Cellular Physiology Practice Questions

Q: Which is the longest phase of meiosis?

Prophase I. **Explanation:** **Prophase I** is the correct answer because it is the most complex and time-consuming stage of meiosis. Unlike mitosis, where prophase is brief, Prophase I involves critical genetic processes that ensure diversity. It is subdivided into five distinct substages: **Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.** The primary reason for its duration is the occurrence of **synapsis** (pairing of homologous chromosomes) and **crossing over** (exchange of genetic material). Specifically, the **Pachytene** stage is often cited as the longest within Prophase I, as this is where recombination occurs. In females, Prophase I is exceptionally long; primary oocytes remain arrested in the Diplotene stage (Dictyotene) from fetal life until ovulation begins at puberty. **Analysis of Incorrect Options:** * **Prometaphase:** This is a short transitional phase where the nuclear envelope breaks down and spindles attach to kinetochores. * **Metaphase I:** A brief period where homologous pairs align at the equatorial plate. * **Anaphase I:** A rapid phase characterized by the migration of homologous chromosomes to opposite poles. **High-Yield Facts for NEET-PG:** * **Pachytene:** Stage where crossing over occurs (mediated by the recombinase enzyme). * **Diplotene:** Stage where **Chiasmata** (X-shaped structures) become visible. * **Oocyte Arrest:** Primary oocytes are arrested in **Prophase I (Diplotene)** until puberty; secondary oocytes are arrested in **Metaphase II** until fertilization. * **Nondisjunction:** Most chromosomal abnormalities (like Down Syndrome) occur due to errors during Anaphase I.

Q: Centrosome duplication takes place in which phase of the cell cycle?

S phase. ### Explanation The correct answer is **S phase (Synthesis phase)**. **1. Why S phase is correct:** The cell cycle requires precise coordination between DNA replication and the duplication of the microtubule-organizing center (the centrosome) to ensure genomic stability. Centrosome duplication begins at the **G1/S transition** and is completed during the **S phase**, occurring concurrently with DNA replication. This synchronization ensures that when the cell enters mitosis, it possesses exactly two centrosomes to form the poles of the bipolar mitotic spindle. **2. Analysis of Incorrect Options:** * **M phase (Mitosis):** This is the stage where the duplicated centrosomes separate and migrate to opposite poles to organize the spindle fibers. No duplication occurs here; rather, the existing centrosomes are utilized. * **G0 phase:** This is a quiescent or "resting" phase where the cell has exited the cycle. Cells in G0 have only one centrosome (comprising two centrioles). * **G2 phase:** While the centrosomes undergo "maturation" (recruiting more gamma-tubulin) during G2 to prepare for mitosis, the actual duplication process is already complete by the end of the S phase. **3. High-Yield NEET-PG Pearls:** * **The Trigger:** Centrosome duplication is triggered by the same cyclin-dependent kinase complex that initiates DNA replication: **Cyclin E-CDK2**. * **Semiconservative:** Like DNA, centrosome duplication is semiconservative; each daughter centrosome contains one "old" (mother) centriole and one newly formed "young" (daughter) centriole. * **Clinical Correlation:** Centrosome **amplification** (having >2 centrosomes) is a hallmark of many cancer cells, leading to multipolar spindles and chromosomal instability (aneuploidy). * **Key Protein:** **PLK4** (Polo-like kinase 4) is the master regulator of centriole duplication.

Q: What is the primary second messenger through which nitric oxide exerts its action?

