Translocation
Cards (17)
- Vascular bundles are made up of xylem and phloem
- Translocation- the movement of organic molecules and some mineral ions up and down the phloem's sieve tube elements from source to sink
- Transporting sugars as sucrose increases stability and prevents unwanted reactions
- Sources= leaves, produce sugars
- Sinks= growing apex, flower, fruit, seeds, stem, roots: anything that uses sugars
- Sources create sugars, sinks use them
- Phloem:
- living
- Sieve tubes
- companion cells
- The mass flow hypothesis:
- tries to explain how sucrose is transported from the leaves to the rest of the plant
- 3 phrases of mass flow hypothesis:
- loading
- moving
- unloading
- Loading:
- sucrose moves from photosynthesising cell to the companion cell via facilitated diffusion
- ATP is hydrolysed to provide energy to actively transport hydrogen into the sieve tube element, creating a high concentration
- High conc of H+ allows for co-transport of sucrose, decrease water potential, water moves in
- Moving:
- moves from high to low hydrostatic pressure
- water and sucrose moves from the phloem into sink cells
- Unloading:
- sucrose moves from sieve tube into companion cell and then into sink cells
- Evidence:
- Loading: aphids
- Moving: Ringing experiment
- Unloading: Tracer experiments
- Aphids
- can be used to tap into the phloem of a plant
- excrete phloem fluid
- track phloem content
- leaves have higher sucrose conc than stems: produce sugars directly
- leaf's sucrose conc peaks earlier than stem's: takes time for sucrose to move around
- Ringing experiment:
- take a ring from the bark of a tree
- interrupts phloem
- sugar accumulates above ring, swells
- Tracer experiments
- grow leaf in 14CO2 isotope environment
- 14C incorporated into sugars made in photosynthesis
- Track 14C to see where sucrose goes
- Evidence against mass flow:
- sieve plates impede mass flow
- solutes move at different speeds
- Sucrose is delivered at the same rate to all regions, rather than faster to the lowest concentration