NEET-PG 2020 — Biochemistry

Q: Addition of which Amino Acid will increase UV absorption

Tryptophan. ***Tryptophan*** - **Tryptophan** contains an **indole ring** with a conjugated pi system responsible for strong **UV light absorption** at approximately **280 nm**. - Its unique aromatic structure allows it to absorb UV light, making it a key amino acid for protein quantification using **spectrophotometry**. *Leucine* - **Leucine** is an **aliphatic amino acid** with a non-polar side chain and lacks chromophores. - It does not significantly absorb UV light in the typical range used for protein analysis. *Proline* - **Proline** is an **imino acid** with a unique cyclic structure, but it lacks aromatic rings or conjugated double bonds. - It does not absorb UV light significantly at wavelengths above 230 nm. *Arginine* - **Arginine** is a **basic amino acid** with a guanidinium group, but this functional group does not contribute to UV absorption in the 280 nm range. - Its presence does not enhance the UV absorbance of proteins. [Practice more Biochemistry PYQs on OnCourse]

Q: Why is blood stored in citrate-phosphate-dextrose considered more beneficial for hypoxic patients compared to blood stored in acidic-citrate-dextrose?

The fall in 2,3-DPG is less.. ***The fall in 2,3-DPG is less.*** * **Citrate-phosphate-dextrose (CPD)** better preserves levels of **2,3-bisphosphoglycerate (2,3-DPG)** in stored red blood cells. * Higher 2,3-DPG levels are crucial for **oxygen unloading** from hemoglobin in tissues, which is particularly beneficial for hypoxic patients who need efficient oxygen delivery. *It has a higher pH level than acidic-citrate-dextrose.* * While CPD does maintain a **less acidic pH** than acid-citrate-dextrose (ACD), which is generally favorable for red blood cell viability, the most direct benefit for hypoxic patients relates to 2,3-DPG. * The slightly higher pH indirectly contributes to better 2,3-DPG preservation but isn't the primary reason for improved oxygen delivery. *It is more effective in oxygen delivery.* * While the *consequence* of using CPD is **more effective oxygen delivery** due to better 2,3-DPG preservation, this option describes the outcome rather than the underlying mechanism compared to the more specific answer regarding 2,3-DPG. * The increased efficacy in oxygen delivery is directly attributable to the preserved 2,3-DPG levels. *It has a longer shelf life than acidic-citrate-dextrose.* * The storage solutions primarily impact red blood cell viability and function, but the **shelf life** (typically 21-35 days depending on the anticoagulant/preservative) is generally determined by other factors, including the additive solutions used with the anticoagulant. * While CPD improves red blood cell quality, the primary advantage for hypoxic patients specifically lies in oxygen affinity rather than overall storage duration. [Practice more Biochemistry PYQs on OnCourse]

Q: Which of the following is the richest source of vitamin B12?

Meat. ***Meat*** - **Meat** and other animal products are the primary natural dietary sources of **vitamin B12** (cobalamin). - This is because vitamin B12 is synthesized by bacteria and accumulates in animal tissues. *Green leafy vegetables* - **Green leafy vegetables** are excellent sources of many vitamins and minerals, such as **folate** and **vitamin K**, but they do not contain vitamin B12. - Vitamin B12 is essentially absent in plant-based foods unless they are fortified or contaminated. *Corn oil* - **Corn oil** is a vegetable oil primarily composed of fats and is a source of **vitamin E** and fatty acids, but it does not contain vitamin B12. - Oils derived from plants lack intrinsic vitamin B12 content. *Sunflower oil* - Similar to corn oil, **sunflower oil** is a plant-based oil rich in **vitamin E** and unsaturated fatty acids, but it contains **no vitamin B12**. - Plant oils are not a source of vitamin B12, which is almost exclusively found in animal products. [Practice more Biochemistry PYQs on OnCourse]

Q: Proteoglycan present in the glomerular basement membrane is?

Heparan sulfate. ***Heparan sulphate*** - Heparan sulphate is a key component of the **glomerular basement membrane** (GBM), crucial for its **negative charge and filtration function** [1][2]. - It plays a significant role in **filtration barrier** properties and affects the permeability of the GBM to proteins [1]. *Keratan sulphate 1* - Predominantly found in **cartilage** and **corneal tissue**, not associated with the glomerular basement membrane. - It contributes to **mechanical support** but lacks the essential role in renal filtration. *Keratan sulphate 2* - Similar to Keratan sulphate 1, this variant is involved in **cartilage** but not in the structure of the glomerular basement membrane. - Has distinct functions related to **tissue hydration** rather than the filtration dynamics of the GBM. *Chondroitin sulphate* - Commonly located in **cartilage** and connective tissues, it does not play a significant role in the structure of the glomerular basement membrane. - While it assists in **cell signaling** and regeneration, it does not influence the GBM permeability like heparan sulphate. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Kidney, pp. 905-907. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 34-35. [Practice more Biochemistry PYQs on OnCourse]

24 Previous Year Questions with Answers & Explanations

Questions

Addition of which Amino Acid will increase UV absorption

Why is blood stored in citrate-phosphate-dextrose considered more beneficial for hypoxic patients compared to blood stored in acidic-citrate-dextrose?

