Energy and Metabolism

ID #2337

Can someone help me understand which reactions are endergonic and exergonic in the steps to get glucose to Pyruvate? Here's what I understand: -The ones that are endergonic (energy requiring) and are coupled to the hydrolysis of ATP to ADP + Pi which is an exergonic process (releases energy) -the synthesis of ATP is an endergonic process, meaning the pathway that is coupled to that (ADP to ATP) is exergonic? (Am I correct with this or totally wrong?) -so whenever the coupled reaction is endergonic, the pathway is exergonic? -if there isn't a coupled reaction, then it isn't necessarily endergonic or exergonic, so the energy is lost as heat; it isn't building or breaking anything so it's called ISOMERIZATION Also, Mr. Mehrtens stressed to focus on the pathway reaction, not the coupled reaction. So if there is a test question that asks what type of step is one that is coupled to ADP to ATP? Would the answer be based on the pathway reaction (exergonic) or the coupled reaction (endergonic)? Sorry for the redundancy, I just wanted to make sure. ONE MORE QUESTION -This might be a silly question, but where does the ATP used to fuel the synthesis of ATP in glycolysis come from in the first place? How is it already there if we are making it in glycolysis?

To address each of your questions: 
-Yes, endergonic reactions are coupled to the hydrolysis of ATP, which is exergonic. 
-Yes, the synthesis of ATP is endergonic. ATP can only be made if it is coupled to an exergonic pathway reaction. (So yes, you are correct.   
-Some reactions are not really endergonic or exergonic. These are rearrangements or isomerizations. The conversion of glucose-6-phosphate to fructose-6-phosphate during glycolysis is an isomerization. The rearrangement of citrate to isocitrate during the Krebs Cycle is also an isomerization. It is possible that energy may be lost as heat if the isomerization is slightly exergonic, but generally they are thought of as energetically "neutral." 
Yes, you should focus on pathway reactions. The synthesis/hydrolysis of ATP and the oxidation/reduction of NADH/NAD+ are the coupled or side reactions. The pathway reactions involve the breakdown of glucose to pyruvate during glycolysis and the conversion of pyruvate/acetyl-CoA to CO2 during the Krebs Cycle. Prof. Mehrtens will make it clear if he is asking about pathway reactions or coupled reactions on the exam. For your theoretical question, an exergonic pathway reaction would be coupled to the synthesis of ATP (since that is endergonic). 
For your last question (which isn't silly at all- many students have similar questions), keep in mind that a cell will be more or less continuously undergoing either aerobic or anaerobic respiration (depending on if it is in the presence of oxygen). Unless ATP is in excess, the cell will be continually trying to make more ATP. The ATP utilized in the investment phase of glycolysis would have been synthesized during previous rounds of glycolysis/Krebs Cycle/ETC. To go back further, when a cell divides into two daughter cells, each daughter cell will get half of the parental cell's ATP supply. If a cell has zero ATP, it will likely die. Cells generally always have at least some ATP molecules present. 

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