A scientist is trying to design a drug to modulate cellular metabolism in the treatment of obesity. Specifically, he is interested in understanding how fats are processed in adipocytes in response to different energy states. His target is a protein within these cells that catalyzes catabolism of an energy source. The products of this reaction are subsequently used in gluconeogenesis or β-oxidation. Which of the following is true of the most likely protein that is being studied by this scientist?
Q2
A 2-month-old Middle Eastern female infant from a consanguinous marriage presents with seizures, anorexia, failure to thrive, developmental delay, and vomiting and fatigue after eating. Blood work demonstrated levels of methylmalonic acid nearly 500 times normal levels. A carbon-14 propionate incorporation assay was performed on the fibroblasts of the patient and compared to a healthy, normal individual. Little to none of the radiolabeled carbons of the propionate appeared in any of the intermediates of the Krebs cycle. Which of the following reactions is not taking place in this individual?
Q3
A 10-year-old boy is brought to the emergency department due to vomiting and weakness. He is attending a summer camp and was on a hike with the other kids and a camp counselor. His friends say that the boy skipped breakfast, and the counselor says he forgot to pack snacks for the kids during the hike. The child’s parents are contacted and report that the child has been completely healthy since birth. They also say there is an uncle who would have to eat regularly or he would have similar symptoms. At the hospital, his heart rate is 90/min, respiratory rate is 17/min, blood pressure is 110/65 mm Hg, and temperature is 37.0°C (98.6°F). Physical examination reveals a visibly lethargic child with slight disorientation to time and place. Mild hepatosplenomegaly is observed but no signs of dehydration are noted. A blood sample is drawn, and fluids are started via an intravenous line.
Lab report
Serum glucose 44 mg/dL
Serum ketones absent
Serum creatinine 1.0 mg/dL
Blood urea nitrogen 32 mg/dL
Alanine aminotransferase (ALT) 425 U/L
Aspartate aminotransferase (AST) 372 U/L
Hemoglobin (Hb%) 12.5 g/dL
Mean corpuscular volume (MCV) 80 fl
Reticulocyte count 1%
Erythrocyte count 5.1 million/mm3
Which of the following is most likely deficient in this patient?
Q4
Researchers are experimenting with hormone levels in mice in fasting and fed states. To test hormone levels in the fed state, the mice are given an oral glucose load and various hormones are measured in a blood sample. Researchers are most interested in the hormone whose blood levels track evenly with C-peptide levels. The hormone the researchers are most interested in is responsible for which of the following actions in the body?
Q5
After being warned by the locals not to consume the freshwater, a group of American backpackers set off on a week-long hike into a region of the Ecuadorean Amazon forest known for large gold mines. The group of hikers stopped near a small stream and used the water they filtered from the stream to make dinner. Within the next half hour, the hikers began to experience headaches, vertigo, visual disturbances, confusion, tachycardia, and altered levels of consciousness. Which of the following enzymes was most likely inhibited in this group of hikers?
Q6
A 5-year-old boy is brought to the physician’s office with complaints of being tired constantly, which has limited his ability to walk or play with his friends. Physical examination in the physician’s office is normal. Further testing reveals that the patient has a genetic mutation in an enzyme and muscle biopsy shows high levels of alpha-ketoglutarate and low levels of succinyl-CoA as compared to normal. The enzyme that is most likely deficient in this patient requires which of the following as a cofactor?
Q7
A startup is working on a novel project in which they claim they can replicate the organelle that is defective in MELAS syndrome. Which of the following metabolic processes must they be able to replicate if their project is to mimic the metabolic processes of this organelle?
Q8
During normal respiration in the lungs, oxygen is absorbed into the bloodstream and carbon dioxide is released. The oxygen is used in cells as the final electron acceptor during oxidative phosphorylation, and carbon dioxide is generated during each turn of the tricarboxylic citric acid cycle (TCA). Which of the following steps in the TCA cycle represents the first decarboxylation reaction that generates carbon dioxide?
