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

Compromises between gas exchange & water loss

• The exchange of gases from the atmosphere is essential for the survival of organisms

  • Oxygen is required for respiration

  • Carbon dioxide is required for photosynthesis

• Water has a range of vital functions within organisms, e.g.

  • it is a solvent that facilitates the transport of essential nutrients

  • it is the main component of cell cytoplasm, in which metabolic reactions occur

• Scientists have observed that adaptations which reduce water loss negatively affect gas exchange and vice versa

• As a result, organisms need to compromise between maximising gas exchange and minimising water loss to ensure that there is a sufficient supply of both water and gases

• Examples of this can be seen in:

  • terrestrial insects

  • xerophytic plants

Minimising water loss in terrestrial insects

• Features of insects that minimise water loss include:

  • a waterproof exoskeleton that prevents water loss over most of the insect's body surface

  • the ability to close spiracles

  • hairs around the spiracles to reduce diffusion of water vapour

Minimising water loss in xerophytic plants

• 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

Image showing how sunken stomata protects the escaping water vapour from the external air currents

Example: cacti

• Cacti are well-studied xerophytes

• They have several characteristics adaptations:

  • their leaves have become spines that can no longer photosynthesise

    • photosynthesis occurs in the green stem which possesses chloroplasts

  • the stem has a thick cuticle to prevent water loss

  • the stem is large in diameter which allows it to store water

  • there are both shallow and deep roots which allow it to access all available water

Image showing the adaptations of a Cactus that enables it to survive in dry, hot environments

Example: marram grass

• Sand dunes are another example of a dry environment where plants have evolved to survive

• Marram grass is commonly found on these 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

The adaptations of the leaf causes water vapour to be trapped near the stomata. This humid air near the stomata means the water potential gradient is reduced, so the rate of evaporation decreases.