3.1.6 Gas Exchange vs Water Loss

The Conflict Between Gas Exchange and Water Loss:

1. Gas Exchange Requirements:

• Large surface area and thin membranes maximise oxygen uptake and carbon dioxide removal.
• Openings, such as stomata in plants or spiracles in insects, facilitate gas diffusion but also allow water to escape.

2. Water Loss:

• Water evaporates from moist exchange surfaces, leading to dehydration.
• This is a significant problem in terrestrial organisms exposed to dry environments.

Adaptations to Reduce Water Loss:

1. Insects:

• Use a tracheal system for gas exchange, which minimises water loss:
  • Spiracles can open and close to reduce evaporation.
  • Waterproof exoskeleton made of chitin prevents water loss through the body surface.
  • Small tracheoles deliver oxygen directly to tissues, reducing the need for open spiracles.

2. Plants:

• Adaptations to reduce water loss while maintaining gas exchange:
  • Stomata:
  • Tiny pores on leaves that regulate gas exchange.
  • Guard cells open and close stomata based on water availability.
  • Waxy cuticle:
  • Waterproof layer on leaves to reduce evaporation.
  • Reduced stomatal density:
  • Some plants have fewer stomata to minimise water loss.
  • Xerophytes (plants adapted to arid environments):
  • Sunken stomata trap moist air, reducing the diffusion gradient for water loss.
  • Rolled leaves and hairs on the surface reduce transpiration.

3. Mammals:

• Efficient gas exchange systems (e.g., alveoli) minimise water loss:
  • Moist surfaces are internal, reducing exposure to the external environment.
  • Air is humidified in the nasal passages, reducing evaporation from the lungs.

4. Amphibians and Reptiles:

• Amphibians use their moist skin for gas exchange but lose water rapidly in dry environments.
• Reptiles have scales that reduce water loss but rely on lungs for gas exchange.

5. Fish:

• Do not face water loss issues due to their aquatic environment.
• Use gills for gas exchange, which are efficient in water but unsuitable for air.

Examples of Organisms Balancing Gas Exchange and Water Loss:

3. Cacti (Xerophyte Plants):

• Reduced leaves (spines) and thick waxy cuticles minimise water loss.
• Stomata open only at night (CAM photosynthesis).

4. Desert Insects:

• Close spiracles during periods of inactivity to conserve water.

5. Kangaroo Rats:

• Highly efficient kidneys produce concentrated urine, compensating for water loss through respiration.

Key Adaptations:

Organism	Adaptation	Purpose
Insects	Spiracles that open and close.	Reduce water loss while allowing gas exchange.
Xerophyte Plants	Sunken stomata and thick cuticle.	Minimise transpiration in arid environments.
Mammals	Internal lungs and humidified airways.	Reduce water loss during breathing.
Fish	Gills submerged in water.	Prevent water loss entirely.