NEET-PG 2012

1213 Questions — Page 21 of 122

Biochemistry

9 questions

Q201

What is the specific activity of an enzyme?

Q202

What is the characteristic nitrogenous product of lecithin hydrolysis?

Q203

In ETC NADH generates -

Q204

Which of the following is an amino sugar formed from fructose-6-phosphate?

Q205

The primary site of lipogenesis is:

Q206

Urea & Kreb's cycle are linked at?

Q207

Organ that can utilize glucose, fatty acids and ketone bodies is:

Q208

Which of the following micronutrient deficiencies can lead to anemia?

Q209

Which of the following usually require a RNA intermediate for cloning/replication?

NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs

Question 201: What is the specific activity of an enzyme?

A. Enzyme units per mg of protein (Correct Answer)
B. Concentration of substrate transformed per minute
C. Enzyme units per mg of substrate
D. Limit of enzyme per gram of substrate

Explanation: ***Enzyme units per mg of protein*** - **Specific activity** is defined as the number of **enzyme units** (representing catalytic activity) per milligram of total protein in the sample. - It is a measure of **purity**, indicating the amount of active enzyme relative to other proteins in a preparation. - Formula: Specific activity = Units of enzyme activity / mg of total protein - Used to track enzyme purification progress during isolation procedures. *Concentration of substrate transformed per minute* - This describes the **reaction velocity** or rate of catalysis, but not the specific activity of the enzyme. - While related to enzyme activity, it does not normalize the activity to the amount of **total protein**. - This would be expressed as reaction rate or velocity (V), not specific activity. *Enzyme units per mg of substrate* - This is an incorrect formulation that confuses substrate with protein. - **Specific activity** is normalized to the amount of **protein** in the enzyme preparation, not the substrate. - This option represents a common misconception in enzyme kinetics terminology. *Limit of enzyme per gram of substrate* - This phrase does not correspond to any standard biochemical measure of enzyme activity or concentration. - It does not provide information about the **catalytic efficiency** or **purity** of the enzyme preparation. - The term "limit" is not used in the context of specific activity measurements.

Question 202: What is the characteristic nitrogenous product of lecithin hydrolysis?

A. Fatty acids
B. Choline (Correct Answer)
C. Glucose
D. Phosphoric acid

Explanation: ***Choline*** - Lecithin is a type of **phospholipid** called **phosphatidylcholine**, meaning its head group contains choline. - Therefore, during hydrolysis, the **choline** component is released as the characteristic nitrogenous product. *Glucose* - **Glucose** is a simple sugar and a carbohydrate, not a component of lecithin. - It is a primary source of **energy** for cells but is not released during lipid hydrolysis. *Fatty acids* - While **fatty acids** are indeed components of lecithin (two fatty acid chains are attached to the glycerol backbone), they are not nitrogenous. - Fatty acids are **hydrophobic hydrocarbon chains** that make up a significant part of the lipid structure. *Phosphoric acid* - **Phosphoric acid** (or phosphate) is also a component of lecithin, connecting the glycerol backbone to the choline group. - However, it is an **inorganic acid** and does not contain nitrogen.

Question 203: In ETC NADH generates -

A. 1 ATPs
B. 4 ATPs
C. 3 ATPs (Correct Answer)
D. 2 ATPs

Explanation: ***3 ATPs*** - Each molecule of **NADH** donates electrons to **Complex I** of the electron transport chain (ETC), resulting in the pumping of enough protons to generate approximately **3 ATP molecules** via **oxidative phosphorylation**. - This high yield is due to NADH's ability to activate multiple proton pumps along the ETC, maximizing the **proton gradient** for ATP synthesis. *1 ATPs* - This is an incorrect yield for NADH; **FADH2** typically generates fewer ATPs (around 2) because it enters the ETC at a later stage, bypassing the initial proton pump. - Generating only 1 ATP from NADH would be very inefficient and is not physiologically accurate for oxidative phosphorylation. *2 ATPs* - While closer, 2 ATPs is the approximate yield for **FADH2**, which enters the ETC at **Complex II**, bypassing Complex I and thus pumping fewer protons. - NADH enters at Complex I, which provides enough energy for a higher ATP yield. *4 ATPs* - 4 ATPs is an overestimation of the ATP yield from NADH in the electron transport chain. - The maximum theoretical yield from NADH via oxidative phosphorylation is typically considered to be 3 ATPs.

