The main function of Vitamin C in the body is
Which of the following trace elements has vitamin E-like action?
Which vitamin is primarily associated with the antioxidant properties of glutathione?
Which of the following statements about vitamin E is false?
Which mineral is known for its antioxidant properties similar to those of vitamin E?
What is the blood form of folic acid?
Which vitamin is primarily involved in redox reactions?
Which is not a dietary fiber ?
What is the immediate source of energy for cellular processes?
Which metabolic pathway provides instant energy to muscles?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 81: The main function of Vitamin C in the body is
- A. Cofactor for hydroxylation reactions in collagen synthesis (Correct Answer)
- B. Regulation of lipid synthesis
- C. Involvement as antioxidant
- D. Inhibition of cell growth
Explanation: ***Cofactor for hydroxylation reactions in collagen synthesis*** - Vitamin C (ascorbic acid) serves as an essential **cofactor** for **prolyl hydroxylase** and **lysyl hydroxylase** enzymes. - These enzymes catalyze the **hydroxylation of proline and lysine** residues in collagen, forming **hydroxyproline** and **hydroxylysine**. - This hydroxylation is crucial for the **stability and cross-linking** of collagen triple helix structure. - Deficiency leads to **scurvy**, characterized by defective collagen synthesis, bleeding gums, poor wound healing, and bone abnormalities. - This is the **primary and main function** of Vitamin C in the human body. *Involvement as antioxidant* - While Vitamin C does act as a **water-soluble antioxidant**, protecting cells from oxidative damage by free radicals, this is a **secondary function**. - It can donate electrons to neutralize reactive oxygen species and regenerate other antioxidants like Vitamin E. - This protective role is important but not the main function compared to its role in collagen synthesis. *Regulation of lipid synthesis* - Vitamin C is **not directly involved** in the primary pathways of lipid synthesis or metabolism. - It may play a minor role in **carnitine synthesis** (needed for fatty acid oxidation), but this is not a major function. - Other nutrients like B vitamins play more significant roles in lipid metabolism regulation. *Inhibition of cell growth* - Vitamin C does **not inhibit normal cell growth**; it is essential for cell health, differentiation, and tissue repair. - While high doses may have some anti-proliferative effects in certain cancer cell lines in vitro, this is not a physiological function in the healthy body.
Question 82: Which of the following trace elements has vitamin E-like action?
- A. Iron
- B. Selenium (Correct Answer)
- C. Copper
- D. Zinc
Explanation: ***Selenium*** - Selenium is an essential component of **glutathione peroxidase**, an enzyme that works alongside vitamin E to protect cells from **oxidative damage**. - Its antioxidant properties are similar to **vitamin E**, as both scavenge free radicals and prevent lipid peroxidation. *Iron* - Iron is vital for **oxygen transport** in hemoglobin and myoglobin, and for cellular respiration as a component of cytochromes. - While essential, iron does not have direct **antioxidant properties** akin to vitamin E; in excess, it can even promote oxidative stress. *Copper* - Copper is a cofactor for several enzymes, including **superoxide dismutase (SOD)**, an antioxidant enzyme, but its primary role is not directly analogous to vitamin E's lipid-soluble antioxidant function. - It also plays a role in **energy production**, iron metabolism, and neurotransmission. *Zinc* - Zinc is crucial for **immune function**, wound healing, and DNA synthesis, acting as a cofactor for over 300 enzymes. - Although it has indirect antioxidant effects by stabilizing cell membranes and reducing oxidative damage, its mechanism and direct action are not considered "vitamin E-like."
Question 83: Which vitamin is primarily associated with the antioxidant properties of glutathione?
- A. Vitamin E
- B. Niacin (Correct Answer)
- C. Vitamin C
- D. Vitamin A
Explanation: ***Niacin*** - **Niacin** (Vitamin B3) is the vitamin most directly associated with glutathione's antioxidant properties - Niacin is a precursor to **NAD+** and **NADP+**, which are converted to **NADPH** - **NADPH is the essential cofactor** for **glutathione reductase**, the primary enzyme that reduces oxidized glutathione (GSSG) back to its active reduced form (GSH) - This NADPH-dependent enzymatic pathway is the **main mechanism** for maintaining the body's glutathione antioxidant system - Without adequate niacin → NADPH, glutathione cannot be efficiently regenerated *Vitamin C* - **Vitamin C** can non-enzymatically reduce GSSG to GSH, providing a **secondary backup mechanism** - While vitamin C does support glutathione regeneration, this is an **indirect, non-enzymatic process** - It acts as an antioxidant itself but is not the primary vitamin associated with glutathione's antioxidant function *Vitamin E* - **Vitamin E** is a **lipid-soluble antioxidant** that primarily protects cell membranes from oxidative damage - Works synergistically with other antioxidants but has **no direct role** in glutathione synthesis or regeneration *Vitamin A* - **Vitamin A** (retinol) is crucial for vision, immune function, and cell differentiation - Has some antioxidant properties as a carotenoid derivative but **no direct involvement** in glutathione metabolism
Question 84: Which of the following statements about vitamin E is false?
