Translocation Flashcards
What is translocation?
Transport of amino acids + sucrose + soluble organic material through phloem (made of many cells/phloem tissue). Via the process of active transport.
What are some examples of sources?
Leaf
Germinating seeds
What are some examples of sinks?
Growing roots
Bulb/Tuber
Flowers (Anther + Ovary)
Seed
What are the four types of cells in phloem?
Sieve tubes
Companion cells
Phloem fibers
Phloem Phloem parendym
What are the qualities of sieve tubes?
No nucleus nor ribosomes nor Golgi nor cytoplasm nor mitochondria nor ER.
Made of sieve elements
End wall perforated with pores to make sieve plates.
What are the qualities of companion cells?
Lots of mitochondria + ribosome = metabolically active
Linked via plasmodesmata to sieve elements.
Evidence to support Mass Flow hypothesis
Solution under pressure.
Evidence for concentration gradient.
Observation of sieve tubes.
Movement of virus through plant.
No movement of virus when no photosynthesis.
Evidence against Mass Flow hypothesis
Get the impression of steady flow, this is wrong.
Certain things move different ways.
The mass flow theory
Sucrose loaded from source (leaf) into phloem
by transfer cells (modified parenchyma) which
have folded inner walls to increase SA
Active transport sucrose into sieve tubes
Sucrose too large to diffuse out
At sink (roots) sucrose actively pumped out by
transfer cells into tissues
Sucrose used in sink tissues
Xylem transports water up to leaves
Water moves by osmosis into phloem in
leaves
Increases hydrostatic pressure in phloem
Mass flow fluid in phloem along hydrostatic
gradient
In roots, water moves from phloem back to
xylem
What are the Alternative theories for translocation?
Active process may be involved
Streaming of cytoplasm may be responsible
for movement in two directions but need
method to cross sieve plates
Protein filaments passing through sieve plates
may transport solutes
What are the qualities Radioactive tracing?
Introduce labelled metabolite
Direct sampling of individual sieve tubes using
mouth parts of aphids as micropipette
Demonstrated that translocation too rapid to
be diffusion
What are the qualities for Radioisotope labelling
Labelled carbon dioxide supplied to
illuminated plant leaf
Radioactive carbon fixed into sugars
Radioactive carbon traced to other parts of
plant by placing on photographic film
Sugars move up and down
Only phloem involved