Question 204: Which of the following is an amino sugar formed from fructose-6-phosphate?

A. N-acetylglucosamine-6-phosphate
B. Glucosamine-6-phosphate (Correct Answer)
C. Galactosamine-6-phosphate
D. UDP-N-acetylglucosamine

Explanation: ***Glucosamine-6-phosphate*** - This amino sugar is directly synthesized from **fructose-6-phosphate** via a transamidation reaction, where an amino group replaces a hydroxyl group. - It is a key intermediate in the biosynthesis of other **amino sugars** and **glycosaminoglycans**. *N-acetylglucosamine-6-phosphate* - This is formed from **glucosamine-6-phosphate** by the addition of an **acetyl group**, making it a subsequent product, not the initial amino sugar from fructose-6-phosphate. - The N-acetylation step is crucial for its role in cellular signaling and structural components. *Galactosamine-6-phosphate* - While an amino sugar, **galactosamine-6-phosphate** is derived from UDP-N-acetylglucosamine, not directly from fructose-6-phosphate. - Its formation involves an **epimerization** step of an existing N-acetylglucosamine structure. *UDP-N-acetylglucosamine* - This is an **activated form** of N-acetylglucosamine, formed by the addition of UTP to N-acetylglucosamine-1-phosphate. - It serves as a precursor for the synthesis of complex **carbohydrates** and glycoproteins, far downstream from fructose-6-phosphate.

Question 205: The primary site of lipogenesis is:

A. Liver (Correct Answer)
B. Skeletal muscles
C. Myocardium
D. Lungs

Explanation: ***Liver*** - The **liver** is the principal organ for **de novo lipogenesis**, converting excess carbohydrates into fatty acids and triglycerides. - This process is highly active in response to a high-carbohydrate diet, with the synthesized lipids packaged into **VLDL** for transport. *Skeletal muscles* - **Skeletal muscles** primarily utilize fatty acids for **energy production** rather than synthesizing large amounts of new lipids. - While they can store some triglycerides, their capacity for de novo lipogenesis is significantly lower compared to the liver. *Myocardium* - The **myocardium** (heart muscle) primarily relies on fatty acids for its continuous **energy demands** and has limited capacity for de novo lipogenesis. - Its metabolic focus is on efficient **ATP generation** to maintain cardiac function. *Lungs* - The **lungs** are not a primary site for general lipogenesis, though they are involved in the synthesis of specific lipids like **surfactant**. - Surfactant synthesis is a specialized process crucial for lung function, distinct from general energy storage lipogenesis.

Question 206: Urea & Kreb's cycle are linked at?

A. Arginine
B. Ornithine
C. Oxaloacetate
D. Fumarate (Correct Answer)

Explanation: ***Fumarate*** - **Fumarate** is a key intermediate produced during the **urea cycle** when argininosuccinate is cleaved into arginine and fumarate. - This fumarate then enters the **Krebs cycle** (citric acid cycle) as an intermediate to be converted into malate and then oxaloacetate, thus linking the two cycles. *Arginine* - **Arginine** is an amino acid that participates in the urea cycle, serving as a precursor for the formation of urea. - While arginine is a part of the urea cycle, it does not directly enter the Krebs cycle or serve as its linking metabolite. *Ornithine* - **Ornithine** is another amino acid central to the urea cycle, being regenerated at the end of the cycle to combine with carbamoyl phosphate. - It is a carrier molecule for the nitrogen atoms, but it does not directly link to the Krebs cycle. *Oxaloacetate* - **Oxaloacetate** is a central intermediate in the Krebs cycle, and it can be a precursor for intermediates in the urea cycle (e.g., through aspartate). - However, it is not the direct molecule that links the two cycles in the direction of the urea cycle feeding into the Krebs cycle.

