1. CHAPTER 38 LECTURE SLIDES
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Transport in Plants
Chapter 38

3. Transport Mechanisms Water first enters the roots
Then moves to the xylem
Innermost vascular tissue
Water rises through the xylem because of a combination of factors
Most of that water exits through the stomata in the leaves

4. Water and dissolved minerals travel great distances in xylem
Some “pushing” comes from pressure of water entering roots
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)

6. Movement of water at cellular level
Water can diffuse through plasma membranes
Other substances depend on protein transporters
Facilitated diffusion or active transport
ATP-dependent hydrogen ion pumps often fuel active transport
Unequal concentrations of solutes drive osmosis

8. Water Potential Potentials are a way to represent free energy
Water potential (yw) is used to predict which way water will move
Water moves from higher to lower yw
Measured in units of pressure called megapascals (MPa)

9. Osmosis If a single plant cell is placed into water
Concentration of solutes inside cell greater than solution
Water moves into cell by osmosis
Cell expands and becomes turgid
If cell placed in high concentration of sucrose
Water leaves cell
Cell shrinks – plasmolysis

10. Water potential has 2 components
Physical forces such as plant cell wall or gravity
Contribution of gravity usually not considered
Turgor pressure resulting from pressure against cell wall is pressure potential yp
As turgor pressure increases, yp increases
Concentration of solute in each solution

11. Solutions that are not contained within a vessel or membrane cannot have turgor pressure, and they always have a Ψp of 0 MPa

12. Concentration of solute in each solution
Solute potential (ys) – smallest amount of pressure needed to stop osmosis
Pure water has a ys of 0
As solute concentration increases, ys decreases (< 0 MPa)
A solution with a higher solute concentration has a more negative ys

14. yw = yp + ys
The total water potential of a plant cell is the sum of its pressure potential (yp) and solute potential (ys)
Represents the total potential energy of the water in the cell
When the yw inside the cell equals that of the solution, there is no net movement of water

15. c. Water Potential ψ = ψs + ψp ψcell = – 0.7 MPa + 0.5 MPa = – 0.2 MPa
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Water Potential
ψ = ψs + ψp
ψcell = – 0.7 MPa MPa = – 0.2 MPa
ψsolution = –0.2 MPa (solution has no pressure potential) c.

18. Osmosis is enhanced by membrane water channels called aquaporins
Once thought water moved across plasma membranes only by osmosis through the lipid bilayer
Water moved more rapidly than predicted
Osmosis is enhanced by membrane water channels called aquaporins
Speed up osmosis but do not change direction of water movement

19. 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

20. Symporters contribute to the yw gradient that determines the
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Air
Water
enters
plant
through
roots.
Decreasing water potential
Plant
Soil
H2O
Soil
Cytosol
Symporters contribute
to the yw gradient that
determines the
directional flow of
water. H+
Symporter
Mineral
ions
Soil
Water
-0.5
-1.0
-100
ψw Water potential (MPa)

21. result in higher rate of transpiration than smooth cell surfaces.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Air
Water exits plant
through stomata.
Smooth
surface
Smooth
surface
Rippled
surface
Rippled
surface
H2O
The water film that
coats mesophyll
cell walls evaporates.
Rippled cell surfaces
result in higher rate of
transpiration than
smooth cell surfaces.
Decreasing water potential
Plant
Water moves up plant
through xylem.
Adhesion due to polarity
of water molecules
Cohesion by
hydrogen bonding
between water
molecules
Soil
-0.5
-1.0
-100
ψw Water potential (MPa)

22. Three transport routes exist through cells
Apoplast route – movement through the cell walls and the space between cells
Avoids membrane transport
Symplast route – cytoplasm continuum between cells connected by plasmodesmata
Transmembrane route – membrane transport between cells and across the membranes of vacuoles within cells
Permits the greatest control

24. Eventually on their journey inward, molecules reach the endodermis
Any further passage through the cell walls is blocked by the Casparian strips
Molecules must pass through the cell membranes and protoplasts of the endodermal cells to reach the xylem

