Transport in Plants

1. Transport in Plants Chapter 36

2. Cellular Transport • A. passive transport • Driven by the principles of diffusion • Much of the diffusion is facilitated • Selective channels are usually gated and regulated

3. Cellular Transport • B. Active Transport • Often uses proton pump • Generates membrane potential • may be used to drive active transport of other molecules (cotransport)

5. Cellular Transport • Water Transport • By osmosis water moves from hypotonic to hypertonic solution in cells • BUT in plant cell, cell wall provides a pressure component, therefore water potential must be used

6. Water potential review: • What is ψ of a container of distilled water that is open? • If ψ inside a cell is -0.7 MPa, and outside of the cell is -0.1 MPa, which direction will water move? • If ψS=-0.5MPa and ψP=0.1MPa, will water move into or out of this cell if it is placed in an environment where ψ=-0.2 MPa?

7. Aquaporins • 1990’s Peter Agre (John’s Hopkins) discovers water pores in membranes that facilitate diffusion of water • 2003 Nobel Prize in Chemistry along with Roderick MacKinnon • Water is small and can diffuse through bilayer to some extent despite polarity • Aquaporins speed up this diffusion • May be gated to regulate water diffusion

8. 3 Cellular Compartments • Apoplast: continuum of cell walls and extracellular spaces • Symplast: continuum of cytosol linked by plasmodesmata • Vacuole: bound by membrane called tonoplast

9. Lateral Transport • Short distance horizontal movement • Mechanisms • Transmembrane (through membranes from one cell to the next) – slow • Symplastic (through one membrane into cell, then through plasmodesmata) • Apoplastic (through cell walls)

11. Absorption by Roots • Root hairs increase surface area • Mycorrhizae (fungal partners) enhance absorption • Endodermis functions as selective region • Symplastic continues through plasmodesmata • Apoplastic must cross plasma membrane into cytosol due to Casparian strip • Casparian strip is waxy and hydrophobic, traps water and minerals in the vascular tissue

14. Long Distance Transport in the Xylem • Root Pressure: Pushing • Transpiration: Pulling

15. Root Pressure • Used by some plants, at night • At night, plants still expend energy pumping minerals into xylem • Accumulation of nutrients decreases ψ , causing water to flow in by osmosis • This upward push is root pressure • Guttation: exudation of water droplets seen in the morning

17. Transpiration • Major mechanism of movement • Water is adhesive and cohesive • As one water droplet moves, the next also moves (water in continuous column in xylem) • As water evaporates out of the stomata, water below moves upward

19. Guard Cells • Guard cells regulate water loss. • What conditions will promote the closing of guard cells? • Buckle outward when turgid • Regulate opening and closing by managing K+

20. Stomata

21. Guard Cells • Pump H+ out, K+ enters due to electric charge, water follows due to osmosis • Close when K+ leaves (water follows) • Aquaporins may vary permeability to regulate

22. Some leaf molds, which are fungi that parasitize plants, secrete a chemical that causes guard cells to accumulate potassium ions. How does this adaptation enable the leaf mold to infect the plant?

23. Guard Cell Cues • Light stimulates a blue light receptor in guard cells, activates H+ pumps • CO2 depletion when Calvin cycle starts • Can trick guard cells by placing in environment without CO2 • Internal Clock • Open and close even in 24 hour dark • Circadian rhythm

24. What adaptations are seen for arid climates? • thick leaves (low SA/vol) • Thick cuticle • Stomata on lower leaf surface • Stomata in pores to shield wind • Shed leaves in hot dry season • Cacti have no leaves (adapt to spines) • CAM photosynthesis

25. Phloem Transport • Translocation • Sugar source to sugar sink • Sugar loaded into sieve tube members by active transport • Water follows by osmosis (increases pressure) • At sink, sugar leaves sieve tube members (by diffusion or active transport) • Water follows (decreases pressure) • Water is recycled by xylem

27. Oleander: stomata in “cypts”

28. Old Man cactus