LIFESCI 7C Lecture Notes - Lecture 6: Salt Gland, Osmotic Pressure, Trimethylamine N-Oxide
Week 6
41.1 Water and Electrolyte Balance
● Homeostasis for water and electrolyte concentration si crytical for normal physiological function in all organisms
Osmosis governs the movement of water across cell membraes
● osmosis The net movement of a solvent, such as water, across a selectively permeable membrane toward the
side of higher solute concentration.
○ solute A dissolved molecule such as the electrolytes, amino acids, and sugars often found in water, a
solvent.
○ Sometimes separated by selectively permeable membrane (like lipid bilayer plasma membrane)
■ aquaporin A protein channel that allows water to flow through the plasma membrane more
readily by facilitated diffusion.
● osmotic pressure The tendency of or pressure needed to prevent water from moving from one solution into
another by osmosis.
○ Higher solute concentration, higher osmostic pressure (greater tendency for water to move INTO the
solution)
■ As water moves to higher solute concetration across selectively permeable membrane,
hydrostatic pressure of teh concentrated solution increases (resulting from gravity or container
walls)
■ In a cell,
● Osmotic pressure too high = cell is damaged or lysed
● Osmotic pressure is too low = cell is dehydrated
■ When osmotic pressure=hydrostatic pressure → equillibrium reached
● Movement persists in both directions, but no NET movement,
○ Direction of osmosis depends on relative solute concentration on either side of membrane
■ Water movement is driven by differences in TOTAL solute concentration!
■ Water moves across selectively permeable membrane in response to concentration differences
of ANY solute (e.g. electrolyte, dissolved molecule,waste, etc) as long as the membrane is
impermeable to it
Osmoregulation is the control of osmotic pressure inside cells and organisms
● osmoregulation The regulation of water and solute levels to control osmotic pressure → type of homeostasis
○ Regulation of water content → keeping inside from becoming too concentrated or diolute
○ Allows different animals to adapt to different environments
○ Animal cells can tolerate dehydration more but excessive dehydration impairs metabolic function
(hydrolysis used to break down proteins/nucleic acid! → depend on presence of water)
○ → control of the solute concentration of the inside of the cell relative to outside
○ → achieved by the movement of solutes (esp. electrolytes)
■ Achieved by balancing input/output of water and electrolytes
Osmoconformers match their internal solute concentration to that of the environment
● osmoconformer An animal that matches its internal osmotic pressure to that of its external environment.
○ Reducing movement of water into or out of bodies
○ Don’t spend a lot of energy regulating overall osmotic pressure, but spend energy regulationg
concentration of particular ions and other solutes (e.g. amino acids/glucose)
■ Intracellular space of nearly all multicellular animals usually have high K+ and low Na+ (active
pumping Na+ out and K+ in)
○ Have to adapt to solute concentration of external environment
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■ Tend to live in environments like seawater w/ stable solute concentrations, maintaining osmotic
equilibrium w/ environment → have high concentrations of Na and Cl in cells to match seawater
● Some marine osmoconformers like sharks, rays, coelacanths match solute
concentration by maintaining high concentration of urea
○ urea A waste product of protein metabolism that many animals excrete.
Osmoregulators have internal solute concentrations that differ from that of the environment
● Expend lots of energy pumping ions across membranes to regulate movement of water in/out of bodies
● Allows osmoregulators to live in a variety of environment → seawater, saltwater, land
● Hypertonic environment → (challenge: water loss and electrolyte gain) want to maintain internal solute
concentration lower than outside → organisms need to take in as much water as possible and eliminate excess
electrolytes
○ Obtain water through food and cell resp; avoid water loss by producing concentrated urine
○ Specialized gills in bony fish that allows excretion of electrolytes, their chloride cells pumping Cl- out of
body
■ Chloride cells create electrical gradient balanced by Na+ moving out of the body as well
● Hypotonic environment → (challenge: water gain and electrolyte loss) want to maintain internal solute
concentration higher than outside → organisms want to minimize water intake and maximize water elimination
○ Not drinking water, producing dilute urine
○ Have gill chloride cells w/ reversed polarity so that they pump Cl- (and Na+) ions into the body to
counter the ongoing loss from the gills into the surrounding freshwater
● Terrestrial environment → (challenge: water loss via breathing, sweating, waste) want to gain water by
drinkingwater hypotonic relative to body fluids, produce concentrated urine and produce water during cell resp
○ Marine birds: nasal salt glands to rid themselves of high salt content of diet
■ Allow net water intake by drinking seawater (and excreting salt)
○ Humans cannot drink seawater → cannot eliminate salt in high concentations AND we get diarrhea from
magnesium sulfate (net water loss)
● Salmon → swim in both salt water and freshwater → radical changes in osmoregulation required!
