Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 21: A granular cytoplasmic reticulum is involved in the synthesis of which substance?

A. Protein
B. Lipid (Correct Answer)
C. Vitamin
D. Carbohydrate

Explanation: **Explanation:** The question refers to the **Agranular Endoplasmic Reticulum (AER)**, more commonly known as the **Smooth Endoplasmic Reticulum (SER)**. The term "agranular" signifies the absence of ribosomes on its surface, which is the defining characteristic that dictates its function. **Why Lipid is correct:** The Smooth ER is the primary site for the synthesis of **lipids, phospholipids, and cholesterol**. In specialized cells, it plays a crucial role in synthesizing **steroid hormones** (derived from cholesterol), such as testosterone in Leydig cells and cortisol in the adrenal cortex. It also facilitates the synthesis of triglycerides and is involved in detoxification processes in the liver. **Why other options are incorrect:** * **Protein:** Protein synthesis is the primary function of the **Granular (Rough) Endoplasmic Reticulum (RER)**. The "granules" are ribosomes, which translate mRNA into polypeptide chains. * **Vitamin:** While the liver (which is rich in SER) stores certain vitamins (like Vitamin A), the synthesis of vitamins is generally not a function of the endoplasmic reticulum. Most vitamins are obtained through diet or synthesized via complex pathways (e.g., Vitamin D via UV light and skin/liver/kidney interaction). * **Carbohydrate:** While the SER is involved in **glycogenolysis** (breakdown of glycogen via glucose-6-phosphatase), the primary synthesis of complex carbohydrates usually occurs in the **Golgi apparatus** (glycosylation). **High-Yield NEET-PG Pearls:** * **Sarcoplasmic Reticulum:** A specialized form of SER in muscle cells that stores and releases **Calcium ($Ca^{2+}$)** for contraction. * **Detoxification:** The SER contains the **Cytochrome P450** enzyme system, essential for metabolizing drugs and toxins. * **Nissl Bodies:** These are large clusters of **Rough ER** found in neurons; they are absent in the axon and axon hillock.

Question 22: A bilipid layer is most permeable to which of the following substances?

A. Potassium
B. Sodium
C. Glucose
D. Urea (Correct Answer)

Explanation: **Explanation:** The permeability of a cell membrane (a phospholipid bilayer) is determined by the **physicochemical properties** of the solute. The lipid bilayer is hydrophobic; therefore, substances that are small, non-polar, or uncharged cross most easily via simple diffusion. **Why Urea is Correct:** Urea is a **small, uncharged polar molecule**. While it is polar, its small molecular size and lack of net charge allow it to penetrate the lipid bilayer significantly more easily than ions or large polar molecules. In the hierarchy of membrane permeability, small uncharged molecules (like Urea, $H_2O$, and $CO_2$) have higher permeability coefficients than larger molecules or charged ions. **Analysis of Incorrect Options:** * **Sodium ($Na^+$) and Potassium ($K^+$):** These are small but carry a **net charge**. Charged ions are surrounded by a hydration shell, making them highly hydrophilic. They cannot dissolve in the hydrophobic fatty acid tails of the bilayer and require specific channels or transporters to cross. * **Glucose:** While uncharged, glucose is a **large, polar molecule**. Its size prevents it from squeezing through the lipid bilayer, necessitating facilitated diffusion via GLUT transporters. **NEET-PG High-Yield Pearls:** 1. **Permeability Hierarchy:** Hydrophobic molecules ($O_2, N_2, \text{benzene}$) > Small uncharged polar molecules ($\text{Urea}, H_2O, \text{Glycerol}$) > Large uncharged polar molecules ($\text{Glucose}$) > Ions ($H^+, Na^+, K^+, Cl^-$). 2. **Overton’s Rule:** The more lipid-soluble a substance is (higher oil-water partition coefficient), the greater its permeability. 3. **Clinical Note:** While urea can cross the bilayer slowly by simple diffusion, specialized transporters (**UT-A and UT-B**) exist in the kidneys to facilitate rapid movement, essential for the countercurrent multiplier system.

Question 23: Regarding the transport of calcium (Ca++) across a membrane, which of the following statements is true?

