Lecture 9
Avoidance, conformity, regulation, and tolerance
- animals are restricted in their distributions by three important physical variables 1. temperature 2. osmotic stress and water availability 3. availability of oxygen
2. conformity
- animals can be an osmotic conformer where their body fluids have the same osmotic pressure as their environment - some marine invertebrates are osmoconformers (and may act as ionconformers) with respect to certain ionic concentrations as well) - for conformers, the extracellular fluids are usually in osmotic equilibrium with seawater, in addition, extracellular fluids are usually very similar ionically to seawater - many of the marine organisms that are conformers are also sessile. Sessile invertebrates may not be able to avoid cahnges in the osmotic strength. The marine animals that act as conformers can usually tolerate only a narrow range of osmotic pressures, thus they are stenohaline - essentially all marine invertebrates abd primitive chordates are stenohaline animals, and isomotic with seawater (1000 to 1050 mOsm). they are slightly higer, due to the Donnan effect (behavior of large electrically charged proteins and metabolites in cells and body fluids)
4. tolerance to water loss
- animals differ in their tolerance of water loss (measured as a maximum % weight loss) - water losses have been measured experimentally via weight changes and by labeled water (tritium-labeled water) - invertebrates generally tolerate much more water loss than vertebrates - habitat is also important: marine animals, including the intertidal species, can lose 15-75% of their weight, depending on species - Many desert insects can lose up to 75% of their weight as water - desert animals in general can tolerate a greater water loss compared to their cousins that live in forests and grasslands; this ability to tolerate a high degree of desiccation is an obvious adaptation to desert life - humans can only tolerate 10-12% (by weight) water loss, however, a few mammals are exceptional in their water loss tolerance -camels can tolerate up to 30% water loss
In many species, the water content differs greatly in different compartments, and some compartments can buffer the effects of water loss
- in annelids, mollusks, and arthropods about 50-75% of the water is in extracellular compartments, such as hydrostatic cavities and coeloms. - in most vertebrates, only about 20-25% of the water is in the extracellular compartments (plasma and interstitial fluids); instead, most of the water is inside cells
Donnan effect or Donnan equilibrium
- occurs when small, permeable charged particles near a semi-permeable membrane fail to distribute evenly across the two sides of the membrane - the usual cause is the presence of a different charged substance that is unable to pass through the membrane - when large, electrically charged proteins (anions) are found only on one side of a semipermeable membrnae, that proteins cannot cross - this fact then affects the movement of the smaller permeable cations and anions - positively charged ions move to the side of the membrane with proteins, but negatively charged ions do not - this uneven movement of charge ions eventually causes the osmotic movement of water into the side of the membrane with the impermeable proteins - when in equilibrium, there is separation of charge across the semipermeable membrane, due to the uneven movement of the ions
Osmoregulation
- osmoregulators = are animals that seek to maintain a steady internal body fluid concentration regardless of external concentrations. all terrestrial animals and most freshwater animals are regulators, as are many species found in marine and brackish water systems - osmoconformers = allow internal fluid concentrations to drift somewhat with the external environment. osmoconformers are common in marine environments -euryhaline vs. stenohaline *stenohaline = organisms restricted to narrow ranges of salinites. Most animals cannot tolerate substantial changes in their bodily fluids (whether or not they are regulators or conformers). most are found only under fairly narrow ranges of osmolarity *euryhaline = organisms tolerant of a wider range of environmental salinities (again, they can either be conformers or regulators) - ions and other dissolved materials often are more concentrated in the animals' tissues than in their environment, so organisms must accumulate the nutrients against their concentration gradients. Organisms also produce or accumulate toxic materials that they must excrete or exclude - due to kinetic energy, ions randomly move about, moving from areas of high concentration to areas of low concentration within cells and within extracellular components (via diffusion)
1. avoidance
- some animals may avoid osmotic stress - for example, animals can avoid habitats with stressful osmotic levels or habitats that vary considerably in osmotic pressure over time - many of the intertidal and estuarine animals are periodically exposed to air and/or freshwater inputs from streams and rivers along with exposure to seawater. they avoid the changes by a variety of ways. Some animals burrow into the mud or sealing up shells when the tide goes out. In contrast, some animals climb vegetation or hide in burrows when the tide comes in
3. regulation
