MCQ (Practice) - Practice Cell Respiration (Level 1) - Class 11 MCQ

The importance of respiration in organisms can be explained as follows:
1. Energy production: Respiration is the process by which organisms convert glucose and oxygen into carbon dioxide, water, and energy. This energy is essential for carrying out various metabolic activities and maintaining the overall functioning of the organism.
2. Gas exchange: During respiration, oxygen is taken in and carbon dioxide is released. This exchange of gases is crucial for the survival of organisms as oxygen is required for cellular respiration and carbon dioxide is a waste product that needs to be eliminated.
3. ATP production: The energy produced during respiration is stored in the form of adenosine triphosphate (ATP). ATP is the main energy currency of cells and is used for various cellular processes such as muscle contraction, nerve impulse transmission, and active transport.
4. Growth and development: Respiration provides the energy necessary for growth, repair, and development of organisms. It is vital for the synthesis of new molecules, cell division, and tissue formation.
5. Maintenance of homeostasis: Respiration helps organisms maintain a stable internal environment by regulating the levels of oxygen and carbon dioxide in the body. The exchange of gases ensures that cells receive an adequate supply of oxygen and that waste gases are removed.
In conclusion, respiration is of utmost importance for organisms as it liberates energy, facilitates gas exchange, produces ATP, supports growth and development, and helps maintain homeostasis.

Energy Storage in Cells

When cells obtain energy from catabolic reactions, this energy is immediately stored in the form of ATP (adenosine triphosphate). ATP is a molecule that serves as the main energy currency of the cell. It is a high-energy molecule that can be easily broken down to release energy for various cellular processes.

Explanation:

• ATP (Adenosine Triphosphate): ATP is a nucleotide that consists of an adenosine molecule bonded to three phosphate groups. The energy obtained from catabolic reactions is used to synthesize ATP through the process of cellular respiration.


• Energy Storage: ATP stores energy in the high-energy phosphate bonds between its phosphate groups. When a phosphate group is removed from ATP, it forms ADP (adenosine diphosphate) and releases energy that can be used by the cell.


• Immediate Storage: The energy obtained from catabolic reactions is quickly used to replenish ATP stores in the cell. This allows the cell to have a readily available source of energy for various cellular processes, such as muscle contraction, active transport, and synthesis of macromolecules.

Therefore, the correct answer is option C: ATP. Cells store energy obtained from catabolic reactions in the form of ATP, which serves as the primary energy currency of the cell.

Component of ETS Mobile, e^– Carrier:
- The component of ETS (Electron Transport System) that acts as a mobile, e^– carrier is UQ (Ubiquinone), also known as Coenzyme Q (CO-Q).
Explanation:
- The Electron Transport System, also known as the respiratory chain or the mitochondrial respiratory chain, is a series of protein complexes located in the inner mitochondrial membrane.
- The ETS plays a crucial role in oxidative phosphorylation, the process by which ATP (adenosine triphosphate) is generated in the mitochondria.
- The ETS consists of several protein complexes and small molecules that act as electron carriers.
- One of these electron carriers is Ubiquinone (UQ), also known as Coenzyme Q (CO-Q). It is a mobile electron carrier that shuttles electrons between the protein complexes in the ETS.
- UQ accepts electrons from Complex I (NADH dehydrogenase) and Complex II (succinate dehydrogenase) and transfers them to Complex III (cytochrome bc1 complex).
- UQ undergoes reversible oxidation and reduction as it shuttles electrons, allowing the flow of electrons through the ETS.
- The transfer of electrons through the ETS generates a proton gradient across the inner mitochondrial membrane, which is used to drive ATP synthesis.
- In summary, UQ (Coenzyme Q) is an essential component of the ETS that acts as a mobile, e^– carrier, shuttling electrons between the protein complexes in the ETS.

Source of Respiration:
The source of respiration is stored food in the form of glucose. When we breathe, our body takes in oxygen and glucose from the food we consume. The process of respiration involves the breakdown of glucose into carbon dioxide and water, releasing energy in the form of ATP (adenosine triphosphate). This energy is essential for the functioning of our cells and various bodily processes.
Explanation:
During respiration, glucose is broken down in a series of chemical reactions that occur in the cells of our body. This process can be divided into two main stages: glycolysis and cellular respiration.
1. Glycolysis:
- Occurs in the cytoplasm of the cell.
- Glucose is converted into pyruvate molecules.
- Small amount of ATP is produced.
2. Cellular Respiration:
- Occurs in the mitochondria of the cell.
- Pyruvate molecules are further broken down.
- Carbon dioxide and water are produced.
- Large amount of ATP is generated through a series of reactions known as the Krebs cycle and the electron transport chain.
The stored food in our body, such as carbohydrates and fats, serves as the source of glucose for respiration. This glucose is obtained from the digestion and absorption of food in our digestive system. Once glucose is available, it undergoes the process of respiration to produce ATP, which is utilized by our cells for energy.
In conclusion, the source of respiration is the stored food in our body, which provides the necessary glucose required for the process of cellular respiration and energy production.

