Cellular Physiology Practice Questions

Q: What is ABC transporter?

P glycoprotein. **Explanation:** **ABC (ATP-Binding Cassette) transporters** represent a large family of primary active transporters that utilize energy derived from ATP hydrolysis to move substrates across cellular membranes against their concentration gradients. **Why P-glycoprotein is correct:** **P-glycoprotein (P-gp)**, also known as **MDR1** (Multidrug Resistance Protein 1), is the most well-characterized member of the ABC transporter superfamily. It functions as an efflux pump, actively transporting various hydrophobic drugs and xenobiotics out of cells. In clinical medicine, its over-expression in cancer cells is a primary cause of resistance to chemotherapy, as it pumps drugs like vincristine and doxorubicin out of the cell before they can take effect. **Analysis of Incorrect Options:** * **Membrane sparing:** This is not a recognized physiological term for a transporter type. * **Channel:** Channels (like ion channels) facilitate passive transport (facilitated diffusion) and do not require ATP hydrolysis, whereas ABC transporters are active pumps. * **Adenylyl cyclase:** This is an enzyme (not a transporter) that converts ATP to cyclic AMP (cAMP) in response to G-protein signaling. **High-Yield NEET-PG Facts:** * **CFTR (Cystic Fibrosis Transmembrane Conductance Regulator):** This is a unique member of the ABC family because it functions as a **chloride channel** rather than a pump, though it still requires ATP binding to open. * **Substrate Diversity:** ABC transporters move a wide range of substances, including ions, lipids, peptides, and drugs. * **Clinical Relevance:** Mutations in ABC transporters lead to diseases such as **Cystic Fibrosis** (CFTR), **Tangier disease** (ABCA1), and **Dubin-Johnson syndrome** (MRP2/ABCC2).

Q: During which phase of the cell cycle are chromosomes formed?

M. ### Explanation **Correct Option: D (M Phase)** The formation of distinct, visible chromosomes occurs during the **M (Mitotic) phase**, specifically during **Prophase**. Throughout the interphase (G1, S, G2), DNA exists in a loosely coiled, thread-like state called **chromatin**. For successful cell division, this chromatin must undergo extreme condensation and supercoiling to form the compact structures we recognize as chromosomes. This condensation ensures that the genetic material is organized enough to be separated into two daughter cells without entangling or breaking. **Analysis of Incorrect Options:** * **A (G1 Phase):** This is the "Gap 1" phase characterized by cell growth and RNA/protein synthesis. DNA remains as diffuse chromatin to allow for gene transcription. * **B (S Phase):** This is the "Synthesis" phase where DNA replication occurs. While the amount of DNA doubles (2n to 4n), it remains in an uncoiled chromatin state to allow DNA polymerase access to the strands. * **C (G2 Phase):** This is the "Gap 2" phase where the cell prepares for mitosis by synthesizing tubulin for spindle fibers. Chromatin condensation begins at the very end of G2, but defined chromosomes are the hallmark of the M phase. **High-Yield NEET-PG Pearls:** * **Karyotyping:** Since chromosomes are at their maximum state of condensation during **Metaphase** (a sub-phase of M), this is the stage used for clinical karyotyping to detect numerical or structural abnormalities. * **Colchicine:** A common exam topic; it inhibits mitosis by arresting cells in metaphase (by inhibiting microtubule polymerization), making it useful for chromosomal studies. * **Checkpoints:** The transition from G2 to M is regulated by the **Cyclin B-CDK1** complex (also known as Maturation Promoting Factor).

Q: Which of the following phases represents the restriction point in the cell cycle?

