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Chapter 36

Chapter 36. Vascular System in Plants. Three ways water moves through root hairs. Apoplast: water moves through cell walls and never enter cells Symplast: water moves from one cell to another through the cytoplasm

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Chapter 36

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  1. Chapter 36 Vascular System in Plants

  2. Three ways water moves through root hairs • Apoplast: water moves through cell walls and never enter cells • Symplast: water moves from one cell to another through the cytoplasm • Transmembrane: water repeatedly moves back and forth from cell wall, crossing the membrane, and through the cytoplasm apoplastic symplastic

  3. Pathway of Water Movement • Root hairs use osmosis to soak up water • Water travels via apoplast or symplast through the cortex until it reaches the endodermis that lines the vascular cylinder (stele). • Endodermis has a “casparian strip”, a selectively waxy layer, which BLOCKS the apoplast pathway, so water MUST move into the stele via symplast (toregulate what minerals in the water can enter the stele) • Xylem within the stele transports water to shoots

  4. Transpiration • Definition: the loss of water vapor from leaves and aerial parts of a plant via stomata • Responsible for upward transport of water through the xylem due to: • pushing forces • pulling forces video

  5. Pushing Forces Endodermis accumlates ions in stele, causing these effects: a) Ψw ______________ in stele b) Water moves ____ stele, generating + pressure forcing water up the xylem (root pressure) c) Guttation is a result of root pressure -low transpiration rate at night -more water moves into stele than transpired -water droplets forced out at end of leaves by morning decreases into More solutes • Ψs • Ψw Ψw= 0

  6. Pulling Forces(transpiration, tension, cohesion mechanism) Transpiration: evaporation of water from stomata of leaves creates (-) pressure and develops tension Bulk Flow: Water moves up the xylem to replace the (-) pressure

  7. Pulling Forces(transpiration, tension, cohesion mechanism) Cohesion: Water molecules are hydrogen bonded to each other due to its polarity Water, therefore, moves up the xylem acting as one large polymer-like substance

  8. How is transpiration controlled by plants? There must be a compromise between photosynthesis and transpiration

  9. Pros: Transpiration can occur for PS Gas exchange for CR and PS can occur Cons: Plant risks dessication from excessive transpiration, leading to wilting The Great Debate!Pros and Cons of an Open Stomata

  10. Pros: Minimal dessication and minimal wilting Cons: No transpiration for PS No PS or CR can occur due to lack of gas exchange The Great Debate!Pros and Cons of a Closed Stomata

  11. Plants maintain a transpiration to PS ratio Ratio (in grams)= water loss CO2 assimilated for PS C3 plants  600:1 ratio C4 plants 300:1 ratio (greater CO2 assimilation rate) Goal = maximize PS rate w/ minimal transpiration

  12. When do plants open and close their stomata? Guard Cells guard cells close High temperatures  [CO2] is low  Night  Day  guard cells open guard cells close guard cells open

  13. How do guard cells open and close? K+ is pumped out of guard cells  ψw __________ inside, and water moves out of guard cells making them flacid and close K+ is pumped into guard cells  ψw ________ inside, and water moves into guard cells making them swell, turgid, and open decreases increases

  14. Translocation Definition: the transport of carbohydrates (CHO) in plants from: - the source (normally where CHO is produced) to - the sink (where CHO is used) ** storage organ (ex. bulb) can be a source or a sink video

  15. Translocation occurs in 2 steps • Sugar Loading • Pressure Flow

  16. Sugar Loading symplast CHO are moved from source to sieve-tube members • Symplast pathway or • Apoplast pathway - needs co-transport mechanism to move CHO from apoplast to cytoplasm of sieve tube apoplast H+ H+ H+ H+ Companion cell H+ ATP ADP ST members

  17. Pressure Flow Phloem (STmembers) xylem • CHO move from source to ST members by sugar loading mechanism • Ψ__________ in the phloem (STM), causing water from xylem to flow into the phloem • This increases pressure in the phloem. Increased pressure causes phloem sap to move to an area of less pressure • Carbohydrates move out towards sink • Ψ ____________ in phloem (STM) so water from phloem diffuses back out to xylem), decreasing pressure decreases source sink increases

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