Ions Lecture 5

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  1. soil composition
    • three phases:
    • solid phase - sand, clay or organic matter
    • liquid phase - contains ions and dissolved CO2 and oxygen
    • gaseous phase - contains CO2 and oxygen
  2. cation exchange
    process whereby cations entering the soil replace cations already held on the negative exchange sites
  3. adsorption
    binding of ions to negative sites on clays and organic matter
  4. clay soils
    • nutrient cations are held on negatively-charged cation exchange sites
    • composed of aluminum or silicon oxides
  5. soil organic matter
    cations are held on negative charges created by the dissociation of hydrogen ions from carboxylic acid
  6. anion exchange
    • anion exchange capacity of soils is low because mineral anions are repelled by the negative charges of the soil particles
    • anions are easily lost from the soil
  7. soil pH
    • low pH tends to favor "weathering" - release of ions from the soil, H+ binds to negative sites very strongly and causes release
    • increases solubility
  8. *How do ions move through the root cortex and endodermis?
    • cross both symplast and apoplast spaces
    • an ion that enters the root may enter the symplast immediately by crossing an epidermal cell plasma membrane or it may diffuse between the epidermal cells through the cell walls
    • the apoplast forms a continuous phase from the root surface through the cortex to the endodermis
  9. the Casparian strip
    suberized cell layer in the endodermis that blocks the entry of water and mineral ions into the stele via the apoplast

    ions must enter the symplast before they can enter the stele because of the Casparian strip
  10. movement of ions across the stele into the xylem
    • once ion has entered the stele from the symplastic connections across the endodermis, continues to diffuse from cell to cell in the xylem
    • ion reenters the apoplast in order to diffuse into a xylem tracheid or vessel element
    • the Casparian strip prevents the ion from diffusing back out of the root through the apoplast
    • - b/c of strip, the plant can maintain a higher ionic concentration in the xylem than in the soil water
  11. xylem loading
    • process whereby ions exit the symplast and enter the conducting cells of the xylem
    • ions taken up into the symplast of the root must be loaded into the tracheids or vessel elements of the stele
    • ions must exit the symplast by crossing the plasma membrane a second time
  12. passive diffusion model
    • ions move passively into the stele via the symplast down a gradient of electrochemical potential and leak out of stele into xylem
    • would require energy when crossing the plasma membrane into the symplast at the root epidermal, cortical or endodermal cells

    • evidence:
    • ion-specific microelectrodes measure electrochemical potential
    • ions are taken up actively and maintained in the xylem against the gradient of electrochemical potential - final movement of ions into xylem would be passive
  13. active ion efflux into apoplast hypothesis
    • 1) active ion uptake
    • 2) passive diffusion through the symplast to xylem cells
    • 3) metabolically-driven efflux of ions from cells into vessels and tracheids

    • evidence:
    • using inhibitors and plant hormones, show that ion uptake by the cortex and ion loading operate independently
    • xylem parcenchyma cell plasma membranes contain proton pumps, water and ion channels
    • flux of ions from the xylem parenchyma cells into the tracheids is under metabolic control through regulation of the plasma membrane H+ATPase and ion efflux channels
  14. active ion uptake leads to root pressure
    root pressure occurs when there is no transpiration

    • conditions required for root pressure:
    • wet soil
    • a high conc of ions in the soil
    • humid conditions - no transpiration

    • root pressure builds up as follows:
    • root absorbs ions, which then enter the xylem
    • the solute potential in the xylem decreases
    • water moves into the xylem along a water potential gradient and the pressure potential in the xylem builds up
  15. guttation
    root pressure causes water to leak out of the hydathodes (like modified open stomates) located near tracheids at the ends of the vascular bundles
Card Set
Ions Lecture 5
plant physiology and biochemistry exam 6
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