The question asks for the substance that has a respiratory quotient (R.Q.) of less than one. The respiratory quotient is the ratio of carbon dioxide produced to oxygen consumed during respiration. It provides an indication of the type of substrate being metabolized.
To determine the substance with an R.Q. less than one, we need to consider the metabolic pathways involved in respiration for each option:
A. Starch:
- Starch is a complex carbohydrate made up of glucose units.
- During respiration, starch is broken down into glucose molecules, which are then metabolized to produce energy.
- The complete oxidation of glucose has an R.Q. of 1, indicating that the amount of carbon dioxide produced is equal to the amount of oxygen consumed.
- Therefore, the R.Q. for starch is 1.
B. Sugarcane:
- Sugarcane is also a complex carbohydrate composed of glucose units.
- Similar to starch, sugarcane is broken down into glucose molecules during respiration.
- The R.Q. for sugarcane is also 1.
C. Glucose:
- Glucose is a simple sugar and the primary energy source for cellular respiration.
- The complete oxidation of glucose has an R.Q. of 1.
D. Ground nut:
- Ground nut, also known as peanut, is a legume that contains a moderate amount of carbohydrates, proteins, and fats.
- The metabolic breakdown of ground nut involves the utilization of both carbohydrates and fats.
- Fats have an R.Q. of 0.7, indicating that they produce less carbon dioxide compared to the amount of oxygen consumed.
- Since ground nut contains fats, its overall R.Q. would be less than 1.
Therefore, the substance with an R.Q. less than one is ground nut (peanut).

Number ATP produced from one pyruvic acid during conversion to acetyl Co-A:
- The conversion of pyruvic acid to acetyl Co-A occurs in the mitochondria of the cell during aerobic respiration.
- During this process, one pyruvic acid molecule undergoes several reactions to produce one molecule of acetyl Co-A.
- The reactions involved in this conversion process are collectively known as the pyruvate dehydrogenase complex (PDC).
- As a result of the PDC reactions, the following products are formed:
- One molecule of acetyl Co-A
- One molecule of NADH
- One molecule of carbon dioxide (CO2)
- The molecule of acetyl Co-A produced can then enter the citric acid cycle (Krebs cycle) to further generate ATP.
- During the citric acid cycle, each acetyl Co-A molecule produces three molecules of NADH, one molecule of FADH2, and one molecule of GTP (which can be converted to ATP).
- The NADH and FADH2 molecules produced during the citric acid cycle then enter the electron transport chain, where they are used to generate ATP through oxidative phosphorylation.
- Overall, the complete oxidation of one molecule of glucose (which yields two molecules of pyruvic acid) can produce a total of 36-38 molecules of ATP, depending on the efficiency of the electron transport chain.
- Therefore, the number of ATP molecules produced from one pyruvic acid molecule during its conversion to acetyl Co-A is 3.

Explanation:
In succulent plants, the respiratory quotient (R.Q.) is less than one due to incomplete oxidation. Here's a detailed explanation:
1. Succulent Plants:
- Succulent plants are known for their ability to store water in their leaves, stems, and roots, allowing them to survive in arid environments.
- These plants have adapted to minimize water loss through transpiration, which is the process of water movement through the plant and evaporation from the leaves.
2. Respiratory Quotient (R.Q.):
- The respiratory quotient (R.Q.) is a measure of the ratio of carbon dioxide (CO2) produced to oxygen (O2) consumed during respiration.
- It provides information about the type of respiratory substrate being utilized by the plant or organism.
- R.Q. can range from 0.7 to 1.0, depending on the type of substrate being oxidized.
3. Incomplete Oxidation:
- In succulent plants, the R.Q. is less than one because they undergo incomplete oxidation of respiratory substrates.
- The incomplete oxidation occurs due to the presence of a specialized type of metabolism called CAM (Crassulacean Acid Metabolism).
- CAM plants open their stomata and take in carbon dioxide during the night, storing it in the form of organic acids.
- During the day, when the stomata are closed to prevent water loss, the stored organic acids are broken down, and carbon dioxide is released for photosynthesis.
- This process of storing and releasing carbon dioxide leads to incomplete oxidation during respiration, resulting in an R.Q. less than one.
4. Other Options:
- Option A: Complete oxidation is not applicable as it would result in an R.Q. equal to one.
- Option B: Complete reduction is not relevant to the R.Q. of succulent plants.
- Option C: Incomplete reduction is not a characteristic of succulent plant respiration.
Therefore, the correct answer is D: Incomplete oxidation.

The link between Glycolysis and Krebs cycle is Acetyl co-enzyme-A. Here is a detailed explanation:
Glycolysis:
- Glycolysis is the first step in cellular respiration, which occurs in the cytoplasm of cells.
- It is a series of chemical reactions that convert glucose into pyruvate.
- Glycolysis produces a small amount of ATP and NADH, which are used in the later stages of cellular respiration.
Krebs cycle:
- The Krebs cycle, also known as the citric acid cycle, is the second step of cellular respiration.
- It takes place in the mitochondria of cells.
- The primary function of the Krebs cycle is to generate high-energy molecules such as NADH and FADH2.
- These high-energy molecules are used in the electron transport chain to produce ATP.
Link between Glycolysis and Krebs cycle:
- Acetyl co-enzyme-A is the link between glycolysis and the Krebs cycle.
- After glycolysis, pyruvate molecules are transported into the mitochondria.
- Pyruvate molecules are converted into acetyl co-enzyme-A before entering the Krebs cycle.
- Acetyl co-enzyme-A combines with oxaloacetate to form citric acid, which initiates the Krebs cycle.
In summary, Acetyl co-enzyme-A is the molecule that connects glycolysis with the Krebs cycle. It is formed from pyruvate molecules and enters the Krebs cycle by combining with oxaloacetate to form citric acid. This connection allows for the continued breakdown of glucose and the production of ATP in cellular respiration.

