Cellular Physiology Practice Questions

Q: Which of the following is a marker of the Golgi apparatus?

Galactosyl transferase. **Explanation:** The Golgi apparatus is the "packaging and processing center" of the cell. Its primary function involves the post-translational modification of proteins and lipids, specifically **glycosylation** (adding sugar moieties). **1. Why Galactosyl transferase is correct:** Galactosyl transferase is an enzyme located within the cisternae of the Golgi complex. It facilitates the transfer of galactose residues to proteins and lipids. Because it is highly localized to this organelle, it serves as the definitive **biochemical marker** for identifying the Golgi apparatus in subcellular fractionation studies. **2. Why the other options are incorrect:** * **Glucose 6 phosphatase:** This is the classic marker for the **Smooth Endoplasmic Reticulum (SER)**. It plays a critical role in gluconeogenesis and glycogenolysis by converting Glucose-6-phosphate to free glucose. * **Acid phosphatase:** This is the hallmark marker for **Lysosomes**. These organelles contain acid hydrolases that function optimally at a low pH to digest cellular debris. * **Pseudocatalase (and Catalase):** These are markers for **Peroxisomes** (microbodies), which are involved in the oxidation of very-long-chain fatty acids (VLCFA) and the detoxification of hydrogen peroxide. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Golgi Functions:** Proteolysis of precursors (e.g., proinsulin to insulin), sulfation, and sorting proteins to their final destinations. * **I-Cell Disease:** A clinical correlation where a deficiency in a Golgi enzyme (phosphotransferase) leads to the failure of tagging lysosomal enzymes with **Mannose-6-Phosphate**, causing them to be secreted extracellularly rather than sent to lysosomes. * **Other Markers to Remember:** * **Mitochondria:** ATP synthase / Cytochrome Oxidase. * **Nucleus:** DNA Polymerase. * **Plasma Membrane:** Na⁺-K⁺ ATPase / Adenylate cyclase.

Q: A single cell within a culture of freshly isolated cardiac muscle cells is injected with a fluorescent dye that cannot cross cell membranes. Within minutes, several adjacent cells become fluorescent. The most likely explanation for this observation is the presence of?

Gap junctions. **Explanation:** The observation of a membrane-impermeable dye spreading from one cardiac cell to its neighbors indicates the presence of **Gap Junctions**. These are specialized intercellular channels that allow the direct passage of ions and small water-soluble molecules (typically <1000 Daltons) between adjacent cells without entering the extracellular space. In cardiac muscle, gap junctions are located within the **intercalated discs**, facilitating electrical coupling and ensuring the heart functions as a **functional syncytium**. **Why other options are incorrect:** * **IP3 Receptors:** These are ligand-gated calcium channels located on the **Sarcoplasmic Reticulum (SR)** membrane. They mediate intracellular calcium release but do not provide a pathway for molecules to move between different cells. * **Transverse (T) Tubules:** These are deep invaginations of the sarcolemma that conduct action potentials into the cell interior to reach the SR. They communicate with the extracellular space, not with adjacent cells. * **Desmosomes (Macula Adherens):** Also found in intercalated discs, these provide **mechanical stability** by anchoring intermediate filaments (desmin) between cells. They prevent cells from pulling apart during contraction but do not allow for the exchange of cytoplasmic contents. **High-Yield NEET-PG Pearls:** * **Connexons:** Gap junctions are composed of six protein subunits called **connexins**. * **Velocity:** Gap junctions are most abundant in the **Purkinje fibers**, contributing to their high conduction velocity. * **Clinical Correlation:** Mutations in connexin genes (e.g., Connexin 43) are linked to arrhythmogenic disorders. * **Intercalated Disc Components:** Remember the triad—Gap junctions (electrical), Desmosomes (mechanical), and Fascia adherens (actin anchoring).

Q: Vesicles leaving the trans-Golgi carry on their surfaces a protein that targets them to the appropriate organelle. What is this protein?

