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B3.1.8—Leaf tissue distribution

Dicot leaf tissues are arranged so light capture, gas diffusion, water supply and sugar transport occur in coordinated epidermal, mesophyll and vascular layers.

Syllabus
First assessment 2025
Objective
B3.1.8
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceNovember 2015
Most common paperPaper2
Typical marks1–8

Common command terms

  • Draw
  • Explain

Scoring notes

Common mistake
Drawing individual cell detail when a plan diagram should show tissue distribution and relative positions.

Recent exam appearances

November 2015Paper2 ["HL"] · TZ07(a)[ 5 ]B3.1.8—Leaf tissue distribution
November 2014Paper2 ["HL"] · TZ06(c)[ 8 ]B3.1.8—Leaf tissue distribution
May 2013Paper2 ["HL"] · TZ15(a)[ 4 ]B3.1.8—Leaf tissue distribution
November 2012Paper1 ["HL"] · TZ031[ 1 ]B3.1.8—Leaf tissue distribution
Practice this objective

Coverage 2012–2015 · Updated 15 Jul 2026

Leaf Tissues Place Exchange Routes Close to Photosynthetic Cells

A plan diagram of a dicot leaf transverse section shows the relative distribution of tissues from upper to lower surface, without drawing individual cells.

The usual sequence is cuticle and upper epidermis → palisade mesophyll → spongy mesophyll with air spaces → lower epidermis with stomata. Vascular bundles lie within the mesophyll, with xylem generally nearer the upper surface and phloem nearer the lower.

Use clear single lines, preserve relative layer thickness and label tissue regions. Include cuticle, upper and lower epidermis, palisade and spongy mesophyll, air spaces, a vascular bundle with xylem and phloem, and stomata/guard cells in the epidermis.

Carbon dioxide entering a lower-surface stoma follows connected spongy-mesophyll air spaces to photosynthetic cells, while xylem in a nearby vein supplies water.

A plan diagram records tissue distribution, not fine cellular detail: do not shade, sketch every chloroplast or replace relative positions with a list of structures.

Leaf tissue distribution

Assessment in practice

1–5 marks
How it is assessed

This objective is assessed through structured response, commonly using Draw / Explain.

Command terms

Draw / Explain

What earns marks

Build the answer around this relationship: Palisade mesophyll lies near the upper leaf surface and is rich in chloroplasts.

Watch for

Drawing individual cell detail when a plan diagram should show tissue distribution and relative positions.

Representative question

Question 1

[Maximum number: 8]

Explain how the distribution of tissues in the leaf of a dicotyledonous plant is adapted to production and distribution of products of photosynthesis.

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

  • Palisade mesophyll lies near the upper leaf surface and is rich in chloroplasts.
  • Spongy mesophyll contains air spaces for internal gas diffusion.
  • Xylem supplies water to leaf tissues, while phloem transports photosynthetic products.
  • A plan diagram shows tissue distribution and labels, not detailed organelles.