Date: ___________________________________
Transport in Plants
Section 13.4
Two types of vascular tissue move substances through the plant body:
1. Dissolved carbohydrates – move through PHLOEM.
2. Water and dissolved nutrients – move through XYLEM
Passive and Active Transport
A. Passive transport – Energy is not required
 along solute concentration gradients:
high concentration to low
 water moves, if solutes cannot
 movement equalizes solute/water ratio
 examples: diffusion, osmosis (diffusion of water)
B. Active transport – Energy is required
 substances are moved against their concentration gradients:
low concentration to high
 example: transport of sugars through protein pumps
Complete the Venn diagram below. Include the following points: Is energy required? Do substances
move down or against their concentration gradients? What types of substances are transported?



Passive
No energy required
Substances move down
concentration gradients
Nutrients (digestive –
animals), water (osmosis)
Both
Forms of transport



Active
Energy required
Movement against
concentration gradient
Sugars in plants
Water Transport in the Xylem

Water moves UPWARDS through the plant’s xylem, from soil to the shoot tissues.

90% of the water that reaches the leaf evaporates through the STOMATA
o

This type of evaporation is called TRANSPIRATION.
Guard cells will close the stomata to limit evaporation.
Factor
Condition that causes
stomata to close…
Explanation
Hotter = more evaporation
Temperature
hot
Dry
Less water vapour outside plant causes water vapour to diffuse
out
windy
Wind blows away water vapour barrier layer outside the plant
Humidity
Wind level
Long-Distance Water Transport
 Water is able to move upwards through long vertical distances.
o However, the force of GRAVITY acts against water movement.

Two processes contribute to efficiency:
I.
Root pressure – Generates positive pressure (a PUSHING force)
II.
Transpirational pull – Generates negative pressure (a PULLING force)
Root Pressure



Pushing force produced at the
bottom of the plant, by movement
of water into XYLEM cells.
Only able to move water through
vertical distances of 10 metres.


Transpirational Pull**
Pulling force produced by transpiration of water from
leaves
Accounts for long-distance movement of water
Enhanced by effects of cohesion and adhesion

Cohesion – Force of attraction
between water molecules.

Adhesion – Force of attraction
between water molecules and
xylem walls.
**The additive effect of transpiration, cohesion, and adhesion is described by the cohesion-tension model:
The Cohesion-Tension Model of Water Transport
1. Water evaporates from stomata (transpiration).
2. Negative pressure pulls adjacent water molecules from xylem into leaf cells.
3. Cohesion between water molecules produces a continuous column of water molecules that is pulled
upwards.
4. Adhesion between water molecules and xylem walls enhances the effects of cohesion.
5. Tension in the water column extends all the way to roots, so that water is drawn in by osmosis.
Nutrient Transport in the Phloem (aka “Translocation”)

Glucose is produced by SUCROSE, in the leaves.
o is converted to sucrose for transport through phloem

Sucrose sources vs. sinks
o Source – Any region where sugars are PRODUCED
 example: ground tissue in the leaves
o
Sink – Any region where sugars are USED or STORED
 examples: parts of the plant that are growing; roots

Nutrients always move in a source to sink direction.

Sucrose movement is driven by positive pressure (pushing).
The Pressure-Flow Model of Nutrient Transport
1. Nutrients are pumped into phloem, and their concentration builds up inside the phloem.
2. Water is drawn into phloem by osmosis.
3. Movement of water generates positive pressure at the source end of the tube. This pushes sucrose-rich
solution towards any sink.
Questions:
1. What process produces the pulling force that draws water up the xylem? TRANSPIRATION
2. In what way do cohesion and adhesion aid in the long distance transport of water? COHESION
PRODUCES A CHAIN OF WATER MOLECULES, WHILE ADHESION PREVENTS THE CHAIN
FROM FALLING DOWN DUE TO GRAVITY
3. Describe how the direction of osmosis is influenced by solute concentration .
WATER TRAVELS TO REGIONS OF HIGH SOLUTE CONCENTRATION
4. Not all leaves are considered sugar sources - why are growing leaves considered sugar sinks?
GROWING LEAVES REQUIRE LOTS OF SUGAR BEFORE THEY CAN PHOTOSYNTHESIZE FOR
THEMSELVES