v-SNARE. **Explanation:** The correct answer is **v-SNARE**. **Underlying Concept:** Vesicular transport is a highly regulated process ensuring that proteins reach their correct destination. When a vesicle buds off from the trans-Golgi network, it incorporates specific integral membrane proteins called **v-SNAREs** (vesicle-SNAREs) into its membrane. These act as "molecular addresses." When the vesicle reaches its target organelle, the v-SNARE binds specifically to a complementary **t-SNARE** (target-SNARE) on the destination membrane. This interaction forms a "SNARE complex," which provides the energy required for membrane fusion and cargo delivery. **Analysis of Incorrect Options:** * **t-SNARE:** These are located on the **target membrane** (e.g., plasma membrane or lysosome), not on the surface of the traveling vesicle. * **Coatomer (COP I/II):** These are protein complexes that help in the **formation/budding** of vesicles (COP II for anterograde and COP I for retrograde transport). They usually dissociate shortly after the vesicle is formed. * **Clathrin:** This is a coat protein involved in **endocytosis** and the formation of vesicles from the trans-Golgi destined for lysosomes. Like coatomers, clathrin helps shape the vesicle but does not serve as the primary targeting/docking signal. **High-Yield Clinical Pearls for NEET-PG:** * **Tetanus and Botulinum toxins:** These act by proteolytically cleaving SNARE proteins (like Synaptobrevin/v-SNARE or Syntaxin/t-SNARE), thereby inhibiting neurotransmitter release at the neuromuscular junction or spinal cord. * **Rab proteins:** These are small GTPases that help the initial "tethering" of the vesicle before the SNAREs lock together. * **Anterograde transport:** Mediated by **COP II** (Golgi → ER). * **Retrograde transport:** Mediated by **COP I** (ER → Golgi).

Q: Which of the following best describes exocytosis?

Extrusion of cell-bound vesicles. **Explanation:** **Exocytosis** is the process of bulk transport where a cell exports materials into the extracellular fluid. This occurs when intracellular membrane-bound vesicles (containing proteins, hormones, or neurotransmitters) migrate to the cell membrane, fuse with it, and release their contents into the external environment. This is an **energy-dependent (active)** process often triggered by an increase in intracellular calcium levels. **Analysis of Options:** * **Option A (Correct):** Exocytosis literally means "out of the cell." It involves the **extrusion** of materials via vesicles. Examples include the release of insulin from pancreatic beta cells or neurotransmitters at the synaptic cleft. * **Option B (Incorrect):** The inclusion (taking in) of liquid particles is called **Pinocytosis** ("cell drinking"), which is a form of endocytosis. * **Option C (Incorrect):** The inclusion of solid particles (like bacteria or cell debris) is called **Phagocytosis** ("cell eating"), another form of endocytosis. * **Option D (Incorrect):** Since B and C describe endocytosis (internalization), they are functionally opposite to exocytosis. **High-Yield NEET-PG Pearls:** 1. **SNARE Proteins:** These are critical for exocytosis. **v-SNAREs** (on the vesicle) and **t-SNAREs** (on the target membrane) interact to facilitate docking and fusion. 2. **Clinical Correlation:** **Tetanus and Botulinum toxins** act by cleaving SNARE proteins, thereby inhibiting the exocytosis of neurotransmitters (GABA/Glycine and Acetylcholine, respectively). 3. **Transcytosis:** A combination of endocytosis and exocytosis used to move substances across an entire cell (e.g., IgA secretion).

Q: IP-3 facilitates the entry of which of the following ions into the cytoplasm?

Ca2+. **Explanation:** The correct answer is **C. Ca2+**. **Mechanism of Action:** Inositol triphosphate (IP3) is a key second messenger in the **G-protein coupled receptor (GPCR) signaling pathway**, specifically the Gq pathway. When a ligand binds to a Gq-coupled receptor, the enzyme **Phospholipase C (PLC)** is activated. PLC cleaves the membrane phospholipid PIP2 into two fragments: **Diacylglycerol (DAG)** and **IP3**. IP3 is water-soluble and diffuses into the cytoplasm, where it binds to specific **IP3 receptors** located on the membrane of the **Endoplasmic Reticulum (ER)** (or Sarcoplasmic Reticulum in muscle). These receptors act as ligand-gated calcium channels. Upon binding, they open and allow Ca2+ to flow down its concentration gradient from the ER lumen into the cytoplasm, leading to various cellular responses such as muscle contraction, secretion, or enzyme activation. **Why other options are incorrect:** * **A (Na+) & B (K+):** These ions are primarily regulated by voltage-gated channels or ion pumps (like Na+/K+ ATPase) and are involved in membrane potential and action potentials, not the IP3 signaling pathway. * **D (Mg2+):** Magnesium acts as a cofactor for many enzymes (especially those using ATP) but is not mobilized from intracellular stores by the IP3 pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Gq-coupled receptors (Mnemonic: HAV 1 M&M):** **H**1, **A**lpha-1, **V**1, **M**1, and **M**3 receptors all utilize the IP3/DAG pathway. * **DAG's Role:** While IP3 increases intracellular Ca2+, DAG remains in the membrane to activate **Protein Kinase C (PKC)**. * **Termination:** The IP3 signal is terminated by dephosphorylation into IP2 or phosphorylation into IP4.

