1. Transport in Plants Chapter 36
Modified from: Kim Foglia, Explore Biology

2. Transport in plants
H2O & minerals
Sugars
Gas exchange

3. Transport in plants H2O & minerals Sugars Gas exchange
transport in xylem
transpiration
evaporation, adhesion & cohesion
negative pressure
Sugars
Gas exchange

4. 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

5. Transport in plants H2O & minerals Sugars Gas exchange
transport in xylem
transpiration
Sugars
transport in phloem
bulk flow
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?

6. 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

7. Cellular transport Active transport proton pumps
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

8. Short distance (cell-to-cell) transport
Compartmentalized plant cells
cell wall
cell membrane
cytosol
vacuole
Movement from cell to cell
move through cytosol
plasmodesmata junctions connect cytosol of neighboring cells
symplast
move through cell wall
continuum of cell wall connecting cell to cell
apoplast
In most plant tissues, the cell walls and cytosol are continuous from cell to cell. Plasmodesmata connect the cytosolic compartments of neighboring cells, thereby forming a continuous pathway for transport of certain molecules between cells. This cytoplasmic continuum is called the symplast. The continuum of cell walls plus the extracellular spaces is called the apoplast. The third cellular compartment, the vacuole, is not shared with neighboring cells.
apoplast
symplast

9. Routes from cell to cell
Moving water & solutes between cells
transmembrane route
repeated crossing of plasma membranes
slowest route but offers more control
symplast route
move from cell to cell within cytosol
apoplast route
move through connected cell wall without crossing cell membrane
fastest route but never enter cell
Functions of the Symplast and Apoplast in Transport
How do water and solutes move from one location to another within plant tissues and organs? For example, what mechanisms transport water and minerals from the root hairs to the vascular cylinder of the root? Such short–distance transport is sometimes called lateral transport because its usual direction is along the radial axis of plant organs, rather than up and down along the length of the plant.
Three routes are available for this transport. By the first route, substances move out of one cell, across the cell wall, and into the neighboring cell, which may then pass the substances along to the next cell in the pathway by the same mechanism. This transmembrane route requires repeated crossings of plasma membranes as the solutes exit one cell and enter the next.
The second route, via the symplast, the continuum of cytosol within a plant tissue, requires only one crossing of a plasma membrane. After entering one cell, solutes and water can then move from cell to cell via plasmodesmata.
The third route for short–distance transport within a plant tissue or organ is along the apoplast, the pathway consisting of cell walls and extracellular spaces. Without entering a protoplast, water and solutes can move from one location to another within a root or other organ along the byways provided by the continuum of cell walls.

10. Long distance transport
Bulk flow
movement of fluid driven by pressure
flow in xylem tracheids & vessels
negative pressure
transpiration creates negative pressure pulling xylem sap upwards from roots
flow in phloem sieve tubes
positive pressure
loading of sugar from photosynthetic leaf cells generates high positive pressure pushing phloem sap through tube
Diffusion in a solution is fairly efficient for transport over distances of cellular dimensions (less than 100 μm), but it is much too slow to function in long–distance transport within a plant. For example, diffusion from one end of a cell to the other takes seconds, but diffusion from the roots to the top of a giant redwood would take decades or more. Long–distance transport occurs through bulk flow, the movement of a fluid driven by pressure. In bulk flow, water and solutes move through the tracheids and vessels of the xylem and through the sieve tubes of the phloem.
In the phloem, for example, the loading of sugar generates a high positive pressure at one end of a sieve tube, forcing sap to the opposite end of the tube.
In xylem, it is actually tension (negative pressure) that drives long–distance transport. Transpiration, the evaporation of water from a leaf, reduces pressure in the leaf xylem. This creates a tension that pulls xylem sap upward from the roots.

11. 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
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

12. Water & mineral uptake by roots
Mineral uptake by root hairs
dilute solution in soil
active transport pumps
this concentrates solutes (~100x) in root cells
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.

13. Root anatomy
monocot
dicot

15. Route water takes through root
Water uptake by root hairs
a lot of flow can be through cell wall route
apoplasty
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.

16. Controlling the route of water in root
Endodermis
cell layer surrounding vascular cylinder of root
lined with impervious Casparian strip
forces fluid through selective cell membrane & into symplast
filtered & forced into xylem vessels
Aaaaah…
Structure-Function
yet again!

17. 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

18. Mycorrhizae
The hyphae of mycorrhizal fungi extend into soil, where their large surface area and efficient absorption enable them to obtain mineral nutrients, even if these are in short supply or are relatively immobile. Mycorrhizal fungi seem to be particularly important for absorption of phosphorus, a poorly mobile element, and a proportion of the phosphate that they absorb has been shown to be passed to the plant.

19. Ascent of xylem “sap”
Transpiration pull generated by leaf

20. Rise of water in a tree by bulk flow
Transpiration pull
adhesion & cohesion
H bonding
brings water & minerals to shoot
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
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.

21. 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

22. 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
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
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

23. 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
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.

24. Transport of sugars in phloem
Loading of sucrose into phloem
flow through symplast via plasmodesmata
active cotransport of sucrose with H+ protons
proton pumps

25. Pressure flow in sieve tubes
Water potential gradient
“source to sink” flow
direction of transport in phloem is variable
sucrose flows into phloem sieve tube decreasing H2O potential
water flows in from xylem vessels
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.
What plant structures are sources & sinks?

26. Experimentation Testing pressure flow hypothesis
using aphids to measure sap flow & sugar concentration along plant stem
Pressure Flow: The Mechanism of Translocation in Angiosperms
Phloem sap flows from source to sink at rates as great as 1 m/hr, much too fast to be accounted for by either diffusion or cytoplasmic streaming. In studying angiosperms, researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure (thus the synonym pressure flow.
The building of pressure at the source end and reduction of that pressure at the sink end cause water to flow from source to sink, carrying the sugar along. Xylem recycles the water from sink to source.
The pressure flow hypothesis explains why phloem sap always flows from source to sink.

27. Maple sugaring