PLANT TRANSPORT “WATER POTENTIAL” Remember Osmosis??? It results in the net uptake or loss of water by the plant cell and depends on whether the cell or the extracellular fluid has a higher or lower…… “WATER POTENTIAL”

So…. What exactly is WATER “POTENTIAL”? Here is the “Helm Definition”: The potential to….. MOVE!!!! In other words: Water will always move across a membrane from an area of high to low potential.

Let’s see if you get it: More HYPERtonic = More HYPOtonic = LESS Potential More HYPOtonic = MORE Potential More Pressure = More Potential Less Pressure = LESS Potential So… Potential results from a combination of BOTH solute concentration AND pressure. Negative Pressure (or Tension) pulls in water ….and don’t forget about the role of the central vacuole!!

Lateral Transport: 3 main routes: 1. Across the membrane/cell walls: (you know… through the cellulose & phospholipid bilayer) 2. The SYMPLAST- through the continuum of cytoplasm formed by plasmodesmata. 3. The APOPLAST- between the actual plant cells -never enters the cells, just works its way through the matrix of cell walls Campbell Animation

Long Distance Transport… Xylem for Water Phloem for “Phooood” (sap)

Transpiration “Plant Sweat” Transpiration / Photosynthesis Ratio- g water lost / g CO2 assimilated The __________ the better?? LOWER! Example: 600:1 in C3 plants 300:1 in C4 plants Benefits of Transpiration: Assists in mineral transfer from root to shoot Evaporative cooling keeps enzymes from overheating Drawbacks: if transpiration exceeds delivery of water… Stomata close, photosynthesis ceases, plants wilt.

Stomata & Guard Cells When guard cells are turgid, they buckle due to radially arranged microfibrils. Turgid = Buckled = OPEN!! Flaccid = Unbuckled = CLOSED!!! Caused by the reversible uptake & loss of K+ ions by Guard Cells. More K+, higher [solute], lower p , water goes IN, microfibrils buckle, STOMATA OPEN! (and vise- versa, of course) K+ movement is triggered by H+ out by proton pumps.

Open or Closed?? OPEN: LIGHT: stimulates proton pumps by making ATP available from photosynthesis. DECREASE in CO2: from increased Photosynthesis. CIRCADIAN RHYTHMS: internal clock (plants are often on a 24 hour cycle even in the dark)

Open or Closed? Closed: WATER DEFICIENCY- makes guard cells flaccid ABSCISIC ACID PRODUCTION- hormone that is activated in response to low water. HIGH TEMPERATURES- increases CO2 due to increased respiration. http://www.tvdsb.on.ca/westmin/science/sbioac/plants/stoma.htm

Campbell Animation OK… so what about what’s goin’ on with the PHLOEM?? Translocation: food/phloem transport Sugar source: sugar production organ (mature leaves) Sugar sink: sugar storage organ (growing roots, tips, stems, fruit) 1- loading of sugar into sieve tube at source reduces water potential inside; this causes tube to take up water from surroundings by osmosis 2- this absorption of water generates pressure that forces sap to flow along the tube 3- pressure gradient in tube is reinforced by unloading of sugar and consequent loss of water from tube at the sink 4- xylem then recycles water from sink to source Campbell Animation