Gas Exchange vs Water Loss (AQA A Level Biology): Revision Note

Compromises between gas exchange & water loss

• Adaptations that aid gas exchange tend to increase the potential for water loss, meaning that organisms need to compromise between maximising gas exchange and minimising water loss

• Examples of this can be seen in:

  • terrestrial insects

  • xerophytic plants

Terrestrial insects

• Insects have a waterproof exoskeleton that prevents water loss by evaporation, but the spiracles provide openings through which water vapour can be lost

• Insect features that minimise water loss include:

  • the ability to close spiracles

  • hairs around the spiracles to reduce diffusion of water vapour

Xerophytic plants

• Plants need to keep their stomata open to allow gas exchange to occur, but open stomata also allow the loss of water vapour

• Plants that live in conditions where fresh water is limited have evolved adaptations to conserve water, including:

  • few stomata

  • stomata that are in pits

  • hairs surrounding stomata

  • needle-shaped leaves with a reduced surface area

  • a thickened waxy cuticle

• Plants with these adaptations are described as xerophytic

Example: cacti

• Adaptations in cacti that reduce water loss include:

  • leaves reduced to spines to reduce surface area for water loss

  • the stem has a thick cuticle to prevent water loss

Example: marram grass

• Marram grass is commonly found on sand dunes

• Their leaves are well adapted to minimise water loss:

  • Leaves can roll up to reduce the exposure of surfaces to the wind

  • The rolling of the leaf provides deep grooves which protect the stomata

  • The exposed surface has no stomata and a thick cuticle

  • The inner surface of the leaf possesses a large number of hairs