G1 to S. **Explanation:** The **Restriction Point (R point)**, also known as the Start point in yeast, occurs during the **late G1 phase**. It is the critical "point of no return" where the cell commits to the cell cycle independent of external growth factors. **1. Why G1 to S is correct:** Before this point, the cell requires continuous stimulation from growth factors to progress. Once the cell passes the R point, it activates **Cyclin D-CDK4/6 complexes**, which phosphorylate the **Retinoblastoma (Rb) protein**. Phosphorylated Rb releases the transcription factor **E2F**, triggering the expression of genes required for DNA synthesis (S phase). After this transition, the cell is committed to completing the division even if growth factors are removed. **2. Analysis of Incorrect Options:** * **G0 to G1:** This represents "re-entry" into the cell cycle from a quiescent state (e.g., hepatocytes after partial hepatectomy), but it is not the commitment point for DNA replication. * **S to G2:** This transition ensures DNA replication is complete. The major checkpoint here is the DNA damage checkpoint, not the restriction point. * **G2 to M:** This is the second major regulatory threshold (regulated by **Cyclin B-CDK1** or MPF). It ensures the cell is large enough and DNA is fully replicated before mitosis begins. **3. NEET-PG High-Yield Pearls:** * **Rb Protein:** Known as the "Guardian of the Restriction Point." Hypophosphorylated Rb is active (stops cycle); Hyperphosphorylated Rb is inactive (allows cycle). * **p53:** Acts primarily at the G1-S checkpoint by inducing **p21** (a CDK inhibitor) in response to DNA damage. * **Clinical Link:** Mutations in the Rb gene or p16 (inhibitor of CDK4) lead to uncontrolled passage through the R point, a hallmark of many cancers (e.g., Retinoblastoma, Osteosarcoma).

Q: Which of the following substances moves most rapidly across cell membranes?

Carbon dioxide (CO2). The movement of substances across a cell membrane is primarily governed by the **lipid solubility** and the **molecular size** of the substance. ### Why Carbon Dioxide (CO2) is Correct Carbon dioxide is a small, non-polar, and highly **lipid-soluble** gas. According to **Fick’s Law of Diffusion**, the rate of diffusion is directly proportional to the lipid solubility of the substance. Because the cell membrane is a phospholipid bilayer, CO2 dissolves instantly into the lipid matrix and diffuses across the entire surface area of the membrane without requiring specialized channels or transporters. It moves significantly faster than water or any solute. ### Why Other Options are Incorrect * **Water (H2O):** Although water is a small molecule, it is highly **polar** (insoluble in lipids). It can only cross the membrane through specialized protein channels called **aquaporins** or via slow simple diffusion through membrane gaps. * **Glucose:** Glucose is a large, polar molecule. It is completely lipid-insoluble and requires **facilitated diffusion** via specific carrier proteins (GLUT transporters). This process is slower as it is limited by the number of available transporters (saturation kinetics). * **Urea:** Urea is a small, polar molecule. While it is slightly more lipid-soluble than glucose, it is far less soluble than CO2 and typically requires **urea transporters (UT)** to move efficiently. ### High-Yield NEET-PG Pearls * **Permeability Order:** Hydrophobic molecules (O2, CO2, N2) > Small uncharged polar molecules (H2O, Urea) > Large uncharged polar molecules (Glucose) > Ions (Na+, K+). * **Gases:** CO2 is approximately **20 times more soluble** than Oxygen (O2) in the lipid bilayer, explaining its rapid diffusion in alveolar gas exchange. * **Fick’s Law:** Diffusion rate is inversely proportional to the **thickness** of the membrane and the **square root of the molecular weight**.

Q: Which of the following processes involves the internalization of material and subsequent lysosomal digestion?

