Test: Respiration in Plants - Aerobic Respiration (July 6) - NEET MCQ

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 is named after Hans Krebs.
The TCA (tricarboxylic acid) cycle, also known as the citric acid cycle or Krebs cycle, is a series of chemical reactions that occur in the mitochondria of cells to generate energy. It was discovered and named after Hans Krebs, a German biochemist, in the 1930s. Here are some key points about Hans Krebs and his contribution to the understanding of the TCA cycle:
- Hans Krebs:
- Hans Adolf Krebs was born on August 25, 1900, in Hildesheim, Germany.
- He studied medicine at the University of Göttingen and later obtained a Ph.D. in medicine from the University of Hamburg.
- Krebs made significant contributions to the field of biochemistry, particularly in the study of intermediary metabolism.
- He is best known for his discovery of the citric acid cycle, for which he was awarded the Nobel Prize in Physiology or Medicine in 1953.
- Krebs also conducted research on the urea cycle and the metabolism of amino acids.
- He died on November 22, 1981, in Oxford, England.
- The Discovery of the TCA Cycle:
- In the 1930s, Hans Krebs and his colleague, William Arthur Johnson, conducted research to understand how cells produce energy.
- Through their experiments, Krebs and Johnson discovered a series of chemical reactions that occur in the mitochondria and produce energy-rich molecules such as ATP.
- They found that citric acid, a molecule with three carboxyl groups (tricarboxylic acid), plays a central role in these reactions.
- Krebs elucidated the complete cycle of reactions involving citric acid and other molecules, which became known as the citric acid cycle or TCA cycle.
- Significance of the TCA Cycle:
- The TCA cycle is an essential part of cellular respiration, which is the process by which cells convert nutrients into usable energy.
- It provides the necessary intermediates and electron carriers for oxidative phosphorylation, the final step in energy production.
- The cycle also generates molecules that are important for biosynthesis, such as amino acids, nucleotides, and lipids.
- Understanding the TCA cycle has been crucial for unraveling the mechanisms of cellular metabolism and has paved the way for advances in biochemistry and medicine.
In conclusion, the TCA cycle is named after Hans Krebs, a renowned biochemist who discovered and elucidated the series of reactions involved in this essential metabolic pathway. His contributions have had a significant impact on our understanding of cellular energy production and metabolism.

Explanation:
The correct answer is C: Tricarboxylic acid cycle and citric acid cycle.
Explanation:
In the given options, the names "Tricarboxylic acid cycle" and "citric acid cycle" refer to the same metabolic pathway.
Here is a brief explanation of the different options:
- Krebs cycle and Calvin cycle: These are two separate metabolic pathways involved in cellular respiration and photosynthesis, respectively. They are not the same thing.
- Citric acid cycle and Calvin cycle: The Calvin cycle is involved in photosynthesis, while the citric acid cycle (also known as the tricarboxylic acid cycle) is involved in cellular respiration. They are not the same thing.
- Tricarboxylic acid cycle and citric acid cycle: These two names refer to the same metabolic pathway. The citric acid cycle is also known as the tricarboxylic acid cycle. This cycle takes place in the mitochondria and is an important part of cellular respiration.
- Tricarboxylic acid cycle and urea cycle: These are two separate metabolic pathways. The tricarboxylic acid cycle is involved in cellular respiration, while the urea cycle is involved in the metabolism of nitrogenous waste. They are not the same thing.
Therefore, the correct answer is C: Tricarboxylic acid cycle and citric acid cycle.

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.'

• Oxygen sits at the end of the electron transport chain, where it accepts electrons, hydrogen and picks up protons to form water.
• Pyruvate is formed in the cytoplasm.
• During fermentation glucose is partially broken down by glycolysis.
• During the conversion of succinyl CoA to succinic acid a molecule of GTP is synthesized.

So, the correct option is 'Oxygen is vital in respiration for removal of hydrogen'.

Key Intermediate Compound Linking Glycolysis to the Krebs Cycle:

• Answer: C. Acetyl CoA

Explanation:

• Glycolysis is the initial step in cellular respiration where glucose is broken down into pyruvate.


• The pyruvate molecules then enter the mitochondria for further processing in the Krebs cycle (also known as the citric acid cycle or the tricarboxylic acid cycle).


• In order for pyruvate to enter the Krebs cycle, it needs to be converted into acetyl coenzyme A (Acetyl CoA).


• Acetyl CoA is formed by a reaction that involves the removal of a carbon dioxide molecule from pyruvate and the transfer of a high-energy CoA molecule.


• Acetyl CoA acts as a key intermediate compound by delivering the acetyl group to the Krebs cycle, where it combines with oxaloacetate to form citrate.


• The citrate is then further metabolized in the Krebs cycle to generate ATP and reduced electron carriers (NADH and FADH2) for the electron transport chain.


• Acetyl CoA is crucial in linking glycolysis, which occurs in the cytoplasm, to the Krebs cycle, which occurs in the mitochondria.

Note: Please note that ATP, malic acid, and NADH are also important molecules in cellular respiration, but they are not the key intermediate compound linking glycolysis to the Krebs cycle.

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’.

Mitochondria except succinate dehydrogenase.