Q9
A 12-month-old child passed away after suffering from craniofacial abnormalities, neurologic dysfunction, and hepatomegaly. Analysis of the child’s blood plasma shows an increase in very long chain fatty acids. The cellular analysis demonstrates dysfunction of an organelle responsible for the breakdown of these fatty acids within the cell. Postmortem, the child is diagnosed with Zellweger syndrome. The family is informed about the autosomal recessive inheritance pattern of the disease and their carrier status. Which of the following processes is deficient in the dysfunctional organelle in this disease?
Q10
Steroid hormone synthesis, lipid synthesis, and chemical detoxification are activities of which of the following?
Fatty acid oxidation (beta-oxidation) US Medical PG Practice Questions and MCQs
Question 1: A scientist is trying to design a drug to modulate cellular metabolism in the treatment of obesity. Specifically, he is interested in understanding how fats are processed in adipocytes in response to different energy states. His target is a protein within these cells that catalyzes catabolism of an energy source. The products of this reaction are subsequently used in gluconeogenesis or β-oxidation. Which of the following is true of the most likely protein that is being studied by this scientist?
A. It is stimulated by epinephrine (Correct Answer)
B. It is inhibited by glucagon
C. It is inhibited by acetylcholine
D. It is inhibited by cortisol
E. It is stimulated by insulin
Explanation: ***It is stimulated by epinephrine***
- The protein described is likely **hormone-sensitive lipase (HSL)**, which catabolizes **triglycerides** in adipocytes to **glycerol** and **fatty acids**.
- **Epinephrine** (and norepinephrine) stimulates HSL activity via a **cAMP-dependent protein kinase A (PKA)** pathway, leading to increased fatty acid release for energy.
*It is inhibited by glucagon*
- **Glucagon primarily acts on the liver** to promote gluconeogenesis and glycogenolysis, but it does **not directly inhibit HSL** in adipocytes.
- While glucagon has a lipolytic effect, it doesn't inhibit the enzyme that releases fatty acids.
*It is inhibited by acetylcholine*
- **Acetylcholine** is a neurotransmitter involved in the **parasympathetic nervous system**, which generally promotes energy storage.
- It does **not directly inhibit HSL**; its effects on lipid metabolism are indirect and typically involve other pathways.
*It is inhibited by cortisol*
- **Cortisol**, a glucocorticoid, generally **promotes lipolysis** (breakdown of fats) in certain contexts, particularly during stress to provide energy substrates.
- Therefore, it would **not inhibit HSL**; rather, it often enhances its activity or provides a permissive effect for other lipolytic hormones.
*It is stimulated by insulin*
- **Insulin** is an **anabolic hormone** that promotes energy storage, including **lipogenesis** (fat synthesis) and inhibits lipolysis.
- Insulin **inhibits HSL activity** by activating phosphodiesterase, which reduces cAMP levels, thus deactivating PKA and preventing HSL phosphorylation.
Question 2: A 2-month-old Middle Eastern female infant from a consanguinous marriage presents with seizures, anorexia, failure to thrive, developmental delay, and vomiting and fatigue after eating. Blood work demonstrated levels of methylmalonic acid nearly 500 times normal levels. A carbon-14 propionate incorporation assay was performed on the fibroblasts of the patient and compared to a healthy, normal individual. Little to none of the radiolabeled carbons of the propionate appeared in any of the intermediates of the Krebs cycle. Which of the following reactions is not taking place in this individual?
A. Pyruvate --> acetyl-CoA
B. Acetyl-CoA + CO2 --> Malonyl-CoA
C. Methylmalonyl-CoA --> Succinyl-CoA (Correct Answer)
D. Propionyl-CoA --> Methylmalonyl-CoA
E. Acetyl-CoA + Oxaloacetate --> Citrate
Explanation: ***Methylmalonyl-CoA --> Succinyl-CoA***
- The significantly elevated levels of **methylmalonic acid** and the failure of **radio-labeled propionate** (which is metabolized to methylmalonyl-CoA) to enter the Krebs cycle strongly indicate a defect in the succinyl-CoA mutase enzyme.
