Identify dissolved substances transported in plants
Water transports some mineral ions and organic compounds through plants as solutes; different tissues provide the routes.
AS Level · Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; identifying tissues from real slides, drawing biological sections, supervised practical competence and human subject-teacher review remain necessary.
Before you begin
- Distinguish plant cell walls from cell membranes and recognise that vascular tissues form pathways through roots, stems and leaves.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which statement about plant transport solutions is correct?
What you will learn
- State that some mineral ions and organic compounds are transported dissolved in water
- Distinguish xylem water/mineral-ion transport from assimilate movement in phloem
A transported solution contains a solvent and solutes
Water is the solvent in plant transport solutions. Some mineral ions absorbed by roots and some organic compounds can be carried dissolved in water. In xylem, water and dissolved mineral ions usually move from roots towards leaves.
In phloem, assimilates such as sucrose and amino acids are dissolved in water and move from sources to sinks. Naming the solute and the tissue makes an answer more precise than saying only that 'food moves around the plant'.
Keep transport tissues distinct
Xylem is the main long-distance pathway for water and dissolved mineral ions; phloem is the pathway for assimilates. A statement that some substances are dissolved in water does not mean every transported material follows the same pathway or direction.
The lesson models transport conceptually. A written answer cannot show that a student has prepared or interpreted a real vascular-tissue specimen correctly.
Pause and explain
Which pair are examples of assimilates transported in phloem?
Worked example
Name one mineral ion that can be dissolved in xylem water and two assimilates that can be dissolved in phloem water.
Show the worked solution
- A mineral ion such as nitrate can be dissolved in xylem water.
- Sucrose and amino acids are examples of phloem assimilates.
Answer Nitrate ions can be transported dissolved in xylem water. Sucrose and amino acids can be transported dissolved in water in phloem sieve tubes.
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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.
State what is meant by a mineral ion being transported dissolved in water in a plant.
Give me a hint
Identify the solvent and the solute.
Compare my reasoning
- Water is the solvent.
- The mineral ion is carried as a solute in the water.
The mineral ion is present as a solute carried in water through a plant transport pathway; water is the solvent.
Look for these in your work
- Water identified as solvent
- Ion identified as solute
Give two assimilates that can be transported in phloem sieve tubes and state their physical form in the transport stream.
Give me a hint
Use the named syllabus examples.
Compare my reasoning
- Sucrose and amino acids are assimilates.
- They move dissolved in water.
Sucrose and amino acids can move as solutes dissolved in water in phloem sieve tubes.
Look for these in your work
- Both assimilates named
- Dissolved state stated
A learner says that because mineral ions and sucrose can both be dissolved in water, they must travel together in the same direction. Explain the error.
Give me a hint
Distinguish solubility from tissue and source/sink route.
Compare my reasoning
- Both may be transported in aqueous solution.
- Xylem and phloem have different roles; phloem direction depends on source and sink.
Being dissolved in water does not determine a shared route or direction. Mineral ions are commonly transported in xylem water, whereas sucrose and amino acids move as assimilates in phloem from sources to sinks.
Look for these in your work
- Solvent point acknowledged
- Tissue routes distinguished
- Direction qualified
Common mistakes
- Calling water itself an assimilate
- Saying every dissolved solute uses the same tissue and direction
- Treating phloem as a water-free solid tube
What can you explain now?
Which statement about plant transport solutions is correct?
Compare with the explanation
Some mineral ions and organic compounds are transported dissolved in water
Water can carry solutes through plant transport tissues.
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Make it stick
Which substances can be transported dissolved in water, and how do xylem and phloem routes differ?
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