Learning Objective :
Synthesising the concept of transport of substances in plants.

Learning Outcomes :
A student is able to :
* describe the pathway of water from the soil to the leaves,
* explain the role of root pressure in the movement of water in plants
* explain the role of cohesion and adhesion of water in the movement of water in plants,
* conceptualise the transport mechanism in plants.

The pathway of water from the soil to the leaves.
1. Water is forced up from cell to cell by the collective push of incoming water from behind. This 'push' is called root pressure.

* The root hair is hypotonic to the adjacent cells.
* Eventually, water flows into cortex until it reaches endodermis

2. Capillary action due to the adhesive and cohesive forces, causes water to be 'pulled' up through the xylem vessels.


* Once the water reaches the endodermal cells, it will move through the cytoplasm and vacuoles.
* This is because there are Casparian strips which line the sides of the (endodermal) cells. The strips are impermeable to water, so the water movement through the cell walls is blocked. The water will move from the cytoplasm and the vacuole in the endodermal cells to the xylem vessels.
* The upward movement of water through the xylem vessels is driven by the forces of adhesion and cohesion of water.
* The water molecules adhere to one another by cohesive forces.The water molecules adhere the walls of xylem vessels by adhesive forces.
* The hydrogen bonding results in the adhesion and cohesion of water molecules.
* The water molecules adhere to one another by cohesive forces.
* The water molecules adhere the walls of xylem vessels by adhesive forces.
* The hydrogen bonding results in the adhesion and cohesion of water molecules.

3. Water moves through leaf tissues via transpirational pull. (a process triggered by water - vapour loss).

* After water molecules evaporate into the atmosphere, more water evaporates from the thin layer of water on the mesophyll cells.
* As the water has cohesive properties, the loss of water creates a tension which pulls a column of water up continuously.
* The transpirational pull draws water from the xylem in the stems and leaves and finally from the xylem in the root.



# The stomata will open in order to obtain sufficient carbon dioxide for photosynthesis.
# However, if the stomata are open, the water will be lost by transpiration through these stomata.

* The closing of the stomata will stop transpiration and reduces water loss
* It prevents the carbon dioxide from entering the leaf.
* Allows the plant to carry out photosynthesis and at the same time prevents the excessive loss of water.
* The stomata will open in response to :

(a) high light intensity
(b) a decrease in the carbon dioxide level.

# The root of some small plants continues to transport ions and water actively into the xylem during night-time.
# Root pressure starts to increase.
# At night, the transpiration rate is low.
# The root pressure will push water all the way up the stem and out of special pores called hydathodes.
# This natural process is known as guttation.
# Guttation is normally occurs on cool humid mornings when the air is too saturated for the water droplets to evaporate from the leaves.

Source from : http://ckgbio.blogspot.com/search/label/Chapter%201%20Form%205