Which of the following is the richest source of vitamin B12?

Which of the following statements about protein structures is most accurate?

Proteoglycan present in the glomerular basement membrane is?

Which pigment is responsible for the greenish-black color of neonatal stool?

Ochronosis is due to the accumulation of?

Which absorbs least water?

Enzyme activated by decrease in Insulin: glucagon ratio:

Q10

Which of the following is the rich source of Vitamin B12?

NEET-PG 2020 - Biochemistry NEET-PG Practice Questions and MCQs

Question 1: Addition of which Amino Acid will increase UV absorption

A. Tryptophan (Correct Answer)
B. Leucine
C. Proline
D. Arginine

Explanation: ***Tryptophan*** - **Tryptophan** contains an **indole ring** with a conjugated pi system responsible for strong **UV light absorption** at approximately **280 nm**. - Its unique aromatic structure allows it to absorb UV light, making it a key amino acid for protein quantification using **spectrophotometry**. *Leucine* - **Leucine** is an **aliphatic amino acid** with a non-polar side chain and lacks chromophores. - It does not significantly absorb UV light in the typical range used for protein analysis. *Proline* - **Proline** is an **imino acid** with a unique cyclic structure, but it lacks aromatic rings or conjugated double bonds. - It does not absorb UV light significantly at wavelengths above 230 nm. *Arginine* - **Arginine** is a **basic amino acid** with a guanidinium group, but this functional group does not contribute to UV absorption in the 280 nm range. - Its presence does not enhance the UV absorbance of proteins.

Question 2: Why is blood stored in citrate-phosphate-dextrose considered more beneficial for hypoxic patients compared to blood stored in acidic-citrate-dextrose?

A. The fall in 2,3-DPG is less. (Correct Answer)
B. It has a higher pH level than acidic-citrate-dextrose.
C. It is more effective in oxygen delivery.
D. It has a longer shelf life than acidic-citrate-dextrose.

Explanation: ***The fall in 2,3-DPG is less.*** * **Citrate-phosphate-dextrose (CPD)** better preserves levels of **2,3-bisphosphoglycerate (2,3-DPG)** in stored red blood cells. * Higher 2,3-DPG levels are crucial for **oxygen unloading** from hemoglobin in tissues, which is particularly beneficial for hypoxic patients who need efficient oxygen delivery. *It has a higher pH level than acidic-citrate-dextrose.* * While CPD does maintain a **less acidic pH** than acid-citrate-dextrose (ACD), which is generally favorable for red blood cell viability, the most direct benefit for hypoxic patients relates to 2,3-DPG. * The slightly higher pH indirectly contributes to better 2,3-DPG preservation but isn't the primary reason for improved oxygen delivery. *It is more effective in oxygen delivery.* * While the *consequence* of using CPD is **more effective oxygen delivery** due to better 2,3-DPG preservation, this option describes the outcome rather than the underlying mechanism compared to the more specific answer regarding 2,3-DPG. * The increased efficacy in oxygen delivery is directly attributable to the preserved 2,3-DPG levels. *It has a longer shelf life than acidic-citrate-dextrose.* * The storage solutions primarily impact red blood cell viability and function, but the **shelf life** (typically 21-35 days depending on the anticoagulant/preservative) is generally determined by other factors, including the additive solutions used with the anticoagulant. * While CPD improves red blood cell quality, the primary advantage for hypoxic patients specifically lies in oxygen affinity rather than overall storage duration.

Question 3: Which of the following is the richest source of vitamin B12?

A. Meat (Correct Answer)
B. Green leafy vegetables
C. Corn oil
D. Sunflower oil

Explanation: ***Meat*** - **Meat** and other animal products are the primary natural dietary sources of **vitamin B12** (cobalamin). - This is because vitamin B12 is synthesized by bacteria and accumulates in animal tissues. *Green leafy vegetables* - **Green leafy vegetables** are excellent sources of many vitamins and minerals, such as **folate** and **vitamin K**, but they do not contain vitamin B12. - Vitamin B12 is essentially absent in plant-based foods unless they are fortified or contaminated. *Corn oil* - **Corn oil** is a vegetable oil primarily composed of fats and is a source of **vitamin E** and fatty acids, but it does not contain vitamin B12. - Oils derived from plants lack intrinsic vitamin B12 content. *Sunflower oil* - Similar to corn oil, **sunflower oil** is a plant-based oil rich in **vitamin E** and unsaturated fatty acids, but it contains **no vitamin B12**. - Plant oils are not a source of vitamin B12, which is almost exclusively found in animal products.

