Cellular Ultrastructure — MCQs
Which of the following factors acts in vesicle targeting?
Tay-Sachs disease is due to a deficiency of which enzyme?
What is the main component of a bilayer cell membrane?
Which of the following is a major component of the plasma membrane?
Diffusion of lipid-insoluble substances across the cell membrane depends on all of the following factors except which one?
A child presented with hypotonia and seizures. It was confirmed to be Zellweger syndrome. Which of the following accumulates in brain?
Which of the following is not a component of a mature sperm cell?
Which type of RNA is primarily involved in gene silencing?
Which one of the following statements about chromatin is not true?
Which cells are present in the collecting duct?
Cellular Ultrastructure Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following factors acts in vesicle targeting?
- A. Sec 12
- B. Rab (Correct Answer)
- C. Ras
- D. SNARE
Explanation: ***Rab*** - **Rab GTPases** are small G proteins that regulate many steps of membrane trafficking, including vesicle formation, cargo selection, vesicle budding, uncoating, motility, and fusion. - They act as molecular switches, cycling between an active GTP-bound state and an inactive GDP-bound state, thereby coordinating the proper targeting of vesicles to their destination membranes. *Sec 12* - **Sec12** is a **GEF (guanine nucleotide exchange factor)** for **Sar1**, which is involved in COPII vesicle formation from the ER. - While it initiates a step in vesicle budding, it does not directly act as a targeting molecule to guide the vesicle to its destination. *Ras* - **Ras GTPases** are primarily involved in cell signaling pathways regulating cell proliferation, differentiation, and survival. - They are not directly involved in the process of **vesicle targeting** in membrane trafficking. *SNARE* - **SNARE proteins** (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) are crucial for the **fusion of vesicles** with their target membranes. - While essential for the later stages of trafficking, they mediate membrane fusion rather than initial vesicle targeting.
Question 2: Tay-Sachs disease is due to a deficiency of which enzyme?
- A. Hexosaminidase B
- B. α-galactosidase
- C. Hexosaminidase A (Correct Answer)
- D. Sphingomyelinase
Explanation: ***Hexosaminidase A*** - **Tay-Sachs disease** is caused by a genetic deficiency in the lysosomal enzyme **hexosaminidase A (HexA)**. - This deficiency leads to the accumulation of **GM2 ganglioside** in neuronal cells, particularly in the brain, causing progressive neurodegeneration. *Hexosaminidase B* - A deficiency in **hexosaminidase B** is associated with **Sandhoff disease**, a lysosomal storage disorder similar to Tay-Sachs but typically more severe. - While HexA is composed of alpha and beta subunits, a deficiency specifically in the beta subunit is characteristic of Sandhoff disease. *α-galactosidase* - A deficiency in **α-galactosidase** is responsible for **Fabry disease**, an X-linked lysosomal storage disorder. - It leads to the accumulation of **globotriaosylceramide (Gb3)**, primarily affecting the kidneys, heart, and nervous system, and does not present with the neurological symptoms of Tay-Sachs. *Sphingomyelinase* - A deficiency in **sphingomyelinase** causes **Niemann-Pick disease (Type A and B)**, another lysosomal storage disorder. - This leads to the accumulation of **sphingomyelin** in various organs, resulting in hepatosplenomegaly, neurodegeneration (in Type A), and lung disease, distinct from Tay-Sachs.
Question 3: What is the main component of a bilayer cell membrane?
- A. Cholesterol ester
- B. Triacyl glycerol
- C. Cholesterol
- D. Phospholipids (Correct Answer)
Explanation: ***Correct: Phospholipids*** - **Phospholipids** are the primary structural components of cell membranes, forming a **bilayer** due to their amphipathic nature. - The **hydrophilic heads** face the aqueous environment, while the **hydrophobic tails** form the core of the membrane. *Incorrect: Cholesterol* - **Cholesterol** is an important component of animal cell membranes, contributing to fluidity and stability, but it is not the **main structural component**. - It inserts between phospholipids, modulating membrane fluidity by preventing the tight packing of fatty acid tails at lower temperatures and hindering excessive movement at higher temperatures. *Incorrect: Cholesterol ester* - **Cholesterol esters** are storage forms of cholesterol and are primarily found in intracellular lipid droplets or associated with lipoproteins in the bloodstream. - They are generally too **hydrophobic** to be significant structural components within the phospholipid bilayer itself. *Incorrect: Triacyl glycerol* - **Triacylglycerols** (triglycerides) are the primary form of **energy storage** in cells, found in lipid droplets within the cytoplasm. - They are highly **hydrophobic** and do not form a structural part of the cell membrane bilayer.
Question 4: Which of the following is a major component of the plasma membrane?