Phagocytosis. **Explanation:** The process described is **Phagocytosis**, a form of endocytosis often referred to as "cell eating." **1. Why Phagocytosis is correct:** Phagocytosis involves the ingestion of large particulate matter (such as bacteria, dead cells, or tissue debris). The cell membrane extends **pseudopodia** to surround the particle, forming a large vesicle called a **phagosome**. This phagosome then fuses with a **lysosome** to form a **phagolysosome**, where hydrolytic enzymes digest the internalized material. This is a primary function of "professional phagocytes" like macrophages and neutrophils. **2. Why other options are incorrect:** * **Pinocytosis:** Also known as "cell drinking," it involves the non-specific intake of ECF and small solutes via small vesicles. While it involves internalization, it is primarily for fluid balance rather than the digestion of large particles. * **Receptor-mediated endocytosis:** A highly selective process (e.g., uptake of LDL or Iron) using **clathrin-coated pits**. While it involves lysosomal processing, the question specifically points toward the general mechanism of material internalization and bulk digestion characteristic of phagocytosis. * **Exocytosis:** This is the reverse process; it involves the expulsion of materials (like neurotransmitters or hormones) from the cell into the extracellular space. **High-Yield Clinical Pearls for NEET-PG:** * **Opsonization:** Phagocytosis is significantly enhanced when particles are coated with opsonins (e.g., **IgG** or **C3b**). * **Residual Bodies:** Material that remains undigested within the lysosome forms residual bodies (e.g., **Lipofuscin**, the "wear-and-tear" pigment). * **Chediak-Higashi Syndrome:** A clinical condition characterized by a defect in phagosome-lysosome fusion, leading to recurrent infections.

Q: Which of the following is NOT a second messenger?

Protein kinase. **Explanation:** In cellular signaling, **second messengers** are small intracellular molecules or ions that relay signals received by cell-surface receptors (the first messenger) to target effector proteins. **Why Protein Kinase is the correct answer:** Protein kinases (e.g., PKA, PKC) are **enzymes**, not second messengers. They are typically the **effectors** activated by second messengers. For example, cAMP (second messenger) binds to and activates Protein Kinase A. The kinase then phosphorylates specific proteins to elicit a cellular response. Therefore, it is a downstream mediator in the signaling cascade rather than a messenger molecule itself. **Analysis of Incorrect Options:** * **cAMP (Cyclic Adenosine Monophosphate):** The most classic second messenger, produced by Adenylyl Cyclase. It mediates responses for hormones like Glucagon and PTH. * **NO (Nitric Oxide):** A unique gaseous second messenger that diffuses across membranes to activate soluble guanylyl cyclase, increasing cGMP levels. * **CO (Carbon Monoxide):** Similar to NO, CO acts as a gaseous signaling molecule (gasotransmitter) that can activate guanylyl cyclase and modulate ion channels. **High-Yield NEET-PG Pearls:** * **Common Second Messengers:** cAMP, cGMP, $IP_3$, DAG, $Ca^{2+}$, NO, and CO. * **Gaseous Messengers:** NO, CO, and $H_2S$ (Hydrogen Sulfide). * **IP3/DAG Pathway:** Phospholipase C cleaves $PIP_2$ into $IP_3$ (releases $Ca^{2+}$ from ER) and DAG (activates Protein Kinase C). * **Receptor Tyrosine Kinases (RTK):** Insulin and Growth Factors use this pathway, which often bypasses traditional second messengers by using direct protein-protein phosphorylation (e.g., JAK-STAT).

Q: The Na+/Glucose cotransporter functions as which type of transporter?

Secondary active transport. **Explanation:** The **Sodium-Glucose Cotransporter (SGLT)** is a classic example of **Secondary Active Transport**. Unlike primary active transport, it does not utilize ATP directly. Instead, it relies on the **electrochemical gradient** of Sodium ($Na^+$) created by the $Na^+/K^+$ ATPase pump (the primary step). As $Na^+$ moves down its concentration gradient into the cell, it provides the energy to "drag" Glucose against its concentration gradient. **Analysis of Options:** * **Secondary Active Transport (Correct):** It uses the energy stored in the $Na^+$ gradient. Specifically, it is a **Symport** mechanism because both $Na^+$ and Glucose move in the same direction (into the cell). * **Primary Active Transport (Incorrect):** This requires direct hydrolysis of ATP by the transporter itself (e.g., $Na^+/K^+$ ATPase, $Ca^{2+}$ ATPase). * **Antiport (Incorrect):** This is a type of secondary active transport where substances move in *opposite* directions (e.g., $Na^+/H^+$ exchanger). SGLT moves them in the *same* direction. * **Uniport (Incorrect):** This involves the movement of a single substance down its gradient. **GLUT** (Glucose Transporter) is a uniporter that facilitates passive diffusion of glucose. **High-Yield Clinical Pearls 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 responsible for 90% of glucose reabsorption. * **Pharmacology Link:** **SGLT-2 Inhibitors** (e.g., Dapagliflozin, Empagliflozin) are "Gliflozins" used to treat Diabetes Mellitus by inducing glucosuria. * **Oral Rehydration Therapy (ORT):** The physiological basis of ORT is the SGLT-1 receptor, where $Na^+$ absorption enhances water and glucose uptake.

