SLIDE 2 Sources of Nitrogenous Wastes
95% of the waste nitrogen you’re producing is coming from protein catabolism. The other 5%
comes from breakdown of purines and pyrimadines.
SLIDE 3 Protein Catabolism
Animals can’t store surplus amino acids. If you want to store amino acids, you want to build
them into functional proteins.
Breaking amino acids involves cutting the molecule and getting rid of the amine group. The
leftover carbon skeleton is used to build other molecules or oxidized as a source of energy.
Amino acids catabolism yields:
o Carbon skeleton
o Ammonia (NH3/NH4+)
Amino acid catabolism yields ammonia which is toxic for a variety of reasons. Ammonia is toxic
largely to the nervous system of animals because it inhibits metabolism in the nervous system
so ATP levels fall in animals. At the same time, ammonia is good at displacing potassium from
transporters. This disrupts electric signals.
Amino acid catabolism is not a simple process. Only 3 amino acids in our body can have their
amino acids directly removed. One of them is glutamate.
The vast majority of amino acids can’t be deaminated directly. They need to have their amine
group transferred to some other molecule to generate one of these amino acids that can be
deaminated directly like glutamate.
For example an amino acid is transferred to an oxoglutarate to produce glutamate which can
then be deaminated directly.
We liberate an ammonia molecule that we have to get rid of. The main way many cells dump
ammonia is that they take it and build another kind of amino acid. So here we take a glutamate
and use ATP to add an ammonia group to it to form Glutamine. This is the main transport form
of ammonia in the animal body.
Glutamine isn’t toxic so we can transport it in the blood and transport it to the site where we
can excrete it, pull the ammonia off and excrete it.
SLIDE 4 “Phylogeny” of Nitrogen Metabolism
We can take cellular proteins and ingested proteins and hydrolyze them to liberate amino acids.
We can’t store excess amino acids so what do we do with them?
Some marine invertebrates such as crayfish can retain amino acids in their circulatory fluid and
use them to adjust to the salinity of the environment in which they live in.
There are some marine invertebrates that retain excess amino acids and use it to maintain
In most animals, if there’s an excess, you either have to build them into new cellular proteins or
get rid of them. When you break down amino acids and are left with ammonia, one way to simply excrete it as it
is. This is the route most aquatic animals take.
Many other animals take that ammonia and convert it to less toxic molecules such as uric acid
and urea and then excrete those compounds. One exception is that some marine invertebrates
like the squid retain certain amounts of ammonia. They use this ammonia in place of sodium
because ammonia is lighter than sodium. This helps them maintain buoyancy.
SLIDE 5 Ammonia is Toxic!
We have ammonotelic animals, and “non-ammonotelic” animals. Ammonotelic animals do the
same thing that fish do, they simply take any ammonia they produce and get rid of it as fast as
they make it as it is.
o Excrete NH3 directly into the environment
Non-ammonotelic animals take the ammonia and convert it into less toxic form like urea and
uric acid and can build up to a certain extent before they’re excreted.
An advantage of being ammonotelic is that it’s cheap; no energy needs to be invested. Ammonia
is also easy to get rid of because it’s lipid soluble and membrane-permeable.
A disadvantage is that you need to get rid of it with a huge amount of water. So it’s not a good
strategy if you don’t have a good source of water
The advantage of being non-ammonotelic is that you don’t have to excrete uric acid or urea with
large amounts of water. You can allow them to accumulate with a small amount of water, and
get rid of that small amount of water and a lot of urea and uric acid.
The disadvantage is that it takes energy to build urea or uric acid.
So ammonotelic animals are all aquatic. Things like aquatic invertebrates, fish, and amphibians.
Non-ammonotelic include terrestrial organisms like insects, arachnids, mammals, birds, lizards,
Being ammonotelic or non-ammonotelic depends on how much water is in your environment.
SLIDE 6 Ammonia Excretion requires a lot of water
Ammonia excretion requires a lot of water
Takes much less water if excreted as Urea
Even less water if excreted as uric acid
SLIDE 7 Ammonotelism and ”non-ammonotelism” : Not always fixed stategies
A single individual can switch between ammonotelic and non-ammonotelic. An example of this
is slender lungfish.
