1. Vascular tissues in plants
Xylem (water & minerals)
Phloem (organic substances, e.g. sucrose)
Xylem & phloem tissues run close to each other to form Vascular bundle (stem & leaves) or Stele (roots)

2. Cross-section of a leaf
vascular bundle

3. Cross-section of a leaf
xylem
to transport water and mineral salts
towards the leaf

4. Cross-section of a leaf
phloem
to transport organic substances
away from the leaf

5. Transport of water and minerals -Structure of Xylem tissue-
This tissue contains different types of cells:
Fibres – long cells with thickened cell walls (support)
Parenchyma - cells with thin walls generally used as a storage
Xylem vessel elements -
Dead cells (walls made of lignin, impermeable to water)
The end of the walls break down to form an uninterrupted pathway for the water (xylem vessels)
Gaps in the walls called pits to allow movements between vessels and/or living tissues nearby
Picture + page

7. Xylem in a root

8. Xylem in the stem

9. Plants take in water through their roots in the soil…
Transpiration
Plants take in water through their roots in the soil…

10. …..and is lost through the leaves in the transpiration stream.
Corel 178(NT)
…..and is lost through the leaves in the transpiration stream.

13. Transpiration
Transpiration is important to guarantee the water movement
Plants need to balance water uptake with water loss. Dilemma:
Stomata open more gas exchange but also more water loss
Stomata closed less water loss but also less gas exchange

14. Temperature, humidity, air movement, light
Rate of transpiration
The rate of transpiration depends on:
Temperature, humidity, air movement, light
Temperature water molecules kinetic energy
evaporation
Water potential is always greater inside the moist mesophyll than the atmosphere transpiration occurs all the time. Nonetheless:
Humidity evaporation

15. Rate of transpiration
Air movement Still air means saturation on the leaves diffusion Air movement saturation diffusion Light intensity also affects the stomatal opening, therefore the rate of transpiration

16. Mention the Marram Grass
Research time...
Plants can be classified as:
Hydrophytes (water plants)
Xerophytes (plants living in very dry conditions)
Mesophytes (plants living in normal conditions)
Research how these three categories of plants are adapted in terms of transpiration
(timing of water intake, stomatal opening times, stomata positioning, leaves adaptations...)
Mention the Marram Grass

17. How does the water move upwards?
Concentration of water in the air is lower than within the leaves
Through the stomata, diffusion makes water vapour move into the air.
Water loss from the mesophyll cells lowers their water potential, so the nearby xylem vessel replace the lost water (diffusion)
Water pressure in the xylem (in the leaf) drops so water moves upwards from the roots where there is higher pressure
Picture + Page 239

19. Water movement theories
Cohesion-Tension theory
Water in the xylem vessels is pulled (therefore under tension) towards the leaves because of the transpiration (transpiration pull).
Water molecules attract each other, because they are dipoles. This provides cohesion (stickiness) of the molecules, hence the uninterrupted column of water.

20. Water movement theories
Root pressure
In the roots, endodermis cells (around the xylem vessels) actively transport mineral ions into the xylem, reducing its water potential
water is drawn in hydrostatic pressure increases water is pushed upwards
(mainly herbaceous plants)

21. Water movement theories
Capillarity
Water molecules ‘climb up’ in narrow tubes (µm) because they are attracted (adhesion) to polar molecules of the tube.
The narrower the tube the higher the water goes.
(mainly small plants)

22. Movement of water into/across the roots
Water enters the plants through the root hairs (thin, permeable, large surface area) moving down a water potential gradient (by osmosis) Why? Because the vacuoles of root cells contain a strong solution of dissolved substances, hence a lower water potential than the soil
Picture + page 244

24. Possible water pathways
Apoplast
between the root cells along the cell walls
Vacuolar
from vacuole to vacuole
Symplast
through the cell membranes , cytoplasm or plasmodesmata (gaps in the cell walls)
picture

25. No apoplast please!
As the water approaches the xylem, the apoplast pathway is blocked by a layer of cells (endodermis) surrounding the pericycle Endodermis cells have thickened walls with suberin (impermeable) Suberin forms a band around these cells called Casparian strip The water is forced into the symplast pathway and trough pits enters the Xylem

27. Uptake of mineral ions
Plants obtain the mineral from the soil (except carnivorous plants and legumes)
Nitrogen usually enters the plant as nitrates/ammonium ions
Ions move into the roots by diffusion (down the concentration gradient) or active transport
Ions move across the roots in solution in the water.
At the endodermis these ions are actively transported to by-pass the Casparian bands. Plants can so be selective of the ion taken in

28. Transport of organic substances
Phloem diagram
Sieve elements
Sieve tube/plate
Companion cells

29. How does translocation occur???
What is transported?
Phloem tissue transports substances made in the leaves to all other parts of the plant. This transport is called translocation Phloem transports: Sucrose (soluble carbohydrate) Amino acids Hormones Minerals
How does translocation occur???

30. Mass flow hypothesis
Fluids can move freely in sieve tubes just following the hydrostatic pressure gradient Sucrose is actively loaded into the sieve tube from the source cell (leaves or storage organ) Water follows the sucrose (osmosis) so the pressure in the sieve tube A sink cell (respiring/storage tissue) unloads the sucrose (+water because of the osmosis) so the pressure in the sieve tube