Cellular Physiology Practice Questions

Q: Proteins are synthesized by:

Golgi Bodies. **Explanation:** The synthesis of proteins is a multi-step process involving different organelles. While ribosomes are the primary site of translation, the **Golgi Apparatus (Golgi Bodies)** plays a critical role in the final stages of protein synthesis, specifically **post-translational modification** and functional maturation. 1. **Why Golgi Bodies is Correct:** After polypeptide chains are formed on the ribosomes, they are transported to the Golgi apparatus. Here, they undergo essential modifications such as **glycosylation** (adding sugar moieties), sulfation, and phosphorylation. Without these modifications, proteins remain non-functional. Therefore, in the context of producing a *functional* protein ready for secretion or membrane integration, the Golgi is indispensable. 2. **Analysis of Incorrect Options:** * **Ribosomes:** These are the sites of **translation** (mRNA to polypeptide). While they assemble the amino acid chain, they do not complete the synthesis of complex, functional proteins. * **Mitochondria:** Known as the "Powerhouse of the cell," their primary role is ATP production via oxidative phosphorylation. Though they contain their own DNA and mitoribosomes, they synthesize only a tiny fraction (13 proteins) of the cell's total protein requirement. * **Nuclear Membrane:** This acts as a physical barrier protecting the genetic material and regulates nucleocytoplasmic transport; it is not a site for protein synthesis. **NEET-PG High-Yield Pearls:** * **Cis-face vs. Trans-face:** The Golgi has a 'Cis' face (entry/receiving) and a 'Trans' face (exit/shipping). * **I-Cell Disease:** A clinical correlation where a deficiency in phosphorylating enzymes in the Golgi leads to lysosomal storage issues. * **Marker Enzyme:** **Thiamine Pyrophosphatase** is the biochemical marker for the Golgi apparatus.

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: Modification of peptide structure of hormones takes place in which organelle?

Golgi apparatus. **Explanation:** The correct answer is **Golgi apparatus**. **Why Golgi apparatus is correct:** The synthesis of peptide hormones follows the central dogma of molecular biology, but the final functional form is achieved through post-translational modifications. While the initial polypeptide chain is synthesized in the ribosomes and folded in the Rough Endoplasmic Reticulum (RER), the **Golgi apparatus** is the primary site for the "finishing touches." These modifications include **proteolysis** (cleaving pro-hormones into active hormones, e.g., Proinsulin to Insulin), **glycosylation** (adding carbohydrate moieties), **sulfation**, and **phosphorylation**. It also acts as the sorting and packaging center, directing hormones into secretory vesicles. **Why other options are incorrect:** * **Endoplasmic Reticulum (ER):** The RER is the site of **translation** and initial folding of the pre-pro-hormone. It primarily handles the removal of the "signal sequence" to form a pro-hormone, but complex structural modifications occur later in the Golgi. * **Ribosomes:** These are the sites of **protein synthesis (translation)** where amino acids are assembled into a linear polypeptide chain based on mRNA templates. They do not perform structural modifications. * **Nucleolus:** This is a sub-nuclear structure responsible for **rRNA synthesis** and ribosome biogenesis; it is not involved in protein modification. **High-Yield NEET-PG Pearls:** * **Golgi Functions:** Think of the Golgi as the "Post Office" of the cell (Packaging, Post-translational modification, and Polarity/Sorting). * **I-Cell Disease:** A clinical correlation where a deficiency in phosphorylating enzymes in the Golgi leads to failure of lysosomal enzyme targeting. * **Proinsulin to Insulin:** This classic example of peptide modification (cleavage of C-peptide) occurs within the Golgi and maturing secretory granules.

Q: Glucose is co-transported with Na+ ions. This is a type of?

