Describe the process by which food energy is converted into the chemical energy of ATP in living cells. Go through each of the major steps in the process, including the starting and ending product of each step, and any important molecules produced or consumed.
Include one or more labeled diagrams to help explain your answer.
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SOLUTION This solution is FREE courtesy of BrainMass!
I hope this explanation helps you to better understand the topic.
So, to start with the conversion of food energy into chemical energy of ATP is called cellular respiration. This is an aerobic process (oxygen is required to complete the entire sequence). Without oxygen, the breakdown of the food molecules is incomplete and the cell does not make as much ATP molecules as it would if oxygen were present.
Okay, so the food particles, by this time broken down into monomers or polymers by the process of digestion, are taken up by the cell. Different types of molecules (proteins, lipids, nucleic acids, carbohydrates) are introduced into the sequence at different times, but the molecule that is used as the "example" for cellular respiration is often glucose (a monosaccharide [component of polysacchariades, aka carbohydrates]), so we'll use that for now.
A glucose molecule begins to be broken down in the cytoplasm of the cell in the process known as glycolysis (literal meaning, breaking down (or apart) a sugar). During glycolysis the glucose is broken down in a series of steps to form two molecules of pyruvate (also known as pyruvic acid). During glycolysis, two ATP actually have to be used to catalyze the change from glucose to glucose-6-phosphate, and from fructose-6-phosphate to fructose-1,6-bisphosphate. This puts the cell two ATP behind to begin with. Later on in the process one of the intermediates (glyceraldehyde-3-phosphate) is oxidized and an energy-carrier molecule known as NAD+ (important later on!!!) is reduced (reduction means the molecule has taken on a hydrogen ion [H+] and an electron) to form NADH. 1,3-Bisphosphoglycerate is then produced. This molecule is then changed to 3-phosphoglycerate and an ATP is produced from ADP. The last step in glycolysis is to changed phosphoenolpyruvate into pyruvate, which also creates 1 ATP. Since the fructose-1,6-bisphosphate breaks apart to form two molecules that continue down the glycolysis pathway, you produce TWO pyruvate at the end and FOUR ATP. Subtracting the two ATP already used in the process, you end glycolysis with 2 pyruvate molecules, 2 ATP made and 2 NADH molecules. The number of carbon atoms is still the same (glucose = 6; TWO pyruvate = 6 [3 x 2]). See diagram on Glycolysis in attached Word file for further information.
These two pyruvate, before being used in the next pathway of cellular respiration, must be modified and linked up to a coenzyme which facilitates its insertion into the pathway. The two pyruvate are changed into the acetyl-CoA (coenzyme A) by pyruvate dehydrogenase, which also reduces a NAD+ to NADH and produces CO2 as a by-product. The original 6 carbon atoms found in the glucose are now down to four (remember: TWO pyruvates are produced, so TWO acetyl-CoA's are produced). Again see Word file.
Now the acetyl-CoA can enter into the mitochondrial matrix where the enzymes that are a part of the Citric Acid Cycle can be found. The pathway is exactly what its name says: a cycle -- so the ending product of one part of the pathway becomes the reactant for the next step. The beginning molecule is citric acid (citrate), hence the name Citric Acid Cycle. The two carbon atoms attached to the CoA bind to the last product of the cycle (oxaloacetate) to form the citrate to begin the cycle. The Coenzyme-A is then free to return to the cytoplasm and bind to another pyruvate. The whole of the CAC is meant to extract the rest of the energy that the last two carbon atoms (in their bonds) of the glucose contain. The citrate (6C) is converted to isocitrate (6C), the isocitrate is converted to alpha-ketoglutarate (5C) which reduces a NAD+ to NADH and releases a CO2 molecule and a hydrogen ion. The alpha-ketoglutarate is converted to succinyl-CoA (4C), creating another NADH, CO2, and H+. The succinyl-CoA is converted to succinate (4C) which produces a GTP from a GDP (this energy and associated phosphate group is then directly transferred to ADP to form an ATP. The GDP then returns to the cycle. At this point, the goal of the cycle is now to re-produce the end oxaloacetate so that it can again bind to an acetyl group and reform the citrate. The succinate is converted to fumarate, reducing another energy-carrier molecule called FAD to FADH2. Fumarate is then converted to malate, which takes a H20 molecule to complete. The malate is then converted to oxaloacetate which reduces another NAD+ to NADH and produces a H+. The thing to remember is that this whole sequence is for ONE pyruvate...and we had TWO from glycolysis. So, for one glucose molecule (which by this time has been COMPLETELY broken down), the CAC produces 2 ATP, 6 NADH and 2 FADH2, along with some H+ ions. It is these energy-carrier molecules (don't forget the 1 from the creation of the acetyl-CoA and the 2 from glycolysis!) that finish off the process in the next and final pathway of cellular respiration.
In the ETC (electron transport chain) the hydrogen ions produced throughout the CAC are pumped into the intermembrane space of the mitochondria to form a concentration gradient. Where does the energy to do this come from? From the energy-carrier molecules NADH and FADH2 produced during glycolysis and the Citric Acid Cycle of course! The ETC is a series of proteins and enzymes found in close proximity to one another in the inner membrane of the mitochondria. Some of the proteins act as energy-carriers themselves, while others act as proton pumps, moving H+ across the membrane to establish the gradient. The NADH and FADH2 oxidized, producing more H+ and (very important) high-energy electrons. It is these electrons, passed from protein to protein along the ETC, that power the proton pumps. At the end of the ETC, the electrons are accepted by oxygen, and along with two H+ ions form water. That's the main reason oxygen is so important to your cells...to act as an electron acceptor at the end of the ETC! Meanwhile, the H+ ions in the inner membrane space can move back through the membrane along the gradient through an enzyme called ATP synthase. The ATP synthase uses the energy of the H+ gradient to attach free phosphate groups onto ADP to produce ATP.
So, at the end of this whole process, we have produced from the energy stored within the original glucose molecule 38-40 ATP; 2 from glycolysis, 2 from the CAC, and 34-36 from the ETC/electron carrier molecules.
The last diagram on the attached file shows you a simplified version of where protein and lipid products enter into the process.
I hope this helps! Good luck!© BrainMass Inc. brainmass.com December 24, 2021, 9:10 pm ad1c9bdddf>