Biochemistry
5 questionsWhat are digestive enzymes classified as?
Which enzyme is primarily associated with the reduction of NADP+ to NADPH in the pentose phosphate pathway?
Which isoenzyme of lactate dehydrogenase (LDH) is predominantly elevated in liver injury?
What is the specific activity of an enzyme?
Which of the following pairs of compounds has the highest standard reduction potential?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 271: What are digestive enzymes classified as?
- A. Hydrolases (Correct Answer)
- B. Oxidoreductases
- C. Transferases
- D. Ligases
Explanation: ***Hydrolases*** - Digestive enzymes like **amylase**, **lipase**, and **proteases** break down complex food molecules by adding water, a process known as **hydrolysis**. - This class of enzymes catalyzes the cleavage of a chemical bond with the concurrent addition of a water molecule. - All major digestive enzymes belong to this class according to the **EC enzyme classification system**. *Oxidoreductases* - These enzymes catalyze **redox reactions**, involving the transfer of electrons from one molecule to another. - Examples include **dehydrogenases** and **oxidases**, which are not primarily involved in breaking down food molecules in digestion. *Transferases* - Transferases catalyze the transfer of functional groups (such as methyl, acyl, or phosphate groups) from one molecule to another. - Examples include **kinases** and **transaminases**, which are involved in metabolic pathways but not in the digestive breakdown of food. *Ligases* - Ligases are enzymes that catalyze the joining of two large molecules by forming a new chemical bond, typically with the concomitant hydrolysis of ATP. - They are involved in **DNA repair** and **biosynthetic reactions**, not in the breakdown of food during digestion.
Question 272: Which enzyme is primarily associated with the reduction of NADP+ to NADPH in the pentose phosphate pathway?
- A. G6PD (Correct Answer)
- B. APDH
- C. α-keto glutarate dehydrogenases
- D. None of the options
Explanation: ***G6PD*** - **Glucose-6-phosphate dehydrogenase (G6PD)** catalyzes the first committed step in the pentose phosphate pathway, converting **glucose-6-phosphate** to **6-phosphogluconolactone**. - This reaction involves the reduction of **NADP+ to NADPH**, making G6PD the primary enzyme for NADPH production in this pathway. *APDH* - **APDH (adenosine phosphosulfate reductase)** is involved in sulfur metabolism and has no direct role in the pentose phosphate pathway or NADPH production. - This enzyme primarily functions in prokaryotes for the **reduction of APS (adenosine 5'-phosphosulfate)**. *α-keto glutarate dehydrogenases* - **Alpha-ketoglutarate dehydrogenase** is a mitochondrial enzyme part of the **Krebs cycle**, converting **alpha-ketoglutarate to succinyl-CoA**. - This enzyme is crucial for ATP production and generates **NADH**, not NADPH, in its reaction. *None of the options* - This option is incorrect because **G6PD** is indeed the primary enzyme responsible for NADPH generation in the pentose phosphate pathway.
Question 273: Which isoenzyme of lactate dehydrogenase (LDH) is predominantly elevated in liver injury?
- A. LDH-3
- B. LDH-5 (Correct Answer)
- C. LDH-1
- D. LDH-2
Explanation: ***LDH-5 isoenzyme most significant in hepatic conditions*** - **LDH-5** is the predominant isoenzyme found in the **liver** and skeletal muscle. - An elevation of **LDH-5** is highly indicative of **hepatocellular damage** or injury. *LDH-1 isoenzyme associated with cardiac tissue* - **LDH-1** is primarily present in the **heart** and red blood cells. - Its elevation suggests conditions like **myocardial infarction** or hemolytic anemia, not liver injury. *LDH-3 isoenzyme typical in respiratory system* - **LDH-3** is found in the **lungs**, kidneys, and other tissues. - While it can be elevated in **pulmonary embolism** or renal disease, it is not specific for liver injury. *LDH-2 isoenzyme linked to erythrocyte metabolism* - **LDH-2** is abundant in **red blood cells** and also found in the heart and kidneys. - Elevations are often seen in conditions involving **hemolysis** or myocardial damage, similar to LDH-1.
Question 274: 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 275: Which of the following pairs of compounds has the highest standard reduction potential?
