Essay - AP Free Response 2009

Many organisms require a continuing source of oxygen for respiration. Discuss important structural and physiological adaptations for oxygen uptake in THREE of the following:

  • a paramecium (simple diffusion, movent toward O2 rich & away from O2 poor environments)
  • a tree (guard cells, stomata, lenticels, aerated soil, roots that break up soil, pneumatophores)
  • a fish (gills, numerous filaments for surface area, counter current flow of blood & water, positive pressure flow - pump water over gills with mouth - air bladder, movement to O2 rich areas, hemoglobin)
  • a mammal (lungs, alveoli create immense surface area and have abundant capillaries, hemoglobin, diaphragm - negative pressure, cartilage in trachea, filter - warm - moisten air with nasal hairs & mucus membranes, clean debris with cilia)

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.

Respiratory System :

All eukaryotic organisms are dependent upon a source of Oxygen to insure that cellular respiration can make adequate ATP available for the cell. Single cell organisms or organisms that are comparitively usually do not have a respiratory system. Diffusion through the skin or plasma membrane provides all the O2 that is needed. Some very active microorganisms enhance gas exchange simply by circulating water across their surfaces like the larva shown below.

Earthworms are not very active but do have a fair amount of girth. Their moist skin provides sufficient surface area for gas exchange. Their circulatory system absorbs oxygen from the skin and transports it throughout the body. lExtremely active multicellular organisms or multicellular organisms with substantial thickness will require the presence of a respiratory system to insure rapid exchange of O2 & CO2. these systems must have:

  • substantial surface area
  • be associated with the circulatory system if one exists
  • keep their surface moist and free of debris for efficient gas exchange.

Some organisms, like the starfish, have numerous extensions from their skin that offers substantial surface area and promotes gas exchange. Small pincher like structures (pedicellaria) are also present to keep the "skin gills" clean of debris. Organisms that live out of water tend to have their respiratory systems located inside their body in order to keep their gas exchange surfaces moist. Insects have numerous tracheal tubes that run throughout the inside of their body. The tracheal tubes open to the air through tiny pores, generally on the sides of the abdomen, called spiracles.

Above: a close up of the back of a starfish. Numerous skin gills can be seen. The pincher like pedicellaria keep the gill surfaces claen of debris. Below: a close up of tracheal tubes inside an insect
Above : Diagram of the tracheal system of a mosquito larva, The snorkel at the tip of the abdomen draws air into the system of tubes that extends trhoughout the body. Below: diagram of the tracheal system of a grasshopper. Spiracles are tiny openings on the sides of the abdomen that allows air to enter the system

Air contains much higher concentrations of oxygen than water. Even with this, when a fish is taken out of the water it soon suffocates. Their are numerous tiny filaments associated with the fish gill that establishes an immense gas exchange surface area. When out of water these filaments are no longer suspended and the filaments clump together decreasing the surface upon which gas exchange can occur.

Fish protect their gills from silt and debris with a cover on the side of their head called the operculum. Water is pumped over their gills by positive pressure. Their mouth opens wide and fills with water. The mouth closes and narrows forcing water over the gills and out underneath the operculum flaps on the sides of the head.

For some fish, such as tuna, the pumping system is less efficient, so they have to swim constantly to keep water moving over their gills. By swimming quickly, at least 65 centimeters (about 2.1 feet) per second, the fish can force enough water over their gills. In addition, the surface area of tuna fish's gills is up to 30 times larger than that of other fish to make up for their reduced pumping efficiency. Many species of fish never stop swimming their entire life. To do so would mean suffocation.

1- Suppose your were told a new species of animal was discovered and it had no respiartory system or circulatory system. What might you conclude about this organsism size based on this information?

2- Why are earth worms commonly seen in large numbers out from under the ground after a heavy rain?

3- Why would you expect to see few starfish (if any) in very shallow tidepools (a tide pool is trapped water left behind during low tide)?

4-Why do fresh water fish tend to congregate next to rocks or under rock ledges and fallen trees immediately after heavy rain falls?

