Which of the following plant components is not fermented by gastrointestinal microorganisms?
What is the primary reason for the detergent action of bile salts?
Glucagon stimulates
What is the unit for a prolactin level of 20 in blood?
What is the main enzyme involved in glycogen breakdown (glycogenolysis)?
Which organelle produces and destroys H2O2?
Which type of RNA is primarily involved in gene silencing?
Which vitamin is primarily involved in redox reactions?
Which is not a dietary fiber ?
What is the immediate source of energy for cellular processes?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 91: Which of the following plant components is not fermented by gastrointestinal microorganisms?
- A. Lignin (Correct Answer)
- B. Cellulose
- C. Hemicellulose
- D. Pectin
Explanation: ***Lignin*** - **Lignin** is a complex polymer found in plant cell walls that is highly resistant to degradation by digestive enzymes and microbial fermentation in the gastrointestinal tract. - Its complex, cross-linked structure makes it **non-fermentable** by the microorganisms typically present in the human gut. *Cellulose* - **Cellulose** is a major component of plant cell walls and is a type of dietary fiber that can be fermented by certain gut bacteria. - While humans lack the enzymes to digest cellulose, colonic microorganisms possess cellulases that break it down into **short-chain fatty acids (SCFAs)**. *Hemicellulose* - **Hemicellulose** is a diverse group of plant polysaccharides that are a significant source of fermentable fiber for gut microbiota. - It is readily broken down by gastrointestinal bacteria into **SCFAs** and gases, contributing to colonic health. *Pectin* - **Pectin** is a soluble dietary fiber found in fruits and vegetables, known for its gel-forming properties. - It is highly fermentable by gut microorganisms, leading to the production of **SCFAs** like butyrate, propionate, and acetate.
Question 92: What is the primary reason for the detergent action of bile salts?
- A. Hydrophobic properties
- B. Acts as a zwitterion
- C. Amphipathic nature (Correct Answer)
- D. None of the options
Explanation: ***Amphipathic nature*** - Bile salts are **amphipathic molecules**, meaning they have both **hydrophilic (water-loving)** and **hydrophobic (water-fearing)** regions. - This dual nature allows them to emulsify fats by surrounding lipid droplets with their hydrophobic ends dissolving in the fat and their hydrophilic ends facing the aqueous environment, stabilizing the emulsion. *Hydrophobic properties* - While bile salts do possess **hydrophobic regions**, these alone are not sufficient for detergent action. - The ability to interact with both oil and water phases simultaneously is crucial for their role in **emulsification**. *Acts as a zwitterion* - A zwitterion is a molecule with both a **positive and negative charge**, but an overall neutral charge. - This property is not the primary mechanism behind the **detergent action** of bile salts, which relies more on their ability to solubilize fats. *None of the options* - The **amphipathic nature** is indeed the primary reason for the detergent action; therefore, this option is incorrect.
Question 93: Glucagon stimulates
- A. Gluconeogenesis (Correct Answer)
- B. Glycogenesis
- C. Fatty acid synthesis
- D. Glycolysis
Explanation: ***Gluconeogenesis*** - **Glucagon** is a hormone that primarily acts to raise **blood glucose levels** by stimulating the production of glucose from non-carbohydrate sources. - This process, **gluconeogenesis**, occurs mainly in the liver and is initiated by glucagon to counteract hypoglycemia. *Glycogenesis* - **Glycogenesis** is the process of synthesizing **glycogen** from glucose and is primarily stimulated by insulin when blood glucose levels are high. - Glucagon's role is to *inhibit* glycogen synthesis and instead promote glycogen breakdown. *Fatty acid synthesis* - **Fatty acid synthesis** is an anabolic process that primarily occurs when there is an excess of energy and glucose, often stimulated by **insulin**. - Glucagon generally has an **inhibitory effect** on fatty acid synthesis, as its main goal is to mobilize energy stores, not create them. *Glycolysis* - **Glycolysis** is the breakdown of glucose to produce energy, and it is stimulated when glucose is abundant and energy is needed. - Glucagon primarily acts to *inhibit* glycolysis in the liver, thereby conserving glucose for use by other tissues and promoting its release into the bloodstream.
Question 94: What is the unit for a prolactin level of 20 in blood?
