1. 35.1, 35.3, 10.4, 10.5 Regulation and Transport in Plants
Chapter
35.1, 35.3, 10.4, Regulation and Transport in Plants

2. Remember what plants need…
Photosynthesis
light reactions
Calvin cycle
light  sun
H2O  ground
CO2  air
What structures have plants evolved to supply these needs?
AP Link—Ch 08: AP-Photo (not on-line)

3. Interdependent Systems
Both systems depend on the other
roots receive sugars & other nutrients from photosynthetic parts
shoot system depends on water & minerals absorbed from the soil by roots
sugars
water

4. Putting it all together…
Obtaining raw materials
sunlight
leaves = solar collectors
CO2
stomates = gas exchange
H2O
uptake from roots
minerals

5. Leaves Function of leaves? photosynthesis gas exchange transpiration
energy production
CHO production
gas exchange
transpiration
simple vs. compound

7. Stomates
Function of stomates?

8. gas exchange water loss
A look at stomates…
Gas exchange
CO2 in  for Calvin cycle
O2 out  from light reactions
H2O vapor out
photosynthesis
gas exchange water loss O2
CO2
xylem (water)
phloem (sugars)
H2O O2
CO2

9. Controlling water loss from leaves
Hot or dry days
stomates close to conserve water
guard cells
gain H2O = stomates open
lose H2O = stomates close
adaptation to living on land, but…
creates PROBLEMS!
On sunny hot days, balancing 2 processes/ 2 forces:
Lots of sun so guard cells producing glucose, therefore increasing sugar concentration, therefore increase osmosis into guard cells = turgid & open
Lots of sun so plant starts to wilt, therefore guard cells shrink = flaccid & closed

10. The best laid schemes of mice and men…
Stomates
closed stomates lead to…
O2 builds up (from light reactions)
CO2 is depleted (in Calvin cycle)
causes problems in Calvin Cycle
The best laid schemes of mice and men…
and plants

11. Inefficiency of Rubisco: CO2 vs O2
Rubisco in Calvin cycle
carbon fixation enzyme
normally bonds C to RuBP
reduction of RuBP
building sugars
when O2 concentration is high
Rubisco bonds O to RuBP
O2 is alternative substrate
oxidation of RuBP
breakdown sugars
photosynthesis
photorespiration
RUBISCO: ribulose bisphosphate carboxylase/oxygenase

12. Calvin Cycle Review 1C 5C 6C 3C 3C 3C 2x x2 2x
RuBP = ribulose bisphosphate 1C
CO2
1. Carbon fixation
3. Regeneration 5C
RuBP
rubisco 6C
unstable
intermediate
3 ADP
3 ATP
PGAL
to make
glucose
RUBISCO ribulose bisphosphate carboxylase/ oxygenase 3C 2x
(PGA)
sucrose
cellulose
etc. 3C x2
PGAL
(3GP)
RuBP = ribulose bisphosphate
Rubisco = ribulose bisphosphate carboxylase
PGA = phosphoglycerate
PGAL = phosphoglyceraldehyde
2. Reduction
6 NADP
6 NADPH
6 ADP
6 ATP 3C 2x

13. Calvin Cycle Review C3 plants 1C 5C 3C 3C 2x x2
RuBP = ribulose bisphosphate 1C
CO2
C3 plants 5C
RuBP
rubisco
PGAL
to make
glucose
RUBISCO ribulose bisphosphate carboxylase/ oxygenase 3C 2x
(PGA) 3C x2
PGAL
(3GP)
RuBP = ribulose bisphosphate
Rubisco = ribulose bisphosphate carboxylase
PGA = phosphoglycerate
PGAL = phosphoglyceraldehyde

14. Calvin Cycle—with  [O2]
RuBP = ribulose bisphosphate 5C
RuBP 2C
rubisco 3C
a good enzyme goes bad…
RUBISCO ribulose bisphosphate carboxylase/ oxygenase
to mitochondria
lost as CO2 without making ATP
RuBP = ribulose bisphosphate
Rubisco = ribulose bisphosphate carboxylase
PGA = phosphoglycerate
PGAL = phosphoglyceraldehyde
photorespiration

