section4_ak - MIT Department of Biology 7.014 Introductory...

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MIT Department of Biology 7.014 Introductory Biology, Spring 2005 Recitation Section 4 Answer Key February 14-15, 2005 Biochemistry—Energy and Glycolysis A. Why do we care In lecture we discussed the three properties of a living organism: metabolism, regulated growth, and replication. Today we will focus on metabolism and biosynthesis. 1. It was said in lecture that chemical reactions are the basis of life. Why do we say that? Being alive implies being able to change your state in response to a change in internal or environmental conditions. We discussed previously that any change in the observable characteristics of a cell begins as and is propagated by molecular interactions. Molecular interactions propagate the signal by changing the state of molecules, i.e. by reactions. 2. Why is metabolism required for life? A cell is subject to all laws of chemistry and physics, including the first and second law of thermodynamics. Changing the state of molecules dissipates energy along the way, so getting or making new energy is essential to life. The energy is then used for many purposes, including making the building blocks and precursors and then using them to build macromolecules that make up cells—biosynthesis. 3. Can an entity that performs no chemical reactions be considered “alive?” In general, no. But there are special cases of the cells that do not perform reactions right at the moment, but have the potential to perform reactions if they encounter particular conditions. Some examples of such cells are spores in nature or frozen permanents in laboratory. If they experience certain conditions, such as availability of food for spores, or defrosting and food source for frozen permanents, these cells will again perform metabolism. 4. Most reactions necessary for life are unfavorable, or do not proceed at an appreciable rate under physiological conditions. How do cells overcome this problem? Several mechanisms exist to allow necessary but unfavorable reactions to proceed or to speed up to levels required for cellular function. Below we explore the following mechanisms: -Enzymes, -concentration gradients, and -coupling unfavorable reactions with favorable ones, including -using the common energy currency, ATP. B. Thermodynamics 1. What is “free energy”? “Free energy” (defined by Gibbs, so we use the symbol G) is the total amount of energy in a system that can be used to do work. By definition, G = “the total free energy of products” – “the total free energy of reactants”
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2. Where is this energy stored? The energy is stored in the bonds of the reactant and product molecules. We say that G is a thermodynamic property, meaning that it is independent of the way that the conversion of reactants to products might proceed. 3.
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This note was uploaded on 01/19/2012 for the course BIOL 7.014 taught by Professor Walker during the Spring '05 term at MIT.

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section4_ak - MIT Department of Biology 7.014 Introductory...

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