- some organisms, however, acts as osmoregulators, they can regulate their internal osmotic pressure and keep it constant over a range of ambient osmotic pressures - the larger, more mobile marine vertebrates, as well as most of the terrestrial and freshwater vertebrate animals, are able to regulate their internal osmotic environment over some range of osmotic conditions of their environment. Many terrestrial and freshwater animals are also ionoregulators - the marine vertebrates are osmoregulators because of (in part) their evolutionary history on land, or from freshwater systems - the marine mammals (seals, whales, dolphins) and birds (penguins) have all reinvaded the sea (from land), as have some bony fish (from freshwater systems) - the blood, extracellular fluids, and intracellular fluids of birds, whales, and seals differ from seawater in that ions have a tendency to move into the animals and water tends to move out, so the animals must have adaptations to prevent these fluxes - the sharks are osmoconformers, but they simultaneously regulate the concentrations of many different ions - as we will see with sharks, their blood is in osmotic equilibrium with seawater because of the various osmolytes (like urea and TMAO), but the ionic concentrations are low, like that seen in the freshwater animals - most vertebrates are osmoregulators that maintain the composition of the body fluids within a narrow range - the body fluids of all vertebrates are hypoosmotic to saltwater and hyperosmotic to freshwater - many terrestrial and aquatic invertebrates also osmoregulate to some degree - on land, maintaining water balance is important to animals that are continuously losing water to the air - at the same time, terrestrial animals are producing metabolic wastes, thus they need water to eliminate these toxi xompounds by excretiom - terrestrial animals also have to breather, and thus they lose water from their respiratory organs during respiration
Osmosis
- the special condition where water diffuses down its concentration gradient through a semipermeable membrane - the ability of a substance or solution to attract water by osmosis is called its osmotic potential or osmotic pressure - osmotic pressure is the minimum pressure that has to be applied to a solution on one side of a semipermeable membrane to prevent the inward flow of pure water across the membrane (this actually applies to all solvents, but we generally talk about water solutions in living things). remember that pressure is the amount of force applies per unit of area on an object (such as a semipermeable membrane) - thus, when an animal is in seawater, the osmotic pressure is higher in the seawater, and water diffuses out of the animal (i.e. the animal is in a hyperosmotic environment) - the opposite is true for most freshwater habitats; the osmotic pressure is higher in the animals' tissues and body fluids, and water is drawn inward (the animal now lives in a hypoosmotic environment)
Molal vs. Molar solutions
1. molar solution = a 1 M solution is 1 mole of solute is dissolved in a finals volumr of 1 liter of solution (1mol/L) 6.022x10^23 = avagadros numberr (# of molecules in a mole) 2. molal solution = 1 mole of solute per kilogram of water 3. the osmotic concentration of a substance can be expressed as osmolarity (osmoles per liter) - the osmolarity depend on the number of dissolved particles and can be stated without knowing exactly what molecules are present
Four colligative properties of solution are properties dependent on the number of solute particles and not the type of solutes
1. the osmotic pressure of a solution is the difference in the pressure between the solution (water plus solute) and the pure liquid solvent (water) when the two are in equilibrium across a semipermeable membrane - water moves into the solution - the membrane allows the solvent molecules to move across it, but not the solute particles - if the two sides of the membrane are at the same initial pressure, there is a net transfer of solvent (osmosis) across the membrane into the solution - the hydrostatic pressure is in reverse of the osmotic pressure: the pressure between the solution (water plus solute) and the pure liquid water on the opposite side of the membrane - hydrostatic pressure is forcing the water molecules against the direction of osmosis - the process stops and equilibrium is attained when the hydrostatic pressure difference equals the osmotic pressure 2. freezing point depression = an ideal (nondissociating or associating) solute depresses the freezing points by 1.86C - pure water freezes at 0C, a 1 molal aqueous solution of an ideal nondissocaiting solute freezes at a slightly colder temperature: -1.86C - a 1.86C freezing point depression of a solution is defined as equivalent to an osmotic concentration of 1 osmolal 3. boiling point elevation: an ideal solution elevates the boiling point by 0.54C - for both the freezing point depression and boiling point elevation, the addition of solute stabilizes the solvent in the liquid phase so the solvent molecules have less of a tendency to move to the gas or solid phases, if the solute remains in the liquid phase 4. vapor-pressure lowering = the vapor pressure of a liquid is the pressure of a vapor phase that is in equilibrium with the liquid phase - the vapor pressure of a solvent is lowered by addition of any non-volatile solute
There are four distinct patterns to dealing with osmotic stress
An organism may behave or adapt in four ways 1. avoidance 2. conformity 3. regulation 4. tolerance to water loss
In theory, the osmolarity and molarity are equivalent for solutions of ideal nondissociating solutes
however, this equivalence does not hold for ionic solutions, because of the variable degree of the dissociation of many salts into two or more ions - the dissociating electrolyte solution will contain more dissolved molecules than the nonelectrolyte solution of the same molarity
