Controlling the Internal Environment II: Salt and water balance Keywords (reading p. 936-949) Ammonia toxicity Urea Uric acid Osmoconformer Osmoregulator Passive transport Facilitated diffusion Active transport Osmoregulation by an aquatic invertebrate Osmoregulation in marine fish Osmoregulation in freshwater fish Water loss on land Permeable and impermeable body surfaces Kangaroo rate water balance anhydrobiosis The internal environment In most animals, the majority of cells are bathed by internal fluids rather than the environment This is advantageous since there can be control of substrates needed for metabolism Consider the origin of life: started out as enzymes in the primordial sea Rates of reactions were determined by the concentrations of substrates in the environment The first proto-organism enclosed it s enzymes inside a membrane and became a cell 1
Control of substrate concentration Products do not diffuse away Good because reactions will work better and you don t lose the products Good because you can keep out molecules that you don t want Bad because there can be osmotic problems Bad because hazardous by products can stay in the cell Hazardous products Therefore the internal chemical environment is controlled Most species of most phyla live in the ocean Some live in freshwater Fewer live on land A. Avoiding buildup of toxic chemicals Dealing with ammonia B. Osmoregulation - controlling internal solutes Hazardous products A. Avoiding buildup of toxic chemicals A major source of hazardous products is the production of nitrogenous wastes Ammonia (NH 3 ) is a small and very toxic molecule that is normal product of protein and amino acid breakdown If you are an aquatic organism, ammonia can readily diffuse out of the body and this is not a problem 2
Ammonia toxicity is a problem for terrestrial animals Ammonia does not readily diffuse away into the air. The strategy of terrestrial animals is to detoxify it then get rid of (excrete) it. Ammonia can be converted to urea which is 100,000 times less toxic Mammals, most amphibians, sharks, some body fishes The drawback of using urea Takes energy to synthesize Still need to use water to flush it out Some animals cannot afford to use water to excrete urea These animals use excrete uric acid instead Uric acid Since uric acid is not very soluble in water, it can be excreted as a paste. Less water is lost Disadvantages: Even more costly to synthesize. Loss of carbon 3
Who uses uric acid? Birds, insects, many reptiles, land snails Related to water use, but also reproduction Eggs - N wastes from embryo would accumulate around it if ammonia or urea are used. Uric acid precipitates out. B. Osmoregulation - controlling internal solutes Osmolarity Osmolarity = # of solutes per volume solution Often expressed in moles (6.02 x 10 23 atoms/molecules) per liter. 1 mole of glucose = 1 mole of solute 1 mole of NaCl = 2 moles of solute Osmotic problems Humans have internal solute concentration (osmolarity) of 300 milliosmoles per liter (mosm/l) The ocean is 1000 mosm/l What would happen if your body surface is water permeable and you fall into the sea 300 mosm/l 1000 mosm/l http://www.yout ube.com/watch? v=ym1rvwppo4&feature=rel ated http://www.yout ube.com/watch? v=gwkcfuhhuk&feature= related Jellyfish in the ocean Keep solutes at 1000 mosm/l no water loss or gain. A relatively simple solution 1000 mosm/l jellyfish 1000 mosm/l 4
Optimal cell conditions Na+ is detrimental to cell function K+ less detrimental than Na+ Life in freshwater - hydra living in a pond Can the same strategy of matching the environmental osmolarity be used? 0 mosm/l 0 mosm/l Green hydra Hydra living in a pond If external osmolarity is very low like 0 mosm/l, hydra cannot maintain an internal osmolarity of 0 mosm/l Why is this? Consequently freshwater animals will most likely have a higher osmolarity than the environment. What happens to freshwater organisms? Water from the environment is continually entering tissues. The diffusion gradient favors loss of solutes Therefore there is a need to regulate solutes and water Two ways to deal with osmotic problems Keep your internal concentrations the same as the environment (osmoconformer) Regulate your internal concentrations (osmoregulator) Solute regulation Transport solutes across the body surface Note: even in the jellyfish example, there is ion regulation. Although the internal fluids have the same osmolarity as seawater, they do not have the same composition 5