Cyclic guanosine monophosphate (cGMP). **Explanation:** Nitric Oxide (NO) is a potent vasodilator and gasotransmitter. Its mechanism of action is a high-yield topic for NEET-PG. **Why cGMP is correct:** Nitric oxide is produced in endothelial cells and diffuses into adjacent smooth muscle cells. Once inside, it binds to and activates the enzyme **soluble Guanylyl Cyclase (sGC)**. This enzyme catalyzes the conversion of GTP into **Cyclic Guanosine Monophosphate (cGMP)**. Increased levels of cGMP activate Protein Kinase G (PKG), which leads to dephosphorylation of myosin light chains and sequestration of calcium, ultimately resulting in **smooth muscle relaxation (vasodilation)**. **Why other options are incorrect:** * **cAMP:** This is the second messenger for hormones like Glucagon, PTH, and catecholamines acting on $\beta$-receptors. It activates Protein Kinase A (PKA). * **Calcium ions (Ca++):** While NO synthesis is often calcium-dependent (via the Calmodulin complex), calcium itself acts as a messenger for muscle *contraction* and neurotransmitter release, opposing the primary relaxant effect of the NO-cGMP pathway. * **Tyrosine:** This is an amino acid. Tyrosine kinases are receptors for insulin and various growth factors, not second messengers for NO. **High-Yield Clinical Pearls for NEET-PG:** * **Sildenafil (Viagra):** Inhibits **Phosphodiesterase-5 (PDE-5)**, the enzyme that breaks down cGMP. This prolongs NO-mediated vasodilation. * **Nitroglycerin:** Acts as a prodrug that is metabolized to release NO, used in angina to cause venodilation and reduce preload. * **Enzyme Source:** NO is synthesized from the amino acid **L-arginine** by the enzyme Nitric Oxide Synthase (NOS).

Q: The p unit of the Na+ - K+ pump contains a binding site for which of the following?

Glycosylation. ### Explanation The **Na⁺-K⁺ ATPase (Sodium-Potassium Pump)** is a P-type transport ATPase consisting of three subunits: **Alpha (α)**, **Beta (β)**, and **Gamma (γ)**. **Why Glycosylation is Correct:** The **$\beta$ subunit** (often referred to as the "p unit" in some texts or shorthand for the glycosylated polypeptide) is a transmembrane glycoprotein. Its primary role is the **proper folding, assembly, and membrane trafficking** of the enzyme complex to the plasma membrane. The extracellular domain of the $\beta$ subunit contains essential **glycosylation sites**. Without this glycosylation, the $\alpha$ subunit cannot be stabilized or correctly inserted into the cell membrane. **Analysis of Incorrect Options:** * **A (Na⁺) & B (K⁺):** The binding sites for both Sodium (3 ions) and Potassium (2 ions) are located exclusively on the **Alpha ($\alpha$) subunit**, which is the large catalytic subunit. * **C (ATP):** The ATP binding site and the phosphorylation site (Aspartate residue) are also located on the **Alpha ($\alpha$) subunit**. This is why the $\alpha$ subunit is known as the "catalytic" subunit. **High-Yield Clinical Pearls for NEET-PG:** * **Stoichiometry:** The pump moves **3 Na⁺ OUT** and **2 K⁺ IN** for every 1 ATP hydrolyzed, making it **electrogenic** (creates a net negative charge inside). * **Inhibitors:** **Cardiac glycosides** (e.g., Digoxin, Ouabain) bind to the extracellular side of the **Alpha subunit**, specifically when it is in the phosphorylated state. * **Energy Consumption:** In a resting state, this pump accounts for approximately **20-30%** of the total energy expenditure in most cells (up to 70% in neurons). * **Subunit Function:** Remember: **$\alpha$ = Action** (Catalytic/Binding); **$\beta$ = Biogenesis** (Folding/Targeting).

Q: Which of the following causes an increase in intracellular cAMP?

Beta receptor activation. ### Explanation The concentration of intracellular cyclic Adenosine Monophosphate (cAMP) is regulated by the **G-protein coupled receptor (GPCR)** signaling pathway. **1. Why Beta receptor activation is correct:** Beta-adrenergic receptors ($\beta_1, \beta_2, \beta_3$) are coupled to **Gs (stimulatory) proteins**. When a ligand (like epinephrine) binds, the Gs alpha subunit activates the enzyme **Adenylate Cyclase**, which catalyzes the conversion of ATP into **cAMP**. Increased cAMP then activates Protein Kinase A (PKA) to mediate cellular effects like increased heart rate or bronchodilation. **2. Why the other options are incorrect:** * **Somatostatin:** Acts via **Gi (inhibitory) proteins**. Activation of Gi inhibits Adenylate Cyclase, leading to a **decrease** in intracellular cAMP levels. * **Alpha receptor activation:** * $\alpha_2$ receptors are coupled to **Gi**, which **decreases** cAMP. * $\alpha_1$ receptors are coupled to **Gq**, which activates Phospholipase C (PLC) to increase $IP_3/DAG$ and Calcium, not cAMP. * **Acetylcholine:** * At **$M_2$ receptors** (heart), it acts via **Gi** to **decrease** cAMP. * At **$M_1/M_3$ receptors**, it acts via **Gq** ($IP_3/DAG$ pathway). **Clinical Pearls for NEET-PG:** * **Gs-coupled (Increase cAMP):** Beta receptors, Glucagon, TSH, ACTH, PTH, Vasopressin ($V_2$). * **Gi-coupled (Decrease cAMP):** $\alpha_2$, $M_2$, Somatostatin, Dopamine ($D_2$). * **Gq-coupled ($IP_3/DAG$):** $\alpha_1$, $M_1$, $M_3$, Vasopressin ($V_1$), Oxytocin. * **Phosphodiesterase (PDE) inhibitors** (e.g., Theophylline, Sildenafil) increase cAMP/cGMP by preventing their breakdown.

