MIT Department of Biology 7.014 Introductory Biology, Spring 2005 7.014 Quiz III Solutions Class Average = 61.8 Median = 62 Grade Range % A 80 - 100 10 B 68 - 79 43 C 49 - 61 27 D 37 – 48 9 F 0 – 36 8Name:________________________________________ TA:____________________ Question 1 (20 points) The table below lists types of metabolism in the left column. Type of metabolism Electron source Carbon source Energy source fermentation sugar sugar sugar sulfur oxidation chemosynthesis H2S CO2 heat, H2S oxygenic photosynthesis H2O CO2 light anaerobic respiration sugar sugar sugar nitrification chemosynthesis NH3 CO2 heat, NH3 aerobic respiration sugar sugar sugar anoxygenic photosynthesis H2S CO2 light a) For each type of metabolism, i) use the following list to fill in Electron source (column 2) and Carbon source (column 3) in the table above. A. sugar (Note, “sugar” here is a generic term for any organic carbon compound (CH2O)n). B. Water (H2O) C. hydrogen sulfide (H2S) D. methane (CH4) E. carbon dioxide (CO2) F. NH3 ii) fill in Energy source (column 4) in the table above. For this part, you are not limited to the list above. b) For a number of the metabolisms above, the electron source is the same as the energy source. Explain why they are not the same for photosynthesis. The energy source in photosynthesis is light. While other energy sources are also sources of electrons (e.g. sugar), the energy source of photosynthesis, light, does not contain electrons. Therefore, organisms using light as an energy source must use a different source of electrons. donor c) Organisms carrying out respiration need a final electron acceptor to keep the electron transport chain (ETC) functional. (Circle the correct term above.) Explain how this compound enables the ETC to remain functional. The final electron acceptor removes the electrons after they have traveled the length of the ETC. This then frees the last protein in the ETC to pull the next pair of electrons from the next to last protein in the chain. The newly freed protein can in turn pull the electrons from (only slightly) less electronegative protein. If no final acceptor existed, the ETC would get “backed up” by accumulating electrons to capacity on ever less electronegative proteins. This back up would propagate to eventually saturate all proteins in the chain. Having a final electron acceptor relieves this potential problem. 2Name:________________________________________ TA:____________________ Question 1, continued d) Using an argument based on redox and energetics, explain how aerobic respiration generates more ATP per molecule of glucose than anaerobic respiration. O2 is the electron acceptor used in aerobic respiration. All the possible electron acceptors used in anaerobic respiration are less electronegative than O2. That means that each of these acceptors would operate with a shorter electron transport chain than O2. A shorter chain would include fewer proton pumps, so a smaller number of protons will be pumped in such a chain per molecule of NADH (and, consequently, per molecule of glucose). ATP synthase makes a new molecule of ATP for each proton pumped by the ETC. Therefore, a chain with as a final acceptor will produce more ATP per molecule of glucose than a chain with any acceptor used in anaerobic respiration. Question 2 (22 points) Baby Jill is stuck on an uninhabited island. The food web quickly comes to look like this: In this web, Plants Mice Ferrets Baby Jill Detritus NPP (plants)=200 kg/day IM (Mice ingestion) = 20kg/day Mice AE (assimilation efficiency) = 20% Mice PE (production efficiency) =10% Ferret EE (exploitation efficiency) = 50% Ferret AE = 80% Ferret PE = 10% Baby Jill EE = 40% Baby Jill AE = 80% Baby Jill PE =10% 3Name:________________________________________ TA:____________________ Question 2, continued a) Suppose that shortly after baby Jill’s arrival the flow into the refractory carbon pool in the system decreased. Why was there a decrease? Jill starts consuming ferrets, which prevents these ferrets from ending up in detritus. Jill herself spends much of the energy she gets from the ferrets in respiration, so that energy is lost to the system. Since the detritivores now have less biomass to start with, less ends up in refractory carbon. Note that this analysis works only in the short term. Longer term, mice not eaten by ferrets eaten by Jill may provide enough biomass to the detritus trophic level to increase the flow to refractory carbon. b) How many grams/day of carbon are stored in the form of ferrets? Show your work. IM x AEM x PEM x EEF x AEF x PEF=20,000g/day x .2 x.1 x .5 x .8 x .1 =16 g/day c) Given only the information on the previous page, is it possible to calculate the mean residence time of carbon in the ferret trophic level? Why or why not? Mean Residence Time (MRT) is defined as pool/flux. While it is possible to calculate flux (as we did above), we do not know how big the ferret biomass pool is. Therefore, we can not calculate MRT given only the information on the previous page. Suppose baby Jill gave ferrets a drug that lets them increase biomass faster on the same amount of food. d) What system parameter(s) could have changed as a result? (circle all that apply) NPP PEJ AEJ GPP EEF PEF AEF Why? Ferrets are consuming the same amount of food as before, so their Exploitation efficiency, or any other parameter that precedes it in the web would not be affected. The two possibilities are that the drug enables the ferrets to assimilate the food they consume more efficiently (AEF) or that it enables the ferrets to utilize the energy derived from food more efficiently (PEF). Baby Jill finds some squash seeds and plants them on a previously vegetation-free lot. She is tired of eating ferrets, so she plans to eat squash exclusively when it is ready. Note that Jill is the only one on the island who will get to eat squash. e) In the table below, for each system parameter listed, circle Yes or No to indicate whether that parameter will change once Jill plants and begins eating squash. For ONLY the parameters you believe will change, explain why in the space provided. Parameter Change? Justify NPP Yes No There is a new primary producer in the system, so the NPP increases. EEJ Yes No Jill can consume entire squash, as opposed to portions of ferrets. PEJ Yes No AEJ Yes No Ferrets are closer in composition to Jill
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