Question 207: Organ that can utilize glucose, fatty acids and ketone bodies is:

A. Liver
B. Brain
C. Skeletal muscle (Correct Answer)
D. RBC

Explanation: ***Skeletal muscle*** - Skeletal muscle is highly adaptable and can utilize **glucose**, **fatty acids (FAs)**, and **ketone bodies** as fuel sources, especially during prolonged exercise or starvation. - Its metabolic flexibility allows it to switch between these substrates depending on their availability and the body's energy demands. *Liver* - The liver is central to metabolism but primarily **produces ketone bodies** from fatty acids rather than utilizing them as a major fuel source for its own energy needs. - While it uses glucose and FAs, its role in ketone body metabolism is largely synthetic. *Brain* - The brain preferentially uses **glucose** as its primary fuel. - During prolonged starvation, it can adapt to utilize **ketone bodies** as an alternative fuel source, but it does not significantly use fatty acids directly. *RBC* - Red blood cells (RBCs) lack mitochondria and therefore rely exclusively on **anaerobic glycolysis** for energy, metabolizing only **glucose**. - They cannot utilize fatty acids or ketone bodies.

Question 208: Which of the following micronutrient deficiencies can lead to anemia?

A. Molybdenum
B. Copper (Correct Answer)
C. Fluorine
D. Selenium

Explanation: ***Copper*** - **Copper** is essential for **iron metabolism** and red blood cell formation; its deficiency can lead to **sideroblastic anemia** (often with microcytic or normocytic features) that may be accompanied by neutropenia. - Copper is required for **ceruloplasmin** function, which is necessary for iron mobilization from stores and incorporation into hemoglobin. - It also plays a role in the function of **superoxide dismutase** and **cytochrome c oxidase**, enzymes involved in antioxidant defense and energy production. *Molybdenum* - **Molybdenum** is a cofactor for several enzymes, including **xanthine oxidase** and **sulfite oxidase**, crucial for purine metabolism and detoxification. - While essential, its deficiency does not typically lead to **anemia** in humans. *Selenium* - **Selenium** is a component of selenoproteins, such as **glutathione peroxidase**, which protect cells from oxidative damage. - Deficiency is associated with conditions like **Keshan disease** (cardiomyopathy) but not primary anemia. *Fluorine* - **Fluorine** (as fluoride) is primarily known for its role in **bone and tooth mineralization**, protecting against dental caries. - It does not directly participate in **hematopoiesis** or iron metabolism, and its deficiency is not linked to anemia.

Question 209: Which of the following usually require a RNA intermediate for cloning/replication?

A. Cosmids
B. Retroviruses (Correct Answer)
C. Plasmids
D. Transposons

Explanation: ***Retroviruses*** - **All retroviruses require an RNA intermediate** for their replication cycle, making this the correct answer. - Retroviruses have an **RNA genome** that must be **reverse transcribed into DNA** by reverse transcriptase enzyme before integration into the host genome. - The integrated DNA (provirus) is then transcribed back to RNA, which serves both as mRNA for viral proteins and as genomic RNA for new virions. - Examples include **HIV, HTLV**, and other retroviruses that definitively use this RNA → DNA → RNA replication strategy. *Transposons* - This option is **too broad** to be correct. Only **retrotransposons** (Class I transposons) use RNA intermediates via a "copy-and-paste" mechanism involving reverse transcription. - However, **DNA transposons** (Class II) move by a "cut-and-paste" DNA mechanism **without any RNA intermediate**. - Since the question asks what "usually requires" RNA intermediate, and many common transposons (like bacterial Tn5, Tn10) are DNA transposons, this answer is imprecise. *Cosmids* - Cosmids are **hybrid cloning vectors** containing cos sites from bacteriophage lambda combined with plasmid sequences. - They replicate as **DNA plasmids** in bacteria using DNA-dependent DNA polymerase. - No RNA intermediate is involved in their replication mechanism. *Plasmids* - Plasmids are **extrachromosomal circular DNA molecules** that replicate independently within bacterial or yeast cells. - Replication occurs via **DNA-to-DNA synthesis** using DNA polymerase. - No RNA intermediate is required for plasmid propagation.

Physiology

1 questions

Q201

Which of the following statements about gastric secretion is true?