- A. Protein
- B. Carbohydrate
- C. Cholesterol
- D. Phospholipid (Correct Answer)
Explanation: ***Phospholipid*** - **Phospholipids** form the fundamental **phospholipid bilayer** structure of the plasma membrane, acting as a barrier. - Their **amphipathic nature** (hydrophilic head and hydrophobic tails) allows them to spontaneously form this bilayer in an aqueous environment. *Carbohydrate* - **Carbohydrates** are present on the outer surface of the plasma membrane, forming the **glycocalyx**, but are not a major structural component of the bilayer itself. - They primarily function in **cell recognition** and adhesion. *Protein* - **Proteins** are embedded within or associated with the phospholipid bilayer, facilitating various functions like **transport**, signaling, and adhesion. - While crucial for function, they do not form the basic structural framework of the membrane. *Cholesterol* - **Cholesterol** is a type of lipid that helps regulate the **fluidity** and stability of the plasma membrane. - It is interspersed within the phospholipid bilayer but is not the primary structural component.
Question 5: Diffusion of lipid-insoluble substances across the cell membrane depends on all of the following factors except which one?
- A. Hydrated radius
- B. Electrical charge
- C. Lipid solubility (Correct Answer)
- D. Shape
Explanation: ***Lipid solubility*** - This property is crucial for substances that **readily diffuse directly through the lipid bilayer**. - Lipid-insoluble substances, by definition, **cannot diffuse through the lipid bilayer based on their lipid solubility**, requiring other mechanisms or factors like channels or carriers. *Hydrated radius* - The **size of a hydrated ion or molecule** is a critical determinant for its ability to pass through specific protein channels or pores in the cell membrane. - A larger hydrated radius impedes passage through narrow channels, directly affecting the diffusion of lipid-insoluble substances. *Electrical charge* - For **charged lipid-insoluble substances** (ions), their movement across the membrane is significantly influenced by the **transmembrane electrical potential difference**. - The electrical gradient can either facilitate or hinder the diffusion of these substances through channels or transporters. *Shape* - The **three-dimensional configuration** of a lipid-insoluble substance can affect its ability to bind to and pass through specific protein channels or carrier proteins. - A substance's shape must complement the architecture of the transport mechanism for efficient diffusion.
Question 6: A child presented with hypotonia and seizures. It was confirmed to be Zellweger syndrome. Which of the following accumulates in brain?
- A. Very long chain fatty acid (Correct Answer)
- B. Lactic acid
- C. Glucose
- D. Triglycerides
Explanation: ***Long chain fatty acid*** - Zellweger syndrome is a **peroxisomal biogenesis disorder**, leading to non-functional peroxisomes. - Peroxisomes are crucial for the **beta-oxidation of very long-chain fatty acids (VLCFAs)**; their dysfunction causes VLCFA accumulation in tissues, including the brain. *Lactic acid* - Accumulation of **lactic acid** is typically associated with **mitochondrial disorders** or conditions leading to anaerobic metabolism, which are not the primary pathology in Zellweger syndrome. - While lactate levels might be altered in metabolic stress, it is not the hallmark accumulating substance for this condition. *Glucose* - **Glucose** accumulation or dysregulation is primarily seen in disorders like **diabetes mellitus** or specific **glycogen storage diseases**, which involve carbohydrate metabolism, not peroxisomal function. - High glucose levels do not directly result from the peroxisomal defect in Zellweger syndrome. *Triglycerides* - **Triglyceride** accumulation is often linked to disorders of **lipid synthesis, transport, or degradation** in adipocytes or hepatocytes, or conditions like obesity and metabolic syndrome. - While peroxisomes participate in lipid metabolism, the primary accumulation in Zellweger syndrome due to impaired beta-oxidation is **very long-chain fatty acids**, not bulk triglycerides.
Question 7: Which of the following is not a component of a mature sperm cell?
- A. Lysosome
- B. Golgi apparatus
- C. Mitochondria
- D. Endoplasmic reticulum (Correct Answer)
Explanation: ***Endoplasmic reticulum*** - The **endoplasmic reticulum** is prominent in spermatogonia and spermatocytes but largely absent in **mature sperm** as organelles are shed during spermiogenesis to reduce cell volume. - Its primary functions of protein synthesis and lipid metabolism are not required in a terminally differentiated, motile cell like a mature sperm. *Golgi apparatus* - The **Golgi apparatus** reorganizes during spermiogenesis to form the **acrosome**, which is a crucial structure for fertilization. - While the distinct Golgi stacks are not present, its modified derivative, the acrosome, is an essential component. *Mitochondria* - **Mitochondria** are abundant in the midpiece of the sperm tail, arranged in a spiral sheath. - They are vital for generating the **ATP** required for the flagellum's motility, enabling the sperm to swim. *Lysosome* - Although typical lysosomes are not found, the **acrosome** of the sperm is considered a modified lysosome. - The acrosome contains **hydrolytic enzymes** similar to lysosomes, which are critical for penetrating the egg's outer layers during fertilization.
