Explain how the adaptations of a named xerophyte allow it to reduce water loss in an arid environment.

IB DP Biology Standard Level (2023 syllabus) — B4.1 Adaptation to environment · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A suitable xerophyte is the cactus (Opuntia). Its adaptations reduce water loss by:
1. Stomata are sunken in pits, limiting water vapour diffusion.
2. A thick waxy cuticle on the epidermis reduces cuticular transpiration.
3. Leaves are reduced to spines, so the stem performs photosynthesis, greatly reducing the total surface area for transpiration.

Examiner tips

Use the exact terminology (sunken stomata, thick waxy cuticle, reduced leaves/spines).
Show the causal link: each adaptation limits water loss.
Mention the cactus (Opuntia) explicitly.

Common mistakes

Failing to name a specific xerophyte.
Using vague terms like "small leaves" instead of "spines".

Mark scheme (4 marks)

Named xerophyte correctly identified (e.g. cactus / Opuntia, marram grass / Ammophila, euphorbia, agave, Welwitschia)
Stomata are sunken in pits / rolled leaves (marram grass) or reduced in number, limiting water vapour diffusion out of the leaf by trapping humid air near the stomatal opening
Thick waxy cuticle on the epidermis reduces evaporation of water directly through the leaf/stem surface (cuticular transpiration)
Leaves are modified into spines (cacti) / leaves are reduced in surface area, so the stem carries out photosynthesis, greatly reducing the total surface area available for transpiration

Key terms in this question

xerophyte