Transport in Plants

1. Transport in Plants

2. What is the tallest tree on the planet? Sequoia sempervirens - The coastal redwood (115m = 379 feet) Seems like it would require a pump, like you and I have, but a much larger one to transport substances from roots to leaves. Trees as we know do not have any “pumps” of that nauture. So how do they do it?

3. Maybe Plants Push Xylem Sap: Root Pressure • Water flows in from the root cortex generating a positive pressure that forces fluid up the xylem. This is upward push is called root pressure • Root pressure sometimes results in guttation, (the exudation of water droplets on tips of grass blades or the leaf margins of some small, herbaceous dicots in the morning). More water enters the leaves than is leaves it (transpired), and the excess is forced out of the leaf.

4. Plant transport mechanisms solve a fundamental biological problem: • The need to acquire materials from the environment and distribute them throughout the entire plant body

5. Activity1 • Clear nail polish • Leaves • Activity 2 • Flaccid carrot and cucumber slices • Bowl • dH2O, bottled water, tap water • Salt

6. Precursor 1: Water chemistry and characteristics • Polarity *H-bonds (Strong or weak? Can you draw and H-bond between 2 or more water molecules?) • Consequences include: Cohesion, Adhesion, Surface Tension…etc (properties of water)

7. Activity 3: A mini-experiment/demonstration • Indirect and relative measure of H-bond strength (as well as cohesion andadhesion) • Glass slides • Plastic cups • Water • Pennies • Masking Tape (Thumbs)

8. Precursor 2. Selective Permeability of Membranes • The selective permeability of a the plasma membrane controls the movement of solutes into and out of the cell AND the role of: • Specific transport proteins are involved in movement of solutes (and water too!) • Passive Transport – Diffusion, Facilitated Diffusion, Osmosis (Differences?) • Active Transport (Features of?)

9. EXTRACELLULAR FLUID CYTOPLASM – + H+ + – ATP H+ – + H+ Proton pump generates membrane potential and H+ gradient. H+ H+ H+ – H+ + H+ – + Proton Pumps Proton pumps create a hydrogen ion gradient that is a form of potential energy that can be harnessed to do work They contribute to a voltage known as a membrane potential (Plant cytoplasm is (-) compared to extracellular fluid) Consequences include: Fac diffusion of other cations Cotransport: symport and antiport (secondary active transport)

10. + – CYTOPLASM EXTRACELLULAR FLUID + – Cations ( , for example) are driven into the cell by themembrane potential. K+ K+ + – K+ K+ K+ K+ K+ – + K+ Transport protein – + (a) Membrane potential and cation uptake Membrane potential and cation uptake • Plant cells use the proton gradient and membrane potential to drive the transport of many different solutes (e.g. cation (+) uptake: opposites attract)

11. + – H+ H+ NO3 – + – NO3– + – Cell accumulates anions (, for example) by coupling their transport to theinward diffusion H+ H+ H+ NO3– H+ H+ H+ H+ of through a cotransporter. NO3– – NO3 – + NO3 – – + H+ NO3– – H+ + H+ H+ (b) Cotransport of anions Cotransport (symport) • In cotransport a transport protein (known as a symport) couples the passage of one solute to the passage of another in the same direction

12. Cotransport (Antiport) • Energy released as a molecule (e.g.H+) diffuses back into the cell and powers the active transport of a second molecule (ex. Ca++ or Na+) out of the cell

13. + – H+ H+ H+ S + – Plant cells can also accumulate a neutral solute, such as sucrose ( ), by cotransporting down the steep proton gradient. H+ – + H+ H+ S H+ – S H+ H+ H+ S S S – H+ + – + H+ S H+ + – (c) Cotransport of a neutral solute Sucrose uptake • The cotransport is also responsible for the uptake of the sugar sucrose (a neutral solute) by plant cells

14. An important membrane protein side note

15. Water Potential • To survive plants must balance water uptake and loss • Water potential is a measurement that combines the effects of solute concentration and physical pressure (due the presence of the plant cell wall) It is a measurement of the FREE amount of water molecules and the direction of movement of water (i.e. water’s potential to do work). • Water flows from regions of high water potential (areas of more free water molecules) to regions of low water potential (less free water molecules)