- This reaction, catalyzed by **methylmalonyl-CoA mutase** (with vitamin B12 as a cofactor), is essential for converting methylmalonyl-CoA to succinyl-CoA, a key intermediate in the Krebs cycle.
*Pyruvate --> acetyl-CoA*
- This reaction is catalyzed by the **pyruvate dehydrogenase complex** and is a critical step for glucose metabolism entering the Krebs cycle.
- The patient's symptoms and lab findings are related to **propionate metabolism**, not primarily glucose, and the experiment directly tests propionate, not pyruvate.
*Acetyl-CoA + CO2 --> Malonyl-CoA*
- This reaction, catalyzed by **acetyl-CoA carboxylase**, is the committed step in **fatty acid synthesis**.
- While important for metabolism, it is not directly related to the breakdown of propionate or the accumulation of methylmalonic acid.
*Propionyl-CoA --> Methylmalonyl-CoA*
- This reaction, catalyzed by **propionyl-CoA carboxylase**, converts propionyl-CoA to D-methylmalonyl-CoA, eventually isomerized to L-methylmalonyl-CoA.
- The fact that methylmalonic acid is nearly 500 times normal suggests that **propionyl-CoA carboxylase is functioning**, leading to the accumulation of its product.
*Acetyl-CoA + Oxaloacetate --> Citrate*
- This reaction is the first step of the **Krebs cycle**, catalyzed by **citrate synthase**.
- While propionate metabolism feeds into the Krebs cycle, the immediate block is upstream at succinyl-CoA formation from methylmalonyl-CoA, preventing its entry into the cycle, rather than the initial step of the cycle itself.
Question 3: A 10-year-old boy is brought to the emergency department due to vomiting and weakness. He is attending a summer camp and was on a hike with the other kids and a camp counselor. His friends say that the boy skipped breakfast, and the counselor says he forgot to pack snacks for the kids during the hike. The child’s parents are contacted and report that the child has been completely healthy since birth. They also say there is an uncle who would have to eat regularly or he would have similar symptoms. At the hospital, his heart rate is 90/min, respiratory rate is 17/min, blood pressure is 110/65 mm Hg, and temperature is 37.0°C (98.6°F). Physical examination reveals a visibly lethargic child with slight disorientation to time and place. Mild hepatosplenomegaly is observed but no signs of dehydration are noted. A blood sample is drawn, and fluids are started via an intravenous line.
Lab report
Serum glucose 44 mg/dL
Serum ketones absent
Serum creatinine 1.0 mg/dL
Blood urea nitrogen 32 mg/dL
Alanine aminotransferase (ALT) 425 U/L
Aspartate aminotransferase (AST) 372 U/L
Hemoglobin (Hb%) 12.5 g/dL
Mean corpuscular volume (MCV) 80 fl
Reticulocyte count 1%
Erythrocyte count 5.1 million/mm3
Which of the following is most likely deficient in this patient?
A. Acyl-CoA dehydrogenase (Correct Answer)
B. α-glucosidase
C. Glucose-6-phosphatase
D. Acetyl-CoA carboxylase
E. Nicotinic acid
Explanation: ***Acyl-CoA dehydrogenase***
- This patient presents with **hypoglycemia** (44 mg/dL) and **absent ketone bodies** after prolonged fasting, along with elevated **liver transaminases** and **hepatosplenomegaly**, which are classic signs of a **fatty acid oxidation disorder**.
- A deficiency in **acyl-CoA dehydrogenase**, particularly **medium-chain acyl-CoA dehydrogenase (MCAD)**, prevents adequate fatty acid breakdown for energy and ketone production, leading to **hypoketotic hypoglycemia** during periods of fasting.
*α-glucosidase*
- A deficiency in **α-glucosidase** (Pompe disease) leads to the accumulation of **glycogen** in lysosomes, primarily affecting muscles, heart, and liver.
