LG 3 – Plant Transport

1. LG 3 – Plant Transport Plant Material Transport Material Transport – Passive and Active Transport – Water Movement in Plants – Transport in Roots Water in Roots – Mineral Active Transport – Transport of Water and Minerals in Xylem – Mechanical Properties of Water Transpiration – Cohesion-tension Mechanisms of Water Transport – Cohesion-Tension in Tallest Trees - Leaf Anatomy Stomata – Physiology of Stomata- Arid Adaptations – Transport of Organic Substances in Phloem Organic Compounds – Sources and Sinks -

2. Unit IIPlants Learning Goal 3 Examine how materials are transported throughout the body of a plant.

3. Plant Material Transport • Material transport • Short distances between cells • Long distances between roots and leaves (xylem and phloem)

4. b. Transport in vascular tissues Cells load and unload organic molecules (including CO2) into and out of phloem (purple arrows to/from phloem). Phloem: transport of sugars Sugar from photosynthesis Vascular tissue distributes substances throughout the plant, sometimes over great distances. Xylem: transport of H2O and O2 a. Short distance transport across cell membranes into roots Water and solutes from soil enter plant roots by passive or active transport through the plasma membrane of root hairs. Sugar from photosynthesis Minerals Mineral ions c. Long distance transport throughout the plant Water and mineral ions travel from root hairs into xylem vessels by passing through or between cells (black arrow into/out of xylem). Stepped Art Fig. 32.2, p. 739

5. Passive and Active Transport • Passive transport requires no metabolic energy • Substance moves down concentration or electrochemical gradient or by membrane potential • Active transport requires metabolic energy (ATP) • Substance moves against gradient

6. Water Movement in Plants • Bulk flow of water due to pressure differences • Xylem sap • Dilute water movement from roots to leaves • Osmosis • Passive movement of water across cell membrane • Water potential (Ψ) • Driving force (pressure and/or solutes)

7. Transport in Roots Water in Roots Apoplastic pathway: Water does not cross cell membrane, includes dead xylem transport Symplastic pathway: Water moves through plasmodesmata (openings between plant cells). Transmembrane pathway: Water moves between living cells through cell membranes. Casparian strip in root endodermis forces apoplastic water to symplast

8. In the apoplastic pathway (red), water moves through nonliving regions–the continuous network of adjoining cell walls and tissue air spaces. However, when it reaches the endodermis, it passes through one layer of living cells. Cell wall Tonoplast Plasmodesma Air space Endodermis with Casparian strips Root hair Xylem vessel in stele In the symplastic pathway (green), water passes into and through living cells. After being taken up into root hairs water diffuses through the cytoplasm and passes from one living cell to the next through plasmo-desmata. In the transmembrane pathway (black), water that enters the cytoplasm moves between living cells by diffusing across cell membranes, including the plasma membrane and perhaps the tonoplast. Root cortex Epidermis Fig. 32.6, p. 743

9. Casparian Strips in Roots

10. a. Root b. Stele in cross section (stained) Exodermis Primary xylem Primary phloem Root cortex Stele Endodermis Abutting walls of endodermal cells c. Casparian strip (from above) Stele Endodermal cells with Casparian strip In root cortex, water molecules move through the apoplast, around cell walls and through them (arrows). Fig. 32.7, p. 744

11. d. Movement of water into the stele Tracheids and vessels in xylem Stele Sieve tubes in phloem Pericycle (one or more cells thick) Endodermis (one cell thick) Radial wall region impregnated with suberin Wall of endodermal cell facing root cortex Transverse wall regions impregnated with suberin Route water takes into the stele Waxy, water-impervious Casparian strip (gold) in abutting walls of endodermal cells that control water and nutrient uptake Fig. 32.7, p. 744

12. Mineral Active Transport • Most minerals for growth are more concentrated in root than in soil • Active transport into symplast • Active transport at Casparian strip across membrane • Minerals loaded into apoplast of dead xylem in root stele • Transported long distance to other tissues

13. Transport of Water and Minerals in the Xylem • Mechanical properties of water have key roles in its transport • Leaf anatomy contributes to cohesion-tension forces • In the tallest trees, the cohesion-tension mechanism may reach its physical limit