Explanation:
To calculate the energy produced by aerobic respiration of glucose, we need to know the number of ATP molecules produced during aerobic respiration.
1. Glucose undergoes glycolysis to produce 2 ATP molecules.
2. Pyruvate, produced during glycolysis, enters the mitochondria and undergoes the Krebs cycle.
3. During the Krebs cycle, 2 ATP molecules are produced through substrate-level phosphorylation.
4. The electron transport chain, located in the inner mitochondrial membrane, generates the majority of the ATP molecules during aerobic respiration.
5. For every NADH molecule produced during the Krebs cycle, 3 ATP molecules are generated in the electron transport chain.
6. For every FADH2 molecule produced during the Krebs cycle, 2 ATP molecules are generated in the electron transport chain.
The total number of ATP molecules produced during aerobic respiration of glucose is calculated as follows:
2 ATP (glycolysis) + 2 ATP (Krebs cycle) + (3 ATP × number of NADH molecules) + (2 ATP × number of FADH2 molecules)
Since one molecule of glucose produces 10 NADH molecules and 2 FADH2 molecules during aerobic respiration, the total ATP production is:
2 ATP (glycolysis) + 2 ATP (Krebs cycle) + (3 ATP × 10 NADH) + (2 ATP × 2 FADH2) = 38 ATP
One mole of glucose produces 38 moles of ATP, and the standard energy released per mole of ATP is 7.3 kcal.
Therefore, the total energy produced by aerobic respiration of glucose can be calculated as:
38 moles ATP × 7.3 kcal/mole ATP = 277.4 kcal
Rounded to the nearest whole number, the energy produced by aerobic respiration of glucose is 277 kcal.
However, the given answer choices are in kilocalories (K.cal), so we need to convert the answer to kilocalories:
277 kcal ÷ 1000 = 0.277 K.cal
Therefore, the correct answer is C: 686 K.cal.

Succinyl Co-A is related to Krebs cycle:
The Krebs cycle, also known as the citric acid cycle or the tricarboxylic acid cycle, is a series of chemical reactions that occur in the mitochondria of cells. Succinyl Co-A is an important molecule in the Krebs cycle as it plays a crucial role in the production of energy in the form of ATP. Here is a detailed explanation of the relationship between Succinyl Co-A and the Krebs cycle:
1. Succinyl Co-A:
- Succinyl Co-A is a compound that is formed during the breakdown of carbohydrates, fats, and proteins in the body.
- It is an intermediate molecule in the metabolic pathway that converts food into energy.
- Succinyl Co-A is derived from the amino acid methionine and is also a key component in the biosynthesis of heme, which is an essential component of hemoglobin.
2. Krebs Cycle:
- The Krebs cycle is the second stage of cellular respiration, which is the process by which cells generate energy from glucose.
- It takes place in the mitochondria and involves a series of reactions that convert acetyl Co-A, a molecule derived from the breakdown of glucose, into carbon dioxide.
- The cycle produces high-energy molecules such as ATP, NADH, and FADH2, which are used to fuel various cellular processes.
3. Role of Succinyl Co-A in the Krebs cycle:
- Succinyl Co-A enters the Krebs cycle by combining with oxaloacetate to form citrate, the first compound in the cycle.
- Through a series of reactions, citrate is converted back into oxaloacetate, generating energy in the form of ATP, NADH, and FADH2.
- Succinyl Co-A is directly involved in the conversion of succinate to fumarate, a key step in the cycle that generates another molecule of ATP.
- The cycle continues, producing more ATP and reducing agents (NADH and FADH2) that will be used in the electron transport chain to generate even more ATP.
In conclusion, Succinyl Co-A is an important molecule in the Krebs cycle. It plays a crucial role in the production of energy by participating in various reactions that generate ATP and other high-energy molecules.

Chemiosmotic Theory and ATP Synthesis
The chemiosmotic theory, proposed by Peter Mitchell in 1978, explains how ATP (adenosine triphosphate) is synthesized on membranes. According to this theory, ATP synthesis occurs due to the proton gradient across the membrane.
Explanation:
To provide a more detailed solution, let's break it down into bullet points:
Chemiosmotic Theory:
- Proposed by Peter Mitchell in 1978.
- Explains how ATP is synthesized on membranes.
ATP Synthesis:
- ATP is the primary energy currency of cells.
- It is synthesized through a process called oxidative phosphorylation.
Proton Gradient:
- During oxidative phosphorylation, electrons are transferred through a series of protein complexes in the mitochondrial inner membrane.
- This transfer of electrons creates a proton gradient across the membrane.
ATP Synthesis Mechanism:
- The proton gradient generated by the electron transfer drives ATP synthesis.
- ATP synthase, an enzyme complex, utilizes the energy from the proton gradient to convert ADP (adenosine diphosphate) and inorganic phosphate (Pi) into ATP.
- The flow of protons through ATP synthase provides the energy required for the synthesis of ATP.
Summary:
According to the chemiosmotic theory proposed by Peter Mitchell, ATP synthesis occurs on membranes due to the proton gradient. The flow of protons through ATP synthase drives the conversion of ADP and inorganic phosphate into ATP, which serves as the primary energy source for cellular processes.

