Sugar Transport in Plants: Phloem Flashcards
Translocation
movement of sugar through the plant phloem from where they are produced to where they are needed for growth
Source
any structure in a plant that either produces (EX: leaf) or releases (EX: storage bulb) sugars for the growing plant
Sink
any location where sugar is delivered for use in a growing tissue or storage for later use
Sink: Growing Tissues
- apical + lateral meristems
- developing leaves, flowers, seeds, and fruits
Sink: Storage Locations
- roots
- tubers
- bulbs
Source or Sink: Middle of Growing Season
- SOURCES: include mature leaves + stems actively photosynthesizing and producing excess sugars
- SINKS: areas of active growth meristems, new leaves, reproductive structures (flowers, seeds), and sugar storage locations like roots, tubers, or bulbs
Source or Sink: End of Growing Season
plant drops leaves and no longer has actively photosynthesizing tissues
- depending on local climate, the season may end due to onset of winter or a drying season
Source or Sink: Start of Next Growing Season
plant can resume growth after dormancy
- SOURCE: since there are no leaves, it gets sugar for growth from storage in roots, tubers, or bulbs from last growing season
- SINKS: actively developing leaves (once they mature they turn into sources during the growing season)
Pressure Flow Model
movement of sugars (translocation) in the phloem:
1. fluid in phloem is under high positive pressure
2. translocation stops if the phloem tissue is killed
3. translocation proceeds in both directions simultaneously (but not in the same tube)
4. translation is inhibited by compounds that stop ATP production in the sugar source
Pressure Flow Model STEPS
- A high concentration of sugar at the source creates a low Ys
- Low Ys draws water into the phloem from the adjacent xylem
- Water movement creates a high Yp in the phloem
- High turgor pressure forces movement of phloem sap from source to sink through “bulk flow”
- Sugars move via bulk flow are rapidly removed from the phloem at the sink
- Removal of sugars increases Ys, causing water to leave the phloem and return to the xylem - decreasing the Yp of the sink
Transport Pathways
- Diffusion
- Proton Pumps
- Cotransporters (Symporters + Antiporters)
Cotransporters: Symporters
type of co-transporter that transports 2 molecules in the same direction (both in or both out of the cell)
Cotransporters: Antiporters
type of co-transporter that transports 2 molecules in opposite directions (one in and one out of the cell)
Movement of Sucrose: STEP ONE
- photosynthates are produced in lead parenchyma cells and are actively transported into sieve-tube companion cells using ATP
- CC create an ECG via an ATP powered proton pump, allowing a proton-sucrose cotransporter to move sucrose into cells against its CG
- allows higher sugar accumulation in CC compared to photosynthesizing leaves
Movement of Sucrose: STEP TWO
- once inside the CC, the sugar DIFFUSES down its CD from the CC into the phloem sieve-tube elements through the plasmodesmata
- Phloem STE have reduced cytoplasmic contents and are connected by pores that allow for pressure-driven bulk flow (translocation) of phloem sap
Movement of Sucrose: STEP THREE
- presence of high sugar concentrations in STE reduces Ys, moving water via osmosis from xylem to phloem cells
- causes Yp to increase, which increases the phloem turgor pressure + Ytotal at the source
- INCREASE Ytotal DRIVES THE BULK FLOW OF PHLOEM FROM SOURCE TO SINK
Movement of Sucrose: STEP FOUR
unloading at the phloem sink end can occur by either:
- Diffusion, if sucrose concentration is lower at the sink than in the phloem
- Active Transport, if sucrose concentration is higher at the sink
Movement of Sucrose: Sink Areas
Sink area influences sucrose concentration:
- Active Growth site
- Storage where sugar is converted to starch site
- Storage where sugar is stored as sucrose site
Sink Areas: Active growth
sucrose concentration is lower in sink cells compared to phloem STE since the sink sucrose is rapidly metabolized for growth
- EX: new leaf or reproductive structure
Sink Areas: Conversion to Starch Site
sucrose concentration is lower in the sink cell compared to the phloem STE since the sink sucrose is rapidly converted to starch for storage
- EX: root or bulb
Sink Areas: Stored as Sucrose
sucrose concentration is higher in sink cells compared to phloem STE; active transport by a proton-sucrose cotransporter is used to transport sugar from CC into storage vacuoles in storage cells
- EX: sugar beet / sugar cane
Movement of Sucrose: STEP FIVE
- once sugar is unloaded at sink cells, Ys increases causing water to diffuse from phloem back into the xylem
- this causes Yp to decrease, reducing the turgor pressure in the phloem at the sink and maintaining bulk flow direction from source to sink
Xylem VS Phloem: Driving Force
- X: transpiration from leaves + cohesion-tension of water in the vessel elements and tracheids (passive)
- P: active transport of sucrose from source cells into phloem STE (energy required)
Xylem VS Phloem: Cells Facilitating Movement
- X: Non-living vessel elements and tracheids
- P: living STE, supported by CC
