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?
During starvation, muscle uses?
Which of the following organs does not primarily utilize fatty acids for energy?
Metabolic changes seen in starvation include all of the following except?
What is the primary action of metalloproteinases in the extracellular matrix?
In which of the following conditions is protein catabolism MOST increased?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 51: 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 52: 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 53: 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 54: 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 55: 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.
Question 56: During starvation, muscle uses?
- A. Fatty acids (Correct Answer)
- B. Ketone bodies
- C. Glucose
- D. Proteins
Explanation: ***Fatty acids*** - During **early and moderate starvation**, muscle tissue primarily uses **fatty acids** released from adipose tissue as its main energy source. - This preserves **glucose** for essential organs like the brain and red blood cells, which have an obligate need for it. *Ketone bodies* - While muscle can utilize **ketone bodies** during prolonged starvation, they are predominantly a fuel source for the **brain** once fatty acid stores are depleted. - The brain's adaptation to using ketones helps reduce the reliance on gluconeogenesis and preserves muscle protein. *Glucose* - Muscle primarily uses **glucose** as its main energy source in the fed state or during high-intensity exercise. - However, during starvation, muscle significantly reduces its glucose uptake to conserve it for other vital organs. *Proteins* - Muscle protein can be broken down into **amino acids** for gluconeogenesis in the liver to maintain blood glucose levels during prolonged starvation. - However, this is a **catabolic process** and not the primary preferred fuel source for muscle activity itself, as it leads to muscle wasting.
Question 57: Which of the following organs does not primarily utilize fatty acids for energy?
- A. Brain (Correct Answer)
- B. Muscle
- C. Liver
- D. Kidney
Explanation: ***Brain*** - The **brain primarily uses glucose** as its main energy source because fatty acids cannot efficiently cross the **blood-brain barrier**. - During prolonged starvation, the brain can adapt to use **ketone bodies**, which are derived from fatty acid breakdown in the liver. *Muscle* - **Skeletal muscle** can utilize both **glucose and fatty acids** for energy, with fatty acids becoming a more prominent fuel source during prolonged exercise and at rest. - **Cardiac muscle** (heart) heavily relies on **fatty acid oxidation** as its primary energy substrate, especially during basal conditions. *Liver* - The **liver is highly metabolically flexible** and readily oxidizes fatty acids for its own energy needs, particularly during fasting states. - It also plays a key role in **fatty acid metabolism**, including synthesis, breakdown, and conversion into ketone bodies. *Kidney* - The **renal cortex** is rich in mitochondria and has a high metabolic rate, primarily utilizing **fatty acid oxidation** to meet its significant energy demands for filtration and reabsorption. - While the renal medulla can use glucose, the cortex's reliance on fatty acids makes it a significant consumer.
Question 58: Metabolic changes seen in starvation include all of the following except?
- A. Ketogenesis
- B. Protein degradation
- C. Increased gluconeogenesis
- D. Increased glycolysis (Correct Answer)
Explanation: ***Increased glycolysis*** - In starvation, the body's primary goal is to conserve **glucose** for essential organs like the brain, as glucose supply is limited. Therefore, glycolysis, the breakdown of glucose, is *decreased*, not increased. - The body shifts to using alternative fuels such as **fatty acids** and **ketone bodies** to spare glucose. *Increased gluconeogenesis* - **Gluconeogenesis**, the synthesis of glucose from non-carbohydrate precursors like amino acids and glycerol, is *increased* during starvation to maintain blood glucose levels. - This process is crucial for providing glucose to tissues that primarily rely on it, such as the brain and red blood cells. *Ketogenesis* - **Ketogenesis**, the production of ketone bodies from fatty acids, is significantly *increased* during prolonged starvation. - **Ketone bodies** become a major energy source for the brain and other tissues when glucose is scarce, helping to spare muscle protein. *Protein degradation* - **Protein degradation** (proteolysis) is *increased* during starvation, especially in the initial phases, to provide amino acids for gluconeogenesis. - Muscle protein is a primary source of these amino acids, contributing to muscle wasting observed in prolonged starvation.
Question 59: What is the primary action of metalloproteinases in the extracellular matrix?
- A. Modification of collagen structure
- B. Degradation of extracellular matrix components, including collagen (Correct Answer)
- C. Formation of collagen
- D. Activation of collagen synthesis
Explanation: ***Degradation of extracellular matrix components, including collagen*** - **Metalloproteinases (MMPs)** are a family of zinc-dependent endopeptidases that are crucial for breaking down various components of the **extracellular matrix (ECM)**. - This degradation is essential for processes like **tissue remodeling**, development, wound healing, and also plays a role in disease pathogenesis such as metastasis and inflammation. *Formation of collagen* - The formation of collagen is primarily mediated by **fibroblasts** and involves a complex process of synthesis, hydroxylation, glycosylation, and assembly of procollagen molecules, not MMPs. - MMPs act to break down existing collagen, not to create new collagen fibers. *Modification of collagen structure* - While collagen undergoes post-translational modifications (e.g., hydroxylation, glycosylation) within cells, MMPs are involved in cleaving the peptide bonds, leading to **degradation**, rather than structural modification of intact collagen. - Enzymes like **lysyl hydroxylase** and **prolyl hydroxylase** are responsible for modifying collagen structure. *Activation of collagen synthesis* - Collagen synthesis is primarily regulated by various **growth factors (e.g., TGF-β)** and hormones that stimulate fibroblasts to produce collagen. - MMPs are involved in the breakdown of collagen, which is the opposite of activating its synthesis.
Question 60: In which of the following conditions is protein catabolism MOST increased?
- A. Burns (Correct Answer)
- B. Surgery
- C. Starvation
- D. Fever
Explanation: ***Burns*** - Severe burns lead to a profound **hypermetabolic state** with the highest increase in **protein catabolism** among all the options listed. - The extensive tissue damage triggers massive breakdown of muscle protein to provide amino acids for **wound healing**, **acute phase protein synthesis**, and **immune response**. - Burns can increase metabolic rate by **100-200%**, with protein catabolism far exceeding that of other stress conditions. *Starvation* - While starvation initially increases protein catabolism, the body adapts within days by shifting towards **ketone body utilization** to spare protein. - After adaptation, protein breakdown decreases to **20-30 grams per day** to preserve lean body mass. - The goal is survival through metabolic adaptation, not tissue repair. *Surgery* - Major surgery induces a **stress response** that increases protein catabolism, but it is typically less severe and shorter-lived than burns. - The degree of catabolism is proportional to the **magnitude of surgical trauma** and usually resolves within days. - Protein catabolism increases by **50-75%** in major surgery compared to **100-200%** in severe burns. *Fever* - Fever increases basal metabolic rate by approximately **13% per degree Celsius** rise in body temperature. - While metabolism is elevated, protein catabolism is **modest** compared to the massive tissue destruction and repair demands of severe burns. - The increase is primarily in energy expenditure, not protein breakdown.