ps1s - MIT Department of Biology 7.014 Introductory...

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MIT Department of Biology 7.014 Introductory Biology, Spring 2004 Solutions to 7.014 Problem Set 1 Question 1 a) Describe the conditions of the atmosphere on prebiotic earth and how these conditions differ from what is found today. Prebiotic earth had an atmosphere that lacked oxygen, but was rich in H 2 , N 2 , CO 2 , H 2 O, and sulfurous gases. In the absence of O 2 and O 3 , the UV light penetrated in far greater strength than it does now. Today, we have an atmosphere rich in oxygen and an ozone that protects us from the sun’s UV light. b) Describe the proposed first organism as it compares to modern organisms; include the type of genetic material and the source of energy it may have used. The first cells were anaerobic and likely absorbed free organic compounds from the primordial seas. They are thought to have used RNA as their genetic material. Some modern anaerobic prokaryotic organisms may resemble these early cells. However, these modern cells do not freely absorb the organic compounds needed, they make them. They also use DNA as their genetic material. c) Describe the process that led to an O 2 atmosphere on earth and how this change in atmospheric conditions influenced the course of evolution. At some point, photosynthetic organisms developed the ability to use the highly abundant H 2 O instead of H 2 S as a source of electrons. This released oxygen into the seas that reacted with dissolved ions and precipitated as oxides (predominantly iron oxide). As the ions were exhausted, the seas became saturated with O 2 , and finally, O 2 escaped, began oxidizing the iron on land and finally began accumulating in the atmosphere. The change from a reducing environment to a more oxidizing one took hundreds of millions of years after the evolution of oxygenic photosynthesis. This change likely resulted in the extinction of many organisms adapted to the reducing conditions, and created an ozone shield that allowed the introduction of many aerobic life forms. Question 2 You are given four test tubes, each contains cells from a different organism. One tube contains bacterial cells, one contains yeast cells (eukaryotic), one contains human cells and the last contains dinosaur cells. Can you identify the cells from each tube if you are given a light microscope? Explain your answer. With a light microscope you could easily distinguish the prokaryotic bacteria from the other cell types.
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This note was uploaded on 05/02/2009 for the course BIOL 7.014 taught by Professor Walker during the Spring '05 term at MIT.

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ps1s - MIT Department of Biology 7.014 Introductory...

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