Case 7: Can the loss of water and electrolytes in exercise be exploited as a strategy to hunt prey?
● African bushman hunter-gatherers hydrate well before hunting prey, select a large animals, and chase it into the
open sun
○ Chase prey until prey is dehydrated and hyperthermic → unable to escape
41.2: Excretion of Wastes
● excretion The elimination of waste and toxic compounds from the body generated by metabolism or ingested
○ Intimately tied to water and electrolyte balance
● Excretory organs maintain water and electrolyte balance AND eliminate waste products
○ Gastrointestinal tract=continuous w/ outside world
○ Vertebrate kidneys and other excretory organs=separate toxic compounds from essential nutrients,
electrolytes, and cells circulating in the blood
The excretion of nitrogenous wastes is linked to an animal’s habitat and evolutionary history
● nitrogenous waste Waste in the form of ammonia, urea, and uric acid, which are toxic to organisms in varying
degrees.
○ Ammonia (NH3)= byproduct of protein/nucleic acid breakdown, esp. Protein breakdown
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■ Can disturb pH balance of cells and damage neurons → either eliminated or converted to less
toxic form
■ Found in most aquatic animals
■ Most toxic but easily released in water (simple diffusion), so does not accumulate to toxic
levels→ not energetically expensive but requires lots of water in surrounding environment to
dilute highly toxic ammonia
○ Urea
■ Found in mammals, amphibians, sharks, and some bony fishes
■ Ammonia converted to less toxic form bc it cannot be excreted w/ simple diffusion into aquatic
environment (takes place on land) → allows storage before elimination
■ Produced in the liver and carried by blood to kidneys, from which it is eliminated
■ Less toxic than ammonia but requires energy to produce and water to eliminate
○ Uric acid
■ Found in birds, insects, reptiles, land snails (convergent evolution)
■ Ammonia converted to less toxic form bc it cannot be excreted w/ simple diffusion into aquatic
environment (takes place on land)--> allows storage before elimination
■ Least toxic (can be stored at higher concentrations) but energetically costly to produce
■ Not dissolved in water, does not exert osmotic pressure and is eliminated w/ minimal water loss
→ ideal for hot/dry environments
● Some animals can adjust the kind of nitrogenous waste they excrete throughout their lifetime
● Other forms of nitrogenous wastes: trimethylamine oxide, creatine, creatinine, amino acids; form of nitrogenous
waste excreted is linekd to the animal’s environment and evolutionary history
Excretory organs work by filtration, reabsorption, and secretion
● filtration The separation of solids from fluids, as when circulatory pressure pushes fluid containing wastes
through specialized filters into an extracellular space. (blood is filtered into an extracellular space, some
substances pass through and some are retained) → PRODUCES FILTRATE OF BLOOD
○ Energetically inexpensive and and allows removal of novel compounds but also removes stuff the body
needs (water, electrolytes, nutrients)
○ In simple organisms, wastes are isolated in a contractile vacuole, which eliminates waste by
exocytosis
○ Multicellular animals isolate wastes from the body by filtration,
■ Filtrate contains waste products along w/ water, electrolytes, solutes, etc.
■ Filtrate drains into excretory tubule that connects to the outside of the body.
■ Excretory filter=selectively permeable to small ions/solutes and water (retains larger solutes like
proteins and cells)
○ Isolates most waste, but nonspecific, so reabsorption is required
■ reabsorption essential molecules are transported back into the blood.
● Can be active or passive trasnport
● Water is reabsorbed into cells by osmosis, driven by osmotic pressure established by
active transport of solutes into cell
● secretion In renal systems, an active process that eliminates substances that were not previously filtered from
the blood. → ADDS SOLUTES TO THE FILTRATE
○ Involves the active transport of molecules from the blood into the intracellular space
○ Uses ATP to pump molecules from blood into excretory tubules → Eliminates substances that were not
filtered from blood as well as helps fine tune electrolyte levels in blood
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Document Summary
Homeostasis for water and electrolyte concentration si crytical for normal physiological function in all organisms. Osmosis governs the movement of water across cell membraes. Osmosis the net movement of a solvent, such as water, across a selectively permeable membrane toward the side of higher solute concentration. Solute a dissolved molecule such as the electrolytes, amino acids, and sugars often found in water, a solvent. Sometimes separated by selectively permeable membrane (like lipid bilayer plasma membrane) Aquaporin a protein channel that allows water to flow through the plasma membrane more. Osmotic pressure the tendency of or pressure needed to prevent water from moving from one solution into readily by facilitated diffusion. another by osmosis. Higher solute concentration, higher osmostic pressure (greater tendency for water to move into the solution) As water moves to higher solute concetration across selectively permeable membrane, hydrostatic pressure of teh concentrated solution increases (resulting from gravity or container walls)