When it’s in water, it excretes nitrogen as ammonia.
Every once in a while, the pond dries up and the lung fish forms a caccoon. Now it becomes non-
ammonotelic and converts ammonia into urea and stores it until the water returns. After the
water returns it pees out all the urea that accumulated. In the graphs if you look at the last two bars, the 40-day control means it’s living in water, or
aestivated meaning it isn’t living in water.
If you look down and see all the different tissues, 40 days living in water shows little urea in the
tissues, but 40 days living without water accumulates lots of urea in the tissues.
SLIDE 8 Basic Chemistry
Ammonia is a base and can bind up protons (NH3). When ammonia picks up a proton it becomes
The pK is the pH at which the equilibrium is such that it has equal amounts of the acid and base.
If you put ammonia in water, if the pH is brought to 9.5 you’ll have equal amounts of
ammonium and ammonia.
If the pH drops below 9.5, ammonium becomes preferable. If the pH is above 9.5 the ammonia
The pH in the body is about 7, so ammonium predominates in the body, so most of the
ammonia is in the ammonium form in the cells and the circulatory system.
SLIDE 9 Ammonotelic Animals: Mechanisms of ammonia excretion- Freshwater
What is the mechanism a fresh water fish uses to excrete ammonium?
Most fresh water fish excrete their ammonia across their gills. So in the blood we have
ammonium and ammonia. Ammonium is the dominant form, but it’s not membrane permeable.
Ammonia is membrane permeable. Ammonia is able to pass across the gills and into
The problem here is that the water is divided into two spaces. The layer of water right up
against the fish gill doesn’t get mixed well with the bulk water. There’s always a little layer of
water around the gills that’s not exchanging materials with the rest of the water.
As this ammonia is diffusing into the boundary layer, it’s building up and not much of it is getting
exchanged with the bulk water. This makes it hard to get rid of more ammonia and it doesn’t
make a favorable diffusion gradient.
To solve this problem, the fish take CO2 from the blood and take it into the gill cells, the gill cells
have enzymes called carbonic anhydrase which convert CO2 into 2 things, a bicarbonate ion and
The proton is pumped actively by ATPase into the boundary layer. Now the proton and
ammonia come together to form ammonium. Now there’s a favorable diffusion gradient to
allow more ammonia to diffuse from the gills.
The fish is acidifying water right next to the gill to convert ammonia into ammonium and keep
the diffusion gradient favorable
SLIDE 10 Ammonotelic Animals: Mechanisms of ammonia excretion- Marine
In marine fish a different mechanism is used. Sea water tends to be much more buffered than
fresh water. Secreting protons into a buffered solution won’t do much to change the pH of the solution.
Because seawater is well buffered, you can’t pump ions into it and expect it to change the pH
and have the same reaction as the fresh water fish take place so you have to use a different
SLIDE 11 Ammonotelic Animals: Mechanisms of ammonia excretion- Marine
To a very small degree ammonia is diffused is lost across the gills. This accounts for about 5-10%
of the total ammonia loss across the gills in marine fish.
Active excretion of ammonium ions accounts for most of the ammonia loss. This involves the
Na+/K+ ATPase in the gill cells actively pump sodium into the blood of the fish and pumping
potassium in the gill cells. There’s very little sodium now in the gill cell and lots of it in the blood.
This sets up a diffusion gradient for sodium to get back in the gill cell.
This involves the NKCC which stands for sodium potassium two chloride transporter. This
transporter allows sodium to pass into the cell and couples that diffusion with the uptake of
both potassium and chloride.
But you don’t see any potassium coming into the cell, you see ammonium coming in. Remember
that one of the reasons ammonium is toxic because it can use any potassium transporter to act
like potassium. This is how it replaces potassium and enters the gill cell.
Now we pump the sodium back out again, so in addition to the sodium coming into the gill cell
from the blood, it also comes in from the water via an exchange known as the sodium
This exchanger allows the sodium to come into the cell and exchanges that sodium with an
ammonium. This is how marine fish excrete ammonia at their gills, in the form of ammonium.
All of this occurs because we can’t acidify sea water because it’s well buffered.
SLIDE 12 “Non-ammonotelic” animals: Urea (ureotelic animals)- Adult amphibians, mammals,