2. Transport in Plants

3. Transport in plants writing

5. Transport Mechanisms
1) Water first enters the roots (higher water potential to lower potential)
2) Then moves to the xylem
Innermost vascular tissue
Water rises through the xylem because of a combination of factors
3) Most of that water exits through the stomata in the leaves

6. Most of the force is “pulling” created by transpiration
Evaporation from thin films of water in the stomata
Occurs due to cohesion (water molecules stick to each other) and adhesion (stick to walls)

7. Water flow through root
Porous cell wall
water can flow through cell wall route & not enter cells
plant needs to force water into cells
Casparian strip
The endodermis, with its Casparian strip, ensures that no minerals can reach the vascular tissue of the root without crossing a selectively permeable plasma membrane. If minerals do not enter the symplast of cells in the epidermis or cortex, they must enter endodermal cells or be excluded from the vascular tissue. The endodermis also prevents solutes that have been accumulated in the xylem sap from leaking back into the soil solution. The structure of the endodermis and its strategic location in the root fit its function as sentry of the border between the cortex and the vascular cylinder, a function that contributes to the ability of roots to transport certain minerals preferentially from the soil into the xylem.

8. Controlling the route of water in root
Endodermis
cell layer surrounding vascular cylinder of root
lined with impermeable Casparian strip
forces fluid through selective cell membrane
filtered & forced into xylem cells
Aaaah…
Structure–Function yet!

9. Most of the water absorbed by the plant comes in through the region of the root with root hairs
Surface area further increased by mycorrhizal fungi
Once absorbed through root hairs, water and minerals must move across cell layers until they reach the vascular tissues
Water and dissolved ions then enter the xylem and move throughout the plant

10. Mycorrhizae increase absorption
Symbiotic relationship between fungi & plant
symbiotic fungi greatly increases surface area for absorption of water & minerals
increases volume of soil reached by plant
increases transport to host plant

11. Water Absorption through Roots
STRUCTURE and FUNCTION

12. Transport in plants H2O & minerals Sugars Gas exchange
transport in xylem
transpiration
evaporation, adhesion & cohesion
negative pressure
Sugars
transport in phloem
bulk flow
Calvin cycle in leaves loads sucrose into phloem
positive pressure
Gas exchange
photosynthesis
CO2 in; O2 out
stomates
respiration
O2 in; CO2 out
roots exchange gases within air spaces in soil
Why does over-watering kill a plant?

13. Leaf structures

14. Control of Stomates Uptake of K+ ions by guard cells
Epidermal cell
Guard cell
Chloroplasts
Nucleus
Uptake of K+ ions by guard cells
proton pumps
water enters by osmosis
guard cells become turgid
Loss of K+ ions by guard cells
water leaves by osmosis
guard cells become flaccid K+ K+
H2O
H2O
H2O
H2O K+ K+ K+ K+
H2O
H2O
H2O
H2O K+ K+
Thickened inner
cell wall (rigid)
H2O
H2O
H2O
H2O K+ K+ K+ K+
Stoma open
Stoma closed
water moves into guard cells
water moves out of guard cells

15. Control of transpiration
Balancing stomate function
always a compromise between photosynthesis & transpiration
leaf may transpire more than its weight in water in a day…this loss must be balanced with plant’s need for CO2 for photosynthesis

16. Rate of Transpiration
Over 90% of the water taken in by the plant’s roots is ultimately lost to the atmosphere
At the same time, photosynthesis requires a CO2 supply from the atmosphere
Closing the stomata can control water loss on a short-term basis
However, the stomata must be open at least part of the time to allow CO2 entry

17. Active pumping of sucrose out of guard cells in the evening leads to loss of turgor and closes the guard cell

18. Guard cells Only epidermal cells containing chloroplasts
Have thicker cell walls on the inside and thinner cell walls elsewhere
Bulge and bow outward when they become turgid
Causing the stomata to open
Turgor in guard cells results from the active uptake of potassium (K+), chloride (Cl)
Water enters osmotically

20. Rate of Transpiration
Transpiration rates increase with temperature and wind velocity because water molecules evaporate more quickly
Several pathways regulate stomatal opening and closing
Abscisic acid (ABA) initiates a signaling pathway to close stomata in drought
Opens K+, Cl– channels
Water loss follows

22. Other pathways regulating stomata
Close when CO2 concentrations are high
Close when temperature exceeds 30º–34ºC and water relations unfavorable
Alternative photosynthetic pathways, such as Crassulacean acid metabolism (CAM), reduce transpiration

23. Water Stress Responses
Many morphological adaptations allow plants to limit water loss in drought conditions
Dormancy
Loss of leaves – deciduous plants
Covering leaves with cuticle and wooly trichomes
Reducing the number of stomata
Having stomata in pits on the leaf surface

25. Plants have adapted to flooding conditions which deplete available oxygen
Flooding may lead to abnormal growth
Oxygen deprivation most significant problem
Plants have also adapted to life in fresh water
Form aerenchyma, which is loose parenchymal tissue with large air spaces
Collect oxygen and transport it to submerged parts of the plant

26. Water & mineral absorption
Water absorption from soil
osmosis
aquaporins
Mineral absorption
active transport
proton pumps
active transport of H+
aquaporin
root hair
proton pumps
H2O

27. Transport of sugars in phloem
Loading of sucrose into phloem
flow through cells via plasmodesmata
proton pumps
cotransport of sucrose into cells down proton gradient

28. Pressure flow in phloem
Mass flow hypothesis
“source to sink” flow
direction of transport in phloem is dependent on plant’s needs
phloem loading
active transport of sucrose into phloem
increased sucrose concentration decreases H2O potential
water flows in from xylem cells
increase in pressure due to increase in H2O causes flow
can flow 1m/hr
In contrast to the unidirectional transport of xylem sap from roots to leaves, the direction that phloem sap travels is variable. However, sieve tubes always carry sugars from a sugar source to a sugar sink. A sugar source is a plant organ that is a net producer of sugar, by photosynthesis or by breakdown of starch. Mature leaves are the primary sugar sources. A sugar sink is an organ that is a net consumer or storer of sugar. Growing roots, buds, stems, and fruits are sugar sinks. A storage organ, such as a tuber or a bulb, may be a source or a sink, depending on the season. When stockpiling carbohydrates in the summer, it is a sugar sink. After breaking dormancy in the spring, it is a source as its starch is broken down to sugar, which is carried to the growing tips of the plant.
A sugar sink usually receives sugar from the nearest sources. Upper leaves on a branch may send sugar to the growing shoot tip, whereas lower leaves export sugar to roots. A growing fruit may monopolize sugar sources around it. For each sieve tube, the direction of transport depends on the locations of the source and sink connected by that tube. Therefore, neighboring tubes may carry sap in opposite directions. Direction of flow may also vary by season or developmental stage of the plant.
On a plant… What’s a source…What’s a sink?

29. Maple sugaring