- A. NADH/NAD+
- B. Succinate/Fumarate
- C. Ubiquinone/Ubiquinol
- D. Fe³⁺/Fe²⁺ (Correct Answer)
Explanation: ***Fe³⁺/Fe²⁺*** - The **Fe³⁺/Fe²⁺ couple** has a **standard reduction potential (E'0)** of **+0.77 V**, making it the highest among the given options. - A higher positive E'0 indicates a stronger tendency for the oxidized form to accept electrons and be reduced. *NADH/NAD+* - The **NADH/NAD+ couple** has a **standard reduction potential** of **-0.32 V**, indicating it is a strong reducing agent. - Its negative reduction potential means it readily donates electrons during metabolic processes. *Succinate/Fumarate* - The **succinate/fumarate couple** has a **standard reduction potential** of **+0.03 V**. - This pair is involved in the **TCA cycle**, where succinate is oxidized to fumarate, releasing electrons. *Ubiquinone/Ubiquinol* - The **ubiquinone/ubiquinol couple** has a **standard reduction potential** varying around **+0.05 to +0.10 V**, depending on the specific state. - It acts as a mobile electron carrier in the **electron transport chain**, accepting electrons from NADH and FADH2.
Physiology
5 questionsWhat triggers the cephalic phase of gastric secretion?
Which of the following statements about gastric secretion is true?
Ptyalin is secreted by?
What is the body's first physiological response to hypoglycemia?
What is the process by which water moves from the extracellular space to the intracellular space?
NEET-PG 2012 - Physiology NEET-PG Practice Questions and MCQs
Question 271: What triggers the cephalic phase of gastric secretion?
- A. On food entering stomach
- B. On food entering intestine
- C. On seeing food (Correct Answer)
- D. On stress
Explanation: ***On seeing food*** - The **cephalic phase** of gastric secretion is initiated by sensory input such as the sight, smell, taste, or even the thought of food. - This phase is mediated by the **vagus nerve**, stimulating gastric acid and enzyme secretion in anticipation of food arrival. *On food entering stomach* - This describes the initiation of the **gastric phase** of digestion, where mechanical stretch and chemical presence of food in the stomach stimulate further secretions. - The gastric phase primarily involves local reflexes and hormonal mechanisms (like **gastrin** release), rather than purely sensory input from the head. *On food entering intestine* - This marks the beginning of the **intestinal phase** of digestion, which involves both stimulatory and inhibitory signals for gastric secretion. - The primary role of the intestinal phase is to regulate the rate at which chyme enters the small intestine and to coordinate bile and pancreatic enzyme release. *On stress* - While stress can impact digestive function, it typically affects the **autonomic nervous system** in a generalized way, often leading to inhibition of digestion or altered motility. - Stress does not specifically trigger the cephalic phase of gastric secretion, which is a physiological response linked to nutrient anticipation.
Question 272: Which of the following statements about gastric secretion is true?
- A. Inhibited by curare
- B. Stimulated by nor adrenaline
- C. Increased by stomach distention (Correct Answer)
- D. Stimulated by an increase in tonic activity
Explanation: ***Increased by stomach distention*** - **Stomach distention** activates local reflexes and the **vagovagal reflex**, leading to the release of **acetylcholine** and **gastrin**, which stimulate gastric acid secretion during the gastric phase. - This is a physiological response that prepares the stomach for digestion of incoming food. *Inhibited by curare* - **Curare** is a **nicotinic acetylcholine receptor antagonist** that primarily affects neuromuscular junctions, causing muscle paralysis. - It does not directly inhibit the primary mechanisms of gastric acid secretion, which are largely mediated by **muscarinic acetylcholine receptors**, histamine, and gastrin. *Stimulated by nor adrenaline* - **Noradrenaline** (norepinephrine) is a neurotransmitter of the **sympathetic nervous system**, which generally **inhibits** gastric motility and secretion. - Activation of alpha-2 adrenergic receptors can decrease gastric acid secretion. *Stimulated by an increase in tonic activity* - This statement is vague; "tonic activity" can refer to various physiological processes. If it refers to **sympathetic nervous system** tonic activity, it would **inhibit** gastric secretion. - If it implies increased vagal tone (parasympathetic activity), then secretion would be stimulated, but the phrasing is not precise enough to be unequivocally true for gastric secretion in general.
