ps5s_05 - MIT Department of Biology 7.014 Introductory...

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- - MIT Department of Biology 7.014 Introductory Biology, Spring 2005 7.014 Solution Set 5 Question 1 Organisms can use a variety of carbon and energy sources, depending on their availability in the environment. Oxidation and reduction reactions play critical roles in metabolism, and the relative favorability of these reactions often dictates the type of metabolism an organism will employ in a given situation. For example, organisms that conduct aerobic respiration (like humans) use organic carbon compounds (like glucose) as both their carbon source and their energy source. a) What compound is reduced by the transfer of electrons from glucose? What are the oxidized and reduced forms of this compound? Electrons in glucose are transferred to reduce NAD + (oxidized) to NADH (reduced). Electrons are then passed down the electron transport chain, sequentially reducing proteins in the chain, in the process creating a proton gradient for ATP synthesis. b) What is the final electron acceptor for organisms performing aerobic respiration? Why do organisms need a final electron acceptor? The terminal electron acceptor for aerobes is oxygen. The electrons that are being passed down ETC have to go somewhere so the chain doesn’t get backed up, and that is why the final electron acceptor is needed. The reduction of oxygen is a very favorable reaction; in fact oxygen is the most favorable electron acceptor. Organisms conducting respiration in the absence of oxygen do so anaerobically, using other compounds as electron acceptors. c) Consider the following environments where different compounds are present. Circle the compound in each environment most likely to be used as an electron acceptor. Top portion of the table from the Redox handout has been reproduced on page 6 (last page) to help you decide. Environment 1 Environment 2 Environment 3 3+ 3 - O 2 Æ Fe 3+ F e N O - NO 3 C H 3 OH NO 2 N O 2 - N O 3 f u m a r a t e Just as electron acceptors are selected in evolution based on thermodynamic favorability, so are electron donors. Whereas respiration uses organic compounds as electron donors, and photosynthesis uses water as an electron donor (to replace the electron excited by light), many organisms use inorganic compounds as electron donors and sources of energy. d) For each electron donor below, give the compound it will be converted to upon oxidation. Circle the compound that will yield the most energy upon oxidation. H 2 S Æ S or SO 4 2- NH 3 Æ NH 2 OH or N 2 or NO 2 - - NO 2 - Æ NO 3 Fe 2+ CH 4 Æ CH 3 OH or CH 2 O or HCOO - or CO 2
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Question 1, continued e) Organisms using the above compounds as electron donors are employing what type of metabolism? Are these organisms most likely prokaryotes, archea, or eukaryotes (pick all that apply)? These organisms are employing chemosynthesis. These organisms are likely to be bacteria or archea.
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ps5s_05 - MIT Department of Biology 7.014 Introductory...

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