Reduction of NADP to NADPH₂
The reduction of NADP to NADPH₂ is an important process in various metabolic pathways. Let's discuss the different metabolic pathways associated with this reduction:
HMP-shunt:
- The hexose monophosphate (HMP) shunt pathway, also known as the pentose phosphate pathway (PPP), is involved in the generation of NADPH₂.
- In the oxidative phase of the HMP-shunt, glucose-6-phosphate is oxidized, leading to the production of NADPH₂.
- The NADPH₂ generated in this pathway plays a crucial role in various cellular processes, including biosynthesis of fatty acids, cholesterol, and nucleotides.
Calvin cycle:
- The Calvin cycle, also known as the light-independent reactions of photosynthesis, is another pathway where NADP is reduced to NADPH₂.
- During the Calvin cycle, NADP acts as a hydrogen acceptor and is reduced to NADPH₂ during the light-independent reactions.
- NADPH₂ produced in the Calvin cycle is used in the synthesis of glucose and other carbohydrates.
Glycolysis:
- Glycolysis is a metabolic pathway that involves the breakdown of glucose to pyruvate.
- Although glycolysis primarily produces ATP and NADH, a small amount of NADPH₂ can also be generated through the conversion of 3-phosphoglycerate to 1,3-bisphosphoglycerate.
EMP-pathway:
- The EMP (Embden-Meyerhof-Parnas) pathway, also known as glycolysis, is the main pathway for glucose metabolism in most organisms.
- While glycolysis primarily generates NADH, a small amount of NADPH₂ can be produced through the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate.
From the above explanations, it is clear that the reduction of NADP to NADPH₂ is associated with the HMP-shunt pathway.

Why does the cut surface of fruit and vegetables often become dark?
There are several factors that can cause the cut surface of fruits and vegetables to become dark. However, the most common reason is the oxidation of tannic acid in the presence of a trace of iron from the knife. Let's break down the explanation:
1. Oxidation of tannic acid:
- Tannic acid is a natural compound found in many fruits and vegetables.
- When the cut surface of a fruit or vegetable is exposed to the air, the tannic acid present in the cells reacts with oxygen.
- This oxidation process can lead to discoloration and darkening of the cut surface.
2. Presence of iron from the knife:
- Knives used to cut fruits and vegetables often contain small amounts of iron.
- When the iron comes into contact with the tannic acid in the cut surface, it can accelerate the oxidation process.
- This interaction between iron and tannic acid contributes to the darkening of the cut surface.
3. Other factors:
- While the presence of a dirty knife or dust in the air may contribute to the discoloration of the cut surface, they are not the primary causes.
- Dust particles or dirt on the knife may introduce additional contaminants, but the main reason is the oxidation process.
In conclusion, the cut surface of fruits and vegetables often becomes dark due to the oxidation of tannic acid in the presence of a trace of iron from the knife.

Explanation:
Competitive inhibition is a type of enzyme inhibition where an inhibitor molecule competes with the substrate for binding to the active site of the enzyme. This type of inhibition can be overcome by increasing the concentration of the substrate.
- Succinic dehydrogenase: It is an enzyme involved in the citric acid cycle and plays a crucial role in the production of ATP. Malonic acid is a competitive inhibitor of succinic dehydrogenase.
- Cytochrome oxidase: It is an enzyme involved in cellular respiration and is responsible for the final step in the electron transport chain. Cyanide is a non-competitive inhibitor of cytochrome oxidase.
- Hexokinas: Hexokinases are enzymes that catalyze the phosphorylation of glucose, the first step in glucose metabolism. Glucose-6 phosphate is the product of this reaction and acts as an allosteric inhibitor of hexokinase.
- Carbonic anhydrase: It is an enzyme that catalyzes the reversible hydration of carbon dioxide to bicarbonate ions. Carbon dioxide is not a competitive inhibitor of carbonic anhydrase.
Based on the information provided, the correct answer is A: Succinic dehydrogenase by malonic acid.

The hexose monophosphate shunt, also known as the pentose phosphate pathway, is an alternative pathway to glycolysis for glucose metabolism. It serves several important functions, including the production of NADPH and ribose-5-phosphate for nucleotide synthesis.
In the hexose monophosphate shunt, a series of reactions occur that ultimately result in the production of NADPH and the conversion of glucose-6-phosphate to ribose-5-phosphate. During these reactions, carbon dioxide (CO2) is released as a byproduct.
The number of CO2 molecules evolved in the hexose monophosphate shunt is more than in glycolysis. This is because one molecule of glucose-6-phosphate is converted into two molecules of NADPH and one molecule of ribose-5-phosphate. In the process, two molecules of CO2 are released.
In contrast, in glycolysis, glucose is metabolized to produce two molecules of pyruvate without the release of CO2. Therefore, the number of CO2 molecules evolved in the hexose monophosphate shunt is greater than in glycolysis.
Key Points:
- The hexose monophosphate shunt is an alternative pathway to glycolysis for glucose metabolism.
- It produces NADPH and ribose-5-phosphate.
- The number of CO2 molecules evolved in the hexose monophosphate shunt is more than in glycolysis.
- Two molecules of CO2 are released during the conversion of glucose-6-phosphate to ribose-5-phosphate in the hexose monophosphate shunt.