Q: Which one of the following does NOT primarily involve the action of K+ channels?

Dantrolene in the treatment of Malignant Hyperthermia. **Explanation:** The correct answer is **Dantrolene in the treatment of Malignant Hyperthermia**, as its mechanism of action involves Calcium (Ca²⁺) signaling rather than Potassium (K⁺) channels. **1. Why Option D is Correct:** Malignant Hyperthermia is caused by a mutation in the **Ryanodine Receptor (RyR1)** in the sarcoplasmic reticulum of skeletal muscle. This leads to excessive Ca²⁺ release, causing sustained muscle contraction and hypermetabolism. **Dantrolene** acts by binding to the RyR1 receptor, inhibiting the release of Ca²⁺. It has no direct primary action on K⁺ channels. **2. Why the other options are incorrect:** * **Hypoxic Pulmonary Vasoconstriction (HPV):** Hypoxia inhibits **Voltage-gated K⁺ channels** in pulmonary artery smooth muscle cells. This leads to depolarization, opening of L-type Ca²⁺ channels, and subsequent vasoconstriction to shunt blood to better-ventilated areas. * **Insulin secretion by Sulfonylureas:** Sulfonylureas bind to the SUR1 subunit of **ATP-sensitive K⁺ (Kₐₜₚ) channels** in pancreatic beta cells, closing them. This causes depolarization, Ca²⁺ influx, and insulin exocytosis. * **Carotid body discharge:** In response to low PO₂, **O₂-sensitive K⁺ channels** in Type I (glomus) cells close. The resulting depolarization triggers neurotransmitter release, stimulating the glossopharyngeal nerve to increase ventilation. **Clinical Pearls for NEET-PG:** * **Malignant Hyperthermia triggers:** Volatile anesthetics (e.g., Halothane) and Succinylcholine. * **Kₐₜₚ Channels:** These are the "metabolic sensors" of the cell; they close when the ATP/ADP ratio increases. * **Dantrolene** is also used in **Neuroleptic Malignant Syndrome (NMS)**, though its primary indication remains Malignant Hyperthermia.

Q: Which of the following ions is least diffusible?

Na+. **Explanation:** The diffusibility of an ion across a cell membrane depends on its **permeability**, which is determined by the presence of specific leak channels and the size of the ion’s **hydration shell**. **1. Why Na+ is the correct answer:** Although the sodium ion ($Na^+$) is physically smaller than the potassium ion ($K^+$), it has a much higher **charge density**. This causes it to attract a larger shell of water molecules (hydration shell). This large hydrated radius, combined with a **very low density of sodium leak channels** in the resting membrane, makes $Na^+$ the least diffusible among the options. In a resting state, the membrane is roughly 50–100 times less permeable to $Na^+$ than to $K^+$. **2. Analysis of Incorrect Options:** * **K+ (Potassium):** The resting cell membrane is highly permeable to $K^+$ due to the abundance of **K+ leak channels**. It is the most diffusible cation and is the primary determinant of the Resting Membrane Potential (RMP). * **Cl- (Chloride):** Many cells have high permeability to $Cl^-$, and it can diffuse relatively easily through specific channels to follow electrochemical gradients. * **Urea:** Urea is a small, uncharged polar molecule. While it is slower than water, it can cross the lipid bilayer via simple diffusion or through specialized transporters (UT-A), making it significantly more diffusible than the charged $Na^+$ ion. **High-Yield Clinical Pearls for NEET-PG:** * **Relative Permeability:** $K^+ > Cl^- > Na^+$. * **Gibbs-Donnan Effect:** Large intracellular anions (proteins) are non-diffusible, leading to the unequal distribution of diffusible ions. * **RMP Calculation:** The **Goldman-Hodgkin-Katz equation** is used instead of the Nernst equation when considering the permeability of multiple ions ($K^+$, $Na^+$, and $Cl^-$). * **Hydration Rule:** Smaller ions (like $Na^+$) have larger hydration shells than larger ions (like $K^+$), making the hydrated $Na^+$ physically bulkier.

