AP-Biology Essay on vertebrate structural adaptations for terrestrial life. (From an actual past AP-BIOLOGY test)

The problems of survival of animals on land are very different from those of survival of animals in aquatic environment. Describe four problems associated with animal survival in terrestrial environments but not in aquatic environments. For each problem, explain a physiological of structural solution.

Four problems faced by animals on land are breathing (respiration), water conservation in excretions, successful reproduction, and the producing an egg which can survive outside of the water.

All animals need to respire, but I have no idea why. Maybe you would like to answer that? Aquatic animals use gills, which are outgrowths from the body which increase surface area over which gas exchange can occur. Inside the gills of aquatic animals, the circulatory system removes oxygen, and delivers waste carbon dioxide. Land vertebrates have developed a different approach to the problem of gas exchange, as water is not present in all of the terrestrial environment. Terrestrial vertebrates have developed lungs to solve this problem. Air enters through the nasal passages, or the mouth, passes through the trachea, then branches off at the two bronchi, and goes through many branching passages called bronchioles, which end in alveoli. Alveoli are sack-like structures where the circulatory system meets the respiratory system.

Since terrestrial vertebrates do not live in water, they need to develop a means of conserving water. One way we do this is through our excretions. Nitrogen forms a major waste product in animals. When amino acids and nucleic acids are broken down, they release toxic ammonia (NH3). To rid the body of this toxin, several mechanisms have evolved, each appropriate to the habitat or survival of the animal. Aquatic animals secrete NH3 directly into the surrounding water. Land animals cannot do this because of the toxicity of NH3. Instead, NH3 is converted into urea in our livers. Urea is significantly less toxic than NH3, and thus requires less water to excrete in the urine. The reason we need the water to excrete this is because the water is needed to dilute the urea (or NH3 if we did excrete it in that form), in order to make it less toxic. Birds excrete ammonia in the form of uric acid, that's what they're always dropping on our heads. Those mangy little rats with wings... have you ever wondered why we let those

Little pests run free in the cities, but we wont let dogs and cats free, even though most people consider the birds more of a nuisance? I didn't think so, anyways:

A third adaptation to terrestrial life is internal fertilization. In aquatic animals, many eggs are laid, usually allowing the water, and chance to fertilize the eggs. We can't do this on land, because the eggs and sperm would dry out, and would stay in the same place, unless they could walk (he he he). To solve this problem, we have developed a system of internal fertilization. The sperm are released directly inside the female, providing an increased chance of fertilization.

The amniotic egg of birds and reptiles represents a transition to terrestrial life. The egg provides conditions similar in some ways to the aquatic environment. In the aquatic environment, eggs have soft, usually permeable shells, which do not have to worry about losing water. The amniotic cavity formed by the amnion is fluid-filled, protecting the embryo. The egg case often leathery in reptiles, and calcified in birds protects the contents, while permitting gas to be exchanged with the surroundings. This egg also prevents the evaporation of water from the embryo, since the egg cannot walk to the store and buy some Evian, it needs all to water it has.

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