15. Impact of Photorespiration
Oxidation of RuBP
short circuit of Calvin cycle
decreases photosynthetic output by siphoning off carbons
no ATP (energy) produced
no C6H12O6 (food) produced
loss of carbons to CO2
can lose 50% of carbons fixed by Calvin cycle
if photorespiration could be reduced, plant would become 50% more efficient
strong selection pressure

16. Why the C3 problem? Possibly evolutionary baggage
we’ve all got baggage…
Possibly evolutionary baggage
Rubisco evolved in high CO2 atmosphere
there wasn’t strong selection against active site of Rubisco accepting both CO2 & O2
Today it makes a difference
21% O2 now vs % O2 then
photorespiration can drain away 50% of carbon fixed by Calvin cycle on a hot, dry day
strong selection pressure to evolve better way to fix carbon & minimize photorespiration

17. Regulation of Stomates
Microfibril mechanism
guard cells attached at tips
microfibrils in cell walls
elongate causing cells to arch open = open stomate
shorten = close when water is lost
Ion mechanism
uptake of K+ ions by guard cells
ion pumps
water enters by osmosis
guard cells become turgid
loss of K+ ions by guard cells
water leaves by osmosis
guard cells become flaccid
Leaves generally have broad surface areas and high surface area–to–volume ratios. The broad surface area is a morphological adaptation that enhances the absorption of light needed to drive photosynthesis. The high surface area–to–volume ratio aids in the uptake of carbon dioxide during photosynthesis as well as in the release of oxygen produced as a by–product of photosynthesis. Upon diffusing through the stomata, CO2 enters a honeycomb of air spaces formed by the spongy parenchyma cells. Because of the irregular shape of these cells, the internal surface area of the leaf may be 10 to 30 times greater than the external surface area we see when we look at the leaf.
Although broad surface areas and high surface area–to–volume ratios increase photosynthesis, they also have the serious drawback of increasing water loss by way of the stomata. Thus, a plant’s tremendous requirement for water is part of the cost of making food by photosynthesis. By opening and closing the stomata, guard cells help balance the plant’s requirement to conserve water with its requirement for photosynthesis

18. Regulation of Stomates
Other cues
light trigger
blue-light receptor in plasma membrane of guard cells triggers ATP-powered proton pumps causing K+ uptake
stomates open
abscisic acid
plant hormone released by mesophyll cells that cause the stomata to close if the mesophyll water potential is too negative (too dry)
depletion of CO2
CO2 is depleted during photosynthesis (Calvin cycle)
circadian rhythm = internal “clock”
automatic 24-hour cycle
Guard cells arbitrate the photosynthesis–transpiration compromise on a moment–to–moment basis by integrating a variety of internal and external stimuli. Even the passage of a cloud or a transient shaft of sunlight through a forest canopy can affect the rate of transpiration.

19. Transport in Plants
H2O & minerals
Sugars
Gas exchange

20. Vascular Tissue Transports materials in roots, stems & leaves Xylem
carry water & minerals up from roots
tube-shaped dead cells
only their walls provide a system of microscopic water pipes
Phloem
carry nutrients throughout plant
sugars (sucrose), amino acids…
tube-shaped living cells

21. water-conducting cells of xylem
vessel
elements
Xylem
dead cells 
water-conducting cells of xylem
tracheids

22. Why does over-watering kill a plant?
Transport in Plants
H2O & minerals
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
Why does over-watering kill a plant?