Conversion of pyruvic acid into ethyl alcohol is mediated by:
A: Phosphatase
- Phosphatase is not involved in the conversion of pyruvic acid into ethyl alcohol.
B: Dehydrogenase
- Dehydrogenase is involved in the conversion of pyruvic acid into ethyl alcohol, but it is not the sole mediator.
C: Decarboxylase & dehydrogenase
- The conversion of pyruvic acid into ethyl alcohol is mediated by decarboxylase and dehydrogenase enzymes working together.
- Decarboxylase removes the carboxyl group from pyruvic acid, forming acetaldehyde.
- Dehydrogenase then reduces acetaldehyde to ethyl alcohol.
D: Catalase
- Catalase is not involved in the conversion of pyruvic acid into ethyl alcohol.
Therefore, the correct answer is C: Decarboxylase & dehydrogenase.

Introduction: In the absence of oxygen, some organisms have the ability to carry out respiration using alternative metabolic pathways. These pathways allow them to generate energy without the need for oxygen. Among the given options, yeast is the commonest living organism that can respire in the absence of oxygen.
Explanation:
- Fish: Fish are unable to respire without oxygen. They rely on gills to extract oxygen from water for respiration.
- Yeast: Yeast is a single-celled fungus that can undergo fermentation in the absence of oxygen. This process, known as anaerobic respiration, allows yeast to generate energy by breaking down sugars into alcohol and carbon dioxide.
- Potato: Potatoes are plant organs and do not respire. They undergo cellular respiration in the presence of oxygen to generate energy.
- Chlorella: Chlorella is a type of algae that can carry out photosynthesis and respiration. However, it requires oxygen for respiration and cannot respire in the absence of oxygen.
Conclusion: Among the given options, yeast is the commonest living organism that can respire in the absence of oxygen. It has the ability to undergo fermentation, allowing it to generate energy anaerobically.

Formation of Acetyl Co-A from Pyruvic Acid
The formation of Acetyl Co-A from pyruvic acid occurs through a process called oxidative decarboxylation. Here is a detailed explanation:
1. Pyruvic Acid
- Pyruvic acid is a three-carbon molecule that is produced during glycolysis, which is the initial step of glucose metabolism.
- It is formed in the cytoplasm of cells and serves as a key intermediate in several metabolic pathways.
2. Transport into Mitochondria
- Pyruvic acid needs to be transported from the cytoplasm into the mitochondria, the powerhouse of the cell where Acetyl Co-A is formed.
- This transport occurs through a specific transport protein called the pyruvate transporter.
3. Decarboxylation
- Once pyruvic acid enters the mitochondria, it undergoes decarboxylation, which involves the removal of a carboxyl group (CO2).
- This step is catalyzed by a multi-enzyme complex called the pyruvate dehydrogenase complex (PDC).
4. Coenzyme A Binding
- After decarboxylation, the remaining two-carbon molecule, known as an acetyl group, binds to coenzyme A (Co-A) to form Acetyl Co-A.
- Coenzyme A is derived from the B vitamin pantothenic acid and plays a crucial role in various metabolic reactions.
5. NADH Production
- The decarboxylation of pyruvic acid also leads to the production of NADH, an energy-rich molecule that carries high-energy electrons to the electron transport chain.
- NADH is an important source of ATP synthesis during cellular respiration.
6. Acetyl Co-A as a Key Intermediate
- Acetyl Co-A serves as a crucial intermediate in various metabolic pathways, including the citric acid cycle (Krebs cycle) and fatty acid synthesis.
- In the citric acid cycle, Acetyl Co-A enters the cycle and undergoes a series of reactions to generate energy-rich molecules such as ATP and NADH.
In conclusion, the formation of Acetyl Co-A from pyruvic acid occurs through the process of oxidative decarboxylation. This process involves the removal of a carboxyl group from pyruvic acid, resulting in the formation of Acetyl Co-A, which serves as a key intermediate in several metabolic pathways.

The link between carbohydrate and fat metabolism is Acetyl Co-A.
- Carbohydrate metabolism involves the breakdown of carbohydrates into glucose, which is then used as a source of energy by the cells.
- Glucose is converted into pyruvic acid through a process called glycolysis in the cytoplasm of the cell.
- Pyruvic acid enters the mitochondria and undergoes further conversion into Acetyl Co-A through a process called pyruvate decarboxylation.
- Acetyl Co-A is a key molecule that links carbohydrate metabolism with fat metabolism.
- Acetyl Co-A can either enter the citric acid cycle (also known as the Krebs cycle) to produce energy or be used as a precursor for fatty acid synthesis.
- When energy needs are high, Acetyl Co-A enters the citric acid cycle to produce ATP, which is the energy currency of the cell.
- However, when energy needs are low and there is an excess of Acetyl Co-A, it can be used for fatty acid synthesis, leading to the production of fat molecules.
- This process is known as lipogenesis and is an important step in fat metabolism.
- Therefore, Acetyl Co-A acts as a link between carbohydrate metabolism and fat metabolism, allowing for the conversion of excess carbohydrates into fat molecules for storage.

