1. Transpiration

2. Slide 2 of 32 Transport Overview  Plants need CO 2, Sunlight and H 2 O in the leaves  ONLY H 2 O needs to be transported to the leaves  CO 2 gets in via stomata  Water is most of the mass of a plant  Carbon accounts for most of the mass of a dried plant

3. Slide 3 of 32 Fundamental Forces  Physical forces drive transport of materials in plants  Movement by concentration gradient -- Movement due to random molecular motion -- Diffusion or facilitated diffusion for things other than water -- Osmosis is for water -- Solutes move independently of water concentration  Movement by pressure gradient -- Bulk Flow – movement of water and solvents due to pressure gradient

4. Slide 4 of 32 3 Types of Transport in Vascular Plants 1. Transport of water & solutes by individual cells -- ALWAYS accomplished by diffusion -- Example: from soil to root hair cell -- Example 2: from one tracheid to another tracheid 2. Short-Distance transport of substances between cells at the tissue level -- ALWAYS accomplished by diffusion 3. Long-distance transport within the xylem & phloem among the entire plant -- ALWAYS accomplished by bulk flow (pressure gradient)

5. Slide 5 of 32 Individual Cell Movement  Passive Transport – movement down a gradient  Does NOT require energy  Simple diffusion, osmosis or facilitated diffusion  Active Transport – Movement against a electrochemical gradient  Requires energy  Most solutes must use transport proteins  Aquaporin – channel (transport) protein for water

6. Slide 6 of 32 Water Potential (Ψ)  Water moves from High concentration (of water, not solute concnetration) to Low concentration via osmosis  Water mover from high pressure to low pressure via bulk flow  Water potential is the combined effect of  Solute Concentration  Physical Pressure  Ψ = Ψs + Ψp  Conclusion: water moves from high water potential to low water potential

7. Slide 7 of 32 Solute Potential (Ψs)  Solute potential (Ψs) is proportional to the number of dissolved solute particles  Also called Osmotic Potential  Ψs = -iCRT  Ψs of water = 0  Addition of solute  Decrease in water potential  More solute = less water (realtively) = lower water potential  Ψs ≤ 0

8. Slide 8 of 32 Pressure Potential (Ψp)  Pressure Potential (Ψp)  Physical pressure on a solution  Created by placing physical pressure (+) or by vacuum/sucking (-)  Water is usually under a positive pressure potential  Turgor pressure – when cell contents press the plasma membrane against the cell wall  Drying out = Negative pressure potential

9. Slide 9 of 32 Water Potential Examples

10. Slide 10 of 32 Short-Distance Transport Symplast  Cytoplasmic continuum (called Symplast) consists of the cytosol of cells and the plasmodesmata connecting the cytosols.  Crosses membrane early in the process Apoplast  Continuum of cell walls + extracellular spaces  Only crosses a membrane at endodermis Transmembrane  Self-evident & highly inefficient

11. Slide 11 of 32 Long Distance Transport  Accomplished by Bulk Flow  Water movement from regions of high pressure to regions of low pressure  Movement in both xylem and phloem is driven by pressure differences between opposite ends of vessels or sieve tubes.  Diffusion is a poor driver over long distances (roots to leaves)  In xylem, water & minerals travel by negative pressure  Transpiration and root push  In phloem, hydrostatic pressure forces materials down

12. Follow a molecule of water or mineral…

13. Slide 13 of 32 Roots & Water Absorption  Root hairs = absorption of water  Root hairs increase surface area for absorption  Hydrophilic cell walls absorbs soil solution (water and minerals)  Mycorrhizae are important for absorption as well  Root epidermis  cortex  vascular cylinder (xylem)  Called Lateral Transport (Short Distance Transport)  To rest of plant via xylem

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15. Slide 15 of 32 Casparian Strip  In the endodermis  Waxy material encircling the cells of the endodermis  Ensures that any water or solutes must pass through a plasma membrane before entering xylem  Impedes apoplastic transfer  Critical control point  Again, plasma membrane controls what can enter the xylem

