Essay - AP Free Response 1998
production is an ongoing occurrence during cellular metabolism. If wastes were
to accumulate a toxic condition would be established ultimately killing the
cell(s). Environmental factors as well as anatomy will impact the chemical makeup of the waste and it's method of removal. Describe waste removal strategies
employed by three of the following making sure to include key structures involved, the
makeup of the waste released, and how osmotic balance is maintained in the process.
Using your text book and the Internet
for help, answer the questions below.
- Sponge (no excretory system, microbodies, wastes diffuse from cells into their environment)
- flatworm (flame cell system, nephridopore, mostly osmotic balance, dilute)
- Grasshopper (malphigian tubules, empty waste into digestive canal, concentrated uric acid)
- bird (kidneys, nephrons, cloaca, concentrated uric acid)
- mammal (kidneys, nephrons, ureters, urinary bladder, urine, dilute)
You may want to check one or more of the many animations available at MedBroadcast.com
or Visual Content for
Health Care for additional support.for additional support.
Excretory System :
All cells process an assortment of organic compounds and eventually break
down some molecules to products that have no use to the cell. These wastes
can be potentially toxic if they become too concentrated. It is essential
that a living cell have a method by which these cellular wastes can be
removed. An excretory system is involved in the collection and removal
of cell wastes.
Amino acids are often processed for energy and to build other organic
compounds. The "Deamination" of amino acids results
in the formation of an acetyl group that may enter the krebs cycle
in the Mitochondria and used to make ATP. Ammonia (NH3) will
also be produced. Ammonia can be toxic to the cell if too concentrated.
Organisms that are very thin and live in extremely moist environments
(water is needed to dilute cell wastes) may have no specilaized
excretory system (sponges, jelly fish, roundworms). Simple diffusion
may be all that is needed. The more complex the tissues and the greater their volume in an organism
the more ammonia will be produced and will likely require
the presence of an excretory system.
In addition to an excretory system many multicellular organisms
have evolved enzymes that bind the ammonia to other substances to
decrease toxicity. The Urea cycle has been established in
mammals as a means of dealing with the production of ammonia.
UREA CYCLE - Ammonia is bound to Ornitihine using CO2 and
removing H2O (dehydration sysnthesis). The chemical formed is called
Citruline. An additional amonia group is bound to the Citruline
making a compound called Arginine. The Arginine has a Urea group
(CN2H4O) removed converting back into ornithine to repeat the cycle.
The beauty of this cycle is that Urea is formed which is much less
toxic than ammonia and requires less water to dispose of.
Urea can be converted chemicaly to Uric Acid which is even less
toxic than urea. Mammals unfortunately can not efficiently export uric
acid. It is a non soluble material that tends to consolidate into crystals
that can block the action of nephrons in the kidney. Kidney stones
can form and lead to kidney failure. Gout occurs when uric acid
begins to accumulate in the tissues.
The pictureabove is a Scanning Electrom Micrograph of encapsullated
uric acid crystals
Birds and reptiles have evolved the ability to encapsulate the
uric acid crystals with a protein material that allows easy export
from the body. As a result birds and reptiles do not have a bladder.
They instead excrete uric acid out through a common cavity (Cloaca)
shared with the rectum. Birds and reptiles do not urinate. When
they have a bowel movement what passes from their body will be part
feces (black - nondigestable material from the rectum) and uric
acid (white - concentrated cell waste from the kidney). The excretion
of waste in the form of uric acid requires hardly any water. Therfore
birds and reptiles can manage a diet or an environment that has
limited water available.
Imagine lunchtime with the seagulls if they had bladders......
umbrellas for everyone.
1- What is the main advantage for removing
cell waste as uric acid?
2- Why can't mammals efficiently remove their cell waste in the form
of uric acid?
3- What is Guano
and why is it so valuable to humans?
The anatomy and physiology of excretory systems varies rather dramaticaly.
