Cellular Respiration and Enzymes

Adenosine-5'-triphosphate (ATP) is comprised of an adenine ring, a ribose sugar, and three phosphate groups. ATP is often used for energy transfer in the cell. ATP synthase produces ATP from ADP or AMP + Pi. The energy consumed during the conversion of ADP into ATP is 7300 cal/mole.

A total 38 ATP molecules (2 ATP molecules from glycolysis, 2 from Kreb's cycle and 34 from electron transport system) are produced per molecule of glucose oxidised in aerobic prokaryotes. Whereas, the net gain in most of the eukaryotes is 36 ATP molecules (as 2 ATP molecules are consumed intransporting $NADH _2$ into mitochondria). 

ATP stands for adenosine triphosphate. It is complex molecule that contains the nucleoside adenosine and a tail consisting of three phosphates. It is the primary energy currency. A single ATP produce -7.3 kcal.

$NAD^+$ and $NADP^+$ are coenzymes, that function in oxidation-reduction reactions. 3 ATP are formed from the conversion of  $NADPH^+$ to $NAD^+$.

 Aerobic respiration takes place through a series of reactions such as Glycolysis, Oxidative decarboxylation of pyruvates, Citric acid cycle and Oxidative phosphorylation. Being efficient in extracting chemical energy, the total free energy produced during the process is 686 kcal. Thus, the correct answer is option C.

38 molecules can be made per oxidised glucose molecule during cellular respiration. It is possible by the yield of ATP molecules from Krebs cycle, glycolysis and electron transport system. 

If the food substances in living cells are oxidized in presence of oxygen, it is called aerobic respiration. It is the step-wise complete breakdown of the respiratory substrates like glucose into simpler molecules like $CO _2$ and water with the release of ATP. It is mostly seen in eukaryotic type of organisms. Each glucose molecule on complete oxidation gives 38 ATP molecules. 

Glycolysis of one molecule of glucose produces 2 PGAL, thus of three molecules will produce 6 PGAL.
Respiration of one molecule of glucose or 2 PGAL produces 38 ATP molecules, thus, of 6 PGAL will produce 114 ATP molecules. Out of the given options 120 ATP is the nearest correct answer. Thus the correct answer is option D.

Adenosine triphosphate is a nucleoside triphosphate used in cells as a coenzyme. It is often called the molecular unit of currency of intracellular energy transfer. ATP transports chemical energy within cells for metabolism. It is one of the end products of photophosphorylation, cellular respiration and fermentation and used by enzymes and structural proteins in many cellular processes, including biosynthetic reactions, motility and cell division. One molecule of ATP contains three phosphate groups.

In glycolysis 4 ATP and 2NADH2 molecules are formed. These 2NADH2  molecules go to electron transport chain. In oxidative decarboxylation no ATP molecule is formed but two molecules of 2NADH2  are formed from two molecules of pyruvate. These two NADH2 go to electron transport chain. In Kreb's cycle 2 ATP, 6NADH2 and 2FADH2 molecules are formed from two molecule of acetyl Co-A. These NADH2 and 2FADH2  go to electron transport chain. In electron transport chain all 2NADH2   and 2FADH2  pass to electron carriers and yield 3 ATP and 2 ATP molecules per 2NADH2   and 2FADH2  respectively. Thus,  4 ATP are formed in glycolysis, 2 ATP in Krebs cycle and 34 ATP from electron transport chain. 40 ATP and 2 ATP molecules are used during glycolysis. So, net gain of ATP molecules during one complete oxidation of a glucose molecule is 38 ATP.

Substrate level phosphorylation involves the conversion of 1,3 bisphosphoglycerate to 3 phosphoglycerate producing ATP.

In aerobic respiration, the malate-aspartate shuttle is more efficient than the glycerol-3-phosphate shuttle. It results in the production of 38 ATP molecules per glucose molecule in prokaryotes or specific eukaryotic tissues.

For synthesis of 1 ATP, 4 H+ are required. ATP synthase synthesizes 1 ATP for every 4 H+ that passes through it. 

During aerobic respiration, ATP is synthesized through both substrate-level phosphorylation (such as during glycolysis and the Krebs cycle) and oxidative phosphorylation (via the electron transport chain and chemiosmosis). Therefore, a combination of both mechanisms is responsible.