3 Transport Processes 1. Transport is a fundamental aspect of life. 2. Diffusion is the movement of solutes from areas of high solute concentration to areas of low solute concentration.3. No energy required (passive)4. If solutes only moved by diffusion……….Need coordinated transport processes in order to move solutes & water faster and farther.
4 Active transport1. Solutes move against their gradient (i.e. to a higher solute concentration)2. Requires energy: driven by breaking ATP’s high–energy phosphate bonds.3. Facilitated by membrane proteins called proton pumps – trans-membrane proteins. ATP gives energy to pump H+ out of a cell.A. Creates a membrane potential (+ outside, - inside) that drives positive ions into the cell (like K+).B. The diffusion of H+ back down the gradient (into cell) can also co-transport anions & neutral substances against their concentration gradient.
6 Transport Processes1. Occur at virtually every level of biological organization.2. Enzymes transport electrons, protons, acetyl groups.3. Membranes transport material across themselves.4. Cells transport material to and from other cells, and within themselves.5. Whole organisms transport water, etc from one organ to another
7 Transport in Plants1. Plants adapted to the division of resources in the land environment (soil & air) by the differentiation of the plant body into roots and shoots.2. But this created a new dilemma, the need to transport materials between roots and shoots.3. Sometimes up to 100m away, in all kinds of environmental conditions.4. What’s a poor plant to do?
8 Three levels of transport in plants: 1. Cellular – uptake of H2O and solutes by individual cells.2. Short-distance - between cells3. Long-distance – throughout whole plant (xylem & phloem)
9 Transport at the Cellular Level 1. Diffusion2. Osmosis – diffusion of H20 from areas of low solute concentration into areas of high solute concentration (i.e. water always acts to dilute)3. Facilitated Diffusion - same as diffusion but with through trans-membrane proteins4. Active TransportReview of cellular transport:
10 An artificial cell consisting of an aqueous solution enclosed in a selectively permeable membrane has just been immersed in a beaker containing a different solution. The membrane is permeable to water and to the simple sugars glucose and fructose but completely impermeable to the disaccharide sucrose.Source: Campbell/Reece - Biology, Sixth Edition, EOC Self-Quiz Question #14-18
11 Which solute(s) will exhibit a net diffusion into the cell? Which solute(s) will exhibit a net diffusion into the cell?sucroseglucosefructoseAnswer: cSource: Campbell/Reece - Biology, Sixth Edition, EOC Self-Quiz Question #14-18
12 Which solute(s) will exhibit a net diffusion out of the cell? Which solute(s) will exhibit a net diffusion out of the cell?sucroseglucosefructoseAnswer: bSource: Campbell/Reece - Biology, Sixth Edition, EOC Self-Quiz Question #14-18
13 Which solution is hypertonic to the other? Which solution is hypertonic to the other?the cell contentsthe environmentAnswer: aSource: Campbell/Reece - Biology, Sixth Edition, EOC Self-Quiz Question #14-18
14 In which direction will there be a net osmotic movement of water? In which direction will there be a net osmotic movement of water?out of the cellinto the cellneitherAnswer: bSource: Campbell/Reece - Biology, Sixth Edition, EOC Self-Quiz Question #14-18
15 After the cell is placed in the beaker, which of the following changes will occur?The artificial cell will become more flaccid.The artificial cell will become more turgid.Answer: bSource: Campbell/Reece - Biology, Sixth Edition, EOC Self-Quiz Question #14-18
16 Directional movement of water is driven by: Water Potential 1. Water potential refers to the free energy of water, its capacity to do work.2. By definition pure free water has a water potential of zero.3. Water potential can be increased byheatingpressureelevation4. Water potential can be decreased by ?
17 Components of Y Y = Ys + Yp + Ym Yš = osmotic potential Yp = pressure potentialYm= matric potential
18 Ys = osmotic potential1. Osmotic potential is a measure of the effect that solutes have on water potential.2. Pure water has a osmotic potential of zero.3. Adding solutes decreases the osmotic potential because water interacts with solutes. (more solutes = more negative)4. Thus Ys is always negative (if solute is present)
19 Yp = pressure potential 1. A measure of the effect that pressure has on water potential.2. Pressure can be positive (when something is compressed). Pushing3. Pressure can be negative (when something is stretched or pulled). Called tension.4. Water can handle large amounts of tension because of cohesion – the tendency of water to stick to itself (H bonding)
21 Ym= matric potential1. Matric potential is a measure of water’s adhesion to non-dissolved but hydrophilic structures such as cell walls, membranes, soil particles etc.2. Adhesion can only decrease water’s free energy.3. So matric potential is always negative
22 Why would water move to where it is less free? 1. Water moves from regions of high Ψ (less negative) to regions of lower Ψ (more negative).Why would water move to where it is less free?Water acts to dilute, hydrate, decrease tensioni.e. water acts to stabilize water potentialsIf two Ψ’s are equal, no net movement of water
25 Cells and water movement 1. A flaccid cell (Yp = 0). No pressure is being exerted against the inside of the cell wall. Cell is not firm.Why? Solute concentration within the cell is lower than surroundings. Water leaves the cell.2. A turgid cell (Yp = +) is filled with water, exerting pressure against its cell walls. Cell is firm.Why? Solute concentration within the cell is higher than surroundings. Water moves into the cell.
