1. Chapter 36
Transport in Plants

2. Plants require: CO2 Light Sugar H2O O2 Minerals Macronutrients
(besides C, H, O)
Ca, K, Mg, N, P, S
Micronutrients
B, Cl, Cu, Fe, Mn, Mo, Ni, Zn
Oxygen is taken in by roots, and is a by-product of PSN.
Fig. 36.2

3. Here we explore how these substances are transported within plants…
but first we need some more background…
Fig. 36.2

4. Background…
Cell wall helps maintain a cell’s shape, but the plasma membrane regulates the traffic of molecules into and out of a cell
Fig. 36.8

5. Plasmodesmata provide cytosolic connections among cells
Background…
Plasmodesmata provide cytosolic connections among cells
Cytosol = cytoplasm minus organelles
Fig. 36.8

6. Background…
Vacuoles often account for 90% of a plant cell’s volume, but they are never shared by adjacent cells
Fig. 36.8

7. Background…
Substances can move from cell to cell via the symplastic, apoplastic, or transmembrane routes
Fig. 36.8

8. Solutes tend to diffuse down concentration gradients
Background…
Solutes tend to diffuse down concentration gradients
Passive transport is diffusion across a membrane
Passive transport is generally slow, unless solutes travel through transport proteins in the membrane
Some transport proteins are selective channels
Some selective channels are “gated” – environmental stimuli open or close them

9. E.g., proton pumps are very common active transport proteins
Background…
Active transport requires energy to move solutes up their concentration or charge gradient
E.g., proton pumps are very common active transport proteins
Fig. 36.3
Proton pumps create membrane potentials; potential energy can be used to perform cellular work

10. The K+ ions pass through transport proteins
Background…
The membrane potential provides the energy to uptake some minerals, e.g., K+ ions
The K+ ions pass through transport proteins
Fig. 36.4

11. The coupled ions pass through [co-]transport proteins
Background…
The membrane potential provides the energy for cotransport of ions up their concentration gradients, as H+ ions move down theirs
Fig. 36.4
The coupled ions pass through [co-]transport proteins

12. Background…
The membrane potential provides the energy for cotransport of some neutral molecules (e.g., sugar) up their concentration gradients, as H+ ions move down theirs
Fig. 36.4

13. Osmosis is diffusion of water across a membrane
Background…
Osmosis is diffusion of water across a membrane
To predict the movement of water across a membrane alone (e.g., an animal cell), it is sufficient to know whether the inside solute concentration is < or > the outside solute concentration
To predict the movement of water across a membrane plus cell wall (e.g., a plant cell), we must know both the solute concentration difference and the pressure difference

14. Ψ (psi)  measured in MPa (megapascals; 1 MPa ≈ 10 atmospheres)
Background…
Water potential is a combined measure of solute concentration and pressure
Ψ (psi)  measured in MPa (megapascals; 1 MPa ≈ 10 atmospheres)
Pure water in an open container: Ψ = 0 MPa
Solutes reduce the value of Ψ
Pressure increases the value of Ψ
Negative pressure (tension) decreases the value of Ψ

15. Ψ = ΨP + ΨS ΨP can be positive, 0, or negative ΨS is always  0
Background…
Water potential = pressure potential + solute (osmotic) potential
Ψ = ΨP + ΨS
ΨP can be positive, 0, or negative
ΨS is always  0
Water will always move across a membrane from higher to lower Ψ

16. Let’s consider 4 examples
Background…
Let’s consider 4 examples
Add solutes alone to one side.
Fig. 36.5

17. Let’s consider 4 examples
Background…
Let’s consider 4 examples
Add solutes and pressure simultaneously; the two in correct amounts cancel each other, since solute concentration is inversely proportional to water potential and pressure is directly proportional to water potential.
Fig. 36.5

18. Let’s consider 4 examples
Background…
Let’s consider 4 examples
Add solutes and even more pressure…
Fig. 36.5

19. Let’s consider 4 examples
Background…
Let’s consider 4 examples
Add solutes to one side, but even more negative pressure to the other; in this case the negative pressure on one side overrides what would otherwise be a tendency for water to move towards the solutes.
Fig. 36.5

20. Background…
Let’s consider real cells
Flaccid cell:
ΨP = 0
Fig. 36.6

21. Background…
Let’s consider real cells
Flaccid cell:
ΨP = 0
Fig. 36.6

22. ΨP = 0 Background… Let’s consider real cells Flaccid cell:
If a cell loses water to the environment, it plasmolyzes
Fig. 36.6

23. Background…
Let’s consider real cells
Flaccid cell:
ΨP = 0
Fig. 36.6

24. Background…
Let’s consider real cells
Flaccid cell:
ΨP = 0
Fig. 36.6

25. ΨP = 0 Background… Let’s consider real cells Flaccid cell:
If a cell gains water from the environment, it becomes turgid
Turgor = pressure that keeps cell membrane pressed against cell wall
Fig. 36.6

26. ΨP = 0 Background… Let’s consider real cells Flaccid cell:
If a cell gains water from the environment, it becomes turgid
Aquaporins are transport proteins that form channels for water
Fig. 36.6

27. Armed with this background…
How do roots absorb water and minerals?

28. How do roots absorb water and minerals?
Solutes pass into roots from the dilute soil solution
Note: See the early part of the lecture for the details of how active transport creates membrane potentials that fuel the uptake of ions.
Fig. 36.9