Q: All of the following events occur during mitosis EXCEPT:

DNA replication. **Explanation:** The cell cycle is divided into two main phases: **Interphase** (preparation) and **M-phase** (mitosis/division). **Why DNA Replication is the correct answer:** DNA replication occurs during the **S-phase (Synthetic phase)** of Interphase, not during Mitosis. During this stage, the DNA content of the cell doubles (from 2n to 4n) to ensure that each daughter cell receives an identical set of chromosomes. By the time a cell enters Mitosis (Prophase), the DNA has already been replicated. **Analysis of other options:** * **Cytokinesis (Option A):** This is the final step of the M-phase, involving the physical division of the cytoplasm into two distinct daughter cells. * **Chromatids separate (Option B):** This is the hallmark of **Anaphase**. The centromeres split, and sister chromatids are pulled toward opposite poles of the cell. * **Kinetochore becomes evident (Option D):** Kinetochores are protein structures on chromatids where spindle fibers attach. They become clearly visible and functional during **Prometaphase/Metaphase**. **High-Yield Clinical Pearls for NEET-PG:** * **G0 Phase:** A quiescent stage where cells (like neurons or mature muscle cells) exit the cycle and stop dividing. * **Checkpoints:** The **G1-S checkpoint** (Rate-limiting step) is regulated by p53; mutations here are linked to most human cancers. * **Colchicine:** A drug used in Gout that inhibits mitosis by interfering with microtubule formation (arrests cells in Metaphase). * **Mnemonic for Mitosis:** **PMAT** (Prophase, Metaphase, Anaphase, Telophase).

Question 1

What is ABC transporter?

Accepted Answer

P glycoprotein

Answer

Membrane sparing

Answer

Channel

Answer

Adenylyl cyclase

Question 2

Simple diffusion and facilitated diffusion share which of the following characteristics?

Accepted Answer

Do not require adenosine triphosphate (ATP)

Answer

Can be blocked by specific inhibitors

Answer

Require a transport protein

Answer

Saturation kinetics

Question 3

During which phase of the cell cycle are chromosomes formed?

Accepted Answer

M

Answer

G1

Answer

S

Answer

G2

Question 4

Which of the following phases represents the restriction point in the cell cycle?

Accepted Answer

G1 to S

Answer

G0 to G1

Answer

S to G2

Answer

G2 to M

Question 5

Which of the following substances moves most rapidly across cell membranes?

Accepted Answer

Carbon dioxide (CO2)

Answer

Water (H2O)

Answer

Glucose

Answer

Urea

Question 6

Which of the following processes involves the internalization of material and subsequent lysosomal digestion?

Accepted Answer

Phagocytosis

Answer

Pinocytosis

Answer

Receptor-mediated endocytosis

Answer

Exocytosis

Question 7

Which of the following is NOT a second messenger?

Accepted Answer

Protein kinase

Answer

cAMP

Answer

NO

Answer

CO

Question 8

The Na+/Glucose cotransporter functions as which type of transporter?

Accepted Answer

Secondary active transport

Answer

Primary active transporter

Answer

Antiport

Answer

Uniport

Question 9

If at resting potential, the concentrations of permeable ions X and Y are represented as -50 and -30 respectively, what is the value of ion Z?

Accepted Answer

80

Answer

20

Answer

-20

Answer

-80

Question 10

All of the following events occur during mitosis EXCEPT:

Accepted Answer

DNA replication

Answer

Cytokinesis

Answer

Chromatids separate

Answer

The kinetochore becomes evident

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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