A. It is a passive transport.
B. It is a symport.
C. It maintains a high intracellular Ca++ concentration.
D. It requires hydrolysis of ATP. (Correct Answer)

Explanation: **Explanation:** The transport of Calcium ($Ca^{2+}$) across cell membranes is a fundamental process in cellular physiology, primarily governed by **Active Transport** mechanisms. **Why Option D is Correct:** Intracellular calcium levels are kept extremely low (~$10^{-7}$ mol/L) compared to extracellular levels (~$10^{-3}$ mol/L). Moving $Ca^{2+}$ out of the cytosol against this steep electrochemical gradient requires energy. This is achieved via **Primary Active Transport**, specifically the **$Ca^{2+}$-ATPase pump** (SERCA in the sarcoplasmic reticulum and PMCA on the plasma membrane). These pumps directly utilize the **hydrolysis of ATP** to transport calcium ions. **Analysis of Incorrect Options:** * **A. It is a passive transport:** Passive transport (facilitated diffusion) only occurs when $Ca^{2+}$ enters the cell through channels (e.g., voltage-gated channels). However, the *maintenance* of the gradient and the bulk of transport out of the cytosol is an active process. * **B. It is a symport:** Calcium transport is typically an **antiport** mechanism (Secondary Active Transport), such as the **Sodium-Calcium Exchanger (NCX)**, which moves 3 $Na^+$ ions into the cell in exchange for 1 $Ca^{2+}$ ion moving out. * **C. It maintains a high intracellular $Ca^{2+}$ concentration:** This is factually incorrect. The transport mechanisms function specifically to maintain a **low** intracellular $Ca^{2+}$ concentration to prevent cytotoxicity and allow for precise signaling. **High-Yield NEET-PG Pearls:** * **SERCA Pump:** The Sarcoplasmic/Endoplasmic Reticulum $Ca^{2+}$ ATPase is inhibited by **Phospholamban** (in its unphosphorylated state). * **Calmodulin:** The primary intracellular binding protein for $Ca^{2+}$ that mediates various cellular effects. * **Digitalis Mechanism:** It inhibits $Na^+$-$K^+$ ATPase, leading to increased intracellular $Na^+$, which subsequently slows the $Na^+$-$Ca^{2+}$ exchanger (NCX), increasing intracellular $Ca^{2+}$ and cardiac contractility.

Question 24: In the physiologic system, nitric oxide is known to act through which second messenger system?

A. Cyclic AMP
B. Calcium ions
C. Cyclic GMP (Correct Answer)
D. Prostacyclins

Explanation: **Explanation:** Nitric Oxide (NO), also known as Endothelium-Derived Relaxing Factor (EDRF), is a potent vasodilator that functions as a gaseous signaling molecule. Unlike most hormones, NO is lipophilic and diffuses directly across cell membranes. **Why Cyclic GMP is Correct:** Once NO enters the target cell (typically vascular smooth muscle), it binds to and activates the enzyme **soluble Guanylyl Cyclase (sGC)**. This enzyme catalyzes the conversion of GTP to **cyclic GMP (cGMP)**. Increased levels of cGMP activate **Protein Kinase G (PKG)**, which leads to the dephosphorylation of myosin light chains and sequestration of intracellular calcium, resulting in smooth muscle relaxation and vasodilation. **Why Other Options are Incorrect:** * **Cyclic AMP (cAMP):** This is the second messenger for hormones like Glucagon, Epinephrine (via $\beta$ receptors), and PTH. While cAMP also causes vasodilation (e.g., via $\beta_2$ receptors), it is not the pathway for NO. * **Calcium ions:** Calcium usually acts as a messenger for contraction or secretion (via the $IP_3/DAG$ pathway). NO actually works to *decrease* cytosolic calcium to induce relaxation. * **Prostacyclins:** These are arachidonic acid derivatives ($PGI_2$) that act as local hormones. While they also cause vasodilation, they do so by increasing **cAMP**, not cGMP. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Nitrates:** Drugs like Nitroglycerin work by being converted into NO, thereby increasing cGMP. * **Sildenafil (Viagra):** Inhibits **Phosphodiesterase-5 (PDE-5)**, the enzyme that breaks down cGMP. This prolongs the effect of NO in the corpus cavernosum. * **NO Synthase (NOS):** NO is synthesized from the amino acid **L-Arginine** by the enzyme Nitric Oxide Synthase. * **ANP/BNP:** Atrial Natriuretic Peptide also uses cGMP as a second messenger, but it activates a *membrane-bound* (particulate) guanylyl cyclase rather than the soluble form used by NO.

