Energy Production and Metabolism Practice Questions

Q: Which of the following hormones can cause hyperglycemia without known effects on glycogen or gluconeogenesis?

Thyroxine. **Explanation:** The correct answer is **Thyroxine (T4)**. While most "anti-insulin" hormones increase blood glucose by stimulating glycogenolysis or gluconeogenesis, Thyroxine has a unique primary mechanism for inducing hyperglycemia. **1. Why Thyroxine is Correct:** Thyroxine increases blood glucose levels primarily by **increasing the rate of glucose absorption from the gastrointestinal tract**. While it does have some permissive effects on catecholamines, its direct and most distinct hyperglycemic action—independent of hepatic glucose production pathways—is the acceleration of intestinal hexose transport. **2. Why the Other Options are Incorrect:** * **Epinephrine:** This is a potent stimulator of **glycogenolysis** in both the liver (increasing blood glucose) and muscle (increasing lactate). It also stimulates gluconeogenesis. * **Glucocorticoids (e.g., Cortisol):** These are classic "diabetogenic" hormones that primarily increase blood glucose by inducing the synthesis of key enzymes involved in **gluconeogenesis** (e.g., PEPCK) and by decreasing peripheral glucose uptake. * **Epidermal Growth Factor (EGF):** This is a signaling protein involved in cell growth and proliferation; it does not play a primary or significant role in systemic glucose homeostasis. **3. NEET-PG High-Yield Pearls:** * **Hyperthyroidism & Diabetes:** Patients with hyperthyroidism often show abnormal glucose tolerance tests (GTT) because the rapid absorption of glucose leads to a high postprandial peak (lag storage curve). * **Growth Hormone:** Also causes hyperglycemia by decreasing peripheral glucose utilization (anti-insulin effect). * **Glucagon:** The primary hormone for acute glucose elevation via hepatic glycogenolysis and gluconeogenesis. * **Key Enzyme:** Glucocorticoids increase the expression of **Glucose-6-Phosphatase**, the final common enzyme for both gluconeogenesis and glycogenolysis.

Q: How many ATP molecules are generated in the Krebs cycle?

24. **Explanation:** The Krebs cycle (TCA cycle) is the final common pathway for the oxidation of carbohydrates, lipids, and proteins. To understand why **24 ATP** is the correct answer, we must calculate the yield per molecule of **Glucose**. 1. **Per Turn of the Cycle:** One molecule of Acetyl-CoA entering the cycle generates: * 3 NADH (3 × 2.5 = 7.5 ATP) * 1 FADH₂ (1 × 1.5 = 1.5 ATP) * 1 GTP/ATP (Substrate-level phosphorylation) * **Total per Acetyl-CoA = 10 ATP** (Modern yield) or **12 ATP** (Classic yield). 2. **Per Glucose Molecule:** One glucose molecule produces **two** Acetyl-CoA molecules via glycolysis and the pyruvate dehydrogenase complex. Therefore, the cycle turns twice per glucose. * **12 ATP × 2 = 24 ATP.** **Analysis of Options:** * **Option A (12):** This represents the ATP yield for a **single turn** of the cycle (one Acetyl-CoA). * **Option B (24):** Correct. It represents the total yield from the Krebs cycle for **one glucose molecule** (two turns). * **Option C (15):** This is the yield of one pyruvate molecule being completely oxidized to CO₂ and H₂O (Pyruvate to Acetyl-CoA = 3 ATP + Krebs cycle = 12 ATP). * **Option D (30):** This is the total net ATP yield of the **entire aerobic respiration** process (Glycolysis + Link Reaction + Krebs Cycle) using modern shuttle calculations. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Isocitrate Dehydrogenase. * **Substrate-level phosphorylation:** Occurs at the step of **Succinate Thiokinase** (Succinyl-CoA to Succinate). * **Inhibitors:** Fluoroacetate (inhibits Aconitase), Arsenite (inhibits α-ketoglutarate dehydrogenase), and Malonate (competitive inhibitor of Succinate dehydrogenase). * **Amphibolic nature:** The TCA cycle serves both catabolic (energy production) and anabolic (providing precursors for amino acids and heme) functions.

Q: In oxidative phosphorylation, the oxidation of one NADH to NAD+ produces how many ATPs?