Pyruvate dehydrogenase complex is used in converting pyruvate to acetyl Co-A.
The Pyruvate dehydrogenase complex is an enzymatic complex that plays a crucial role in the conversion of pyruvate to acetyl Co-A. Here is a detailed explanation of the process:
1. Pyruvate: Pyruvate is a three-carbon compound that is produced during glycolysis, which is the first step of glucose metabolism.
2. Transport into mitochondria: Pyruvate needs to be transported from the cytoplasm into the mitochondria, where the pyruvate dehydrogenase complex is located. This transport is facilitated by specific transport proteins.
3. Decarboxylation: Once inside the mitochondria, the pyruvate dehydrogenase complex catalyzes the decarboxylation of pyruvate. This process involves the removal of a carboxyl group from pyruvate, resulting in the release of carbon dioxide.
4. Formation of acetyl Co-A: The decarboxylated pyruvate then undergoes a series of enzymatic reactions within the pyruvate dehydrogenase complex. These reactions result in the formation of acetyl Co-A, a two-carbon compound that plays a crucial role in the citric acid cycle (also known as the Krebs cycle).
The conversion of pyruvate to acetyl Co-A is an important step in cellular respiration as it allows for the further breakdown of glucose and the production of ATP, the energy currency of the cell. This process is essential for the efficient utilization of glucose as a source of energy in aerobic organisms.

The first compound of TCA cycle is:

• Citric acid (Answer: C)

Explanation:

• The TCA (tricarboxylic acid) cycle, also known as the Krebs cycle or citric acid cycle, is a series of chemical reactions that occur in the mitochondria of cells.


• It is an important metabolic pathway that generates energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins.


• The TCA cycle starts with the condensation of acetyl-CoA and oxaloacetate to form citric acid (citrate).


• Citrate is then converted to isocitrate through a series of enzymatic reactions.


• Isocitrate is further oxidized to α-ketoglutarate, which generates NADH and CO2 in the process.


• α-Ketoglutarate is then converted to succinyl-CoA, which generates another molecule of NADH.


• Succinyl-CoA is converted to succinate, which generates GTP (guanosine triphosphate).


• Succinate is then converted to fumarate, which generates FADH2 (flavin adenine dinucleotide).


• Fumarate is further converted to malate, which generates another molecule of NADH.


• Finally, malate is converted back to oxaloacetate, completing the cycle.

Therefore, the first compound formed in the TCA cycle is citric acid (citrate).

End product of glycolysis is Pyruvic acid
Glycolysis is the metabolic pathway that converts glucose into pyruvate. During glycolysis, glucose is broken down into two molecules of pyruvate through a series of enzymatic reactions. The end product, pyruvate, can then be used in various metabolic pathways depending on the cell's energy needs.
Here is a detailed explanation of the glycolysis pathway and the end product:
1. Glucose phosphorylation: Glucose is phosphorylated by the enzyme hexokinase, using ATP as a phosphate donor. This step traps glucose inside the cell and converts it into glucose-6-phosphate.
2. Isomerization: Glucose-6-phosphate is converted into fructose-6-phosphate through an isomerization reaction catalyzed by the enzyme phosphoglucose isomerase.
3. Phosphorylation: Fructose-6-phosphate is phosphorylated by ATP to form fructose-1,6-bisphosphate. This reaction is catalyzed by the enzyme phosphofructokinase.
4. Cleavage: Fructose-1,6-bisphosphate is cleaved into two three-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). The enzyme responsible for this step is aldolase.
5. Isomerization: DHAP is converted into another molecule of G3P by the enzyme triose phosphate isomerase. Both G3P molecules can proceed to the next steps of glycolysis.
6. Oxidation and phosphorylation: G3P is oxidized by NAD+ to form 1,3-bisphosphoglycerate (1,3-BPG), and NADH is produced in the process. The enzyme responsible for this step is glyceraldehyde-3-phosphate dehydrogenase.
7. Substrate-level phosphorylation: 1,3-BPG is converted into 3-phosphoglycerate (3-PG) by transferring a phosphate group to ADP, forming ATP. The enzyme responsible for this step is phosphoglycerate kinase.
8. Conversion: 3-PG is converted into 2-phosphoglycerate (2-PG) by the enzyme phosphoglycerate mutase.
9. Dehydration: 2-PG loses a water molecule to form phosphoenolpyruvate (PEP). The enzyme responsible for this step is enolase.
10. Substrate-level phosphorylation: PEP is converted into pyruvate by transferring a phosphate group to ADP, forming ATP. The enzyme responsible for this step is pyruvate kinase.
At the end of glycolysis, the final product is pyruvate, which is a three-carbon molecule. Pyruvate can then enter different metabolic pathways depending on the cell's energy needs. It can be converted into acetyl-CoA and enter the citric acid cycle (also known as the Krebs cycle) to produce more ATP through oxidative phosphorylation. It can also