Secondary active transport. **Explanation:** The correct answer is **Secondary active transport (A)**. This process occurs when a transport protein moves two different molecules across a membrane simultaneously. In the case of glucose and Na+, the transport protein (SGLT) uses the **electrochemical gradient of sodium** (created by the Na+/K+ ATPase pump) as the energy source to move glucose against its concentration gradient. It is "secondary" because it does not use ATP directly, but relies on the energy stored in a gradient previously established by primary active transport. **Why other options are incorrect:** * **Primary active transport (B):** This involves the direct hydrolysis of ATP to move substances against a gradient (e.g., the Na+/K+ ATPase pump). Glucose transport itself does not hydrolyze ATP. * **Facilitated diffusion (C):** This is a passive process where molecules move down their concentration gradient via a carrier protein (e.g., **GLUT** transporters). It does not require energy. * **Simple diffusion (D):** This is the movement of small, non-polar molecules (like O2 or CO2) directly through the lipid bilayer without the help of a protein. **High-Yield Facts for NEET-PG:** * **SGLT-1:** Located in the small intestine (for glucose absorption) and the late proximal tubule of the kidney. * **SGLT-2:** Located in the early proximal tubule (S1 segment) of the kidney; it is the target for **SGLT-2 inhibitors** (e.g., Dapagliflozin) used in Diabetes Mellitus. * **Oral Rehydration Solution (ORS):** Its efficacy is based on this specific co-transport mechanism; Na+ absorption via SGLT-1 enhances water absorption. * **Symport vs. Antiport:** Glucose-Na+ transport is a **Symport** (both move in the same direction), whereas the Na+-Ca2+ exchanger is an **Antiport**.

Q: Action potentials are generated in which of the following excitable cells?

All of the above. ### Explanation **The Concept of Excitability** Excitability is the physiological ability of a cell to respond to a stimulus by generating an **action potential (AP)**—a rapid change in the resting membrane potential. This property is characteristic of cells with voltage-gated ion channels. **Why "All of the Above" is Correct:** 1. **Nerve Cells (Neurons):** These are the classic examples of excitable tissue. They generate APs primarily at the axon hillock to transmit signals over long distances via neurotransmitter release. 2. **Muscle Cells:** All three types (skeletal, cardiac, and smooth) are excitable. In skeletal muscle, the AP triggers calcium release for contraction (excitation-contraction coupling). Cardiac cells exhibit unique APs with plateaus or spontaneous depolarization (pacemaker activity). 3. **Gland Cells:** While often overlooked, many endocrine and exocrine cells (e.g., **Pancreatic Beta cells**, Anterior Pituitary cells) are electrically excitable. In these cells, an AP triggers the opening of voltage-gated calcium channels, leading to the exocytosis of hormones or enzymes (excitation-secretion coupling). **High-Yield NEET-PG Pearls:** * **Resting Membrane Potential (RMP):** Primarily determined by **K+ efflux** through leak channels. * **Depolarization Phase:** Usually due to **Na+ influx** (nerves/skeletal muscle) or **Ca2+ influx** (SA node/smooth muscle). * **Threshold Stimulus:** The minimum intensity of a stimulus required to generate an AP (typically -55mV to -65mV). * **All-or-None Law:** Once the threshold is reached, the AP occurs at maximum amplitude regardless of the stimulus intensity. This applies to single nerve fibers and muscle fibers, but **not** to whole nerves or whole muscles.

Question 1

Proteins are synthesized by:

Accepted Answer

Golgi Bodies

Answer

Mitochondria

Answer

Ribosomes

Answer

Nuclear Membrane

Question 2

Which is the longest phase of meiosis?

Accepted Answer

Prophase I

Answer

Prometaphase

Answer

Metaphase I

Answer

Anaphase I

Question 3

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 4

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 5

The sodium-potassium pump is a type of:

Accepted Answer

Primary active transport

Answer

Passive transport

Answer

Secondary active transport

Answer

Counter transport

Question 6

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 7

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 8

Modification of peptide structure of hormones takes place in which organelle?

Accepted Answer

Golgi apparatus

Answer

Endoplasmic reticulum

Answer

Ribosomes

Answer

Nucleolus

Question 9

Glucose is co-transported with Na+ ions. This is a type of?

Accepted Answer

Secondary active transport

Answer

Primary active transport

Answer

Facilitated diffusion

Answer

Simple diffusion

Question 10

Action potentials are generated in which of the following excitable cells?

Accepted Answer

All of the above

Answer

Nerve cells

Answer

Muscle cells

Answer

Gland cells

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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