Explain upward and downward translocation routes
Follow sources and sinks through separate phloem pathways without inventing a single fixed direction.
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Recall source/sink roles and vascular tissues.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What determines the route in this translocation account?
What you will learn
- Predict direction from organ roles
- Distinguish different tube pathways from opposite simultaneous flow in one tube
Different destinations can lie in different directions
A source leaf can supply a growing shoot above it and a root below it. Organic nutrients can therefore move upward or downward through different phloem pathways according to the source–sink relationship. “Phloem always flows downward” does not fit these routes.
Draw separate arrows from the source to each destination and name the tissue on each. The arrows describe movement of sucrose/amino acids, not a switch to xylem whenever the page arrow points upward.
Keep the tube claim precise
A plant can support different directions in different phloem tubes or pathways. This does not mean that the same stream in one tube necessarily flows in opposite directions at the same instant. Distinguish whole-plant routing from an individual conducting pathway.
For a supplied tracer released from a source, its arrival at a sink supports a route in that setup. The diagram alone is a model, not a measured allocation fraction or rate. Do not infer equal delivery to all sinks merely because arrows have equal lengths.
Pause and explain
Which statement is careful?
Worked example
A source leaf supplies a bud above and a root below. Name the directions and tissue.
Show the worked solution
- The source-to-bud route is upward in phloem.
- The source-to-root route is downward in phloem, using a different pathway.
Answer Upward and downward phloem routes can serve different sinks; this is not opposite simultaneous flow in one tube.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw routes from a source leaf to a higher bud and lower root, labelling the tissue.
Give me a hint
Each arrow starts at the source and ends at a sink.
Compare my reasoning
- Leaf → higher bud is upward phloem movement.
- Leaf → lower root is downward phloem movement.
Two source-to-sink phloem routes, one upward and one downward.
Look for these in your work
- Give both routes
- Label both as phloem
Correct “sucrose moves upward, therefore it is in xylem”.
Give me a hint
Use cargo and source/sink rather than direction alone.
Compare my reasoning
- Sucrose is a phloem cargo.
- A sink above a source produces an upward phloem route.
Upward sucrose movement can be phloem translocation from a lower source to a higher sink.
Look for these in your work
- Keep the cargo/tissue link
- Explain upward source-to-sink movement
Two equal-length arrows lead to different sinks. Can you conclude each gets half the sucrose? Explain.
Give me a hint
Arrow length is not a measured amount.
Compare my reasoning
- The arrows show destinations/directions.
- No quantitative allocation measurements were supplied.
No. The model shows routes, not equal quantities; measured delivery data would be needed.
Look for these in your work
- Distinguish route from quantity
- State missing allocation evidence
Common mistakes
- Calling upward sucrose movement xylem.
- Reading two whole-plant arrows as opposite streams in one tube.
What can you explain now?
Which statement is careful?
Compare with the explanation
Different phloem pathways can have different directions
The whole plant can supply sinks above and below.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain whole-plant upward/downward routing without claiming opposing streams in one tube.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.