Organisms that require the conservation of water due to its limited uptake
are likely to excrete their cell waste in the form of uric acid and will
have excretory anatomy that accomodates it. Multicellular organisms that
live in water or have a fair amount of water available to them in their
diet are likely to excrete their cell waste in the form of ammonia or
Urea diluted with water (urea + water = urine). This type of system in
most will require a temporary storage organ for the urine (Urinary
4- How might an animal that does produce urea as their key waste product
survive environments with
very limited amounts of water available (California
Spade Foot Toad, Kit
Flatworms are the simplest multicellular organisms to have an excretory
system. They posses a system of tubes that have specialized cells
attached to them called "Flame Cells". The flame
cells have flagella that establishes circulation of fluid out through
a "nephridiapore" on the side of the body. The circulation
insures the passage of wastes out of the body. This system is said
to be composed of "Proto-nephridia" (which =s before
Earthworms have a system a bit more advanced. in almost every segment
there exists "Meta-nephridia". These are surrounded
by their own cappilary network. The nephrostome pumps fluid from
the forward segment into the nephridium. Capillaries reclaim essential
substances leaving only water and cell waste to pass out of the
Insects like the grasshopper have an open circulatory system (no
capillaries) and can not have an excretory system as complex as
that seen in the earthworm. Insects possess a system of tubes that
are attached to the digestive tract and extend into the body cavity.
Wastes are actively transported from body fluids into the tubes
which then pass them into the digestive tract to leave the body
with nondigested food. These excretory tubes are called "malpighian
5- Explain why nephridia and or kidneys would not be possible in organisms
with an open circulatory system?
6- There is a large purple and white jelly fish found off of the California
Coast called "Pelagia".
It is not uncommon to see this species with a bell size that exceeds 70cm.
Offer an explanation for why this organism does not have an excretory
system. (Be complete with your explanation).
Vertebrates have kidneys. (Excellent
The excretory system of mammals is the most complex of all. Special
organs called Kidneys contain millions of individual tubes
called nephrons (diagram above and lower left) . Each
nephron is serviced with two capillary beds. The "glomerulus"
is the first capillary bed located in a bowl shaped depression (Bowmans
Capsule) at the beginning of the nephron. The glomerulus raises
BP enough to cause large amounts of the fluid (filtrate)
that enters it to be forced into the nephron tube.
The second capillary bed surrounds a portion of the nephron that
extends downward in the kidney to the inner medula.ggg
Filtrate includes water, urea, salts, ions, glucose, vitamins,
and amino acids. The function of the nephron is to recover beneficial
substances leaving only water and urea in the tube. Cells in the
nephron actively transport sugar and amino acids back into the capillaries
in the convoluted tubules. Salts, water, and useful ions
will move back into the capillaries in the Loop of Henle.
urea and water will be dumped into a collecting tubule and make
it's way to the renal pelvis. From there it will move down
the ureter to the bladder for temporary storage. When
the time for removal is needed, a sphincter muscle at the
base of the urethra will be relaxed and the smooth muscle
around the bladder will force the contents out of the body. Strecth
receptors imbedded in the walls of the bladder indicates when it
Many animals are able to expel the urine in a precise direction.
This can be done for marking a territory or to temporarily blind
a would be predator.
View A Movie About Nephron Function
7- Why is it important that the interstitial fluid of the kidney maintain
an increasing concentration of solutes as you move from the cortex (outer
region - 300 mosm/L) to the inner medulla (inner most region- 1200 mosm/L).
8- What is the significance of two capillary beds in the kidneys before
the blood makes it's way back to the heart?
kidneys also play a vital role in osmoregulation. If water concentration
is too high than the tissues may be subjected to a hypoosmotic medium
that could lead to the rupturing of a cell.
If soulte concentrations get to high than the tissues could be subjected
to a hperosmotic medium that could cause the cells to shrivel.
The kidneys are capable of expelling excess water or solutes if they
were to get out of balance.
Organisms that have little minimal amounts of water being expelled by
the kidney (as a result of high solute concentration or limited water
uptake) will reveal a rather copius urine (yellow). If the kidney is expelling
large amounts of water (as a result of significant uptake) will reveal
a very dilute urine (clear). For humans, passing large amounts of water
through the kidneys can help to flush out uric acid crystals and salts
and help avoid the formation of kidney stones.
Other interesting osmoregulatory organs
Salt excreting glands in the nasal passageways of marine birds
Contractile vacuole of fresh water protozoans
Salt extracting cells in the gills of marine fish
Production of serum albumin by the liver
Storage of large amounts of urea in the tissues of a shark
9- Why is it so important for marine organisms to have specialized salt
10- Why don't salt water protists possess contractile vaculoes?