25. Because the mineral ion concentration in the soil water is usually much lower than it is in the plant, an expenditure of energy (ATP) is required for these ions to accumulate in root cells
Plasma membranes of endodermal cells contain a variety of protein transport channels, through which proton pumps transport specific ions against even larger concentration gradients

26. Xylem Transport
Root pressure is caused by the continuous accumulation of ions in the roots
When transpiration from leaves is low or absent – at night
Causes water to move into plant and up the xylem despite the absence of transpiration
Guttation (production of dew) is loss of water from leaves when root pressure is high
Root pressure alone, however, is insufficient to explain xylem transport
Transpiration provides the main force

27. Water potential regulates the movement of water through a whole plant
Water moves from the soil into the plant only if water potential of the soil is greater than in the root
Water in a plant moves along a yw gradient from the soil to successively more negative water potentials in the roots, stems, leaves, and atmosphere

29. Water has an inherent tensile strength that arises from the cohesion of its molecules
The tensile strength of a water column varies inversely with its diameter
Because tracheids and vessels are tiny in diameter, they have strong cohesive water forces
The long column of water is further stabilized by adhesive forces

30. Cavitation
An air bubble can break the tensile strength of a water column
A gas-filled bubble can expand and block the tracheid or vessel
Damage can be minimized by anatomical adaptations
Presence of alternative pathways
Pores smaller than air bubbles

32. Tracheids and vessels are essential for the bulk transport of minerals
Ultimately the minerals are relocated through the xylem from the roots to other metabolically active parts of the plant
Phosphorus, potassium, nitrogen, and sometimes iron
Calcium, an essential nutrient, cannot be transported elsewhere once it has been deposited in a particular plant part

33. 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

34. 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–), and malate
Water enters osmotically

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

38. 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–, and malate channels
Water loss follows

40. Other pathways regulating stomata
Close when CO2 concentrations are high
Open when blue wavelengths of light promote uptake of K+ by the guard cells
Close when temperature exceeds 30º–34ºC and water relations unfavorable
Alternative photosynthetic pathways, such as Crassulacean acid metabolism (CAM), reduce transpiration

41. 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

43. Plants have adapted to flooding conditions which deplete available oxygen
Flooding may lead to abnormal growth
Oxygen deprivation most significant problem
Form larger lenticels and adventitious roots
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

45. Plants such as mangroves grow in areas flooded with salt water
Must supply oxygen to submerged roots and control salt balance
Pneumatophores – long, spongy, air-filled roots that emerge above the mud
Provide oxygen to submerged roots
Succulent leaves contain large amount of water to dilute salt
May secrete salt or block salt uptake

47. Halophytes
Plants that can tolerate soils with high salt concentrations
Some produce high concentrations of organic molecules in their roots
This decreases the water potential enhancing water uptake from the soil

48. Phloem Transport
Most carbohydrates produced in leaves are distributed through phloem to rest of plant
Translocation provides building blocks for actively growing regions of the plant
Also transports hormones, mRNA, and other molecules
Variety of sugars, amino acids, organic acids, proteins, and ions

49. Copyright © The McGraw-Hill Companies, Inc
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Phloem
fluid
Stylet
Phloem a.
400 µm b.
25 µm
a: © Andrew Syred/Photo Researchers, Inc.; b: © Bruce Iverson Photomicrography.
Using aphids to obtain the critical samples and radioactive tracers to mark them, plant biologists have demonstrated that substances in phloem can move remarkably fast, as much as 50 to 100 cm/h

50. Pressure-flow theory is a model describing the movement of carbohydrates in phloem
Dissolved carbohydrates flow from a source and are released at a sink
Sources include photosynthetic tissues
Food-storage tissue can be sources or sinks
Sinks include growing root and stem tips as well as developing fruits

51. Phloem-loading occurs at the source
Carbohydrates enter the sieve tubes in the smallest veins at the source
Sieve cells must be alive to use active transport to load sucrose
Water flows into sieve tubes by osmosis
Turgor pressure drives fluid throughout plant
At sink, sucrose actively removed and water follows by osmosis
Water my be recirculated in xylem or lost