2. Uptake and Transport of Sulfur
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- sulfate permease - symporter, sulfate ions are taken up into root cells
- cytosol -> plastids where sulfide reduced to form cysteine
- or sulfate is stored in vacuole, transported through the xylem to the leaf
- sulfate to leaf cytosol - once inside the leaf, sulfate may enter the leaf mesophyll cell through sulfate-permease in the plasma membrane
- chloroplast sulfate reduction - sulfate transported from cytosol into the choloroplast where reduced to sulfide from cys and glutathione
- export of glutathione - from chloroplast into the cytosol then into the phloem for transport back to the root
- export of sulfide - S may also flow from the cholorplast to the mitochondria in the form of sulfide
Is there one or multiple sulfate transporters?
- multiple transporters
- high affinity isoform - in roots
- low affinity isoform - leaf cell
Mechanism of sulfate uptake across the plasma membrane and into root cells?
- sulfate permease - sulfate transporter
- permease structure - single polypeptide chain with 12-membrane spanning regions
- permease functioning - symporter, co-transporting sulfate with protons
- proton gradient - produced by proton-pumping ATPas
transport into the vacuole
- uniporter - transport a single solute across the membrane
- driven by a proton gradient from a tonoplast proton ATPase
- driven by electrical gradient
transport into plastids from root to leaf
- proton-sulfate cotransporter
- triose phosphate translocator
- major species of sulfur transported is sulfate
What are the main steps of the reductive sulfate assimilation pathway?
- Part 1: activation of sulfate
- Part 2: reduction of activated sulfate to sulfide
- Part 3: formation of cysteine from sulfide
- Part X: formation of methionine from cysteine
- sulfate to sulfide - requires a lot of energy
- 1) photosynthesis - choloroplasts
- 2) oxidative pentose phosphate pathway - roots
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