O-Level and SEC G3 Biology K325
B10: Nutrition and Transport in Flowering Plants
Link photosynthesis, leaf and root adaptations, xylem, phloem, transpiration, and limiting factors.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Flowering-plant nutrition and transport links leaf and root structure, photosynthesis, xylem, phloem, and transpiration. Follow each route from structure to movement, then distinguish a measured proxy from the biological rate it estimates.
Photosynthesis and leaf adaptations
Chlorophyll absorbs light energy for making glucose from carbon dioxide and water, releasing oxygen. Glucose can be respired, stored as starch, made into cellulose or fat, or used with absorbed nitrogen to make amino acids and proteins.
A broad thin leaf captures light and gives a short gas-diffusion distance. Palisade cells contain many chloroplasts, spongy mesophyll has internal air spaces, and stomata allow carbon dioxide entry and oxygen and water-vapour exit.
Root uptake, xylem and phloem
Root hair projections increase surface area for water uptake by osmosis and ion uptake by diffusion or active transport. Water and ions enter xylem, whose dead hollow lignified vessels form a continuous root-to-leaf pathway.
Phloem sieve tubes and companion cells translocate mainly sucrose from sources such as photosynthesising leaves to sinks such as growing roots or fruits. Different phloem tubes can carry material in different directions.
Transpiration and limiting factors
Transpiration is water-vapour loss from leaves after evaporation from mesophyll and diffusion through stomata. It helps pull a continuous water column through xylem. Wind, temperature, humidity, light, and stomatal opening affect its rate and can contribute to wilting.
Light intensity, carbon-dioxide concentration, or temperature can limit photosynthesis. Increasing one raises rate only while it is limiting. Control the other factors in an investigation and identify changing limiting ranges from the graph.
Formulae and relationships
| Relationship | Use |
|---|---|
| Represent photosynthesis in the presence of light and chlorophyll. | |
| Calculate a measured photosynthesis or transpiration proxy. |
Worked examples
Example 1: A potometer bubble moves in , then in after a fan is switched on. Compare the measured uptake rates.
- Before the fan, rate is .
- With the fan, rate is .
- State that water uptake is a proxy for transpiration and is not identical to water loss.
Answer: The measured uptake rate increases by , from to .
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Connect photosynthesis inputs, products, chloroplast function, and limiting factors.
- Relate root, leaf, xylem, and phloem structures to uptake and transport.
- Explain transpiration through evaporation, diffusion, and the factors changing the gradient.
Official outcome coverage
K325 B10: 14 mapped outcomes, references B10(a), B10(b), B10(c), B10(d), B10(e), B10(f), B10(g), B10(h), B10(i), B10(j), B10(k), B10(l), B10(m), B10(n). Check the official K325 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Investigate photosynthesis or transpiration with acclimatisation, controlled leaf area and temperature, repeats, and a proxy clearly identified.
Exam traps and retrieval check
Avoid these traps
- Saying plants obtain food from soil instead of making glucose by photosynthesis.
- Treating xylem and phloem as interchangeable transport tissues.
- Calling potometer water uptake an exact direct measure of transpiration.
Check from memory
Which tissue carries water from roots?
Xylem.
What is a phloem sink?
A region that uses or stores transported assimilates.
How does higher humidity affect transpiration?
It usually lowers the water-vapour gradient and reduces the rate.
Pure versus Combined scope
Combined Biology shares the core idea but assesses a narrower outcome set. Use the K327 or K328 component checklist to set the exact boundary.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K325. Open the related IP explanation.