- A. Act as antioxidant
- B. Chemically tocopherol
- C. Water soluble vitamin (Correct Answer)
- D. Prevent lipid peroxidation of cell membrane
Explanation: ***Water soluble vitamin*** - This statement is false because **vitamin E** is a **fat-soluble vitamin**, meaning it dissolves in lipids and is stored in the body's fatty tissues and liver. - Its absorption requires the presence of dietary fat and bile salts, unlike water-soluble vitamins. *Act as antioxidant* - **Vitamin E** is a powerful **lipid-soluble antioxidant**, protecting cell membranes from damage by scavenging **free radicals**. - It helps prevent **oxidative damage** to polyunsaturated fatty acids within cell membranes. *Chemically tocopherol* - The most biologically active form of **vitamin E** is **alpha-tocopherol**, although vitamin E encompasses a group of eight fat-soluble compounds, including tocopherols and tocotrienols. - This term correctly identifies the chemical nature of the prominent forms of vitamin E. *Prevent lipid peroxidation of cell membrane* - As an **antioxidant**, **vitamin E** specifically interferes with the **propagation of lipid peroxidation**, a chain reaction that damages cell membranes by oxidizing their lipid components. - It donates an electron to **free radicals**, thereby neutralizing them and protecting the integrity of the cell membrane.
Question 85: Which mineral is known for its antioxidant properties similar to those of vitamin E?
- A. Calcium
- B. Iron
- C. Selenium (Correct Answer)
- D. Magnesium
Explanation: ***Selenium*** - Selenium is a crucial component of **glutathione peroxidase**, an enzyme with significant **antioxidant properties** that protects cells from oxidative damage. - Its antioxidant function is often compared to that of **vitamin E**, as both work to neutralize **free radicals** in the body. *Calcium* - Calcium is primarily known for its role in **bone health**, muscle contraction, and **nerve transmission**, not for its antioxidant properties. - It does not directly participate in neutralizing free radicals like selenium or vitamin E. *Iron* - Iron is essential for **oxygen transport** in red blood cells and various metabolic processes. - While crucial, excessive free iron can actually promote the formation of **free radicals** (Fenton reaction), making it pro-oxidant rather than antioxidant. *Magnesium* - Magnesium is involved in over 300 enzymatic reactions, including **energy metabolism** and muscle and nerve function. - It does not share direct antioxidant properties similar to vitamin E or selenium.
Question 86: What is the blood form of folic acid?
- A. Folinic acid
- B. Pteroglutamate
- C. Methyltetrahydrofolate (Correct Answer)
- D. None of the options
Explanation: ***Methyltetrahydrofolate*** - **5-methyltetrahydrofolate (5-MTHF)** is the **primary circulating form** of folate in the blood plasma and the most metabolically active form of folate. - It plays a crucial role in various metabolic pathways, especially in **one-carbon metabolism** for DNA synthesis and repair. *Folinic acid* - **Folinic acid** (leucovorin) is a **reduced form of folic acid** that does not require reduction by dihydrofolate reductase for activity. - It is often used as a therapeutic agent, particularly to **counteract the effects of methotrexate** toxicity, but it is not the main physiological circulating form. *Pteroglutamate* - **Pteroglutamate** is a generic term referring to compounds structurally related to folic acid, which is itself chemically known as pteroylglutamic acid. - While it describes the **general structure**, it is not the specific blood form of folic acid. *None of the options* - This option is incorrect because **methyltetrahydrofolate** is indeed the correct answer.
Question 87: Which vitamin is primarily involved in redox reactions?
- A. Pyridoxine
- B. Biotin
- C. Folic acid
- D. Riboflavin (Correct Answer)
Explanation: ***Riboflavin*** - **Riboflavin** (Vitamin B2) is a precursor to **flavin adenine dinucleotide (FAD)** and **flavin mononucleotide (FMN)**, which are crucial coenzymes in many **redox reactions**. - These coenzymes act as electron carriers in metabolic pathways, including the **electron transport chain**, where they accept and donate electrons. *Pyridoxine* - **Pyridoxine** (Vitamin B6) is primarily involved in **amino acid metabolism**, including transamination, decarboxylation, and racemization. - It functions as **pyridoxal phosphate (PLP)**, a coenzyme for many enzymes in these pathways, but not directly in redox reactions. *Biotin* - **Biotin** (Vitamin B7) is a coenzyme for **carboxylase enzymes**, which are involved in carboxylation reactions (addition of a carboxyl group). - Its main roles are in **fatty acid synthesis**, gluconeogenesis, and leucine metabolism, not direct redox reactions. *Folic acid* - **Folic acid** (Vitamin B9) is essential for **one-carbon metabolism**, involved in transferring one-carbon units in the synthesis of nucleotides (DNA/RNA) and amino acids. - It functions as **tetrahydrofolate (THF)**, playing a critical role in cell division and growth, but not as a direct redox agent.
