13: Lecture REACTIONS AND ENZYMES
Summary. Biological metabolism allows matter to become transformed from what is generally available in the environment to what is needed to build and maintain the cell. All chemical reactions within the cell proceed according to the laws thermodynamics. Chemists summarize those laws by the change in free energy as reactants become products. If the change in free energy is negative, the reaction will occur spontaneously (downhill), although it may occur only infrequently. If the change in free energy is positive, the reaction will not occur spontaneously (up hill). The cell stores energy temporarily in the form of ATP, which is used to drive chemical reactions that are uphill. Proteins, acting as enzymes, act as catalysts of chemical reaction by lowering the energy barriers for intermediate chemical states in the reaction. The regulation of such enzymes controls the rate of metabolism according to the needs of the cell. Next we look at some of the reactions, reactants, products, and enzymes important for the cell's energy conversion reactions.
Reading Assignment
Campbell, N. A. and Reece, J. B. Biology, (2008). Chapter 8.
Objectives
1. To understand the concept of energy and how it can take different forms and to be able to discuss the physical units used for energy. 2. To understand the meaning behind the first and second laws of thermodynamics and their application to chemical reactions. What is the meaning of entropy, sometimes referred to as a measure of disorder? 3. To explain how biological molecules, which are ordered, require energy expenditure for assembly. Explain how this does contradict not the second law of thermodynamics. 4. To be able to explain the concept of "free energy" in the context of a chemical reaction. What is the formula for the change in free energy in terms of enthalpy, entropy and temperature? 5. To be able to discuss the difference between reactions that have a net G > 0 and versus G < 0. Explain how the reaction equilibrium shifts as G of reactants and their products changes. 6. To be familiar with the energetic gain from the conversion of glucose to CO2 and water. What are the units, how much energy is this in everyday terms? (Compare the amount of energy in the combustion of a mole of glucose to the amount of energy needed to raise a liter of water from 0 deg. C to 100 deg. C). 7. To be able to draw the structure of ATP, and identify the component parts. 8. To be able to explain why hydrolysis of ATP provides convenient energy for the cell, and explain why ATP is used universally as an energy storage intermediate.
9. To be able explain how ATP can be used to fuel an endergonic reaction and how ATP is regenerated from ADP and inorganic phosphate. 10. To explain why biological reactions require enzymes, explain what an enzyme is, how it functions, and how the specific structure of the enzyme relates to its specific function. 11. To be able to explain how the function of an enzyme can be modified by cell conditions: pH, temperature, and the presence of cofactors, inhibitors, etc.
Study Questions
This topic may be a review to many students. Use this opportunity to read the chapter carefully and answer the study questions at the end of the chapter, page 161.