29. How do roots absorb water and minerals?
Symplastic route:
Active transport occurs through proton pumps, that set up membrane potentials, that drive the uptake of mineral ions
Fig. 36.9

30. How do roots absorb water and minerals?
Apoplastic route:
Some water and dissolved minerals passively diffuse into cell walls
Fig. 36.9

31. How do roots absorb water and minerals?
Solutes diffuse through the cells (or cell walls) of the epidermis and cortex (the innermost layer of which is the endodermis)
Fig. 36.9

32. How do roots absorb water and minerals?
At the endodermis, only the symplastic route is accessible, owing to the Casparian strip
Chapt. 35 observed that the endodermis regulates the passage of substances into the vascular stele
Fig. 36.9

33. How do roots absorb water and minerals?
The final layer of live cells actively transports solutes into their cell walls
Solutes then diffuse into xylem vessels to be transported upward
Fig. 36.9

34. How do roots absorb water and minerals?
The final layer of live cells actively transports solutes into their cell walls
The final layer may be an endodermal cell…
Fig. 36.9

35. How do roots absorb water and minerals?
The final layer of live cells actively transports solutes into their cell walls.
… or a cell of the pericycle (outermost layer of stele)
Fig. 36.9

36. How do roots absorb water and minerals?
Note: In this figure the pericycle is drawn as a continuous layer of cells

37. How do roots absorb water and minerals?
Mycorrhizal mutualism (fungus + roots)
Fungus helps plant obtain water and minerals (e.g., P)
Plant feeds sugars to the fungus
No fungus
With fungus

38. How do roots absorb water and minerals?
Some species (including many legumes) have root nodules that house N-fixing bacteria
Bacteria convert N2 to NH4+ (ammonium), providing plant with fixed N
Plant feeds sugars to the bacteria

39. How does xylem transport xylem sap?

40. In some species of trees the process moves water and nutrients
How does xylem transport xylem sap?
In some species of trees the process moves water and nutrients
> 100 m upwards!
Fig

41. How does xylem transport xylem sap?
Nearly all of the energy to drive the process comes from the sun
Evapor-ation at the top pulls water up from the bottom
Unbroken chains of water molecules (held together by cohesive
H-bonds) fill xylem vessels
Fig

42. The evaporation of water out of leaves is called transpiration
How does xylem transport xylem sap?
The evaporation of water out of leaves is called transpiration
Fig

43. Water vapor escapes through stomata
How does xylem transport xylem sap?
Water vapor escapes through stomata
Fig
Fig

44. Transpiration creates a water pressure gradient
How does xylem transport xylem sap?
Transpiration creates a water pressure gradient
Fig

45. How does xylem transport xylem sap?
Transpiration creates a water pressure gradient
Water flows upward through xylem vessels by bulk flow down the pressure gradient
Lower Ψ at the top is the tension that pulls water up from the bottom
Fig

46. The Transpiration-Cohesion-Tension Mechanism
How does xylem transport xylem sap?
The Transpiration-Cohesion-Tension Mechanism
Fig

47. How do plants regulate the transport of xylem sap?

48. K+ is actively transported into and out of guard cells
How do plants regulate the transport of xylem sap?
Stomata
K+ is actively transported into and out of guard cells

49. How do plants regulate the transport of xylem sap?
Stomata
When [K+] is high, the amount of H20 is high, and guard cells open stomata

50. How do plants regulate the transport of xylem sap?
Stomata
When [K+] is low, the amount of H20 is low, and guard cells close stomata

51. Light stimulates the uptake of K+ by guard cells, opening stomata
How do plants regulate the transport of xylem sap?
Stomata
Light stimulates the uptake of K+ by guard cells, opening stomata

52. Low [CO2] stimulates the uptake of K+ by guard cells, opening stomata
How do plants regulate the transport of xylem sap?
Stomata
Low [CO2] stimulates the uptake of K+ by guard cells, opening stomata

53. How do plants regulate the transport of xylem sap?
Stomata
Low H2O availability inhibits the uptake of K+ by guard cells, closing stomata

54. How does phloem transport phloem sap?

55. How does phloem transport phloem sap?
Sugars manufactured in leaves diffuse to phloem companion cells
Fig

56. How does phloem transport phloem sap?
Companion cells actively transport sugars into sieve-tube members (elements)
Fig

57. How does phloem transport phloem sap?
Food (sugars) are then translocated from sources to sinks according to the Pressure-Flow Theory:
1. At sources, sugars are actively transported into phloem
Fig

58. How does phloem transport phloem sap?
Food (sugars) are then translocated from sources to sinks according to the Pressure-Flow Theory:
2. Water follows by osmosis from source cells and xylem;
this creates high pressure
Fig

59. How does phloem transport phloem sap?
Food (sugars) are then translocated from sources to sinks according to the Pressure-Flow Theory:
3. At the sink, sugars diffuse out of the phloem and water follows by osmosis;
this creates low pressure
Fig

60. How does phloem transport phloem sap?
Food (sugars) are then translocated from sources to sinks according to the Pressure-Flow Theory:
Sugar solution flows from high to low pressure
Fig

61. How does phloem transport phloem sap?
Food (sugars) are then translocated from sources to sinks according to the Pressure-Flow Theory:
4. Water may be taken up by the transpiration stream in the xylem
Fig

62. How does phloem transport phloem sap?
Pressure-Flow Theory

63. Not all herbivores chew leaves… Some exploit sap
E.g., aphids tap sieve-tube
elements for phloem sap