Ways molecules get across membranes Passive transport: Diffusion Works for lipid soluble molecules and gases No good for most water soluble molecules and ions http://www.youtube.com/watch?v=q qsf_ujcfbc&feature=related Passive transport: Facilitated diffusion Generally used for ions, larger molecules, non-lipid soluble molecules. Must be a gradient favoring diffusion http://www.youtube.com/watch?v=s0p1 ztrbxpy&feature=related Active transport Works for ions and molecules like glucose or amino acids Can transport against a gradient. Costs energy, usually ATP http://www.youtube.com/watch?v=st zoirqzzl4 In this diagram, how might sodium get across the membrane? A) diffusion B) active transport C) facilitated diffusion or active transport In this diagram, how might sodium get across the membrane? A) diffusion B) active transport C) facilitated diffusion or active transport 6
In this diagram, how might sodium get across the membrane? - - - - - - - - - - - - - + + + + + + + + + + A) diffusion B) active transport C) facilitated diffusion or active transport In this diagram, how might s get across the membrane? A) diffusion B) active transport C) facilitated diffusion D) all of the above In this diagram, how might s get across the membrane? A) diffusion B) active transport C) facilitated diffusion D) all of the above Responses of soft-bodied invertebrates to changes in salinity Marine invertebrates can often be exposed to salinity changes (e.g., tidepool drying out, estuaries) If salts enter the body, pump them out using transporters If salts are leaving body, take them up from the environment using transporters Or just let your internal concentrations follow changes in the environment Dumping/pumping amino acids One way to respond while keeping internal ion concentrations the same is to pump amino acids out. Often used by bivalves living in estuaries Clams, oysters, mussels 7
Estuary - high tide Estuary - low tide 1000 mosm/l aa aa 1000 mosm/l 1000 mosm/l aa aa 500 mosm/l 500 mosm/l Estuary - low tide 500 mosm/l aa aa aaaa Advantages of amino acid osmoregulation Changing amino acid concentrations is less disruptive on internal processes (enzyme function). Costs: pumping amino acids (can involve ATP), loss of amino acids (carbon and nitrogen) Osmoregulation in other aquatic organisms Example: fishes maintain internal concentration of solutes Body volume does not change Involves energetic cost of active transport In bony fishes this can be 5% of metabolic rate Marine fishes 8
Marine fishes Problem: lower internal osmolarity than seawater Water will leave body, sea salts will go in Solution: Fish drink large amounts of seawater, then transport out ions (, Cl - ) at their gill surface or in urine (Ca ++, Mg ++, SO 4 -- ). Freshwater fishes Freshwater fishes The opposite situation: tendency to lose solutes and gain water Solutions: take up salts in food and by active transport across gills Eliminate water via copious dilute urine production Water balance on land Unlike aquatic animals, terrestrial animals don t lose or gain water by osmosis However, water loss or solute gain can be a major problem Cells are maintained at around 300 mosm/l Humans die if they lose 12% of their body water Why not just prohibit water loss? Impermeable surfaces: waxy exoskeleton (insects), shells of land snails, thick skin (vertebrates). Not all surfaces can be impermeable because gas exchange must also occur. Evaporation across respiratory surfaces is only one of the two main causes of water loss The other is urine production Drinking Replenishes water that is lost Water can also be gained by moist foods What if there is no water to drink? 9
Desert kangaroo rat Desert kangaroo rat does not drink Don t lose much water Special nasal passages Urine doesn t contain much water Recovers almost all of the water that results from cellular respiration Note comparison is relative not absolute Greater proportion of water intake of K rat is from metabolism Low proportion of K rat water loss is in urine Anhydrobiosis: Tardigrades (water bears) Can lose 95% of their body water 10