14. Root pressure contributes to upward water movement in some plants • Stomata regulate the loss of water by transpiration • In dry climates, plants exhibit various adaptations for conserving water

15. Mechanical Properties of Water • Transpiration • Evaporation of water out of plants • Greater than water used in growth and metabolism • Cohesion-tension mechanism of water transport • Evaporation from mesophyll walls • Replacment by cohesion (H-bonded) water in xylem • Tension, negative pressure gradient, maintained by narrow xylem walls, wilting is excess tension

16. Mesophyll Vein Upper epidermis The driving force of evaporation into dry air 1 Transpiration is the evaporation of water molecules from above ground plant parts, especially at stomata. The process puts the water in the xylem sap in a state of tension that extends from roots to leaves. Stoma Vascular cambium Water uptake in growth regions Xylem Phloem Cohesion in the xylem of roots, stems, and leaves Growing cells also remove small amounts of water from xylem. 2 The collective strength of hydrogen bonds among water molecules, which are confined within the tracheids and vessels in xylem, imparts cohesion to the water. Water uptake from soil by roots Water molecule Root hair Stele cylinder Endodermis Cortex 3 As long as water molecules continue to escape by transpiration, that tension will drive the uptake of replacement water molecules from soil water. Fig. 32.8, p. 746

17. Cohesion-Tension in Tallest Trees • Transpiration follows atmospheric evaporation • Driving forces: Dryness and radiation • Tallest trees (>110m) near physical limit of cohesion • Root pressure occurs in moist to wet soils • Moves water up short distances • Guttation • Water movement under pressure out leaves

18. Guttation

19. Leaf Anatomy • Stomata • Transpiration losses of water must be regulated to prevent rapid dessication • Cuticle limits H2O loss but also prevents CO2 uptake • Water is always lost when stomata open for photosynthesis

20. a. Open stoma b. Closed stoma Guard cell Guard cell Chloroplast (guard cells are the only epidermal cells that have these organelles) Stoma Fig. 32.10, p. 748

21. Physiology of Stomata • Stomata must balance H2O loss and CO2 uptake by responding to many signals, biological clock • Stomata open to increase photosynthesis • Increasing light (blue) • Decreasing CO2 concentration in leaf • Stomata close under water stress • Abscisic acid is hormonal signal for closure, synthesized by roots and leaves

22. Arid Adaptations Xenophytes have adaptations to aridity Thickened cuticle, sunken stomata, water storage in stems

23. Transport of Organic Substances in the Phloem • Organic compounds are stored and transported in different forms • Organic solutes move by translocation • Phloem sap moves from source to sink under pressure

24. Transport of Organic Substances in the Phloem • Organic Compounds • Translocation • Long-distance transport of substances via phloem • Phloem flow under pressure, moves any direction • Macromolecules broken down into constituents for transport across cell membranes • Phloem sap composed of water and organic compounds that move through sieve tubes

25. Sources and Sinks • Source: Any region of plant where organic substance is loaded into phloem • Companion and transfer cells, use free energy • Sink: Any region of plant where organic substance is unloaded from phloem • Pressure flow mechanism moves substance by bulk flow under pressure from sources to sinks • Based on water potential gradients

26. Sieve tube of the phloem 1 Active transport mechanisms move solutes into the companion cells and then into the sieve tube, against concentration gradients. Source (for example, mature leaf cells) Solute Water 2 As a result of the increased solute con-centration, the water potential is decreased in the sieve tube, and water moves in by osmosis, increasing turgor pressure. 3 The pressure then pushes solutes by bulk flow between a source and a sink, with water moving into and out of the system all along the way. bulk flow 4 Pressure and solute con-centrations gradually decrease between the source and the sink as substances move into the sink from phloem. 5 Solutes are unloaded into sink cells, and the water potential in those cells is lowered. Water moves out of the seive tube and into sink cells. Sink (for example, developing root cells) Fig. 32.15, p. 752

27. LG 3 Vocab Terms • Passive Transport - • Active Transport - • Osmosis - • Water Potential - • Apoplastic vs Symplastic Pathway - • Casparian Strip - • Cohesion-Tension Mechanism - • Source - • Sink - • Pressure Flow Mechanism -