Water and Life: Terrestrial Challenges

Study Notes

Water is essential for all biochemical reactions in living organisms.
Terrestrial environments present challenges for organisms due to limited water availability.

Water flows from areas of high pressure to low pressure.
Atmospheric water potential is influenced by relative humidity.
When humidity drops below 98%, atmospheric water potential is lower than that of organisms, leading to water loss.

Terrestrial organisms have developed barriers to reduce water loss, such as skin and waxy cuticles in insects and plants.
Plants absorb water from the soil through their roots to replenish water lost to the atmosphere.
Plants lose water through transpiration when stomata open for carbon dioxide uptake.

Water potential (ψ) is the measure of water pressure within a plant or soil.
Units for water potential are megapascals (MPa).
Water flows from areas of high water potential to areas of low water potential. This explains how pure water moves into plant roots or dry soil.
In plants, water potential is determined by three factors:
- Hydrostatic pressure (ψp): Pressure potential
- Osmotic potential (ψπ): Increases with solute concentration, and decreases water potential.
- Matric potential (ψm): Increases with surface area, and decreases water potential.
Water potential gradients typically flow from soil to root to leaf to air.

Stomata are tiny pores on plant leaves that allow carbon dioxide to enter for photosynthesis.
Water vapor is also lost through stomata during transpiration.
Guard cells surrounding stomata control their opening and closing:
- When guard cells shrink, they close the stomata, conserving water.
- Stomata generally close at night and during droughts.