- While it can cause hepatomegaly and muscle weakness, it typically presents with **cardiomyopathy** and does not directly cause hypoketotic hypoglycemia.
*Glucose-6-phosphatase*
- A deficiency in **glucose-6-phosphatase** (Von Gierke disease) is a type of **glycogen storage disease** characterized by severe **fasting hypoglycemia with lactic acidosis**, **massive hepatomegaly**, and **hyperlipidemia**.
- Unlike fatty acid oxidation disorders, Von Gierke disease typically presents with **lactic acidosis** as the predominant metabolic derangement, and patients often have a **doll-like face** and **growth retardation** from chronic presentation.
*Acetyl-CoA carboxylase*
- **Acetyl-CoA carboxylase** is a key enzyme in **fatty acid synthesis**, not fatty acid oxidation.
- A deficiency would primarily impair the body's ability to synthesize fatty acids, which is not consistent with the hypoketotic hypoglycemia observed here.
*Nicotinic acid*
- **Nicotinic acid** (niacin or vitamin B3) is a precursor to **NAD+** and **NADP+**, coenzymes involved in various metabolic reactions, including fatty acid synthesis and breakdown.
- While a deficiency (pellagra) can cause dermatitis, diarrhea, and dementia, it does not directly lead to **hypoketotic hypoglycemia** or fatty liver disease.
Question 4: Researchers are experimenting with hormone levels in mice in fasting and fed states. To test hormone levels in the fed state, the mice are given an oral glucose load and various hormones are measured in a blood sample. Researchers are most interested in the hormone whose blood levels track evenly with C-peptide levels. The hormone the researchers are most interested in is responsible for which of the following actions in the body?
A. Protein catabolism
B. Fatty acid breakdown
C. Fatty acid synthesis (Correct Answer)
D. Ketogenesis
E. Lipolysis
Explanation: ***Fatty acid synthesis***
- The hormone whose blood levels track evenly with **C-peptide** levels after a glucose load is **insulin**.
- Insulin is a key anabolic hormone that promotes **fatty acid synthesis** from excess glucose in the fed state, particularly in the liver and adipose tissue.
*Protein catabolism*
- **Insulin** is an anabolic hormone that generally **inhibits protein catabolism** and promotes protein synthesis.
- Conditions like **glucagon excess** or **cortisol excess** promote protein catabolism, not insulin.
*Fatty acid breakdown*
- **Insulin inhibits fatty acid breakdown** (beta-oxidation) by suppressing hormone-sensitive lipase.
- **Glucagon** and **epinephrine** promote fatty acid breakdown, especially during fasting.
*Ketogenesis*
- **Insulin inhibits ketogenesis** by reducing the supply of fatty acids to the liver and inhibiting the enzymes involved in ketone body formation.
- **Glucagon** and **low insulin levels** (as in uncontrolled diabetes or prolonged fasting) promote ketogenesis.
*Lipolysis*
- **Insulin is a potent inhibitor of lipolysis** (breakdown of triglycerides into fatty acids and glycerol) in adipose tissue.
- **Glucagon**, **catecholamines**, and **growth hormone** stimulate lipolysis.
Question 5: After being warned by the locals not to consume the freshwater, a group of American backpackers set off on a week-long hike into a region of the Ecuadorean Amazon forest known for large gold mines. The group of hikers stopped near a small stream and used the water they filtered from the stream to make dinner. Within the next half hour, the hikers began to experience headaches, vertigo, visual disturbances, confusion, tachycardia, and altered levels of consciousness. Which of the following enzymes was most likely inhibited in this group of hikers?
A. NADH dehydrogenase
B. ATP synthase
C. Cytochrome c oxidase (Correct Answer)
D. Cytochrome bc1 complex
E. Succinate dehydrogenase
Explanation: ***Cytochrome c oxidase***
- The symptoms described (headaches, vertigo, visual disturbances, confusion, tachycardia, altered consciousness occurring within 30 minutes) are characteristic of **acute cyanide poisoning**.
