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B3.1.10—Stomatal density

Stomatal density measures stomata per unit leaf area and helps compare potential gas exchange and water loss between leaf surfaces.

Syllabus
First assessment 2025
Objective
B3.1.10
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1B
Typical marks1

Common command terms

  • Outline

Recent exam appearances

May 2025Paper1B ["SL"] · TZ11(c)(i)[ 1 ]B3.1.10—Stomatal density
Practice this objective

Coverage 2025–2025 · Updated 15 Jul 2026

Stomatal Density Changes Potential Gas Exchange

Stomatal density is the number of stomata per unit leaf area, determined from a micrograph or a leaf-surface cast with a calibrated field of view.

Count only stomata within a known sampled area and divide by that area. Use a consistent boundary rule so stomata touching an edge are not counted twice across adjoining fields.

Stomataldensity=numberofstomatacounted÷sampledarea;report,forexample,instomatamm−2.Stomatal density = number of stomata counted ÷ sampled area; report, for example, in stomata mm⁻².

Repeat counts in several randomly or systematically selected fields from the same leaf surface, calculate each density and report a mean; repeat across leaves when comparing plants.

One field may not represent a biologically variable leaf. Replication increases reliability, while magnification alone is insufficient unless the real field area has been calibrated.

Stomatal density

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Outline.

Command terms

Outline

What earns marks

Build the answer around this relationship: Stomatal density is calculated as the number of stomata divided by leaf area observed.

Representative question

Question 1

[Maximum number: 1]

Outline how stomatal density in busy Lizzie leaves can be estimated within a known field of view.

Gas Exchange Across Animals And Leaves

Core gas-exchange answers link exchange surfaces to diffusion gradients. For animals, exchange surfaces are explained by diffusion properties, ventilation, and blood flow. For plants, leaves allow carbon dioxide entry and oxygen/water vapour exit while controlling water loss through stomata. Spirometry, transpiration, and stomatal density data provide evidence of gradient and surface-area effects.

  • Core animal answer: large, thin, moist, permeable surface plus ventilation and blood flow.
  • Core plant answer: stomata, guard cells, mesophyll air spaces, cuticle, and transpiration factors.
  • Data questions usually test rate, gradient, volume, or density per area.

Concept essentials

  • Stomatal density is calculated as the number of stomata divided by leaf area observed.
  • More stomata on a surface can indicate greater potential transpiration from that surface.
  • A known field of view or calibrated image is needed for a valid density estimate.
  • Density comparisons are meaningful only when the same area basis is used.