1. Transport in Plants
Explain the need for transport systems in multicellular plants in terms of size and surface- area-to-volume ratio.
Describe the distribution of xylem and phloem tissues in roots, stems and leaves of dicotyledonous plants.

2. Transport systems and surface area to volume ratio
Single celled organisms have large surface area: vol ratio and can obtain all their requirements (oxygen, CO2, water and minerals) by diffusion. The sugar they make is available throughout the cell.
Large plants need to transport water and minerals up from the roots. Sugars need to be moved from the leaves where they are made to other areas of the plant. In order to do this they need transport systems

3. Tissues that carry out movement of water, minerals and sugars
A Tissue is group of similar cells that carry out a particular function
Xylem tissue is made of Xylem vessels, fibres and parenchyma cells. Xylem transports water and minerals.
Phloem tissue is made of Sieve tubes and companion cells. Phloem transports sugars.
© Pearson Education Ltd 2008
This document may have been altered from the original

4. Distribution of vascular tissue in plant roots
Xylem forms a cross shape in the centre with areas of phloem between the arms
The vascular (transport) tissue is surrounded by a layer of endodermis
The Pericycle is a layer of meristem (dividing) cells inside the endodermis

5. Distribution of vascular bundles in stem
In the stem the vascular bundles of Xylem, Cambium and Phloem are arranged around the edge of the stem
Xylem is always on the inside of the bundle,
Cambium is a layer of meristem cells in the middle of the bundle
Phloem is always on the outside of the bundle

6. Distribution of vascular tissue in leaves
In leaves the vascular tissue is seen as the midrib and veins in the leaf
The xylem is always closer to the top surface of the leaf
The phloem is below the xylem

7. Describe the structure and function of xylem vessels, sieve tube elements and companion cells.

8. Xylem tissue

9. Phloem (longitudinal section)‏
Week 10
Phloem (longitudinal section)‏
© Pearson Education Ltd 2008
This document may have been altered from the original

10. Pure water has a water potential of zero
Explain, in terms of water potential, the movement of water between plant cells, and between plant cells and their environment.
Pure water has a water potential of zero
Any solute dissolved in water lowers the water potential and makes it more negative
Water always moves from an area of higher water potential to an area of lower water potential

11. Movement of water into plant cells
This cell has a water potential of kPa
It is bathed in pure water with a water potential of 0kPa
So water enters the cell down the water potential gradient (from higher to lower) by osmosis
Eventually the pressure exerted by the water entering the cell equals the pressure exerted by the cell wall on the contents, water stops going into the cell
The cell becomes turgid
A pair of adjacent cells, A and B, have water potentials of -1000kPa and 1200kPa respectively. Which cell will gain water from the other

12. Movement of water out of plant cells
If a plant cell is put into a solution with a much lower water potential than the cell then water will leave the cell by osmosis.
The vacuole shrinks and then the cytoplasm shrinks and becomes smaller in volume
Eventually the cell membrane pulls away from the cell wall
The cell becomes plasmolysed.
What is in the space between the cell wall and the plasma membrane of the plasmolysed cell?

13. Movement of water between plant cells
If adjacent cells have different water potentials then water will move between them, by osmosis, down the water potential gradient
© Pearson Education Ltd 2008
This document may have been altered from the original

14. Complete the question on water potential

15. Movement of water through plants
A, the Apoplast pathway. Water and dissolved ions move through the cell walls between the cellulose molecules
B, the Symplast pathway. Water goes into the cell through the plasma membrane into the cytoplasm. It moves from cell to cell through the plasmodesmata
C, the Vacuolar pathway. Water goes into the cell through the plasma membrane into the cytoplasm and then into the vacuole.

16. Describe, with the aid of diagrams, the pathway by which water is transported from the root cortex to the air surrounding the leaves, with reference to the Casparian strip, apoplast pathway, symplast pathway, xylem and stomata.
Explain the mechanism by which water is transported from the root cortex to the air surrounding the leaves, with reference to adhesion, cohesion and the transpiration stream.
© Pearson Education Ltd 2008
This document may have been altered from the original

17. Describe, with the aid of diagrams, the pathway by which water is transported from the root cortex to the air surrounding the leaves, with reference to:
the Casparian strip,
apoplast pathway
symplast pathway,
xylem and stomata.

18. Water movement into the plant
Water goes into the root cells and moves across the cortex by osmosis, water can move by any of the pathways
At the Casparian strip in the Endodermis water is forced out of the Apoplast pathway and into symplast or vacuolar pathway
Water and ions pass through proteins in the plasma membrane into the cytoplasm
Nitrate ions are actively pumped from the endodermis cells into the xylem
This lowers the water potential in the xylem so water follows by osmosis

19. Movement up the stem and out of the leaves
Pumping ions into the xylem forces water to follow by osmosis
Water can rise up stems about 3 metres by this process
Water evaporates from leaves through the stomata, water moves through the leaf by osmosis down the water potential gradient
Water leaves the xylem creating tension in the xylem, this is why the xylem needs to be strengthened with lignin, to prevent collapse.
Cohesion between water molecules means that the whole column of water is pushed upwards from below and pulled upwards from above

20. Loss of water from leaf by Transpiration
Osmosis moves water from xylem to palisade and spongey mesophyll
Water evaporates from the mesophyll cells into the intercellular spaces
Water diffuses from intercellular spaces out through stomata

21. Factors that increase transpiration rates
Number of leaves
Number of stomata
Cuticle
Light
Temperature
Relative humidity
Air movement/ wind
Water available
More leaves = larger surface area
More stomata = more spaces for evaporation
More cuticle= slower evaporation
Stomata open in sunlight
Higher temp = faster evaporation, faster diffusion through stomata
Lower gradient slows water loss
Maintains water potential gradient
Little water in plant closes stomata and reduces water loss

22. Animation link
spiration.swf