3. **Explanation:** In oxidative phosphorylation, the oxidation of **NADH** occurs via the Electron Transport Chain (ETC). NADH enters the chain at **Complex I** (NADH dehydrogenase). As electrons pass from NADH through the ETC to oxygen, protons are pumped from the mitochondrial matrix into the intermembrane space at three specific sites: * **Complex I:** 4 protons * **Complex III:** 4 protons * **Complex IV:** 2 protons This creates a total gradient of **10 protons** per NADH molecule. According to the classical P:O ratio (used in most standard medical textbooks like Harper’s and Lehninger for competitive exams), it takes approximately 3 protons to drive the ATP synthase and 1 proton for phosphate transport. Thus, 10 protons yield approximately **3 ATPs**. (Note: While modern research suggests a more precise value of 2.5, the traditional value of 3 remains the standard for NEET-PG unless 2.5 is specifically provided as an option). **Analysis of Incorrect Options:** * **Options A & B (5 and 6):** These values are physiologically impossible for a single NADH molecule as the proton gradient generated is insufficient to drive the synthesis of this many ATP molecules. * **Option C (4):** This value is incorrect because the energy released during the transfer of electrons from NADH to Oxygen is only sufficient to pump 10 protons, which correlates to 3 ATPs. **High-Yield Clinical Pearls for NEET-PG:** * **FADH2** enters at **Complex II**, bypassing the first proton pump. It results in the pumping of only 6 protons, yielding **2 ATPs** (Modern value: 1.5). * **Cyanide and Carbon Monoxide** inhibit **Complex IV** (Cytochrome c oxidase), halting ATP production. * **Oligomycin** directly inhibits the **F0 subunit** of ATP synthase. * **Uncouplers** (e.g., 2,4-DNP, Thermogenin) increase oxygen consumption but decrease ATP synthesis by dissipating the proton gradient as heat.

Q: In the TCA cycle, from which molecule is CO2 released?

Isocitrate dehydrogenase. **Explanation:** In the Tricarboxylic Acid (TCA) cycle, carbon dioxide ($CO_2$) is released during **oxidative decarboxylation** reactions. The correct answer is **Isocitrate dehydrogenase**, which catalyzes the conversion of Isocitrate to $\alpha$-Ketoglutarate. This is the first rate-limiting step where $CO_2$ is produced and $NAD^+$ is reduced to $NADH + H^+$. **Analysis of Options:** * **Isocitrate Dehydrogenase (Correct):** It facilitates the decarboxylation of the 6-carbon isocitrate into the 5-carbon $\alpha$-ketoglutarate. * **Thiokinase (Incorrect):** Also known as Succinyl-CoA synthetase, this enzyme converts Succinyl-CoA to Succinate. It is responsible for **substrate-level phosphorylation** (generating GTP/ATP), not $CO_2$ release. * **Citrate Dehydrogenase (Incorrect):** This is a distractor; there is no enzyme by this name in the TCA cycle. Citrate is formed by Citrate Synthase. * **Alpha-ketoglutarate (Incorrect):** This is a *substrate*, not an enzyme. While the enzyme $\alpha$-ketoglutarate dehydrogenase does release $CO_2$, the option lists the molecule itself. **High-Yield NEET-PG Pearls:** 1. **Two $CO_2$ Exit Points:** $CO_2$ is released at two steps in the TCA cycle: * Isocitrate $\rightarrow$ $\alpha$-Ketoglutarate (via Isocitrate Dehydrogenase). * $\alpha$-Ketoglutarate $\rightarrow$ Succinyl-CoA (via $\alpha$-Ketoglutarate Dehydrogenase). 2. **Rate-Limiting Step:** Isocitrate dehydrogenase is the primary rate-limiting enzyme of the TCA cycle, inhibited by high ATP and NADH. 3. **Cofactors:** $\alpha$-Ketoglutarate dehydrogenase requires five cofactors (Tender Loving Care For No-one): **T**PP, **L**ipoic acid, **C**oA, **F**AD, and **N**AD.

Q: Which of the following is an uncoupler of oxidative phosphorylation?

2, 4-Dinitrophenol (2,4-DNP). ### Explanation **Correct Option: A. 2, 4-Dinitrophenol (2,4-DNP)** Uncouplers are substances that dissociate oxidation from phosphorylation. They act by increasing the permeability of the inner mitochondrial membrane to protons ($H^+$). This collapses the proton gradient, allowing electrons to flow through the Electron Transport Chain (ETC) to oxygen without the synthesis of ATP. The energy released is dissipated as **heat**. 2,4-DNP is a classic lipophilic protonophore that carries protons across the membrane, bypassing the $F_0F_1$ ATP synthase. **Analysis of Incorrect Options:** * **B. British Anti-Lewisite (BAL):** Also known as Dimercaprol, this is a chelating agent used in heavy metal poisoning (e.g., arsenic, mercury). In the context of the ETC, it acts as an **inhibitor of Complex III**, not an uncoupler. * **C. Trientine:** This is a chelating agent specifically used to treat **Wilson’s Disease** by removing excess copper. It has no direct role in the inhibition or uncoupling of oxidative phosphorylation. * **D. Rotenone:** This is a classic **inhibitor of Complex I** (NADH dehydrogenase). It blocks the transfer of electrons from Fe-S centers to Ubiquinone, halting the entire respiratory chain. **High-Yield Clinical Pearls for NEET-PG:** * **Physiological Uncoupler:** **Thermogenin (UCP1)** found in brown adipose tissue is essential for non-shivering thermogenesis in neonates. * **Other Uncouplers:** High doses of **Salicylates** (Aspirin), Dicumarol, and FCCP. * **Clinical Presentation:** Uncoupler overdose leads to hyperthermia, tachycardia, and diaphoresis because energy is lost as heat instead of being stored as ATP. * **Key Distinction:** Inhibitors (like Cyanide or Rotenone) stop oxygen consumption; Uncouplers **increase** oxygen consumption while stopping ATP synthesis.