Question 8: 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 9: Which one of the following statements about chromatin is not true?
- A. DNA winds approximately 1.75 times around the nucleosomes
- B. Covalent modification of histones influence chromatin compaction
- C. Non-histone proteins are part of mitotic chromosomes
- D. H2A-H2B bind to both the entry and exit ends of DNA in nucleosomes (Correct Answer)
Explanation: ***H2A-H2B bind to both the entry and exit ends of DNA in nucleosomes*** - This statement is **not entirely true** as presented because while **H2A-H2B dimers** do make contacts with DNA near entry/exit regions, they do not bind **exclusively** at these ends. - In the nucleosome structure, two H2A-H2B dimers flank the central **(H3-H4)₂ tetramer** and interact with DNA throughout approximately **30 base pairs on each side**. - The **entry and exit points** of nucleosomal DNA are primarily stabilized by **linker histones (H1)**, which bind to the dyad axis and linker DNA regions. - The statement oversimplifies the complex three-dimensional interactions within the nucleosome core particle. *DNA winds approximately 1.75 times around the nucleosomes* - This statement is **true**; approximately **1.65 to 1.75 turns** of DNA (about 146-147 base pairs) wrap around the **histone octamer** to form the core nucleosome particle. - This precise winding is crucial for the compaction of DNA into eukaryotic chromatin and represents the fundamental repeating unit of chromatin structure. *Covalent modification of histones influence chromatin compaction* - This statement is **true**; **post-translational modifications** (PTMs) such as acetylation, methylation, phosphorylation, and ubiquitination on histone tails significantly impact **chromatin structure and accessibility**. - For example, **histone acetylation** generally leads to a more open chromatin conformation (euchromatin) by neutralizing positive charges, facilitating gene expression. - **Histone methylation** can lead to either open or compact chromatin depending on the specific residue modified (e.g., H3K4me3 for activation, H3K9me3 for repression). *Non-histone proteins are part of mitotic chromosomes* - This statement is **true**; mitotic chromosomes contain numerous **non-histone proteins** essential for chromosome structure and function. - Examples include **structural maintenance of chromosomes (SMC) proteins** like condensin and cohesin, topoisomerases (DNA topoisomerase II), and kinetochore proteins. - These non-histone proteins are crucial for chromosome condensation, sister chromatid cohesion, segregation, and proper mitotic progression.
Question 10: Which cells are present in the collecting duct?
- A. Principal and intercalated cells (Correct Answer)
- B. Parietal and oxyntic cells
- C. Lacis cells
- D. Podocytes
Explanation: The collecting duct is the final segment of the renal tubule system, playing a critical role in fluid balance and acid-base homeostasis. ### **Explanation of the Correct Answer** The collecting duct is lined by a simple cuboidal epithelium consisting of two distinct cell types: 1. **Principal Cells (P cells):** These are the predominant cells. They possess receptors for **ADH (Vasopressin)**, which regulates water reabsorption via Aquaporin-2 channels [3], and **Aldosterone**, which mediates sodium reabsorption and potassium secretion. 2. **Intercalated Cells (I cells):** These are fewer in number and are primarily involved in acid-base balance [2]. **Type A** cells secrete $H^+$ (acidosis compensation), while **Type B** cells secrete $HCO_3^-$ (alkalosis compensation). ### **Analysis of Incorrect Options** * **B. Parietal and Oxyntic cells:** These are synonyms for the same cell type found in the **stomach lining** (gastric glands) responsible for secreting Hydrochloric acid (HCl) and Intrinsic Factor. * **C. Lacis cells:** Also known as extraglomerular mesangial cells, these are part of the **Juxtaglomerular Apparatus (JGA)** located between the afferent and efferent arterioles. * **D. Podocytes:** These are highly specialized visceral epithelial cells of the **Bowman’s capsule** [1] that wrap around glomerular capillaries to form the filtration slits. ### **High-Yield NEET-PG Pearls** * **Histology Tip:** Principal cells have short microvilli and a single primary cilium, whereas Intercalated cells have prominent microplicae (surface folds). * **Pharmacology Link:** Potassium-sparing diuretics (like Spironolactone and Amiloride) act specifically on the **Principal cells**. * **Embryology:** Unlike the rest of the nephron (derived from Metanephric blastema), the collecting duct develops from the **Ureteric bud**.
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free