1. Plant Anatomy and Transport
Chapter 35 and 36
Plant Anatomy and Transport

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

3. Plant tissues Dermal Vascular Ground
single layer of tightly packed cells that covers & protects plant
endodermis, epidermis, guard cells, root hairs
Vascular
transport materials between roots & shoots
xylem & phloem
Ground
everything else: storage, photosynthetic
bulk of plant tissue
3 cell types – parenchyma, collenchyma, sclerenchyma

4. Plant cell types in tissues
Parenchyma
“typical” plant cells = least specialized
Thin, flexible primary cell walls – no secondary cell wall
photosynthetic cells, storage cells
tissue of leaves, stem, fruit, storage roots
Collenchyma
unevenly thickened primary walls; lack secondary cell wall = support
Sclerenchyma
very thick, “woody” secondary walls = lignin
used for support of plant – fibers and sclerids
rigid cells that can’t elongate
dead at functional maturity

5. Leaves Function of leaves? photosynthesis energy production
sugar production
gas exchange
transpiration

6. Stomates
Function of stomates?

7. 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

8. 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

9. Regulation of stomates
Other cues
Increase in K+ in the guard cells - this lowers water potential, which causes water to diffuse in
light trigger
blue-light receptor in plasma membrane of guard cells triggers ATP-powered proton pumps causing K+ uptake
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.

10. 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

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

12. Xylem Dead at functional maturity Cell elongated into tubes tracheids
long, thin cells with tapered ends
walls reinforced with lignin = support
thinner pits in end walls allows water flow
vessel elements
wider, shorter, thinner walled & less tapered
perforated ends walls allows free water flow

13. 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.

14. Phloem: food-conducting cells
sieve tube elements & companion cells

15. Phloem: food-conducting cells
sieve tube elements & companion cells

16. Phloem Living cells at functional maturity Cells
lack nucleus, ribosomes & vacuole
more room: specialized for liquid food (sucrose) transport
Cells
sieve tubes
end walls, sieve plates, have pores to facilitate flow of fluid between cells
companion cells
nucleated cells connected to the sieve-tube
help sieve tubes

17. Phloem
sieve plate
sieve
tubes

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

19. 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?

20. Vascular tissue in herbaceous stems
dicot
trees & shrubs
monocot
grasses & lilies

21. 1
Roots
Roots anchor plant in soil, absorb minerals & water, & store food
fibrous roots (1)
mat of thin roots that spread out
monocots
tap roots (2)
1 large vertical root
also produces many small lateral, or branch roots
dicots
root hairs (3)
increase absorptive surface area 2 3

22. Root structure: dicot
phloem
xylem

23. Root structure: monocot

24. 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.

25. 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.

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
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. 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.

28. 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

29. 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

30. Meristem Regions of growth perpetually embryonic tissue
regenerate new cells
apical shoot meristem
growth in length
primary growth
apical root meristem
lateral meristem
growth in girth
secondary growth

31. Root structure & growth

32. Growth in woody plants Woody plants grow in height from tip
primary growth
apical meristem
Woody plants grow in diameter from sides
secondary growth
vascular cambium
vascular meristem layer

33. Growth in woody plants Primary growth shoot root
tips of roots & shoots (apical meristem)
restricted to youngest parts of plant
shoot
root

34. Growth in woody plants Secondary growth
thickens & strengthens older part of tree
cork cambium makes bark
growing ring around tree
vascular cambium makes xylem & phloem

35. Woody stem cork cambium bark phloem late vascular cambium early xylem
Phloem produced to the outside
Xylem produced to the inside
cork cambium
bark
phloem
late
vascular cambium
early
xylem

36. Woody stem How old is this tree? cork cambium vascular cambium late
early
xylem
phloem
bark