16. Slide 16 of 32 Xylem moves vertically, how?  After water or minerals gets past the endodermis, most will find its way to the xylem  BULK FLOW, not concentration differences drives this transport  2 PRESSURE differences drive this  Root Pressure or root push  Transpiration (much more important)

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18. Slide 18 of 32 Root Pressure  Water diffusing into the root cortex = positive pressure  This pressure forces fluid up the xylem  Weak force – can only propel fluids up a couple of feet

19. Slide 19 of 32 Transpiration  Your book calls this: transpiration-cohesion-tension mechanism  In leaves, water is lost through stomata  Why? Lower water pressure in air than in leaves  Water is drawn up in to this area of negative pressure  Water molecules pull up other water molecules  Cohesion – water on water action  Adhesion – water to cell wall action  Via Hydrogen bonds

20. Slide 20 of 32 Transpiration (Page 2)  Transmitted all the way from Leaves to the soil solution  Again, due to PRESSURE differential, not concentration  Small diameter of vessel elements and tracheids increases adhesion  Transpiration is ultimately due to stomata  Necessary water loss for CO 2 uptake and O 2 removal  If stomata closed, then less photosynthesis and plant may overheat

21. Slide 21 of 32 Transpiration (Page 3)  1 molecule of H2O evaporates due to transpiration, another molecule is drawn from the roots to replace it.  Factors that influence transpiration  High humidity = DECREASE transpiration  Wind = INCREASE transpiration  Increasing light intensity = INCREASE transpiration  Close stomata = NO transpiration

22. Slide 22 of 32  90% of water lost by plants is through stomata  Stomata account for 1% of leaf surface area  Guard cells control opening & closing of stomata

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24. Slide 24 of 32 Phloem Translocation  Photosynthetic products (Phloem Sap) are translocated through the phloem  Translocation literally means “movement from place to place”  30% of phloem sap is sucrose, but it can be any assimilate form of sugars (G3P)  Translocation is NOT a one-way transport mechanism  Sieve tube elements carry sugar from source to sink  Source – leaves (net producer of sugar)  Sink – roots (net consumer of sugar)

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26. Slide 26 of 32  Sucrose is added at the sugar source (leaves)  Sucrose first moves in by diffusion  H2O follows  Once sucrose concentration is too high, an electrochemical gradient is created to move sucrose into phloem by cotransport  Decreases water potential in phloem, so creates positive pressure  Phloem sap is propelled away from the source  Where sugar is used, negative pressure is found  Used in respiration  Converted to starch or cellulose

27. Slide 27 of 32  Sugar loading into the sieve-tubes is necessary prior to any bulk flow  Movement through the sugar source cells can be either apoplastic or symplastic  Symplastic movement occurs via plasmodesmata

28. Slide 28 of 32  Where sugar is used = sink  Concentration in sink is lower than in phloem  So sugar concentration gradient = diffusion of sugar and then water out of the phloem  So lower pressure at the sink  Sugar may be -- Used in respiration -- Converted to starch

29. Slide 29 of 32 Pressure Flow Hypothesis  Also called mass flow (bulk flow) hypothesis  Phloem sap moves from source to sink at 1 m/hr, which is far faster than diffusion or cytoplasmic streaming  So it is the PRESSURE differential that moves phloem sap  Pressure builds at source  Pressure falls at sink

30. Slide 30 of 32 Sucrose Loading  From cell to cell through the plasmodesmata (Symplast) OR Along cell walls (apoplast)  Surface membranes of companion cells actively pump sucrose into the sieve tube’s cytoplasm.

31. Slide 31 of 32 The accumulation of sucrose and other solutes, such as amino acids, in sieve elements lowers the water potential so that water diffuses in by osmosis from adjacent cells and from the xylem. This creates pressure in the sieve elements causing the liquid (phloem sap) to flow out of the leaf.

32. Slide 32 of 32 Sucrose is unloaded at sinks. This is taken up by the cells and is respired or stored as starch. This reduces the concentration of phloem sap and lowers the pressure, so helping to maintain a pressure gradient form source to sink so the sap keeps flowing in the phloem.