First Reaction in Pentose Phosphate Pathway - Oxidation of Glucose-6-phosphate
The first reaction in the pentose phosphate pathway is the oxidation of glucose-6-phosphate. This pathway is an alternative metabolic pathway to glycolysis and plays a crucial role in the production of NADPH and ribose-5-phosphate. Here is a detailed explanation of this reaction:
1. Glucose-6-phosphate Dehydrogenase (G6PDH) Reaction:
- The first enzyme involved in the pentose phosphate pathway is glucose-6-phosphate dehydrogenase (G6PDH).
- G6PDH catalyzes the conversion of glucose-6-phosphate to 6-phosphogluconolactone.
- This reaction involves the oxidation of glucose-6-phosphate and the reduction of NADP+ to NADPH.
- The reaction is irreversible and is the rate-limiting step of the pentose phosphate pathway.
2. Role of NADPH:
- NADPH produced in this reaction is an essential cofactor in various biosynthetic processes.
- It serves as a reducing agent for the synthesis of fatty acids, cholesterol, and steroids.
- NADPH is also required for the detoxification of reactive oxygen species (ROS) by the enzyme glutathione peroxidase.
3. Generation of Ribose-5-phosphate:
- The oxidation of glucose-6-phosphate in the pentose phosphate pathway generates ribulose-5-phosphate.
- Ribulose-5-phosphate can be converted to ribose-5-phosphate, which is a crucial precursor for nucleotide synthesis.
In conclusion, the first reaction in the pentose phosphate pathway is the oxidation of glucose-6-phosphate by glucose-6-phosphate dehydrogenase. This reaction produces NADPH and is essential for the production of ribose-5-phosphate and other important cellular processes.

To determine the number of ATP molecules produced during the oxidation of one molecule of glucose in aerobic respiration, we need to consider the different stages of cellular respiration where ATP is generated.
1. Glycolysis:
- Glycolysis is the initial stage of cellular respiration that takes place in the cytoplasm.
- It involves the breakdown of one molecule of glucose into two molecules of pyruvate.
- During glycolysis, a net production of 2 ATP molecules is generated through substrate-level phosphorylation.
2. Krebs cycle (Citric Acid Cycle):
- The pyruvate molecules produced during glycolysis enter the mitochondria and undergo further oxidation in the Krebs cycle.
- One glucose molecule results in two turns of the Krebs cycle since two pyruvate molecules are produced during glycolysis.
- Each turn of the Krebs cycle generates 1 ATP molecule through substrate-level phosphorylation.
- Therefore, a total of 2 ATP molecules are produced from the Krebs cycle.
3. Electron Transport Chain (ETC):
- The final stage of aerobic respiration is the electron transport chain, which takes place in the inner mitochondrial membrane.
- During the ETC, the electrons from NADH and FADH2 (produced in glycolysis and the Krebs cycle) are transferred through a series of protein complexes.
- This electron transfer creates a proton gradient across the inner mitochondrial membrane, which drives ATP synthesis.
- The exact number of ATP molecules generated per NADH and FADH2 varies, but on average, the ETC produces approximately 28 ATP molecules.
Overall ATP production:
- From glycolysis: 2 ATP molecules
- From the Krebs cycle: 2 ATP molecules
- From the ETC: Approximately 28 ATP molecules
- Therefore, the total ATP produced from the oxidation of one molecule of glucose in aerobic respiration is approximately 32 ATP molecules.
Answer: A. 36 ATP molecules (Approximately)

Answer:
The cytochrome that donates electrons to O₂ during terminal oxidation in the electron transport chain is Cytochrome-a₃.
Explanation:
The electron transport chain is a series of protein complexes located in the inner mitochondrial membrane. It plays a crucial role in the production of ATP through oxidative phosphorylation. During terminal oxidation, electrons are transferred from reduced coenzyme carriers (such as NADH and FADH₂) to molecular oxygen (O₂) to form water (H₂O).
In the electron transport chain, there are several cytochromes that serve as electron carriers. These include cytochrome b, cytochrome c, cytochrome a, and cytochrome a₃. Among these, cytochrome a₃ is the cytochrome that donates electrons to O₂ during terminal oxidation.
To summarize:
- The cytochrome that donates electrons to O₂ during terminal oxidation is Cytochrome-a₃.

Number of oxygen atoms required for aerobic oxidation of one pyruvate:
To determine the number of oxygen atoms required for the aerobic oxidation of one pyruvate molecule, we need to understand the process of aerobic oxidation and the chemical reactions involved.
1. Pyruvate Decarboxylation:
- Pyruvate, a three-carbon molecule, is first decarboxylated to form acetyl-CoA in the presence of the enzyme pyruvate dehydrogenase.
- During this process, one carbon atom from pyruvate is released as carbon dioxide (CO2).
- Therefore, the number of oxygen atoms required for this step is zero.
2. Krebs Cycle:
- Acetyl-CoA enters the Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle.
- In the Krebs cycle, acetyl-CoA is oxidized, generating energy and producing carbon dioxide (CO2) as a byproduct.
- Two molecules of CO2 are released per pyruvate molecule.
- Since each molecule of CO2 contains one carbon and two oxygen atoms, the number of oxygen atoms required for this step is four.
3. Electron Transport Chain (ETC):
- The final step in aerobic oxidation is the electron transport chain, where the majority of ATP production occurs.
- Oxygen acts as the final electron acceptor in this process, combining with electrons and hydrogen ions to form water (H2O).
- Since two water molecules are formed from four hydrogen ions and four electrons, the number of oxygen atoms required for this step is four.
Total number of oxygen atoms:
- Pyruvate decarboxylation: 0 oxygen atoms
- Krebs cycle: 4 oxygen atoms
- Electron transport chain: 4 oxygen atoms
- Total: 0 + 4 + 4 = 8 oxygen atoms
Therefore, the correct answer is B: 8.