Question 88: Which is not a dietary fiber ?
- A. Lignin
- B. Pectin
- C. Cellulose
- D. Lactulose (Correct Answer)
Explanation: ***Lactulose*** - **Lactulose is NOT a dietary fiber** - it is a synthetic disaccharide used pharmaceutically as an osmotic laxative and for treating hepatic encephalopathy. - Unlike true dietary fibers, lactulose is a manufactured drug, not a naturally occurring food component. - While it is fermented by colonic bacteria (similar to fiber), it does not meet the definition of dietary fiber. *Lignin* - Lignin is a complex aromatic polymer that provides structural support to plant cell walls. - It is classified as a non-polysaccharide dietary fiber that is largely indigestible by human enzymes. - Contributes to fecal bulk and is considered an insoluble fiber. *Pectin* - Pectin is a soluble dietary fiber found naturally in fruits, particularly in apple peels and citrus fruits. - Forms a gel when mixed with water, slowing gastric emptying and aiding digestion. - Beneficial for gut health and blood glucose regulation. *Cellulose* - Cellulose is the most abundant dietary fiber and a major structural component of plant cell walls. - An insoluble fiber composed of β-1,4-linked glucose polymers that cannot be digested by human enzymes. - Contributes to stool bulk and promotes regular bowel movements.
Question 89: What is the immediate source of energy for cellular processes?
- A. Cori's cycle
- B. HMP
- C. ATP (Correct Answer)
- D. TCA cycle
Explanation: ***ATP*** - **Adenosine triphosphate (ATP)** is the direct and immediate source of energy for almost all cellular processes, including **muscle contraction**, **active transport**, and **biosynthesis**. - Its high-energy phosphate bonds release energy upon hydrolysis, driving various cellular functions. *Cori's cycle* - The **Cori cycle** involves the interconversion of **lactate** and **glucose** between the muscle and the liver to regenerate glucose stores. - It is an important metabolic pathway for glucose homeostasis during anaerobic conditions, but it does not directly provide immediate energy for cellular processes. *HMP* - The **Hexose Monophosphate Pathway (HMP)**, also known as the **pentose phosphate pathway**, primarily produces **NADPH** and **ribose-5-phosphate**. - While it generates NADPH for reductive biosynthesis and protects against oxidative stress, it is not an immediate source of energy. *TCA cycle* - The **Tricarboxylic Acid (TCA) cycle**, or Krebs cycle, is a central metabolic pathway that oxidizes **acetyl-CoA** to produce **ATP**, **NADH**, and **FADH2**. - While it is a major producer of ATP, it is not the *immediate* source; instead, it generates the precursors that fuel oxidative phosphorylation to produce ATP.
Question 90: Which metabolic pathway provides instant energy to muscles?
- A. Embden-Meyerhof pathway (Correct Answer)
- B. HMP shunt
- C. Cori cycle
- D. TCA cycle
Explanation: ***Embden-Meyerhof pathway*** - This pathway, also known as **glycolysis**, rapidly breaks down glucose into pyruvate to produce **ATP without oxygen**, providing instant energy to muscles during high-intensity activity. - Generates a net of **two ATP molecules** per glucose molecule, which is crucial for quick bursts of energy. *HMP shunt* - The **hexose monophosphate shunt** primarily produces **NADPH** for reductive biosynthesis and **ribose-5-phosphate** for nucleotide synthesis, not immediate large-scale ATP for muscle contraction. - Plays a role in protecting cells from **oxidative stress** and synthesizing precursors for DNA and RNA. *Cori cycle* - The **Cori cycle** involves the recycling of **lactate** produced in muscles back to glucose in the liver, which is a slower process for maintaining glucose homeostasis rather than providing instant muscle energy. - It helps prevent **lactic acidosis** during strenuous activity but is not a primary pathway for rapid ATP generation. *TCA cycle* - The **TCA cycle (Krebs cycle)** is part of **aerobic respiration** and produces a significant amount of ATP, but it is a slower, more sustained energy production pathway that requires oxygen. - Primarily active during **lower-intensity**, longer-duration activities when oxygen supply is adequate.