16. Ex. of water “doing work” on an organismal level

17. Which has the greatest water concentration? • A or B A or B • Water potential is essentially not much different

18. Getting a little technical - The water potential equation. Don’t freak out! Think Poseidon!

19. By convention, plant physiologists measure water potential in units of pressure called megapascals (MPa). Note: bars is acceptableFor a baseline, the water potential for pure water at 1ATM is expressed as having 0 Mpa or 0 bars

20. Breaking it down….

21. Cont’d

23. Consider this (U-tube Examples – AP Loves them) • An artificial model

24. Cont’d: Addition of Solute example

25. Cont’d – Positive Pressure Example

26. Cont’d: A negative pressure example

27. Connection to plants:

28. AP will not be thrilled however if that was your response to an “Explain what happens” prompt • So what’s a better answer?

29. AP “Explain what happens” prompt possible answers • 1 star = The cell gains water • 2 stars = Since water moves from high water potential to low water potential, it will enter the cell. • 3 stars = (include the data if provided) – Since the water potential for the cell is -0.7 bars and the surrounding environment has a water potential of 0 bars, water moves into the cell. • 4 stars = (include consequences ) Since the water potential for the cell is -0.7 bars and the surrounding environment has a water potential of 0 bars, water moves into the cell making it turgid.

30. Cont’d – Produce 4 star answer for scenario B(At home, not now ) • Note: The original cell has a starting water potential of -0.7 bars

31. Compare each situation with respect to the cytoplasm’s water potential and the surrounding environment’s water potential cell env. cell env. cell env. Water Pot: Bonus info, free of charge: What could you say about each situations: cell env cell env cell env Water concentration? Solute Concentration? Osmotic potential?

32. Collaborative Review/Study Break • On mini-poster paper • 1. Explain the role(s) of a gradient of protons in moving substances across a plant cell’s plasma membrane • 2. How do symports and antiports differ? Give an example of key substances each mechanism transports. • 3. What is “water potential” and discuss why it is important with respect to plant cells

33. CHECK YOUR VEGETABLE AND YOUR FRUIT!! • Evaluate your slices • Explain what has happened to them to a classmate (or to a teacher)

34. Next Step: How do roots take in water and minerals from the soil • Water and mineral salts from the soil enter the plant through the epidermis of roots and ultimately flow to and through the shoot system (xylem tissue) by bulk flow and active transport respectively. • Bulk flow – the group movement of molecules in response to a difference in pressure between two locations (see more later) • Soil solutionRoot Hair EpidermisRoot Cortex Root Xylem

35. Cont’d • Root Hairs • Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and where root hairs are located • Root hairs account for much of the surface area of roots

36. A mutulaistic symbiotic relationship. and a surface area multiplier

37. Cell wall Transport proteins in the plasma membrane regulate traffic of molecules between the cytosol and the cell wall. Transport proteins in the vacuolar membrane regulate traffic of molecules between the cytosol and the vacuole. Cytosol Vacuole (a) Cell compartments. The cell wall, cytosol, and vacuole are the three main compartments of most mature plant cells. Vacuolar membrane (tonoplast) Plasmodesma Plasma membrane Plant Cell Structure- more info for understanding transport • The vacuole is a large organelle that can occupy as much as 90% of more of the protoplast’s volume • The vacuolar membrane (the tonoplast) • Regulates transport between the cytosol and the vacuole

38. Key Symplast Apoplast Transmembrane route Apoplast The symplast is the continuum of cytosol connected by plasmodesmata. The apoplast is the continuum of cell walls and extracellular spaces. Symplast Symplastic route Apoplastic route Transport routes between cells. At the tissue level, there are three passages: the transmembrane, symplastic, and apoplastic routes. Water travels to the root xylem by one of three pathways Water and minerals can travel through a plant by one of three routes • Out of one cell, across a cell wall, and into another cell (transmembrane route) • Via the symplast (symplastic route) • Along the apoplast (apoplastic route)

