Adaptation to Aquatic and Terrestrial Environments

Claude Bernard’s "Milieu Intérieur"

• 19^th Century French animal physiologist Claude Bernard noted importance of animals maintaining a constant internal environment (i.e., constant composition of blood and interstitial fluids)—homeostasis

• we can extend his thinking to the intracellular environment as well, and to all organisms

• internal environment best suits (enzyme-mediated) chemical reactions occurring in organisms; result of past evolution

• simple model for an organism (single-celled):

• can develop a simple model of an animal based on this concept:

• epithelium provides barrier between external environment and extracellular fluid (ECF)—transport across epithelium regulates composition of ECF

• plasma membrane provides barrier between ECF and intracellular fluid (ICF)—transport across plasma membrane regulates composition of ICF

• movement of water across a barrier (epithelium or plasma membrane) occurs only by passive transport (down its concentration gradient)

• movements of solutes across a barrier can occur by either of two mechanisms:

1. passive transport—solutes move down their concentration gradient; does not require expenditure of energy
2. active transport—solutes move up their concentration gradient; requires expenditure of energy (from ATP)

 

A. Salt Balance and Water Balance

• life began in the oceans; all species spent most or all of the evolutionary history in the oceans

• the composition of the body fluids of most marine organisms still reflects this origin:

• sea water has an osmotic pressure of about 1000 mOsm/l (1000 milliOsmoles per liter; 1 osmole = 1 mole of osmotically active particles); [recall salinity of ~35^o/_oo]

• most marine invertebrate animals have body fluids that are also ~1000 mOsm/l—i.e., there is no osmotic (water) gradient across their surface; however, their body fluids typically differ from sea water in the relative amounts of specific ions—they must still maintain gradients of these ions across their surface by active transport

• teleost ("bony") fish have body fluids considerably more dilute (less solutes) than sea water—~300 mOsm/l—interpreted as artifact of their evolution in fresh water (adaptation to limit water gradient across their body surface there):

• fresh water teleosts have body fluids (still) considerably more concentrated (more solutes but less water) than their environment—they tend to lose solutes to (and gain water from) their environment:

• fresh water teleosts do NOT purposefully drink (but some water invariably swallowed when eating)

• excess water eliminated by producing a dilute urine (retains solutes)

• solutes replaced by active uptake (expends energy from ATP) inwards across gill epithelium

• primary effect of acidic waters (i.e., due to "acid rain") on teleost fish is to increase their permeability to Na⁺ (and Cl^-)—these ions then decline in the blood—ultimately causes death

• marine teleosts (which re-invaded sea water later in their evolution, retaining ~300 mOsm/l) have body fluids considerably more dilute (less solutes but more water) than their environment (sea water)—therefore, they tend to take up solutes but lose water to their environment—live in a desiccating environment!:

• to replace the water they lose to their environment, marine teleosts must drink sea water—additional salt loading to their body fluids

• excess solutes (salts) eliminated by active transport (expends energy from ATP) across their gill epithelium; cannot produce concentrated urine

• some marine elasmobranchs (sharks, skates and rays) have solved the problem of water balance by retaining urea (a nitrogenous waste product) in their body fluids to raise their osmotic pressure to nearly 1000 mOsm/l—this eliminates the gradient for water gain

• many fresh water invertebrates have very dilute body fluids to reduce the water and solute gradients across their bodies (regulation largely moved to plasma membranes of cells)

• terrestrial animals regulate their extracellular fluid composition primarily at two sites:

1. kidney—produces copious amounts of dilute urine to eliminate excess water or produces small amounts of concentrated urine to eliminate solutes
2. gut—ingestion of water or solutes (e.g., salty foods) in part determined by "appetites" (physiological responses to changing ECF composition)

Salinity Relations of Aquatic Organisms

• different organisms respond differently to salinity changes in their environment:

• conformers (osmoconformers)—body fluid composition varies with that of environment

• regulators (osmotic regulators)—body fluid composition keep constant despite changes in environment

• organisms also differ in the range of salinities they can tolerate:

• euryhaline—can tolerate a wide range of salinities

• stenohaline—can tolerate only a narrow range of salinities

B. Nitrogen Excretion

• heterotrophs use the organic molecules they obtain for:

1. building blocks for growth and reproduction
2. a source of energy—liberated by respiration

• respiration of carbohydrates and lipids leaves only CO₂ and H₂O as end products

• respiration of proteins and nucleic acids (nitrogen-containing compounds) results additionally in production of the nitrogenous waste ammonia

• proteins and nucleic acids are obtained in excess of their needs for building blocks—these are first deaminated (i.e., the –NH₂ (amine) groups are removed) before respiration and are converted to ammonia (NH₃), which is toxic in high concentrations

• small aquatic animals simply allow ammonia to diffuse out across body surface

• larger aquatic animals produce relatively large amounts of urine (NH₃ concentration kept low)

• but most terrestrial animals cannot afford to lose so much water just to eliminate excess N:

• mammals convert ammonia (NH₃) to urea (O = C(NH₂)₂), which is less toxic—can be concentrated more by the kidney without toxicity problems

• desert mammals (e.g., kangaroo rat) produce most concentrated urine—about 14 x more solutes than blood

• how much kidney concentrates urine varies over time and is under hormonal control—kidney is major site of regulation of body water and solute concentration in terrestrial animals as well as site of nitrogenous waste excretion

• birds and reptiles conserve even more water by producing uric acid (C₅H₄N₄O₃):

• uric acid is relatively insoluble in water so its crystallizes out into a slurry ("paste")—milky white part of bird droppings

• due to their carbon content, both urea and (especially) uric acid production costs the animal—but benefit in is conserving valuable water

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