Question 25: Meiotic crossover occurs in which of the following stages?

A. Leptotene
B. Zygotene
C. Pachytene (Correct Answer)
D. Diplotene

Explanation: ### Explanation The question refers to the stages of **Prophase I** of Meiosis I, which is the most critical phase for genetic diversity. **Correct Answer: C. Pachytene** Crossing over (recombination) is the exchange of genetic material between non-sister chromatids of homologous chromosomes. This process occurs specifically during the **Pachytene** stage. It is mediated by the formation of **recombination nodules** on the synaptonemal complex. By the end of this stage, the homologous chromosomes are linked at the points of crossing over, known as chiasmata. **Analysis of Incorrect Options:** * **A. Leptotene:** This is the first stage where chromatin begins to condense into visible threads ("thin thread" stage). No pairing or crossing over occurs here. * **B. Zygotene:** This stage is characterized by **Synapsis**, the physical pairing of homologous chromosomes to form a bivalent or tetrad. While the synaptonemal complex forms here, actual crossing over has not yet begun. * **D. Diplotene:** In this stage, the synaptonemal complex dissolves, and homologous chromosomes begin to separate. They remain attached only at the **Chiasmata** (the physical manifestation of previous crossing over). **High-Yield NEET-PG Pearls:** * **Sequence mnemonic:** **L**ittle **Z**ebra **P**lays **D**irty **D**ogs (**L**eptotene, **Z**ygotene, **P**achytene, **D**iplotene, **D**iakinesis). * **Oocyte Arrest:** Primary oocytes are arrested in the **Diplotene** stage (specifically the Dictyotene stage) from fetal life until ovulation. * **Synaptonemal Complex:** Forms in Zygotene, is fully functional in Pachytene, and disappears in Diplotene. * **Nondisjunction:** Most chromosomal errors (like Down Syndrome) occur due to failure of proper separation during Meiosis I.

Question 26: All of the following transport processes follow 'saturation kinetics' except:

A. Facilitated diffusion
B. Na+-Ca2+ exchanger
C. Simple diffusion (Correct Answer)
D. Na+-coupled active transport

Explanation: ### Explanation The concept of **saturation kinetics** (also known as the $V_{max}$ effect) applies to transport mechanisms that rely on **carrier proteins**. Since there are a finite number of carriers in a cell membrane, the rate of transport increases with solute concentration only until all binding sites are occupied. **1. Why Simple Diffusion is the Correct Answer:** Simple diffusion occurs either through the lipid bilayer or through open protein channels. It does **not** require a carrier protein to bind the solute. Therefore, the rate of transport is directly proportional to the concentration gradient and does not reach a plateau. It follows a linear relationship rather than a hyperbolic saturation curve. **2. Why the Other Options are Incorrect:** * **Facilitated Diffusion (Option A):** Uses specific carrier proteins (e.g., GLUT transporters) to move substances down their concentration gradient. Because it is carrier-mediated, it exhibits saturation, specificity, and competitive inhibition. * **Na+-Ca2+ Exchanger (Option B):** This is a form of secondary active transport (specifically counter-transport). It relies on a membrane protein to exchange ions; thus, it is limited by the number of available exchangers. * **Na+-coupled Active Transport (Option D):** This refers to secondary active transport (e.g., SGLT in the kidneys/intestine). These symporters have specific binding sites for sodium and the cotransported molecule, making them saturable. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Fick’s Law:** Governs simple diffusion. The rate is determined by surface area, concentration gradient, and membrane permeability, but **not** by carrier density. * **GLUT-4:** A classic example of facilitated diffusion; its translocation to the membrane is insulin-dependent. * **SGLT-2 Inhibitors (Dapagliflozin):** These drugs work by inhibiting a saturable carrier-mediated process in the proximal tubule to treat Diabetes Mellitus. * **Key Distinction:** If a question mentions "Carrier-mediated," it automatically implies **Saturation, Stereospecificity, and Competition.**

Question 27: Which of the following statements about facilitated diffusion is FALSE?

