Coping with Environmental Variation: Water and Solutes

Learning goals:
* be able to predict the movement of water and solutes between an organism and its environment based on:
    1) the water potentials of the organism and its environment
    2) the pathway of water and solutes and the barriers (resistances/ conductances) to its movement
* be able to describe some of the tradeoffs associated with adaptations to variation in water availability and solutes

Water is the medium in which all biochemical reactions necessary for life occur- it has unique properties that make it a universal solvent for biologically important solutes.
Maintaining optimal water content is a challenge primarily for freshwater and terrestrial organisms; Ocean waters maintain the water balance of marine organisms (solute balance is a different matter).
* Terrestrial species lose water to a dry atmosphere.
* Freshwater organisms lose solutes to and gain water from their environment.

Water flows along energy gradients associated with gravity, pressure/tension, solute concentrations, matric forces.  Water potential of a system (amount of energy) is related to the sum of the pressure potential, the osmotic potential, and the matric potential: Ψ = ψ_o + ψ_p + ψ_m
The difference in water potential between an organism and its surrounding environment (or between adjacent cells) will determine whether it will gain or lose water

External cover- e.g. skin, cuticle, decreases movement of water by increasing resistance (a force that impedes water movement along an energy gradient) or decreasing conductance (reciprocal of resistance)
Increasing resistance is an adaptation to lowering water loss in a dry environment

Water balance of single-celled aquatic organisms is mostly determined by osmotic potential.  In most aquatic environments, the osmotic potential doesn’t change much over time, except in tidal pools, estuaries, saline lakes, and soils.  Where salinity changes these organisms must alter their osmotic potential to maintain water balance = osmotic adjustment.  Solute concentration in a cell can be increased by synthesizing organic solutes, or by taking up inorganic salts from the external environment.

Terrestrial plants control water loss by controlling stomatal opening
* transpiration is an important cooling mechanism
* plants gain CO₂ through stomates
* large water potential gradient between plant and air
* too much water loss can damage plant tissues- symptom is wilting, or loss of plant turgor
With time, transpiration and evaporation can deplete soil water if precipitation doesn’t replenish what is lost to the atmosphere.  Eventually stomates must lower water loss to prevent damage to plants

Multicellular animals have complex systems for digestion, gas exchange, and reproduction that effect water balance
For aquatic animals, water can be:
Hyperosmotic - more saline than the animal’s cells.
Hypoosmotic - less saline than the animal’s cells.
Isoosmotic - have the same solute concentration as the animal’s cells
Marine animals tend to be isoosmotic to seawater, but the types of salts they have may be very different than seawater
Marine bony fish (teleosts) and mammals evolved in freshwater and their blood is hypoosmotic (less saline) relative to seawater; these fish exchange salts across the gills, and by eating and drinking
In contrast, freshwater teleosts have blood that is hyperosmotic (more concentrated) than their environment

Terrestrial animals must balance gas exchange and water loss
To minimize water loss, some live in moist environments (e.g. amphibians), while some increase skin resistance (e.g. reptiles).
Increasing resistance to water loss lowers amount of gas exchange possible, particularly when skin is part of gas exchange system- selection for specialized respiratory structures (e.g. trachea/ lungs)
Insect cuticles have a waxy coating with very high resistance to water loss- constrains amount of growth, requiring molting as animal grows
Desert rodents show several adaptations to low water availabilty; exemplified by kangaroo rats
* obtain most of their water from burning carbohydrates and fats (oxidative water) and eating leaves, insects, and dry seeds- don’t need to drink liquid water
* thick, oily skin reduces water loss
* active only at night during the hottest times of year, and remain in deep, cool burrows during the day
* remove most of the water from their urine and feces