39. Casparian strip Endodermal cell Pathway along apoplast Pathway through symplast 1 Uptake of soil solution by the hydrophilic walls of root hairs provides access to the apoplast. Water and minerals can then soak into the cortex along this matrix of walls. Casparian strip 2 Plasma membrane 1 Minerals and water that cross the plasma membranes of root hairs enter the symplast. Apoplastic route 2 Vessels (xylem) 3 As soil solution moves along the apoplast, some water and minerals are transported into the protoplasts of cells of the epidermis and cortex and then move inward via the symplast. Root hair Symplastic route Epidermis Endodermis Vascular cylinder Cortex 5 4 Endodermal cells and also parenchyma cells within the vascular cylinder discharge water and minerals into their walls (apoplast). The xylem vessels transport the water and minerals upward into the shoot system. Within the transverse and radial walls of each endodermal cell is the Casparian strip, a belt of waxy material (purple band) that blocks the passage of water and dissolved minerals. Only minerals already in the symplast or entering that pathway by crossing the plasma membrane of an endodermal cell can detour around the Casparian strip and pass into the vascular cylinder. Lateral transport of minerals and water in roots

40. The Endodermis • Is the innermost layer of cells in the root cortex • Surrounds the vascular cylinder and functions as the last checkpoint for the selective passage of minerals from the cortex into the vascular tissue • Water can cross the cortex via the symplast or apoplast • The waxy Casparian strip of the endodermal wall blocks apoplastic transfer (but not symplastic) of water and minerals from the cortex to the vascular cylinder

41. Ascent of Xylem Sap • Plants lose an enormous amount of water through transpiration (the loss of water vapor through the stomata) and the transpired water must be replaced by water transported up from the roots • Xylem sap rises to heights of more than 100 m in the tallest plants

42. Pulling Xylem Sap The Transpiration-Cohesion-Tension Theory • Transpirational Pull • Water transport begins as water evaporates from the walls of the mesophyll cells inside the leaves and into the intercellular spaces • Driven by the

43. Cohesion and Adhesion in the Ascent of Xylem Sap • The transpirational pull on xylem sap: • Solar Powered • Bulk Flow (pressure differences created by water potential differences) Is transmitted all the way from the leaves to the root tips and even into the soil solution It is facilitated by the cohesion and adhesion properties of water Narrow diameter of xylem

44. Cont’d • Transpiration produces negative pressure (tension) in the leaf which exerts a pulling force on water in the xylem, pulling water into the leaf • This water vapor escape through the stomata

45. The Transpiration Dance and • Transpiration animations • https://www.youtube.com/watch?v=U4rzLhz4HHk

46. 20 µm Stomata and Transpiration Control • Stomata help regulate the rate of transpiration • Leaves generally have broad surface areas and high surface-to-volume ratios. Good and bad: •  increase photosynthesis; •  Increase water loss through stomata

47. Check your nail-polished spinach leaves • Tear your leaf as to produce a “lip” of dried nail polish • Peel off as large a section of the dried nail polish only • Microscopic observation reveals imprint of the organization of the leaf surface – specifically stomata (guard cell) arrangement

48. Cells turgid/Stoma open Cells flaccid/Stoma closed Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell Stomata cont’d • About 90% of the water a plant loses escapes through stomata (lenticel, cuticle other 10%) • Each stoma is flanked by guard cells which control the diameter of the stoma by changing shape Guard Cells

49. H2O H2O H2O H2O Role of potassium in stomatal opening and closing. The transport of K+ (potassium ions, symbolized here as red dots) across the plasma membrane and vacuolar membrane causes the turgor changes of guard cells. H2O K+ H2O H2O H2O H2O H2O Shape changes due to multiple factors including: • Changes in turgor pressure that open and close stomata result primarily from the reversible uptake and loss of potassium ions (K+) by the guard cells • Creates water potential differences

50. Cuticle Upper epidermal tissue Lower epidermal tissue Trichomes (“hairs”) 100 m Stomata Xerophyte Adaptations That Reduce Transpiration • Xerophytes are plants adapted to arid climates • They have various leaf modifications that reduce the rate of transpiration • The stomata of xerophytes • Are concentrated on the lower leaf surface • Are often located in depressions that shelter the pores from the dry wind • Possess thicker waxy cuticles • Sunken stomata • Trichomes (“hair) Stomata in recessed crypts of Oleander plant