26 Cells and water movement 1. Hypotonic solution – low solute concentration2. Hypertonic solution – high solute concentration3. A flaccid cell placed in a hypotonic solution will: ?4. A turgid cell placed in a hypertonic solution will:?This causes the cell membrane to shrink away from the cell wall (i.e. plasmolyze)
29 Short Distance (Lateral) Transport – between cells Three routes for lateral transport:1. Transmembrane – water & solutes move across the plasma membranes and cell walls of adjacent cells2. Symplast – movement through a continuum of cytoplasm connected by the plasmodesmata of cell walls.3. Apoplast – extracellular pathway; movement through the continuous matrix of cell walls
31 Examples of Short Distance Transport 1. Guard cells2. Motor cells3. Transfer cells4. Absorption of water & minerals by roots
32 Guard cells 1. control stomatal diameter by changing shape. A. Take up water, become turgid, buckle => stomata are openB. Lose water, become flaccid, stomata close
33 Guard cells C. Opening Mechanism: a. K+ is pumped into GC by active transport. Proton pump creates membrane potential that drives K+ in.b. Thus Ψ inside cell is lower than outside cell.c. Water diffuses into the guard cell.d. Sunlight, circadian rhythms, & low CO2 concentration in leaf air spaces stimulate the proton pumps & thus stomatal opening
34 Guard cells D. Closing mechanism: a. Proton pumps no longer active (darkness)b. K+ is lost from the GC, creating lower water potential outside cell.c. Water flows out of GC and cells become flaccidd. Stomatal closure during the day stimulated by water stress – not enough water to keep GCs turgid
36 Motor Cells 1. Examples: prayer plant Oxalis, Venus’ flytrap 2. Leaves of these plants can flex & fold in response to stimuli3. Motor cells are the “joints” where this flexing occurs.4. Accumulate or expel potassium to adjust their Ψ & thus turgidity.5. Oxalis – leaves fold in sunlight to minimize transpiration; open in shade6. Transpiration = loss of water vapor from the stomata
39 Transfer Cells1. Cell walls have many finger–like projections on the inner surface.2. The plasma membrane is pressed firmly against these convolutions, creating an increase in surface area3. Greater surface area means more molecular pumps & thus high – volume solute transport4. Found in areas of rapid, high volume transport: salt-excreting glands, sugar loading into phloem
40 Absorption of water & minerals by roots 1. Soil particles coated with water, minerals; adhere to hydrophyllic epidermal cells of root hair2. Soil solution moves freely through epidermal cells & cortex via symplast and apoplast pathways3. Endodermis – selective barrier to soil solution between cortex & stele. Sealed together by the waxy Casparian strip – forces soil solution in apoplast to pass through the selectively permeable membrane of the endodermis.4. Once through the endodermis, soil solution freely enters the xylem
43 Mechanisms of Long Distance Transport 1. Xylem: Transpiration (evaporation from leaves) creates a tension which pulls sap up from the roots, in direction of lower Ψ.2. Phloem: Hydrostatic pressure at one end of the sieve tube forces sap to the other end of the tube (= bulk flow).
44 Transport of xylem sap 1. Xylem sap both pushed & pulled up the stem 2. Pushed by root pressureA. Stele has high concentration of minerals, decreasing Ψ. Water flows in, creating pushing pressureB. Guttation – exudation of water droplets by leaves during the night when transpiration is low. Caused by root pressure
46 Pulling xylem sap 1. Transpiration – cohesion – tension mechanism 2. Transpirational pull: Ψ of air typically << than Ψ of leaf, thus evaporation.3. Water remaining in leaf is tightly adhered to cell walls in the mesophyll4. This adhesion & surface tension of the water creates negative pressure – the pulling force
48 Ascent of xylem sap against gravity 1. Aided by:A. Cohesiveness of water – allows water to be pulled up in a continuous column without breakingB. Adhesion of water to hydrophyllic cell walls of the xylem, creates tension (negative pressure/ pull)C. Diameters of tracheids & vessel elements are small, so lots of surface area for adhesion2. Since movement is by bulk flow (i.e. no membranes to pass through), Ψs is not involved in the overall process
50 Speed of Xylem Sap Translocation Examples Max. Speed (cm/hr)Conifers 120Angiosperm TreesHerbs 6,000Vines 15,000 (0.6 mi/hr)
51 Why is xylem transport in trees much slower than in herbs? Forces that act against transpirational pull:1. Gravity2. Hydraulic resistance
52 Control of Transpiration 1. Guard cells! – balance two contrasting needs of the plant:Conserve waterCO2 for photosynthesis2. Water use efficiency (WUE) =g H2O lost / g CO2 assimilated by photosynthesisaverage = 600/1
53 3. Desert plants have adaptations to increase their WUE: A. Small thick leaves (less SA for water loss)B. Thick cuticleC. Stomata only on bottom of leavesD. High-volume water storage (cacti)E. Crassulacean Acid Metabolism (CAM) – plants take in CO2 only at night, so that stomata only have to be open at night.
54 Wilting1. Transpirational pull is greater than the delivery of water by the xylem. Cells lose turgor pressure & stomata close
56 Translocation of Phloem Sap 1. Phloem sap = sucrose, amino acids, hormones2. Sieve tubes carry sap from sugar source (e.g. leaves) to sugar sink (e.g. growing roots, shoot tips, stems, flowers, fruits)3. Thus not unidirectionale.g. tubers can be source in spring and sink in fall
57 Phloem loading1. Sap moves into sieve tubes via companion cells by symplast and apoplast pathways2. Loading is by active transport only – why?Sap in sieve tubes is highly concentrated with solutesThus phloem unloading is passive (source to sink)
59 Mechanism of phloem translocation 1. Pressure-flow hypothesis:A. Phloem loading creates high sugar concentration at source, decreasing Ψ.B. Thus water flows into sieve tubes, creating hydrostatic pressure (pushing pressure: positive).C. Less pressure at sink end, where sugar is leaving sieve tube for consumptionD. Thus movement from source to sink