Cellular Respiration and Enzymes

The Krebs cycle is a major metabolic pathway that releases energy in the form of NADH, FADH2, and ATP/GTP. While it is a central hub, it is primarily responsible for the oxidation of acetyl-CoA.

The citric acid cycle starts with the condensation of acetyl group with oxalo acetic acid and water to yield citric acid. The reaction is catalysed by the enzyme citrate synthase and a molecule of CoA is released.

So the correct option is 'citric acid'.

Glycolysis has 1 oxidation step (G3P to 1,3-BPG), the link reaction has 1 (pyruvate to acetyl-CoA), and the Krebs cycle has 4 (isocitrate to alpha-ketoglutarate, alpha-ketoglutarate to succinyl-CoA, succinate to fumarate, and malate to oxaloacetate).

The Kreb’s Cycle gives 3 moles of NADH, 1 mole of FADH and 1 mole of ATP per cycle of substrate i.e per 1 mole of Acetyl CoA. However net ATP i.e after ETS which would make the answer to 12 ATP(9+2+1) respectively.

TCA or Krebs' cycle is amphibolic (both catabolic and anabolic) because it provides a number of intermediates for anabolic pathways.

The conversion of pyruvate to acetyl-CoA is a linker step between glycolysis and Krebs cycle. After the entry of pyruvate in the mitochondria the acetylation takes place in the perimitochondrial space which is the space between the inner and outer mitochondrial membrane. Each molecule of pyruvic acid is oxidatively decarboxylated to acetyl coA by the action of an enzyme complex pyruvate dehydrogenase. The acetyl coA then enters the Krebs cycle.  Thus the correct answer is option C.

The pyruvic acid enters Krebs cycle in form of acetyl CoA where it is oxidatively decarboxylated into CO$ _2$. The complete Kreb cycle leads to the net formation of 2 molecules of ATP by substrate-level phosphorylation, 8 molecules of NADH plus 2 molecules of FADH$ _2$ (via succinate dehydrogenase). The NADH and FADH$ _2$ then enter the ETS to drive ATP production by substrate level phosphorylation. 

Kreb's cycle consists of 8 cyclic steps which take place in the mitochondrial matrix. Each step is catalyzed by a specific enzyme. In Kreb's cycle, the entrant molecule is 2- carbon acetyl coenzyme A and the receptor molecule is 4- carbon oxaloacetate. Since the process involves both catabolic and anabolic reactions. it is an amphibolic pathway. It degrades acetyl coenzyme A and yields carbon dioxide and energy which is used to produce NADH, H+, and FADH+

In the Krebs cycle, two decarboxylation steps occur per acetyl-CoA molecule, releasing two molecules of CO2.

The end products of aerobic metabolism of carbohydrates are carbon dioxide and water. Carbon dioxide remains dissolved in the blood and is carried by the veins to the lungs for oxygenation. If carbon dioxide is not removed from the metabolizing tissues, it will be toxic for that tissue and stop metabolism.

When muscles work continuously enough, oxygen is not reached to muscles to burn glucose aerobically in such conditions glucose is converted into lactic acid i.e anaerobic glycolysis. The lactic acid produced in muscles is moved to the liver and converted to glucose. In ureotelic organisms, lactic acid is converted into Glucose through the Cori cycle. This metabolic cycle was discovered by Carl Ferdinand Cori and  Gerty Cori, hence named Cori cycle.

Proton pumps create a proton motive force, which consists of a pH gradient (chemical gradient) and an electrical potential gradient across the membrane, providing energy for various cellular processes.

Muscles have two major proteins actin and myosin. These are mostly contractile proteins. The energy from the hydrolysis of ATP by myosin head is directly used in the power stroke i.e., flexing of myosin head during muscular contraction. ATP is also used during muscle relaxation in the absence of ATP the myosin head will continue to bind with actin head and the muscle fibre shall remain contracted. So, the correct answer is option C.