Alternate name of Krebs cycle is Citric acid cycle.
The Krebs cycle, also known as the citric acid cycle, is a series of chemical reactions that occur in the mitochondria of cells. It is an essential metabolic pathway that is responsible for the oxidation of acetyl-CoA, a molecule derived from carbohydrates, fats, and proteins, to produce energy in the form of ATP.
The cycle was first discovered and described by Sir Hans Krebs in 1937, hence the name Krebs cycle. However, it is also commonly referred to as the citric acid cycle because of the key role played by citric acid, or citrate, in the series of reactions.
Here are some key points about the Krebs cycle:
1. Overview: The Krebs cycle is an aerobic process that takes place in the mitochondria of eukaryotic cells and the cytoplasm of prokaryotic cells.
2. Steps: The cycle consists of a series of eight enzymatic reactions that result in the complete oxidation of acetyl-CoA to carbon dioxide (CO2) and the production of energy in the form of ATP.
3. Key molecules: The cycle begins with the condensation of acetyl-CoA with oxaloacetate to form citrate. Citrate undergoes a series of transformations, leading to the regeneration of oxaloacetate and the release of CO2.
4. Energy production: The oxidation reactions in the Krebs cycle generate high-energy electrons that are transferred to electron carriers, such as NADH and FADH2. These electron carriers then participate in the electron transport chain, where ATP is generated through oxidative phosphorylation.
5. Role in metabolism: The Krebs cycle is a central hub in cellular metabolism. It not only generates ATP but also provides intermediates for other metabolic pathways, such as amino acid synthesis and the production of other important molecules.
In conclusion, the alternate name of the Krebs cycle is the citric acid cycle. It is a fundamental metabolic pathway that plays a crucial role in energy production and the overall metabolism of cells.

Respiration in plants
Respiration is an essential biological process that occurs in all living cells, including plants. It is the process by which cells break down organic compounds to release energy. Here is a detailed explanation of respiration in plants:
1. Occurrence:
- Respiration in plants occurs both during the day and night.
- While photosynthesis (a process that occurs only during the day) produces glucose and oxygen, respiration utilizes these products to release energy.
2. Formation of vitamins:
- Respiration in plants does not directly result in the formation of vitamins.
- Vitamins are organic compounds that are synthesized by plants through various metabolic pathways, not solely through respiration.
3. Characteristic of all living cells:
- Respiration is a characteristic feature of all living cells, including plant cells.
- It is a vital process that provides energy for cellular activities, growth, and development.
4. Requirement of CO2:
- Respiration in plants often requires CO2 (carbon dioxide).
- During respiration, plants take in oxygen and release CO2 as a byproduct.
- This CO2 can be used in photosynthesis by plants during the day.
In conclusion, respiration in plants is a fundamental process that occurs in all living cells. It takes place both during the day and night and is essential for energy production. While it does not directly result in the formation of vitamins, it is a characteristic feature of all living cells. Additionally, respiration in plants often requires CO2, which can be used in photosynthesis.

Respiration

Respiration is a vital process that occurs in living organisms, including plants and animals. It involves the exchange of gases, particularly the intake of oxygen (O₂) and the release of carbon dioxide (CO₂). This process is necessary to provide energy for the functioning of cells and various bodily processes.

Importance of Respiration

The key features and importance of respiration can be summarized as follows:

• Liberates energy: One of the most crucial functions of respiration is the liberation of energy. Through a series of biochemical reactions, energy is released from the breakdown of glucose molecules. This energy is then utilized by cells to perform various metabolic activities and sustain life.


• Provides oxygen (O₂): Respiration is responsible for the intake of oxygen, which is necessary for the process of cellular respiration. Oxygen is required in the final stage of respiration, known as oxidative phosphorylation, to generate ATP (adenosine triphosphate) – the energy currency of cells.


• Utilizes carbon dioxide (CO₂): Respiration helps in the removal of carbon dioxide, a waste product produced during cellular metabolism. The respiratory system removes CO₂ from the body through exhalation, maintaining the proper balance of this gas in the bloodstream.


• Synthesizes complex compounds: While respiration primarily involves the breakdown of glucose to release energy, it also plays a role in the synthesis of complex compounds. For example, during cellular respiration, intermediate molecules such as pyruvate can be used as building blocks for the synthesis of other compounds like amino acids and lipids.

Therefore, while all the options provided (A, B, C, and D) are related to respiration, the most important feature is that it liberates energy. Respiration is essential for the survival and functioning of cells, as it provides the energy needed for various biological processes.