Question

How many ATP molecules are produced in the Krebs cycle?
A. 36
B. 38
C. 40
D. 24
E. 2

Answer 1

Hint: Krebs cycle also known as the citric acid cycle is a series of reactions that occurs in the matrix of mitochondria. It involves the oxidation of acetyl-CoA. It converts acetyl-CoA into carbon dioxide. Coenzymes get reduced yielding ATP in the ETC. One glucose molecule gives two acetyl-CoA each gives one ATP.

Complete answer: The Krebs cycle was given by Hans Krebs. It is a series of eight steps that involve enzyme-catalyzed reactions. It begins with glucose molecules yielding two acetyl-CoA molecules that undergo oxidation one by one individually. The oxidation of acetyl-CoA gives two carbon dioxide molecules. This is further followed by the reduction of coenzymes that yields high energy compounds like NADH, FADH$${}_2$$, and ATP. The Krebs cycle is a major part of the cellular respiration process. Cellular respiration is a biochemical process through which nutrients are broken down to produce energy in form of ATP along with waste products. It is a four-stage step and the Krebs cycle is the third step.
The various steps of the Krebs cycle are:
a) In the first step, condensation of acetyl CoA with oxaloacetate occurs to form 6C citrate. A coenzyme is released in this step. The enzyme involved is citrate synthase.
b) In the second step, the citrate produced in the first step is converted to an isomer called isocitrate. The enzyme used for this is aconitase.
c) In the third step, the isocitrate undergoes the process of dehydrogenation and decarboxylation. This results in the formation of 5C $\alpha$-ketoglutarate. A molecule of carbon dioxide is released. The enzyme employed here is isocitrate dehydrogenase. This enzyme is $$NA{D}^{+}$$ dependent. Here $$NA{D}^{+}$$ is converted to NADH.
d) In the fourth step, oxidative decarboxylation of $\alpha$-ketoglutarate occurs to form succinyl CoA which is a 4 carbon compound. The reaction uses a complex enzyme called $\alpha$-ketoglutarate dehydrogenase. One carbon dioxide molecule is released and NADH is formed from $$NA{D}^{+}$$.
e) In the fifth step, succinyl CoA makes succinate. Succinyl CoA synthetase enzyme is used. GDP to GTP conversion happens by substrate-level phosphorylation of GDP. The GTP formed transfers its phosphate to ADP so that ATP can be formed.
f) In the sixth step, oxidation of succinate occurs by succinate dehydrogenase enzyme to make Fumarate. FAD converts to $$FAD{H}_2$$.
g) In step seven, the Fumarate gets converted to malate. The enzyme involved here si fumarase.
In the eighth or last step, malate is dehydrogenated so as to form Oxaloacetate. This combines with another molecule of acetyl CoA resulting at the beginning of a new cycle.
Thus, one acetyl CoA forms one ATP molecule. Hence, the two will form 2 ATP molecules in one cycle of the Krebs cycle.
So, the correct answer is option E.

Note: Krebs cycle is the final pathway for oxidation of glucose, amino acids, and fats. Although glucose is the main energy source in many animals. Some animals depend on other nutrients for energy. The Krebs cycle is the major source of ATP in cells. A large amount of energy is obtained after the completion of the oxidation of nutrients.