Krebs Cycle (Aerobic Respiration) - Free MCQ Practice Test with solutions,

Mitochondria (singular - Mitochondrion) are known as the powerhouse of the cell because they are responsible for the release of energy from food ,i.e, cellular respiration. This energy is released in the form of ATP (adenosine triphosphate), the energy currency of the cell.

While the cells release 2 ATP, mitochondria releases 34 ATP which adds up to 36 ATP. Since a major portion of the ATP is released by mitochondria, they are called the powerhouse of the cell.

Krebs’ cycle is the central pathway for the metabolism of carbohydrates, fats, and proteins. It begins with a citric acid molecule which is a 6- carbon compound and ends at Oxaloacetate which is a 4-carbon compound which indicates the removal of two carboxyl groups.
These carboxyl groups are removed with the release of a CO₂ molecule in two different steps.

- The first oxidative decarboxylation takes place at the fourth step of the TCA cycle where isocitrate is converted to 5-carbon α-ketoglutarate, with the release of a pair of hydrogen atoms and a molecule of carbon dioxide.

- The second oxidative decarboxylation occurs at the fifth step of the Krebs’ cycle where a molecule of coenzyme-A reacts with the α-ketoglutarate to form a 4-carbon compound succinyl- coenzyme A and releasing carbon dioxide and a pair of hydrogen atoms.

The TCA cycle, also known as the Krebs cycle or tricarboxylic acid cycle, has been named after the German biochemist Sir Hans Krebs who discovered the cycle in 1937 in England.

Tricarboxylic acid cycle or the citric acid cycle or the Krebs cycle takes place within the mitochondrial matrix of the cell. The end product of Krebs cycle is 2 GTP, 6 NADH and 4 carbon dioxide.

Pyruvate, the product obtained through glycolysis, gets oxidised with the loss of its carboxy group as CO₂, to give acetyl Co-A, under aerobic condition. This acetyl Co-A is further oxidised completely to CO₂ + H₂O in citric acid cycle. Other options are incorrect as Lactic acid is formed in muscles under anaerobic conditions. Ethanol and CO₂ are products of anaerobic respiration in yeast cells. CO₂ and H₂O are final and complete reaction products released at the end of cellular respiration. 

Glycolysis produces 2 pyruvate molecules. The pyruvate molecules then forms acetyl CoA and one molecule of carbon dioxide is released. The acetyl-CoA molecules then enters the citric acid cycle. The end product of citric acid cycle is 2 carbon dioxide molecules, 1 GTP, 3 NADH and 1 FADH_2. Thus, the correct answer is 'Carbon dioxide + Water.'

A, B, and C are scientifically correct as per NCERT.

Option D is incorrect because glycolysis is an anaerobic process and does not require oxygen; oxygen acts as the terminal electron acceptor only during the electron transport chain.

Acetyl CoA is the key intermediate between the Krebs cycle of glycolysis. After glycolysis, the glucose converts to pyruvic acid which is a three-carbon molecule. It is converted into acetyl coenzyme a by oxidative decarboxylation This, Acetyl CoA enters the Krebs cycle and along with oxaloacetic acid forms the citric acid which is a 6C compound.

1. Krebs cycle is also known as the citric acid cycle because this reaction starts with the six-carbon compound which is citric acid. It occurs in the mitochondrial matrix.
2. Krebs cycle is a closed-loop cycle. And each loop of the cycle generates a molecule of ATP. This cycle consists of eight steps which include redox, dehydration, hydration, and decarboxylation reactions. It is an aerobic pathway because NADH is produced and the electrons released are used up in the next cycle which uses oxygen.
3. The process of the cycle starts with the condensation of acetyl- CoA with oxaloacetate.
4. This reaction is controlled by the amount of ATP present.
5. If the ATP level increases then the rate of the reaction decreases and vice versa. After glycolysis, the pyruvate is then converted into acetyl CoA which enters the citric acid cycle.
6. The Krebs cycle is the pathway that all organisms use to generate energy. The intermediate compound that links pyruvate to the Krebs cycle is Acetyl CoA.
7. So, the answer is option (B) ‘the intermediate compound which links glycolysis with the Krebs cycle is malic acid’.

The majority of the enzymes involved in the Tricarboxylic Acid (TCA) cycle, also known as the Krebs cycle or citric acid cycle, are located in the mitochondrial matrix. This is because the TCA cycle primarily occurs within the matrix of mitochondria, except for succinate dehydrogenase, which is uniquely embedded in the inner mitochondrial membrane.

The mitochondrial matrix provides the necessary environment for these enzymatic reactions to occur efficiently, playing a crucial role in cellular respiration and energy production.