1
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
2
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
• Most species of
most phyla live
in the ocean
• Some live in
freshwater
• Fewer live on
land
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
3
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
4
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
1000 mosm/L300 mosm/L
What would happen if your body
surface is water permeable and you fall
into the sea
http://www.yout
ube.com/watch?
v=Ym1rvwP-
po4&feature=rel
ated
http://www.yout
ube.com/watch?
v=gWkcFU-
hHUk&feature=
related
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
5
Optimal cell conditions
• Na+ is detrimental to cell function
• K+ less detrimental than Na+
Life in freshwater - hydra living
in a pond
0 mosm/L
0 mosm/L
Green hydra
• Can the same strategy of matching the
environmental osmolarity be used?
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
6
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
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+
7
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
steroidsteroid
steroid
steroidsteroid
In this diagram, how might steroids
get across the membrane?
• A) diffusion
• B) active transport
• C) facilitated
diffusion
• D) all of the above
steroidsteroid
steroid
steroidsteroid
steroidsteroid
steroid
steroidsteroid
steroidsteroid
steroid
steroidsteroid
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
8
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1000 mosm/L
1000 mosm/L
Estuary - high tide
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500 mosm/L
1000 mosm/L
Estuary - low tide
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500 mosm/L
500 mosm/L
Estuary - low tideAdvantages 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
9
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?
10
Desert kangaroo ratDesert 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