- **Cyanide** is commonly found in water near **gold mining operations**, where it is used in the gold extraction process and can contaminate local water sources.
- **Cyanide** is a potent inhibitor of **cytochrome c oxidase** (Complex IV) in the electron transport chain, binding to the heme iron (Fe³⁺) and preventing oxygen utilization, leading to **histotoxic hypoxia**.
- This results in cellular energy failure, particularly affecting high-energy-demand organs like the brain and heart, explaining the acute neurological and cardiovascular symptoms.
*NADH dehydrogenase*
- While NADH dehydrogenase (Complex I) is a component of the electron transport chain, it is not the primary target of **cyanide poisoning**.
- Inhibitors of Complex I include rotenone and barbiturates, which cause different clinical presentations and do not produce the rapid onset of symptoms seen with cyanide.
*ATP synthase*
- **ATP synthase** (Complex V) synthesizes ATP using the proton gradient, but it is not directly inhibited by **cyanide**.
- Inhibitors of ATP synthase, such as oligomycin, prevent ATP synthesis by blocking the enzyme directly, whereas cyanide acts upstream at Complex IV.
*Cytochrome bc1 complex*
- The **cytochrome bc1 complex** (Complex III) is involved in electron transfer and proton pumping, but it is not the primary enzyme inhibited by **cyanide**.
- Inhibitors of Complex III include antimycin A, which would disrupt the electron transport chain but do not cause the characteristic rapid-onset symptoms of cyanide poisoning.
*Succinate dehydrogenase*
- **Succinate dehydrogenase** (Complex II) participates in both the citric acid cycle and the electron transport chain, but it is not targeted by **cyanide**.
- Inhibitors of Complex II, such as malonate, competitively block succinate oxidation but do not produce the acute systemic toxicity characteristic of cyanide poisoning.
Question 6: A 5-year-old boy is brought to the physician’s office with complaints of being tired constantly, which has limited his ability to walk or play with his friends. Physical examination in the physician’s office is normal. Further testing reveals that the patient has a genetic mutation in an enzyme and muscle biopsy shows high levels of alpha-ketoglutarate and low levels of succinyl-CoA as compared to normal. The enzyme that is most likely deficient in this patient requires which of the following as a cofactor?
A. Vitamin B6
B. NADH
C. Carbon dioxide
D. ATP
E. Vitamin B1 (Correct Answer)
Explanation: ***Vitamin B1***
- The patient's symptoms of fatigue and exercise intolerance, along with high levels of **alpha-ketoglutarate** and low levels of **succinyl-CoA**, indicate a defect in the **alpha-ketoglutarate dehydrogenase complex**.
- This enzyme complex, crucial for the **Krebs cycle**, requires **thiamine pyrophosphate (TPP)**, a derivative of **Vitamin B1**, as a vital cofactor.
*Vitamin B6*
- **Vitamin B6 (pyridoxine)** is a cofactor for enzymes involved in amino acid metabolism, such as **transaminases** and **decarboxylases**, but not specifically for alpha-ketoglutarate dehydrogenase.
- A deficiency in Vitamin B6 would lead to different metabolic profiles, not the specific accumulation of alpha-ketoglutarate.
*NADH*
- **NADH** is a product of several enzymatic reactions within the Krebs cycle, including the one catalyzed by alpha-ketoglutarate dehydrogenase, rather than a cofactor for this specific enzyme.
- While essential for the electron transport chain, NADH's role is as an electron carrier, not a direct cofactor for the alpha-ketoglutarate dehydrogenase complex's catalytic activity.
*Carbon dioxide*
- **Carbon dioxide** is often a product of decarboxylation reactions within the Krebs cycle (e.g., isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase), but it does not serve as a cofactor.
- Its presence or absence as a cofactor would not directly explain the enzymatic deficiency observed in this patient.
*ATP*
- **ATP** (adenosine triphosphate) is the primary energy currency of the cell, consumed or produced by many metabolic pathways, but it is not a direct cofactor for the alpha-ketoglutarate dehydrogenase complex.
