Translocation

Question 1

What is translocation?

Answer 1

Transport of assimilates from source to sink. Via the process of active transport.

Question 2

What are some examples of sources?

Answer 2

Leaf
Germinating seeds

Question 3

What are some examples of sinks?

Answer 3

Growing roots
Bulb/Tuber
Flowers (Anther + Ovary)
Seed

Question 4

What are the four types of cells in phloem?

Answer 4

Sieve tubes
Companion cells
Phloem fibers
Phloem parendym

Question 5

What are the qualities of sieve tubes?

Answer 5

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.

Question 6

What are the qualities of companion cells?

Answer 6

Lots of mitochondria + ribosome = metabolically active
Linked via plasmodesmata to sieve elements.

Question 7

Evidence to support Mass Flow hypothesis

Answer 7

Evidence For Mass Flow

Aphid Punctures: Aphids feeding on phloem show sap continuing to flow out, even when the insect is removed, demonstrating transport under pressure.

Tracer Studies: Using radioactive tracers confirms sugars move from source (leaves) to sink (roots/fruits) via the phloem.

Osmotic Movement: Water moves into the phloem from xylem due to higher solute (sugar) concentration, creating the pressure gradient.

Question 8

Evidence against Mass Flow hypothesis

Answer 8

Different Speeds/Directions: Different solutes (sugars, amino acids) move at different rates, and sometimes in opposite directions within the same sieve tube, contradicting bulk flow.

Sieve Plates: These pores create resistance, requiring significant pressure to overcome, yet measured pressures aren’t always high enough.

Active Transport: Metabolic poisons (like cyanide) stop phloem transport, indicating active energy (ATP) is used along the phloem’s length, not just at loading points, challenging the passive flow idea.

Multiple Sinks: The theory struggles to explain how sugars efficiently reach various sinks simultaneously.

Question 9

The mass flow theory

Answer 9

Source Loading (High Pressure):
At the source (e.g., a leaf), sucrose is actively loaded into the phloem’s sieve tubes via co-transporters on companion cells then diffuse through plasmodesmata. (use of hydrogen ions pumped out via proton pump using ATP)
This lowers the water potential in the sieve tube elements.
Water moves from the xylem into the phloem by osmosis, increasing hydrostatic pressure.

Mass Flow (Pressure Gradient):
A high-pressure area forms at the source and a lower-pressure area at the sink.
This pressure gradient drives the bulk movement (mass flow) of sugary phloem sap towards the sink.

Sink Unloading (Low Pressure):
At the sink (e.g., a root), sugars are removed from the phloem and converted into starch or used for energy.
This removal increases the water potential, causing water to move out of the phloem by osmosis.
This maintains the pressure gradient, allowing continuous flow.

Question 10

What are the Alternative theories for translocation?

Answer 10

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

Question 11

What are the qualities of Radioactive tracing using an aphid?

Answer 11

Introducing a Tracer: A plant’s leaf is exposed to a controlled atmosphere containing radioactively-labeled carbon dioxide.

Photosynthesis: The plant uses the during photosynthesis to produce radioactively-labeled sugars (e.g., sucrose), which are then loaded into the phloem for transport.Aphid

Feeding: Aphids are placed on the plant stem and encouraged to insert their tubular mouthparts (stylets) into the phloem sieve elements to feed on the sap.

Stylectomy: Once an aphid is steadily feeding, it is humanely anaesthetized, and its body is carefully severed from the stylet. Due to the high hydrostatic pressure within the phloem, the sap continues to exude as a droplet from the detached stylet, which acts as a natural, fine tap.

Question 12

What are the qualities for Radioisotope labelling

Answer 12

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

Question 13

How is surcose moved into the phloem

Answer 13

Hyrogen Ions are actively pumped outside the phloem using ATP, It then returns down the concentration gradient (of protons and sucrose) via a co transport protein with a sucrose molecule,

Question 14

How are companion cells Adapted for the transportation of sucrose into the Phloem

Answer 14

Many infold to increase SA:Vol ratio for AT of sucrose

Lots of mitochondira to produce ATP for the proton pump