures. Large, in order to trap lots of light energy, and
thin so that the carbon dioxide can diffuse into the leaf
from the surrounding air.
As transpiration takes place, water diffusing into the air
spaces from the spongy mesophyll cells takes its place. This
is turn sets up a concentration gradient across which water
moves by osmosis out of the xylem cells and across the
leaf. A similar concentration gradient occurs between the
xylem and palisade layer so that water will also move by
osmosis to the palisade cells in order that it can be used by
these cells in the food manufacturing process of
photosynthesis.