Question 273: Ptyalin is secreted by?
- A. Gastric gland
- B. Salivary gland (Correct Answer)
- C. Duodenal gland
- D. Pancreatic gland
Explanation: ***Salivary gland*** - **Ptyalin**, also known as **salivary amylase**, is an enzyme produced by the salivary glands. - Its primary role is to initiate the **digestion of carbohydrates** (starches) in the mouth. *Gastric gland* - Gastric glands primarily secrete **hydrochloric acid** and **pepsinogen**, involved in protein digestion. - They do not produce ptyalin or enzymes for carbohydrate digestion. *Duodenal gland* - Duodenal glands (Brunner's glands) secrete alkaline mucus to protect the duodenum from acidic chyme. - They are not involved in the production of carbohydrate-digesting enzymes like ptyalin. *Pancreatic gland* - The pancreas produces **pancreatic amylase**, which continues starch digestion in the small intestine. - While it secretes an amylase, it is distinct from salivary amylase (ptyalin) and released into the duodenum, not the mouth.
Question 274: What is the body's first physiological response to hypoglycemia?
- A. Decreased insulin (Correct Answer)
- B. Increased glucagon
- C. Increased cortisol
- D. Increased norepinephrine
Explanation: ***Decreased insulin*** - **Decreased insulin secretion** is the body's **first and earliest** physiological response to falling blood glucose levels, occurring at approximately **80-85 mg/dL**. - This represents the **primary defense mechanism** against hypoglycemia - by reducing insulin release from pancreatic beta cells, the body removes the most potent glucose-lowering stimulus. - This allows blood glucose to stabilize before it drops further, and occurs **before** any active counterregulatory hormones are released. - This is a critical **first-line defense** that prevents the need for more aggressive counterregulatory responses. *Increased glucagon* - **Glucagon** is the **second line of defense** against hypoglycemia, with secretion increasing at glucose levels around **65-70 mg/dL**. - While glucagon is the most important **active counterregulatory hormone** (stimulating glycogenolysis and gluconeogenesis), it is not the *first* response. - The temporal sequence is: insulin suppression occurs first, followed by glucagon release if glucose continues to fall. *Increased cortisol* - **Cortisol** is a late counterregulatory hormone, responding to more severe or prolonged hypoglycemia (glucose <65 mg/dL). - It promotes gluconeogenesis and reduces peripheral glucose utilization over hours, not minutes. - Along with growth hormone, cortisol provides sustained glucose elevation but is not an early response. *Increased norepinephrine* - **Norepinephrine** (and epinephrine) are part of the sympathetic/adrenomedullary response to hypoglycemia at approximately **65-70 mg/dL**. - These catecholamines provide important counterregulation but are activated after insulin suppression has already occurred. - They contribute to both glucose mobilization and the symptomatic (adrenergic) response to hypoglycemia.
Question 275: What is the process by which water moves from the extracellular space to the intracellular space?
- A. Osmosis (Correct Answer)
- B. Diffusion
- C. Filtration
- D. Active transport
Explanation: ***Osmosis*** - **Osmosis** is the movement of water across a **semipermeable membrane** from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration). - In the context of fluid shifts, if the **extracellular fluid** becomes hypotonic relative to the **intracellular fluid**, water will move into the cells to equalize the solute concentration. *Diffusion* - **Diffusion** refers to the net movement of particles from an area of higher concentration to an area of lower concentration, down their **concentration gradient**. - While water molecules can diffuse, **osmosis** specifically describes the net movement of water across a membrane due to **solute concentration differences**, which is the precise mechanism for water moving between fluid compartments. *Filtration* - **Filtration** is the process by which water and solutes move across a membrane due to a **pressure gradient**, typically a **hydrostatic pressure gradient**. - This process is crucial in the kidneys for forming filtrate, but it is not the primary mechanism for water movement between the intra- and extracellular spaces based on solute concentration. *Active transport* - **Active transport** involves the movement of molecules across a membrane against their **concentration gradient**, requiring **energy expenditure** (e.g., ATP). - Water movement between fluid compartments is generally a passive process, relying on **osmotic gradients** rather than direct energy input to pump water molecules.