Q: Which molecule is most permeable to a pure phospholipid bilayer?

O2. **Explanation:** The permeability of a molecule through a pure phospholipid bilayer is determined by its **size, charge, and lipid solubility (hydrophobicity)**. The cell membrane is a semi-permeable lipid barrier that favors the passage of non-polar, uncharged substances. **1. Why O2 is Correct:** Oxygen (O2) is a small, non-polar, and highly lipid-soluble gas. According to **Fick’s Law of Diffusion**, such molecules can dissolve directly into the hydrophobic fatty acid tails of the phospholipid bilayer and diffuse rapidly. Other gases like CO2 and N2 follow the same principle. **2. Why the others are Incorrect:** * **Na+ and Cl- (Options B & D):** These are small ions, but they carry a **net charge**. Charged particles are surrounded by a hydration shell and are strongly repelled by the hydrophobic core of the membrane. They require specific transmembrane proteins (channels or pumps) to cross. * **H2O (Option C):** Water is a small, uncharged molecule, but it is **polar**. While it can leak through the membrane slowly via simple diffusion, its permeability is significantly lower than that of dissolved gases. In physiological systems, water primarily moves through specialized channels called **Aquaporins**. **High-Yield Facts for NEET-PG:** * **Permeability Hierarchy:** Hydrophobic molecules (O2, CO2, Steroids) > Small uncharged polar molecules (H2O, Urea) > Large uncharged polar molecules (Glucose) > Ions (Na+, K+, Cl-). * **Lipid Solubility:** This is the single most important factor determining the rate of simple diffusion (measured by the oil-water partition coefficient). * **Clinical Pearl:** General anesthetics (like Halothane) work on the principle of high lipid solubility (Meyer-Overton theory) to cross the blood-brain barrier and cell membranes rapidly.

Question 1

Which is the longest phase of meiosis?

Accepted Answer

Prophase I

Answer

Prometaphase

Answer

Metaphase I

Answer

Anaphase I

Question 2

Centrosome duplication takes place in which phase of the cell cycle?

Accepted Answer

S phase

Answer

M phase

Answer

G0 phase

Answer

G2 phase

Question 3

What is the primary second messenger through which nitric oxide exerts its action?

Accepted Answer

Cyclic guanosine monophosphate (cGMP)

Answer

Cyclic adenosine monophosphate (cAMP)

Answer

Calcium ions (Ca++)

Answer

Tyrosine

Question 4

The p unit of the Na+ - K+ pump contains a binding site for which of the following?

Accepted Answer

Glycosylation

Answer

Na+

Answer

K+

Answer

ATP

Question 5

What is the effect on the resting membrane potential (RMP) when the extracellular concentration of potassium (K+) is decreased?

Accepted Answer

Decreased magnitude of RMP

Answer

Increased negativity of the membrane

Answer

Increased magnitude of RMP

Answer

Decreased negativity of the membrane

Question 6

Which of the following causes an increase in intracellular cAMP?

Accepted Answer

Beta receptor activation

Answer

Somatostatin

Answer

Alpha receptor activation

Answer

Acetylcholine

Question 7

Which one of the following does NOT primarily involve the action of K+ channels?

Accepted Answer

Dantrolene in the treatment of Malignant Hyperthermia

Answer

Hypoxic Pulmonary Vasoconstriction

Answer

Insulin secretion by Sulfonylurea

Answer

Carotid body discharge with reduced O2

Question 8

Which of the following ions is least diffusible?

Accepted Answer

Na+

Answer

K+

Answer

Cl-

Answer

Urea

Question 9

Which molecule is most permeable to a pure phospholipid bilayer?

Accepted Answer

O2

Answer

Na+

Answer

H2O

Answer

Cl-

Question 10

Various cells respond differentially to a second messenger (such as increased cAMP) because they have different what?

Accepted Answer

Enzymatic composition

Answer

Receptors

Answer

Nuclei

Answer

Membrane lipids

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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