Controlling the Internal Environment II: Salt and water balance

Keywords (reading p. 879-884) Ammonia toxicity Urea Uric acid Osmoconformer Osmoregulator Passive transport Facilitated diffusion Active transport Uniport Antiport symport 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

Control of substrate concentration Products do not diffuse away

Good because you can keep out molecules that you don’t want 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 A. Avoiding buildup of toxic chemicals Dealing with ammonia B. Osmoregulation - controlling internal solutes

A. Avoiding buildup of toxic chemicals

Hazardous products A major source of hazardous products is the production of nitrogenous wastes Ammonia (NH3) 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

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

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 Uric acid

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 1023 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 1000 mosm/L Keep your internal concentrations the same as the environment (osmoconformer) Regulate your internal concentrations (osmoregulator) 300 mosm/L

Jellyfish in the ocean 1000 mosm/L 1000 mosm/L Keep solutes at 1000 mosm/L no water loss or gain. A relatively simple solution 1000 mosm/L 1000 mosm/L jellyfish

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

Ways molecules get across membranes

Passive transport: Diffusion Works for lipid soluble molecules and gases No good for most water soluble molecules and ions

Passive transport: Facilitated diffusion Generally used for ions, larger molecules, non-lipid soluble molecules. Must be a gradient favoring diffusion

Active transport Works for ions and molecules like glucose or amino acids Can transport against a gradient. Costs energy, usually ATP

In this diagram, how might sodium get across the membrane? A) diffusion B) active transport C) facilitated diffusion or active transport Na+ Na+ Na+ Na+

In this diagram, how might sodium get across the membrane? A) diffusion B) active transport C) facilitated diffusion or active transport Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+

In this diagram, how might sodium get across the membrane? A) diffusion B) active transport C) facilitated diffusion or active transport Na+ Na+ Na+ Na+ - - - - - - - - - - - - - + + + + + + + + + + Na+ Na+

In this diagram, how might steroids get across the membrane? A) diffusion B) active transport C) facilitated diffusion D) all of the above steroid steroid steroid steroid steroid

In this diagram, how might steroids get across the membrane? A) diffusion B) active transport C) facilitated diffusion D) all of the above steroid steroid steroid steroid steroid steroid steroid steroid steroid steroid steroid steroid steroid steroid steroid

Types of active transport

What type of active transport is this? A) uniport B) symport C) antiport K+

What type of active transport is this? A) uniport B) symport C) antiport K+ Sodium potassium ATPase Na+

What type of active transport is this? Cl- A) uniport B) symport C) antiport K+

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

Estuary - high tide 1000 mosm/L 1000 mosm/L aa aa aa aa aa aa aa aa

Estuary - low tide 500 mosm/L 1000 mosm/L aa aa aa aa aa aa aa aa

Estuary - low tide 500 mosm/L 500 mosm/L aa aa aa aa aa aa aa aa

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

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 (Na+, Cl-) at their gill surface or in urine (Ca++, Mg++, SO4--).

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?

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