23. Transport in Plants Gas exchange photosynthesis respiration
CO2 in; O2 out
stomates
respiration
O2 in; CO2 out
roots exchange gases within air spaces in soil

24. Transport in Plants
Physical forces drive transport at different scales
cellular
from environment into plant cells
transport of H2O & solutes into root hairs
short-distance transport
from cell to cell
loading of sugar from photosynthetic leaves into phloem sieve tubes
long-distance transport
transport in xylem & phloem throughout whole plant

25. Cellular Transport Active transport
solutes are moved into plant cells via active transport
central role of proton pumps
chemiosmosis
The most important active transport protein in the plasma membranes of plant cells is the proton pump , which uses energy from ATP to pump hydrogen ions (H+) out of the cell. This results in a proton gradient with a higher H+ concentration outside the cell than inside. Proton pumps provide energy for solute transport. By pumping H+ out of the cell, proton pumps produce an H+ gradient and a charge separation called a membrane potential. These two forms of potential energy can be used to drive the transport of solutes.
Plant cells use energy stored in the proton gradient and membrane potential to drive the transport of many different solutes. For example, the membrane potential generated by proton pumps contributes to the uptake of K+ by root cells.
In the mechanism called cotransport, a transport protein couples the downhill passage of one solute (H+) to the uphill passage of another (ex. NO3−). The “coattail” effect of cotransport is also responsible for the uptake of the sugar sucrose by plant cells. A membrane protein cotransports sucrose with the H+ that is moving down its gradient through the protein.
The role of proton pumps in transport is an application of chemiosmosis.
proton pumps

26. Water & Mineral Uptake by Roots
Mineral uptake by root hairs…
dilute solution in soil
active transport pumps
this concentrates solutes (~100x) in root cells
…followed by water uptake by root hairs
flow from high H2O potential to low H2O potential
creates root pressure
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.

27. Movement of Water in Plants
cells are flaccid
plant is wilting
Water relations in plant cells is based on water potential
osmosis through aquaporins
transport proteins
water flows from high potential to low potential
AP Movie—Ch 35: Recovery of a Wilted Coleus
Water potential is the force that moves water across the membranes of plant cells, but how do the water molecules actually cross the membranes? Because water molecules are so small, they move relatively freely across the lipid bilayer, even though the middle zone is hydrophobic. Water transport across biological membranes, however, is too specific and too rapid to be explained entirely by diffusion through the lipid bilayer. Indeed, water typically crosses vacuolar and plasma membranes through transport proteins called aquaporins These selective channels do not affect the water potential gradient or the direction of water flow, but rather the rate at which water diffuses down its water potential gradient. Evidence is accumulating that the rate of water movement through these proteins is regulated by phosphorylation of the aquaporin proteins induced by changes in second messengers such as calcium ions (Ca2+).
cells are turgid

28. Ascent of Xylem “sap”
Transpiration pull generated by leaf

29. Rise of Water in a tree by Bulk Flow
Water potential
high in soil  low in leaves
Root pressure push
due to flow of H2O from soil to root cells
upward push of xylem sap
Transpiration pull
adhesion & cohesion
H bonding
brings water & minerals to shoot
AP Link—Ch 35: Water Transport (Campbell)
The transpiration–cohesion–tension mechanism that transports xylem sap against gravity is an excellent example of how physical principles apply to biological processes. In the long–distance transport of water from roots to leaves by bulk flow, the movement of fluid is driven by a water potential difference at opposite ends of a conduit. In a plant, the conduits are vessels or chains of tracheids. The water potential difference is generated at the leaf end by transpirational pull, which lowers the water potential (increases tension) at the “upstream” end of the xylem.
On a smaller scale, water potential gradients drive the osmotic movement of water from cell to cell within root and leaf tissue. Differences in both solute concentration and turgor pressure contribute to this short–distance transport. In contrast, bulk flow depends only on pressure. Another contrast with osmosis, which moves only water, is that bulk flow moves the whole solution, water plus minerals and any other solutes dissolved in the water.
The plant expends no energy to lift xylem sap by bulk flow. Instead, the absorption of sunlight drives transpiration by causing water to evaporate from the moist walls of mesophyll cells and by lowering the water potential in the air spaces within a leaf. Thus, the ascent of xylem sap is ultimately solar powered.

30. Control of Transpiration
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
a corn plant transpires 125 L of water in a growing season

31. Don’t be a “plant”. Study for NEXT TUESDAY’S EXAM!
Any Questions??
Don’t be a “plant”. Study for NEXT TUESDAY’S EXAM!
AP Movie—Ch 08: Awesome Photosynthesis Animation