A. It occurs down the concentration gradient.
B. It does not require metabolic energy.
C. It can occur against an electrical gradient. (Correct Answer)
D. It is influenced by the charge of the molecule.

Explanation: ### Explanation **Facilitated diffusion** is a form of passive transport that utilizes specific carrier proteins or channels to move substances across the cell membrane. **Why Option C is the Correct (False) Statement:** Facilitated diffusion is strictly a **passive process**. By definition, passive transport moves solutes down their **electrochemical gradient** (a combination of both concentration and electrical gradients). It cannot move a substance against an electrical gradient; doing so would require the input of metabolic energy (ATP), which would classify the process as **active transport**. **Analysis of Incorrect Options:** * **Option A:** Facilitated diffusion occurs **down the concentration gradient** (from high to low concentration). Unlike simple diffusion, it is limited by the number of available carrier proteins (Vmax/saturation). * **Option B:** Since it is a passive process driven by kinetic energy and entropy, it **does not require metabolic energy (ATP)**. * **Option D:** The rate and direction of facilitated diffusion for ions are heavily **influenced by the charge** of the molecule and the membrane potential, as these factors constitute the electrical component of the electrochemical gradient. **NEET-PG High-Yield Pearls:** * **Key Examples:** Glucose transport via **GLUT** transporters (e.g., GLUT4 in muscle/adipose) and water transport via **Aquaporins**. * **Kinetics:** Unlike simple diffusion, facilitated diffusion shows **saturation kinetics** (Vmax) and **specificity**. * **Competitive Inhibition:** Because it uses carriers, it can be inhibited by structurally similar molecules (e.g., galactose can inhibit glucose transport). * **Insulin's Role:** Insulin increases glucose uptake in specific tissues by increasing the number of GLUT4 transporters via facilitated diffusion.

Question 28: According to Fick's law of diffusion, particle flux will decrease if there is an increase in the:

A. Particle's concentration difference across the membrane
B. Thickness of the membrane (Correct Answer)
C. Area of the membrane
D. Temperature of the solution

Explanation: ### Explanation Fick’s Law of Diffusion describes the rate at which molecules move across a biological membrane. The mathematical expression for the law is: **$J = \frac{D \cdot A \cdot \Delta C}{\Delta X}$** Where: * **$J$** = Rate of diffusion (Flux) * **$D$** = Diffusion coefficient (influenced by temperature and molecular size) * **$A$** = Surface area of the membrane * **$\Delta C$** = Concentration gradient * **$\Delta X$** = **Thickness of the membrane** (Diffusion distance) #### Why the Correct Answer is Right: According to the formula, the rate of diffusion ($J$) is **inversely proportional** to the thickness of the membrane ($\Delta X$). Therefore, as the thickness increases, the resistance to movement increases, causing the particle flux to **decrease**. #### Analysis of Incorrect Options: * **A. Concentration difference ($\Delta C$):** Diffusion is directly proportional to the gradient. A higher difference "pushes" more particles across, increasing flux. * **C. Area of the membrane ($A$):** A larger surface area provides more space for molecules to pass through simultaneously, increasing flux. * **D. Temperature:** Increasing temperature increases the kinetic energy of particles (increasing the diffusion coefficient $D$), which increases flux. #### Clinical Pearls & High-Yield Facts: * **Pulmonary Edema/Fibrosis:** In clinical practice, conditions like interstitial lung disease (fibrosis) or pulmonary edema increase the thickness of the blood-gas barrier. This decreases the diffusion of oxygen, leading to hypoxemia. * **Emphysema:** This condition decreases the **surface area** ($A$) available for gas exchange due to alveolar wall destruction, also leading to decreased flux. * **Lipid Solubility:** The diffusion coefficient ($D$) is also affected by the lipid solubility of the substance; CO₂ is 20 times more soluble than O₂, explaining why it diffuses faster despite a smaller pressure gradient.

Question 29: At equilibrium, the concentrations of Cl- inside and outside a cell are 8 mmol/L and 120 mmol/L, respectively. Calculate the equilibrium potential for Cl- at 37oC.

