weirdfacts
Year 11 / Cellular energy, gas exchange and plant physiology

Water rises. Sugars travel.

Xylem carries water and mineral ions from roots toward leaves. Evaporation from leaf surfaces creates tension that pulls a continuous water column upward. Cohesion and adhesion support that column.

Phloem distributes sugars from source regions to sinks that use or store them. Stomata permit gas exchange but also allow water loss, so plants balance photosynthesis against dehydration.

INTERACTIVE MODEL

Stomatal trade-off

CO₂ uptakeWater lossStomataOpening 50% · CO₂ uptake 50% · relative water loss 43%

Explore: Change stomatal opening and observe the water-loss and CO₂-uptake trade-off.

roots→xylemleavessource→phloemsink\text{roots}\xrightarrow{\text{xylem}}\text{leaves}\qquad\text{source}\xrightarrow{\text{phloem}}\text{sink}
WORKED EXAMPLE

A dry, windy day increases evaporation from leaves.

  1. Transpiration pull tends to increase if water remains available.
  2. Stomatal closure can limit water loss but also restrict carbon dioxide uptake.
Assumed knowledge

Plant organs and osmosis.

Learning checkpoints & sourceYOUR LEARNING CHECKPOINT
  • Distinguish xylem from phloem.
  • Explain the transpiration pull.
QCAA Biology · Unit 1 · Cellular energy, gas exchange and plant physiology ↗
READY TO TRY IT?

Put the idea to work.

Back to syllabusNext: Return toward the set point.
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