- While energy is needed for many cellular processes, a direct ATP deficiency would manifest differently and would not specifically cause a buildup of alpha-ketoglutarate.
Question 7: A startup is working on a novel project in which they claim they can replicate the organelle that is defective in MELAS syndrome. Which of the following metabolic processes must they be able to replicate if their project is to mimic the metabolic processes of this organelle?
A. Hexose monophosphate shunt
B. Cholesterol synthesis
C. Glycolysis
D. Fatty acid (beta) oxidation (Correct Answer)
E. Fatty acid synthesis
Explanation: ***Fatty acid (beta) oxidation***
- **MELAS syndrome** (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes) is caused by defects in **mitochondrial function**.
- **Beta-oxidation of fatty acids** is a crucial metabolic process that occurs within the mitochondria, generating ATP.
*Hexose monophosphate shunt*
- The **hexose monophosphate shunt** (pentose phosphate pathway) occurs in the **cytosol** and is primarily involved in producing NADPH and synthesizing nucleotides, not a primary mitochondrial function.
- Its dysfunction is not directly linked to the core metabolic defects seen in MELAS syndrome.
*Cholesterol synthesis*
- **Cholesterol synthesis** primarily occurs in the **cytosol** and the **endoplasmic reticulum**, not within the mitochondria.
- While cholesterol metabolism can be indirectly affected by mitochondrial health, it is not a direct mitochondrial metabolic pathway.
*Glycolysis*
- **Glycolysis** is the metabolic pathway that converts glucose into pyruvate, occurring in the **cytosol**.
- Although it precedes mitochondrial oxidative phosphorylation, glycolysis itself does not occur within the mitochondria.
*Fatty acid synthesis*
- **Fatty acid synthesis** primarily takes place in the **cytosol** and endoplasmic reticulum, utilizing NADPH from the hexose monophosphate shunt.
- It is an anabolic process, while MELAS typically involves defects in catabolic/energy-producing mitochondrial pathways.
Question 8: During normal respiration in the lungs, oxygen is absorbed into the bloodstream and carbon dioxide is released. The oxygen is used in cells as the final electron acceptor during oxidative phosphorylation, and carbon dioxide is generated during each turn of the tricarboxylic citric acid cycle (TCA). Which of the following steps in the TCA cycle represents the first decarboxylation reaction that generates carbon dioxide?
A. Isocitrate to alpha ketoglutarate (Correct Answer)
B. Fumarate to Malate
C. Citrate to isocitrate
D. Malate to oxaloacetate
E. Alpha-ketoglutarate to Succinyl-CoA
Explanation: ***Isocitrate to alpha ketoglutarate***
- This is the **first decarboxylation reaction** in the TCA cycle, catalyzed by **isocitrate dehydrogenase**.
- During this reaction, **isocitrate** is oxidized and a molecule of **carbon dioxide** is released, along with the reduction of NAD+ to NADH.
- This is one of the three irreversible steps in the TCA cycle and a key regulatory point.
*Fumarate to Malate*
- This step involves the **hydration** of **fumarate** to **malate** by the enzyme **fumarase**.
- There is no release of carbon dioxide in this reaction; it's a simple addition of water.
*Citrate to isocitrate*
- This is an **isomerization** reaction, catalyzed by **aconitase**, where **citrate** is rearranged into its isomer, **isocitrate**.
- This step does not involve the removal of carbon atoms or the production of carbon dioxide.
*Malate to oxaloacetate*
- In this step, **malate** is oxidized to **oxaloacetate** by **malate dehydrogenase**, which produces NADH.
- This is an **oxidation** reaction, not a decarboxylation reaction, and no carbon dioxide is released.
*Alpha-ketoglutarate to Succinyl-CoA*
- This is the **second decarboxylation** step in the TCA cycle, catalyzed by the **alpha-ketoglutarate dehydrogenase complex**.
- While this step also produces carbon dioxide and reduces NAD+ to NADH, it occurs after the isocitrate to alpha-ketoglutarate step, making it the second rather than the first decarboxylation reaction.