Frogs use both positive and negative pressure breathing. A small throat pouch pulls down drawing air through the nostrils into the mouth (negative pressure breathing). Valves in the nostrils close and the throat pouch pushes upwards forcing the air into the lungs (positive pressure breathing). With the valves in the nostrils still closed, the throat pouch pulls down sucking the air from the lungs back into the mouth. The valves in the nostrils open, the tongue blocks passage to the lungs, and the throat pouch pushes the air from the mouth back into the atmosphere. A rich supply of blood vessels to its thin skin enables it to absorb large amounst of oxygen while under water.

Birds depend on their respiartory system for temperature control and to provide a tremendous amount of oxygen to support the production of energy neede during flight. To accomodate this need birds have evolved air sacs that extend off their lungs. The air sacs occupy space within the body cavity and therfore help to reduce weight as well. When the bird exhales the air in the air sacs moves into the lungs. This allows the lungs to be filled with large volumes air on both inhalation as well as exhaltion.
Air moves from Posterior air sac->lungs->anterior air sac->out of body

5- Why are frogs able to hibernate underwater at the bottom of a pond?

6- List two reasons why birds tend to have much more rapid throat expansion and contraction (panting) when hot?

Mammals have perhaps the most well developed respiratory system. We will look at the hman respiartory system as an example. The human respiratory system establishes all the necessary requirements for efficient gas excahnge.

Breathing through the nasal passage warms, moistens, and filters the air before it enters the system. warm moist air enhances gas exchange. Filtering out particulate matter in the mucus mebranes of the sinuses limits the accumulation of debris on gas exchange surfaces.

Using the mouth to breathe can allow larger volumes of air to enter during levels of high activity.

The epiglottis folds over the air passage (trachea) during swallowing to prevent food from entering the respistory system

Trachea have numerous cartilage rings to insure that it does not collapse with inhallation.

Cilliated epithelium lining the major airways forces particulate matter, stuck in the thin layer of mucus on the surface of the passageway, back up to the mouth for removal.

The tiniest passageways (bronchioles) terminate in clusters of microscopic air sacs called Alveoli. Each cluster of alveoli is serviced by its own cappilary bed (see diagram below). The lung is not a bag on the end of a tube, it is a mass of millions of tiny air sacs.

A muscle, the diaphragm, draws air into and forces air out of the respiratory system

7- Why do all smokers eventually develop the characteristic "smokers cough"?

8- How might mouth breathing in humans lead to impaired gas exchange in the alveoli?


The diaphragm works by creating a negative pressure area. When pulling downward it makes the thoracic cavity have a substantially lower internal pressure than what exists out side the cavity. Air rushes into the respisrtory system.

When the diaphragm relaxes it pushes upward causing the pressure in the thoracic cavity to become greater than exists outside the cavity. Air is forced out of the respiartory system.

9- Why does breathing become more difficult at high altitudes?

10- If a person were to have the chest wall punctured during an accident or attack (knife wound), how might this compromise the movement of air through the respirstory system?

11- An enlarged region of the trachea is seen in necks of many mammals. What is this area called and what is it's function?

The transport of gasses by the blood is made more efficient in many ways:

Hemoglobin in the red blood cells increases the carrying capacity of oxygen hundreds of times greater than plain water.

CO2 that diffuses into the blood enters red blood cells where an enzyme converts the CO2 into bicarbonate ions (HCO3-). Converting the CO2 into Bicarbonate ions increases the carrying capacity of CO2 molecules.

In addition, formation of bicarbonate ions offers the body an effective method of regulating blood pH. CO2 will react with water to produce carbonic acid. If carbonic acid were to increase (which can occur as a result of increased cellular activity) blood pH would lower which could effect enzyme activity. The fact that red blood cells convert CO2 into Bicarbonate ions, which are basic, enables the body to mainatin a constant pH in the blood.

See a movie showing the formation of bicarbonate ions

12- The brain is constantly monitoring blood chemistry. What changes to the circulatory and respisrtory systems might be made by the brain in response to a lowering of blood pH?

The human respiratory system establishes all the essential characteristics needed to maintain efficient gas exchange

  • substantial surface area (alveoli)
  • surfaces kept moist (inside the body, air passes over mucus membranes)
  • gas exchange surfaces have efficient transposrt of gasses (abundant cappilary beds)
  • a mechanism to promote movement of air over the echange surfaces (diaphragm)
  • method of keeping exchange surfaces free of debris (cilliated epithelium)
  • protection from damage/injury (rib cage)