Water Transport in Plants
Water enters the plant at the roots then passes up the xylem structures to the rest of the plant.
A transverse diagram of a young root is shown below.
Image from http://www.botany.uwc.ac.za/ecotree/root/rootA.htm
The outer layer of the root is the epidermis. This region produces projections called root hairs (not shown on the diagram) which project into the soil increasing surface area for uptake
Below the epidermis is the cortex. Cortical cells are highly permeable to water and dissolved solutes.
Below the cortex is a thin layer called the endodermis
At the centre of the root is the stele. This region contains the conductive tissues (the xylem and phloem) surrounded by a layer of cells called the pericycle. The remainder of the stele is the vascular cambium (not labelled on the diagram) these cells are able to divide to form new xylem and phloem increasing the diameter of the root.
Water passes from the soil into the root hair
Image from http://www.rothamsted.bbsrc.ac.uk/notebook/courses/guide/rthair.htm
Water inside the cells in the root has much higher solute concentrations than does water in the soil so there is a water potential gradient.
Hence water moves into the cell by osmosis
The water moves across the root from the hair cell to the xylem by three different pathways
- apoplast pathway - water passes through the cell walls and from the wall of one cell to the wall of an adjacent cell
- symplast pathway - water moves through the cytoplasm and from the cytoplasm of one cell into the cytoplasm of the next cell via the plasmodesmata
- vacuolar pathway - water passes from the vacuole of one cell into the vacuole of the next cell
Obviously these are slightly artificial categories because water has to move through the plasmodesmata and through the cytoplasm to get to the vacuole. For exam purposes it should be enough to be able to label a simple diagram with the three pathways
Water is not completely free to move from the root hair cell to the xylem.
The apoplastic pathway is blocked when it reaches the endodermis because the endodermal cells have a waxy, waterproof layer which prevents the further passage of water through the wall. This waxy layer is called the Casparian strip and is shown in the diagram below.
It is assumed that the presence of the Casparian strip allows the endodermis to regulate the quantity of water entering the xylem - but no mechanism is known
Image from http://www.puc.edu/Faculty/Gilbert_Muth/art0017a.jpg
Once the water reaches the xylem it moves upwards as a result of transpiration.
Transpiration is the loss of water from the aerial parts of the plant (particularly from the stomata in the leaves) as a result of evaporation
The evaporation of water from the top of the plant creates a pulling force drawing the water up the xylem
This pulling force is called tension
The water molecules are strongly attracted to each. This is called cohesion
So the two forces together give us the cohesion-tension model for the movement of water in xylem
Tension pulls the top of the column, cohesion between the water molecules pulls the lower molecules up
A third force, adhesion, occurs as a result of attraction between the water molecules and the wall of the vessel. Adhesion holds the column of water in place
A useful Flash demonstration of these processes is available here
For tension to successfully pull water to the top of a plant (or a tree) one particular rule must be obeyed: the column of water inside the xylem must be intact - any breaks in the column or bubbles would break the cohesion between the water molecules and hence prevent upwards movement.
Bubbles occur frequently in xylem vessels and any damage to the plant is likely to break the column of water.
However inspection of the diagram of xylem vessels shows the pits between adjacent vessels - these allow lateral movement of water, maintaining an intact column or allowing the column to bypass bubbles
The structure of the vessels are adapted to allowing cohesion-tension; the vessels are narrow enabling the adhesion of the water to the walls and they are rigid preventing the vessel from collapsing under the tension pressure (think how straws can collapse if the sucking pressure is too great). In spite of this the vessels do narrow under the tension pressure and the overall diameter of trees actually narrows as a result.
Factors affecting transpiration
Loss of water by
the plant occurs primarily through the stomata
The Role of Stomata The diagram shows a cross-section of a typical leaf from a mesophyte plant
i.e. a plant which lives in "normal" conditions (not in very wet or
very hot, dry conditions) The lower epidermis of a leaf has more pores in it called stomata Water is lost via the stomata The stomata can open and close. This is the function of the guard cells and
occurs as a result of osmosis of water into and out of the guard cells. As can be seen in the diagram stomata are kidney shaped. The cell wall of the
guard cells is thicker on the pore side than on the other side. This makes the
pore side less elastic so any swelling of the cell, due to the entry of water,
will cause the cell to become more curved opening the pore. It should be obvious from this that, if the plant has lots of water available
to it, the guard cells will become turgid (will swell) and the stomata will
open. An electrochemical gradient is also created which increases the uptake of
water. By an unclear mechanism (involving H+ ions being actively
pumped of the cell) potassium ions (K+) and chloride ions (Cl-)
diffuse into the cell. This makes the cell’s water potential more negative so
water flows in by osmosis. When the stomata close the potassium ions are actively pumped out of the cell
(and chloride ions would tend to diffuse out as a result of electrochemical
differences) so water moves out by osmosis The following conditions affect the rate of transpiration Light Stomata open in the presence of light and so are open during the day and
closed at night This is thought to be caused by the reduction in carbon dioxide level
resulting from the action of photosynthesis Temperature The higher the temperature the greater the evaporation of water from
mesophyll cells in the leaf so the greater the amount of transpiration Humidity Humidity is the measure of water saturation of the air. So the higher the humidity the greater the amount of water vapour there is in
the atmosphere. Higher humidity thus means a lower diffusion gradient between the water in
the leaf and water in the air. So higher humidity means slower transpiration Air movements When the air around a leaf is still a "shell" of air saturated with
water vapour forms immediately surrounding the leaf. There is thus a small
diffusion gradient between the leaf and this "shell" of air so the
diffusion rate is low. Moving air (e.g. in windy conditions) moves the "shell" of air
creating a steeper diffusion gradient The movement of ions Ions enter plants primarily via the roots in
association with water. They are actively transported through the root
hairs because the concentration inside roots is much higher than that found in
soil so, as well as being important to plants as nutrients, they are essential
for the uptake of water by osmosis. Note that calcium, unlike other ions, is able to diffuse into roots because
it is in higher concentration in the soil than in the root. Active transport of ions involves the pumping of protons (hydrogen ions) out
of the cell. This results in an electrochemical gradient across the membrane
favouring the movement of ios into the cell via carrier proteins. Movement within the root is via the symplast pathway. Once at the xylem ion transport is by mass flow in association with water 