Question 9: A 12-month-old child passed away after suffering from craniofacial abnormalities, neurologic dysfunction, and hepatomegaly. Analysis of the child’s blood plasma shows an increase in very long chain fatty acids. The cellular analysis demonstrates dysfunction of an organelle responsible for the breakdown of these fatty acids within the cell. Postmortem, the child is diagnosed with Zellweger syndrome. The family is informed about the autosomal recessive inheritance pattern of the disease and their carrier status. Which of the following processes is deficient in the dysfunctional organelle in this disease?
A. Transcription
B. Phosphorylation
C. Translation
D. Ubiquitination
E. Beta-oxidation (Correct Answer)
Explanation: ***Beta-oxidation***
- Zellweger syndrome is a **peroxisomal disorder** where the peroxisomes are either absent or dysfunctional.
- Peroxisomes are primarily responsible for the **beta-oxidation** of very long chain fatty acids (VLCFAs, >C22) and branched-chain fatty acids.
- Mitochondria handle shorter chain fatty acids, but **only peroxisomes can initiate breakdown of VLCFAs**, which explains why these fatty acids accumulate in the blood when peroxisomes are defective.
*Transcription*
- **Transcription** is the process of synthesizing RNA from a DNA template in the nucleus.
- This process is not directly affected in Zellweger syndrome, which involves organelle dysfunction, not gene expression at the mRNA level.
*Phosphorylation*
- **Phosphorylation** is the addition of a phosphate group to a molecule, a common post-translational modification or energy transfer mechanism.
- While essential for many metabolic pathways, it is not the primary enzymatic process deficient in peroxisomes in Zellweger syndrome.
*Translation*
- **Translation** is the process by which ribosomes synthesize proteins from mRNA in the cytoplasm.
- This cellular process is not the direct cause of the accumulation of very long chain fatty acids in Zellweger syndrome.
*Ubiquitination*
- **Ubiquitination** is a process that tags proteins for degradation by the proteasome.
- While critical for protein turnover, it is not the deficient metabolic pathway within peroxisomes in Zellweger syndrome.
Question 10: Steroid hormone synthesis, lipid synthesis, and chemical detoxification are activities of which of the following?
A. Peroxisomes
B. Nucleolus
C. Rough Endoplasmic Reticulum
D. Smooth Endoplasmic Reticulum (Correct Answer)
E. Golgi bodies
Explanation: ***Smooth Endoplasmic Reticulum***
- The **smooth endoplasmic reticulum (SER)** is rich in enzymes that catalyze the synthesis of **lipids**, including steroid hormones, and is crucial for the detoxification of drugs and poisons, particularly in liver cells.
- Its tubular structure, devoid of ribosomes, differentiates its functions from the rough ER, focusing on metabolic processes like **calcium ion storage** and carbohydrate metabolism.
*Peroxisomes*
- Peroxisomes are primarily involved in the breakdown of **fatty acids** and amino acids, producing hydrogen peroxide as a byproduct.
- They also play a role in detoxification but are not the primary site for steroid hormone or general lipid synthesis.
*Nucleolus*
- The **nucleolus** is a dense structure within the nucleus responsible for synthesizing **ribosomal RNA (rRNA)** and assembling ribosomes.
- It has no direct role in steroid hormone synthesis, lipid metabolism, or chemical detoxification.
*Rough Endoplasmic Reticulum*
- The **rough endoplasmic reticulum (RER)** is studded with **ribosomes** and is primarily involved in the synthesis and modification of **proteins** destined for secretion or insertion into membranes.
- While it's part of the endomembrane system, it does not directly perform lipid synthesis or chemical detoxification as its main functions.
*Golgi bodies*
- **Golgi bodies (or Golgi apparatus)** are responsible for modifying, sorting, and packaging **proteins and lipids** synthesized in the ER into vesicles for secretion or delivery to other organelles.
- They do not perform the initial synthesis of steroid hormones or lipids, nor are they the primary site for chemical detoxification.
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