Solution to oxidative phosphorylation
-By Melissa Reardon
Original Question
What kind of reaction is oxidative phosphorylation referred to as? Where is it seen in metabolism? Explain the process. How is it essential to the example you provided?
Oxidative phosphorylation is referred to as a coupled reaction. It is found in the electron transport system.
Explanation of coupled reaction:
Think of the body as Green party member. It advocates conservation, and therefore shows great effort to not waste during chemical reactions. Think of ATP as a tree branch. If I break off a third of that branch’s length picture that third as the third phosphate within the molecule. It is pretty easy to break the stick. What if I tried to put it back together? It would be silly of me to push either end of the stick together in the attempt to make the stick go back to the way it was before I broke it. The effort would be a waste of my energy. Are you getting the picture? Breaking the stick would equate an exothermic reaction:
ATP → ADP + Pi + energy
Putting the stick back together is an endothermic reaction:
ADP + Pi + energy →ATP
Making ATP is energy-costly, but if production does not occur the body cannot live. So what is the body to do? Coupled reactions. One can think of reactions of these types as coupons for the body:
ADP wants to go into Macys to but a new hat (its third phosphate). This new hat costs $100, but ADP has a credit card limit of $75. As luck would have it, ADP received a 25% coupon in the mail, and is able to cut off $25 from the original price. ADP is a happy molecule!
When a reaction is too energy costly for the body, it couples it with an exothermic reaction, so that the overall energy change within the body is favorable. For example the energy cost of phosphorylating glucose is 3.3 kcal/mol, but releasing a phosphate from ATP releases 7.3 kcal/mol. Add these two together, and there is a net release of energy of 4.0 kcal/mol. This process is essential because reactions would cost too much energy and most likely not occur without it (same concept with enzymes). The electron transport system is the site of ATP synthesis, but we’ve established that production costs a lot of energy. That is where oxidative phosphorylation comes into play. The electron transport system is known as a coupled reaction because phosphorylation of ADP (to yield ATP) is driven by oxidation. The first step of the electron transport system is the oxidation of NADH and FADH2 to yield NAD+, FADH+, H+, and electrons. Electrons travel along this chain where they repeatedly reduce and oxidize intermediates. Reduction occurs when electrons enter an intermediate, while oxidation occurs when electrons leave intermediates for another electron-acceptor. Oxidation provides the energy to pump H+ into the mitochondria’s intermembrane space. This process results in a higher concentration of H+ “outside” the organelle relative to the inside, thus creating a concentration gradient (potential energy!). Once a certain concentration is reached, H+ diffuses back into the mitochondria through ATP syntase. Their return causes the complex to spin which provides the energy to phosphorylate ADP to yield ATP.
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