A. +4.07 mV
B. -4.07 mV
C. +71.7 mV
D. -71.7 mV (Correct Answer)

Explanation: ### Explanation The equilibrium potential ($E_x$) for an ion is the membrane potential at which the electrical gradient exactly balances the chemical concentration gradient, resulting in no net movement of the ion. This is calculated using the **Nernst Equation**. **1. Why Option D is Correct:** The Nernst Equation at body temperature ($37^\circ\text{C}$) is: $$E_{Cl^-} = \frac{-61.5}{z} \times \log_{10} \frac{[Cl^-]_{in}}{[Cl^-]_{out}}$$ *(Where $z$ is the valence of the ion. For Chloride, $z = -1$)*. Alternatively, using the simplified version for anions: $$E_{Cl^-} = -61.5 \times \log_{10} \frac{[Cl^-]_{out}}{[Cl^-]_{in}}$$ Substituting the values: $$E_{Cl^-} = -61.5 \times \log_{10} \frac{120}{8}$$ $$E_{Cl^-} = -61.5 \times \log_{10} (15)$$ Since $\log_{10}(15) \approx 1.176$: $$E_{Cl^-} = -61.5 \times 1.176 \approx -72.3 \text{ mV}$$ The closest value provided is **-71.7 mV**. The negative sign is crucial because the cell must be negatively charged to repel the high concentration of Cl⁻ ions outside and maintain equilibrium. **2. Why Other Options are Incorrect:** * **Options A & B:** These values are mathematically incorrect and do not reflect the significant concentration gradient (15-fold difference). * **Option C (+71.7 mV):** This represents the correct magnitude but the **wrong polarity**. A positive internal potential would pull Cl⁻ into the cell, rather than keeping it out. **3. High-Yield Clinical Pearls for NEET-PG:** * **Resting Membrane Potential (RMP):** In many neurons, the RMP is very close to the $E_{Cl^-}$, meaning Cl⁻ is often at or near equilibrium. * **GABA Receptors:** Activation of $GABA_A$ receptors increases Cl⁻ conductance. If $E_{Cl^-}$ is more negative than the RMP, Cl⁻ enters the cell, causing **hyperpolarization** (Inhibitory Post-Synaptic Potential). * **Nernst vs. Goldman Equation:** Use Nernst for a **single ion**; use the Goldman-Hodgkin-Katz (GHK) equation to calculate the **actual RMP** considering multiple ions (Na⁺, K⁺, Cl⁻).

Question 30: The ligand-receptor complex dissociates in the endosome because:

A. Of its large size
B. The vesicle loses its clathrin coat
C. Of the acidic pH of the vesicle (Correct Answer)
D. Of the basic pH of the vesicle

Explanation: ### Explanation **Concept Overview:** The process described is **Receptor-Mediated Endocytosis**. Once a ligand (e.g., LDL, insulin, or iron-transferrin) binds to its specific cell-surface receptor, the complex is internalized via clathrin-coated pits into an early endosome. The critical step for the intracellular sorting of these molecules is the **acidification** of the endosomal lumen. **Why Option C is Correct:** The membrane of the endosome contains **V-type H+ ATPase pumps** (Proton pumps) that actively transport hydrogen ions into the vesicle. This lowers the internal pH to approximately **5.0–6.0**. This acidic environment induces a conformational change in the receptor and/or ligand, reducing their binding affinity. This causes the ligand to dissociate from the receptor, allowing the receptor to be recycled back to the cell membrane while the ligand is sent to lysosomes for processing. **Why Other Options are Incorrect:** * **Option A:** Size does not dictate dissociation; chemical affinity, governed by pH and molecular structure, is the primary driver. * **Option B:** While the vesicle does lose its clathrin coat (becoming an "uncoated vesicle") shortly after internalization, this step is necessary for fusion with the endosome, not for the dissociation of the ligand from the receptor. * **Option D:** The endosomal environment is acidic, not basic. A basic pH would not trigger the necessary conformational changes for dissociation. **High-Yield NEET-PG Pearls:** * **CURL:** The endosome is often referred to as the **C**ompartment for **U**ncoupling of **R**eceptor and **L**igand. * **Exceptions:** Not all complexes dissociate. For example, the **Transferrin-Receptor** complex stays bound in the endosome; only the iron is released, and the "Apotransferrin-Receptor" complex is recycled together. * **Clinical Correlation:** Familial Hypercholesterolemia can result from defects in the LDL receptor's ability to internalize or dissociate, leading to high serum cholesterol.

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