- A. ng/ml (Correct Answer)
- B. mg/ml
- C. mg/l
- D. ng/l
Explanation: ***ng/ml*** - Prolactin levels in blood are typically measured in **nanograms per milliliter (ng/mL)**, reflecting the very small concentrations of hormones. - A value of 20 ng/mL falls within the typical reference range for prolactin. *mg/mL* - **Milligrams per milliliter (mg/mL)** is a unit used for much higher concentrations, more common for drugs or larger molecules, not hormones like prolactin. - If prolactin were measured in mg/mL, a value of 20 mg/mL would be an astronomically high and physiologically impossible level. *mg/L* - **Milligrams per liter (mg/L)** is also a unit for higher concentrations than those typically seen for hormones in blood. - 20 mg/L is equivalent to 20 µg/mL or 20,000 ng/mL, which would indicate severe hyperprolactinemia. *ng/L* - **Nanograms per liter (ng/L)** is a unit for extremely low concentrations. - A reading of 20 ng/L would be too low for normal physiological prolactin levels, as 1 ng/mL equals 1000 ng/L.
Question 95: What is the main enzyme involved in glycogen breakdown (glycogenolysis)?
- A. Glycogen phosphorylase (Correct Answer)
- B. Glycogen synthase
- C. Glucose-6-phosphatase
- D. Hexokinase
Explanation: ***Glycogen phosphorylase*** - This is the **rate-limiting and primary enzyme** for **glycogenolysis**, the breakdown of glycogen into glucose units. - It cleaves **α-1,4-glycosidic bonds** in glycogen, releasing **glucose-1-phosphate** units. - Regulated by both **allosteric mechanisms** and **hormonal control** (epinephrine, glucagon). - Works until it reaches 4 glucose residues from a branch point, where debranching enzyme takes over. *Glycogen synthase* - This is the main enzyme for **glycogenesis** (glycogen synthesis), not breakdown. - It catalyzes formation of α-1,4-glycosidic bonds to build glycogen chains. - This is the opposite direction of metabolism from what the question asks about. *Glucose-6-phosphatase* - This enzyme is involved in **gluconeogenesis** and the final step of converting **glucose-6-phosphate to free glucose**. - It is NOT directly involved in glycogen breakdown itself, but rather in the subsequent conversion pathway. - Found primarily in **liver and kidney** to release free glucose into blood. *Hexokinase* - This enzyme phosphorylates free glucose to **glucose-6-phosphate** (opposite direction). - It is involved in **glucose utilization**, not glycogen breakdown. - It traps glucose inside cells for metabolism or glycogen synthesis.
Question 96: Which organelle produces and destroys H2O2?
- A. Peroxisome (Correct Answer)
- B. Lysosome
- C. Golgi body
- D. Ribosome
Explanation: ***Peroxisome*** - **Peroxisomes** are organelles that both produce and break down **hydrogen peroxide (H2O2)** during metabolic processes. - They contain **oxidases** (such as D-amino acid oxidase and urate oxidase) that produce H2O2 as a byproduct during oxidation reactions. - They also contain the enzyme **catalase** that converts H2O2 into water and oxygen, protecting the cell from oxidative damage. - This dual function makes peroxisomes unique in H2O2 metabolism. *Lysosome* - **Lysosomes** are responsible for breaking down waste materials and cellular debris through **hydrolytic enzymes**. - They are primarily involved in **cellular digestion** and waste removal, not H2O2 metabolism. *Golgi body* - The **Golgi apparatus** modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles. - It is crucial for **protein trafficking** and glycosylation, but does not produce or destroy H2O2. *Ribosome* - **Ribosomes** are responsible for **protein synthesis** (translation) based on genetic information carried by mRNA. - They are involved in the assembly of amino acids into proteins, not the metabolism of hydrogen peroxide.
Question 97: Which type of RNA is primarily involved in gene silencing?
- A. rRNA
- B. tRNA
- C. miRNA (Correct Answer)
- D. mRNA
Explanation: ***miRNA*** - **miRNA** (microRNA) is a small non-coding RNA molecule that plays a crucial role in **post-transcriptional regulation of gene expression**. - It functions by binding to complementary messenger RNA (mRNA) molecules, leading to **mRNA degradation** or **inhibition of translation**, thereby silencing genes. - miRNA is the primary RNA type involved in **gene silencing** through the RNA interference (RNAi) pathway. *rRNA* - **rRNA** (ribosomal RNA) is a primary component of **ribosomes**, the cellular machinery responsible for protein synthesis. - Its main function is to **catalyze peptide bond formation** and provide structural integrity to the ribosome, not gene silencing. *tRNA* - **tRNA** (transfer RNA) is responsible for carrying specific **amino acids** to the ribosome during protein synthesis. - It acts as an adapter molecule, translating the **genetic code** in mRNA into an amino acid sequence. *mRNA* - **mRNA** (messenger RNA) carries genetic information from **DNA to ribosomes** for protein synthesis. - While mRNA can be targeted by gene silencing mechanisms (like miRNA), it is not the RNA type that performs the silencing function itself.
Question 98: 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 99: 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 100: 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.