Question 4: Which of the following statements about protein structures is most accurate?

A. Secondary structure is stabilized by hydrogen bonds.
B. Denaturation primarily affects secondary and tertiary structures, leaving the primary structure intact.
C. The sequence of amino acids determines the secondary and tertiary structures of proteins. (Correct Answer)
D. The three-dimensional structure of a protein is referred to as its tertiary structure.

Explanation: ***The sequence of amino acids determines the secondary and tertiary structures of proteins.*** - This represents **Anfinsen's principle**, the most fundamental concept in protein folding: the **primary structure (amino acid sequence) contains all the information necessary** to determine the final three-dimensional structure of a protein. - This was demonstrated by **Nobel Prize-winning experiments** showing that denatured proteins can spontaneously refold into their native structure based solely on their amino acid sequence. - This is the **foundational principle** from which all other structural concepts derive - the sequence dictates everything else about protein structure. *Secondary structure is stabilized by hydrogen bonds.* - While this statement is **factually correct**, it describes a *mechanism* of structural stabilization rather than the fundamental principle of protein structure determination. - Hydrogen bonds are **one type of interaction** that stabilizes already-formed secondary structures, but the formation pattern itself is determined by the amino acid sequence. *Denaturation primarily affects secondary and tertiary structures, leaving the primary structure intact.* - This statement is also **factually correct** and describes what happens during denaturation (loss of 3D structure while peptide bonds remain intact). - However, it describes a *consequence* or phenomenon rather than the fundamental organizing principle of protein structure. *The three-dimensional structure of a protein is referred to as its tertiary structure.* - This is a **correct definition** but merely terminology rather than a principle. - It defines what tertiary structure means but doesn't explain the underlying mechanism of how protein structures are determined.

Question 5: Proteoglycan present in the glomerular basement membrane is?

A. Keratan sulfate 1
B. Keratan sulfate 2
C. Heparan sulfate (Correct Answer)
D. Chondroitin sulfate

Explanation: ***Heparan sulphate*** - Heparan sulphate is a key component of the **glomerular basement membrane** (GBM), crucial for its **negative charge and filtration function** [1][2]. - It plays a significant role in **filtration barrier** properties and affects the permeability of the GBM to proteins [1]. *Keratan sulphate 1* - Predominantly found in **cartilage** and **corneal tissue**, not associated with the glomerular basement membrane. - It contributes to **mechanical support** but lacks the essential role in renal filtration. *Keratan sulphate 2* - Similar to Keratan sulphate 1, this variant is involved in **cartilage** but not in the structure of the glomerular basement membrane. - Has distinct functions related to **tissue hydration** rather than the filtration dynamics of the GBM. *Chondroitin sulphate* - Commonly located in **cartilage** and connective tissues, it does not play a significant role in the structure of the glomerular basement membrane. - While it assists in **cell signaling** and regeneration, it does not influence the GBM permeability like heparan sulphate. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Kidney, pp. 905-907. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 34-35.

Question 6: Which pigment is responsible for the greenish-black color of neonatal stool?

A. Biliverdin (Correct Answer)
B. Urochrome
C. Stercobilin
D. Bilirubin (yellow pigment)

Explanation: ***Biliverdin*** - **Biliverdin** is a green pigment formed from the breakdown of heme before it is converted to bilirubin, and it is responsible for the greenish-black color of **meconium**. - The presence of this pigment in the stool indicates the passage of **meconium**, the first stool of a newborn. *Urochrome* - **Urochrome** is responsible for the yellow color of **urine**, not stool. - It is a pigment derived from **bilirubin** that is excreted by the kidneys. *Stercobilin* - **Stercobilin** is responsible for the characteristic **brown color of adult feces**. - It is formed when **bilirubin** is metabolized by bacteria in the intestine. *Bilirubin (yellow pigment)* - **Bilirubin** is typically a **yellow-orange pigment**, not greenish-black. - While bilirubin is the precursor to stercobilin, its yellow form is more associated with **jaundice** when present in high concentrations.

Question 7: Ochronosis is due to the accumulation of?

