Controlling the Internal
Environment II: Salt and water
balance
Keywords (reading p. 879-884)
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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 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
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
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
internal concentrations
the same
300 your
mosm/L
as the environment (osmoconformer)
• Regulate your internal concentrations
(osmoregulator)
Jellyfish in the ocean
• 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?
Na+
Na+
Na+
Na+
• A) diffusion
• B) active transport
• C) facilitated
diffusion or active
transport
In this diagram, how might
sodium get across the membrane?
Na+
Na+
Na+
Na+
Na+
Na+
Na+
+
Na
Na+
Na+
Na+
Na+
• A) diffusion
• B) active transport
• C) facilitated
diffusion or active
transport
In this diagram, how might
sodium get across the membrane?
Na+
Na+
Na+
Na+
- - - - - - - - - - - - + + + + + + + + + +
Na+
Na+
• A) diffusion
• B) active transport
• C) facilitated
diffusion or active
transport
In this diagram, how might steroids
get across the membrane?
steroid
steroid
steroid
steroid
steroid
• A) diffusion
• B) active transport
• C) facilitated
diffusion
• D) all of the above
In this diagram, how might steroids
get across the membrane?
steroid
steroid
steroid
steroid
steroid
steroid
steroid
steroid
steroid
steroid
steroid
steroid
steroid
steroid
steroid
• A) diffusion
• B) active transport
• C) facilitated
diffusion
• D) all of the above
Types of active transport
What type of active transport is
this?
K+
• A) uniport
• B) symport
• C) antiport
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-
K+
• A) uniport
• B) symport
• C) antiport
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
aaaaaa
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