Distinguish phloem transport from xylem transport
Identify sucrose and amino acids as phloem cargo, including transport to growing tissues.
Section B8.1 · Xylem and phloem
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall sugars, amino acids and mineral ions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What are proteins assembled from?
What you will learn
- Identify sucrose and amino acids in phloem
- Choose tissue from the transported material
Organic nutrients move between organs
Phloem transports sucrose and amino acids through a plant. Photosynthetic leaves can provide sugars to organs that need them for respiration, growth or storage. Roots need organic nutrients even though they also absorb water and mineral ions.
Amino acids contribute to protein synthesis in growing cells. They are not interchangeable with nitrate ions: nitrate is a mineral nutrient, whereas an amino acid is an organic molecule made by the plant.
Use the cargo as evidence
A stated movement of sucrose from a leaf to a growing root identifies phloem. A movement of water and dissolved magnesium ions from a root towards a leaf identifies xylem. Both tissues occur in vascular bundles, so finding a bundle alone does not distinguish them.
Do not define phloem as simply “downward transport”. In Extended work, source and sink explain the direction; leaves can supply both shoots above and roots below. The tissue is identified by its transport role rather than one fixed compass direction.
Sources for this lesson
Pause and explain
Which pair is transported in phloem?
Worked example
Which tissue carries amino acids to a growing shoot, and why are they useful there?
Show the worked solution
- Amino acids are organic nutrients carried in phloem.
- Growing cells use amino acids to make proteins.
Answer Phloem carries amino acids for protein synthesis in the growing shoot.
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.
Name the tissue for each: sucrose to a root; amino acids to a bud.
Give me a hint
Both materials are organic nutrients.
Compare my reasoning
- Identify sucrose transport as phloem.
- Apply the same tissue choice to amino acids.
Both use phloem. The substances are sucrose and amino acids respectively.
Look for these in your work
- Name phloem twice
- Keep the two substances distinct
Correct “xylem carries nitrate and amino acids because both contain nitrogen”.
Give me a hint
Chemical composition alone does not select the tissue.
Compare my reasoning
- Nitrate ions move in the xylem water stream.
- Amino acids are transported in phloem.
Xylem carries nitrate ions; phloem carries amino acids. Sharing nitrogen does not make their transport pathways identical.
Look for these in your work
- Separate the cargo
- Explain why the original rule fails
A root receives sucrose while a leaf receives magnesium ions. Explain why both organs can depend on transport.
Give me a hint
State a material, route and use for each organ.
Compare my reasoning
- The root receives phloem sucrose that can support respiration or storage.
- The leaf receives xylem magnesium ions needed for chlorophyll.
The root receives organic nutrients through phloem, while the leaf receives water/mineral ions through xylem. These routes connect different needs.
Look for these in your work
- Link root cargo to phloem
- Link leaf mineral cargo to xylem
Common mistakes
- Calling nitrate and amino acids the same substance.
- Identifying every downward movement as phloem without checking its cargo.
What can you explain now?
Which pair is transported in phloem?
Compare with the explanation
Sucrose and amino acids
Both are organic nutrients transported in phloem.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Give one xylem cargo pair and one phloem cargo pair; do not use direction alone.
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.