Q: Which of the following is NOT a component of the basement membrane?

Rhodopsin. ### Explanation The **Basement Membrane (BM)** is a specialized form of extracellular matrix that provides structural support to tissues and acts as a selective barrier. It is primarily composed of four major glycoproteins: **Type IV collagen, Laminin, Entactin (Nidogen), and Heparan sulfate proteoglycans (Perlecan).** **Why Rhodopsin is the Correct Answer:** **Rhodopsin** is a biological pigment found in the rod cells of the retina. It is a G-protein-coupled receptor (GPCR) responsible for **phototransduction** (vision in low light). It is a transmembrane protein, not a structural component of the extracellular basement membrane. **Analysis of Incorrect Options:** * **Laminin:** This is the most abundant non-collagenous glycoprotein in the basement membrane. It is crucial for cell adhesion and anchoring the epithelium to the BM. * **Entactin (Nidogen):** This is a rod-like molecule that functions as a "molecular bridge." It binds both Laminin and Type IV collagen, stabilizing the basement membrane structure. * **Tenascin:** This is an extracellular matrix glycoprotein involved in cell adhesion. While more prominent during embryogenesis and wound healing, it is a recognized component of the basal lamina/basement membrane complex in various tissues. **High-Yield Clinical Pearls for NEET-PG:** * **Goodpasture Syndrome:** Characterized by autoantibodies against the **α-3 chain of Type IV collagen**, leading to glomerulonephritis and pulmonary hemorrhage. * **Alport Syndrome:** A genetic defect in **Type IV collagen** synthesis, resulting in "split basement membrane" (basket-weave appearance), leading to nephritis and sensorineural deafness. * **Laminin-5 Deficiency:** Associated with **Junctional Epidermolysis Bullosa**, a severe skin blistering disease.

Question 1

Which of the following is a marker of the Golgi apparatus?

Accepted Answer

Galactosyl transferase

Answer

Glucose 6 phosphatase

Answer

Acid phosphatase

Answer

Pseudocatalase

Question 2

A single cell within a culture of freshly isolated cardiac muscle cells is injected with a fluorescent dye that cannot cross cell membranes. Within minutes, several adjacent cells become fluorescent. The most likely explanation for this observation is the presence of?

Accepted Answer

Gap junctions

Answer

IP3 receptors

Answer

Transverse tubules

Answer

Desmosomes

Question 3

Vesicles leaving the trans-Golgi carry on their surfaces a protein that targets them to the appropriate organelle. What is this protein?

Accepted Answer

v-SNARE

Answer

t-SNARE

Answer

Coatomer

Answer

Clathrin

Question 4

Which of the following best describes exocytosis?

Accepted Answer

Extrusion of cell-bound vesicles

Answer

Inclusion of liquid particles

Answer

Inclusion of solid particles

Answer

All of the above

Question 5

IP-3 facilitates the entry of which of the following ions into the cytoplasm?

Accepted Answer

Ca2+

Answer

Na+

Answer

K+

Answer

Mg2+

Question 6

Across the cell membrane of a skeletal muscle cell in the resting state with a non-electrogenic pump (1 Na+ : 1K+), which of the following statements is true?

Accepted Answer

The rate of sodium influx is equal to the rate of potassium efflux.

Answer

Both sodium and potassium are in electrochemical equilibrium.

Answer

The rate of sodium influx is much less than the rate of potassium efflux.

Answer

The rate of sodium influx is much greater than the rate of potassium efflux.

Question 7

All of the following are true about gap junctions except:

Accepted Answer

Opening of gap junctions does not depend on transmembrane voltage.

Answer

Permits passage of molecules up to 1000 Da molecular weight.

Answer

Halothane closes gap junctions reversibly.

Answer

A gap junction is made up of connexons, each made of six identical connexins.

Question 8

Which of the following is NOT a component of the basement membrane?

Accepted Answer

Rhodopsin

Answer

Tenascin

Answer

Laminin

Answer

Enatactin (Nidogen)

Question 9

Which of the following statements is NOT true about microtubules?

Accepted Answer

Do not require GTP

Answer

Exhibit dynamic instability

Answer

Possess polarity

Answer

Are charged

Question 10

All of the following statements about phagocytosis are true, except:

Accepted Answer

Used to ingest particles < 0.5 microns in size.

Answer

Amoeba and other unicellular protozoa make their living out of this process.

Answer

Used to ingest particles > 0.5 microns in size.

Answer

Digestion occurs within phagolysosomes.

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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