A. Homogentisic acid (Correct Answer)
B. Alkapton
C. Xanthurenate
D. Glyoxylate

Explanation: ***Homogentisic acid (Correct)*** - **Ochronosis** is a rare genetic disorder characterized by the accumulation of **homogentisic acid** in connective tissues. - This accumulation results from a deficiency of the enzyme **homogentisate 1,2-dioxygenase**, which is crucial in the catabolism of tyrosine and phenylalanine. - Clinically presents with dark pigmentation of cartilage, sclera, and other connective tissues. *Alkapton (Incorrect)* - While **alkaptonuria** is the disease caused by homogentisic acid accumulation, **alkapton** itself is not the substance that accumulates in tissues in ochronosis. - **Alkapton** refers to the dark-colored urine observed in patients with alkaptonuria, which is due to the oxidation of homogentisic acid in the urine. *Xanthurenate (Incorrect)* - **Xanthurenate** is an intermediate in the metabolism of tryptophan, and its accumulation is associated with certain vitamin B6 deficiencies. - It is not involved in the pathogenesis of ochronosis or alkaptonuria. *Glyoxylate (Incorrect)* - **Glyoxylate** is a metabolic intermediate involved in various pathways, including carbohydrate and amino acid metabolism. - Accumulation of glyoxylate is associated with **primary hyperoxaluria type 1**, but not with ochronosis.

Question 8: Which absorbs least water?

A. Cellulose (Correct Answer)
B. Mucilage
C. Pectin
D. Gums

Explanation: ***Cellulose*** - **Cellulose** is a **polysaccharide** with strong **intermolecular hydrogen bonding** between its linear chains. - These strong bonds form a highly ordered, crystalline structure that makes it **insoluble in water** and resistant to water absorption. *Mucilage* - **Mucilage** consists of **polysaccharides** that have a high capacity to absorb water, forming a slimy, gelatinous mass. - This property is due to its highly branched structure and abundance of **hydroxyl groups**, which readily form hydrogen bonds with water. *Pectin* - **Pectin** is a complex **polysaccharide** found in plant cell walls, known for its ability to absorb significant amounts of water. - It forms **gels** with water, a property widely utilized in food production. *Gums* - **Gums** are a diverse group of **polysaccharides** that are highly soluble in water and have an excellent capacity for water absorption. - They tend to form **viscous solutions** or gels when mixed with water.

Question 9: Enzyme activated by decrease in Insulin: glucagon ratio:

A. PFK
B. Glucose 6 phosphatase (Correct Answer)
C. Glucokinase
D. Hexokinase

Explanation: ***Glucose 6 phosphatase*** - A decreased **insulin:glucagon ratio** signifies a catabolic state, promoting glucose release into the blood. - **Glucose-6-phosphatase** is the key enzyme in **gluconeogenesis** and **glycogenolysis** in the liver, dephosphorylating **glucose-6-phosphate** to **free glucose**, which can then be exported from the liver. *PFK* - **Phosphofructokinase (PFK)** is a key regulatory enzyme in **glycolysis**, which is inhibited in a state of low insulin:glucagon ratio. - Its activity would decrease, not increase, to reduce glucose utilization. *Glucokinase* - **Glucokinase** phosphorylates glucose to **glucose-6-phosphate** in the liver, trapping it for metabolism; its activity is increased by high insulin levels. - In a low insulin:glucagon ratio, its activity would be reduced to conserve glucose. *Hexokinase* - **Hexokinase** phosphorylates glucose in most peripheral tissues but has a lower Km for glucose than glucokinase, becoming saturated even at low glucose concentrations. - Its activity is not primarily regulated by the insulin:glucagon ratio; it is generally involved in glucose uptake for cellular energy needs.

Question 10: Which of the following is the rich source of Vitamin B12?

A. Mango
B. Carrot
C. Meat (Correct Answer)
D. Spinach

Explanation: ***Meat*** - **Vitamin B12** is primarily found in **animal products** because it is synthesized by bacteria in the digestive tracts of animals. - **Meat**, especially red meat and liver, is an excellent source of this vitamin. *Mango* - Mangoes are a good source of **Vitamin C** and **Vitamin A**, but they contain negligible amounts of Vitamin B12. - As a fruit, mangos are a plant-based food and generally **do not contain B12**. *Carrot* - Carrots are rich in **beta-carotene** (a precursor to Vitamin A) and **fiber**, but they are not a source of Vitamin B12. - Being a vegetable, carrots are a **plant-based food** and lack Vitamin B12. *Spinach* - Spinach is known for its high content of **iron**, **folate**, and **Vitamin K**, but it does not contain Vitamin B12. - Like other plant-based foods, spinach **naturally lacks Vitamin B12**.