Q: Which cell in the body primarily utilizes the pentose phosphate pathway?

Red Blood Cells (RBCs). The Pentose Phosphate Pathway (PPP), also known as the Hexose Monophosphate (HMP) Shunt, is the primary source of **NADPH** in the body. **Why Red Blood Cells (RBCs) are the correct answer:** RBCs are uniquely dependent on the PPP because they lack mitochondria. The PPP is their **only source of NADPH**. NADPH is essential for maintaining a pool of **reduced glutathione**, which neutralizes reactive oxygen species (ROS) like hydrogen peroxide. Without this pathway, hemoglobin would undergo oxidative damage, leading to the formation of Heinz bodies and subsequent hemolysis. **Analysis of Incorrect Options:** * **Hepatocytes:** While the liver is a major site for the PPP (used for fatty acid and cholesterol synthesis), hepatocytes have alternative metabolic pathways and mitochondria to manage oxidative stress. * **Muscle cells:** Muscles primarily utilize glucose for ATP production via glycolysis and the TCA cycle. They have very low PPP activity because they do not perform significant lipid synthesis. * **Neurons:** Neurons rely almost exclusively on aerobic glycolysis and the TCA cycle for high energy demands. While they use some NADPH for antioxidant defense, it is not their primary metabolic characteristic compared to RBCs. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to neonatal jaundice and drug-induced hemolytic anemia (e.g., after taking Primaquine or eating Fava beans) due to the inability of RBCs to produce NADPH. * **Tissues with high PPP activity:** Adrenal cortex, Liver, Testes/Ovaries (for steroid synthesis), and Lactating mammary glands (for fatty acid synthesis). * **Products:** NADPH (for reductive biosynthesis) and Ribose-5-phosphate (for nucleotide synthesis).

Question 1

Which of the following hormones can cause hyperglycemia without known effects on glycogen or gluconeogenesis?

Accepted Answer

Thyroxine

Answer

Epinephrine

Answer

Glucocorticoids

Answer

Epidermal growth factor

Question 2

Which of the following is considered the regulated step in steroid hormone synthesis in the zona fasciculata?

Accepted Answer

Cholesterol to pregnenolone

Answer

Corticosterone to aldosterone

Answer

11-Deoxycortisol to cortisol

Answer

Pregnenolone to progesterone

Question 3

How many ATP molecules are generated in the Krebs cycle?

Accepted Answer

24

Answer

12

Answer

15

Answer

30

Question 4

In oxidative phosphorylation, the oxidation of one NADH to NAD+ produces how many ATPs?

Accepted Answer

3

Answer

5

Answer

6

Answer

4

Question 5

In the electron transport chain (ETC), oxidative phosphorylation (ATP formation) is regulated by which of the following components?

Accepted Answer

All of the above

Answer

NADH Co-Q reductase

Answer

Cytochrome C oxidase

Answer

Co-Q-Cytochrome C reductase

Question 6

In the TCA cycle, from which molecule is CO2 released?

Accepted Answer

Isocitrate dehydrogenase

Answer

Thiokinase

Answer

Citrate dehydrogenase

Answer

Alpha ketoglutarate

Question 7

Which of the following is an uncoupler of oxidative phosphorylation?

Accepted Answer

2, 4-Dinitrophenol (2,4-DNP)

Answer

British Anti-Lewisite (BAL)

Answer

Trientine

Answer

Rotenone

Question 8

A scientist was studying the electron transport chain and developed a compound that bound to all the cytochromes in the proteins of the electron transport chain. Once bound, the compound blocked electron acceptance by the iron in the heme group of the cytochrome. Which one of the following would be most likely to occur in the presence of an oxidizable substrate?

Accepted Answer

Cell death would result from a lack of ATP.

Answer

The amount of heat generated from NADH oxidation would increase.

Answer

The rate of succinate oxidation would not be affected.

Answer

ATP could still be generated from the oxidation of NADH by transfer of electrons to O2.

Question 9

All of the following participate in oxidative phosphorylation except?

Accepted Answer

NADH

Answer

FADH2

Answer

NADPH

Answer

ATP

Question 10

Which cell in the body primarily utilizes the pentose phosphate pathway?

Accepted Answer

Red Blood Cells (RBCs)

Answer

Hepatocytes

Answer

Muscle cells

Answer

Neurons

Energy Production and Metabolism — MCQs

Energy Production and Metabolism — MCQs

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