Essay - AP Free Response 1998

Waste 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.

  • 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)
Using your text book and the Internet for help, answer the questions below.
You may want to check one or more of the many animations available at 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 Bladder).

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 Fox)?


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 true nephridia)

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 nephridiapore.

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 tubules"

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 video overview)

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 is full.

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?

The 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 extracting cells?

10- Why don't salt water protists possess contractile vaculoes?