Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 331: Which ion movement is primarily responsible for hyperpolarization of the cell membrane?

A. Chloride (Cl-) influx
B. Potassium (K+) efflux (Correct Answer)
C. Sodium (Na+) influx
D. None of the options

Explanation: ***Potassium (K+) efflux*** - **Potassium efflux** (K+ leaving the cell) is the **primary mechanism** responsible for hyperpolarization of the cell membrane across most cell types. - When K+ channels open, positive charges leave the cell, making the intracellular environment more negative relative to the extracellular space, thereby **hyperpolarizing** the membrane. - This mechanism is responsible for: - **Afterhyperpolarization** following action potentials - Setting the **resting membrane potential** close to the K+ equilibrium potential (-90 mV) - **Repolarization and hyperpolarization phases** of action potentials - Examples include delayed rectifier K+ channels and calcium-activated K+ channels. *Chloride (Cl-) influx* - While Cl- influx can cause hyperpolarization (especially through **GABA-A receptors** in neurons), it is a **secondary or specialized mechanism**, not the primary one. - In many mature neurons, the Cl- equilibrium potential is close to the resting potential, limiting its hyperpolarizing effect. - This mechanism is important in **inhibitory neurotransmission** but not universally across all cell types. *Sodium (Na+) influx* - **Sodium influx** through voltage-gated sodium channels is responsible for the **depolarization phase** of an action potential. - This makes the inside of the cell significantly more positive (+30 to +40 mV), which is the opposite of hyperpolarization. *None of the options* - This option is incorrect because **potassium efflux** is indeed the primary mechanism for membrane hyperpolarization.

Question 332: The process by which differentiated cells can convert into a different cell type, acquiring the characteristics and functions of that new cell type, is referred to as:

A. De-differentiation
B. Re-differentiation
C. Trans-differentiation (Correct Answer)
D. Cellular plasticity

Explanation: ***Trans-differentiation*** - **Trans-differentiation** is the process where a differentiated cell directly converts into another differentiated cell type without first undergoing a pluripotent state (like de-differentiation). - This process involves a complete change in the identity and function of the cell, acquiring features of a new, distinct cell type. *De-differentiation* - **De-differentiation** is the process by which a differentiated cell loses some or all of its specialized characteristics and returns to a more primitive or stem cell-like state. - This process is often a prerequisite for **regeneration** or can be observed in cancer cells, but it does not directly lead to a new differentiated cell type without further differentiation steps. *Re-differentiation* - **Re-differentiation** typically refers to the process where a de-differentiated cell (or a progenitor/stem cell) differentiates again into a specialized cell type. - It implies that the cell previously underwent de-differentiation and is now regaining its specialized functions, which is distinct from changing directly into a different cell type. *Cellular plasticity* - **Cellular plasticity** is a broad term describing the ability of cells to change their phenotype, state, or function in response to various cues. - While trans-differentiation is a form of cellular plasticity, it is a specific mechanism for direct cell conversion, whereas cellular plasticity encompasses a wider range of cellular adaptations including de-differentiation and reprogramming.

Question 333: What is the equilibrium potential for sodium?

A. -70 mV
B. -90 mV
C. +60 mV (Correct Answer)
D. 0 mV

Explanation: ***+60 mV*** - The **equilibrium potential for sodium** (E_Na) is approximately +60 mV. This is calculated using the **Nernst equation**, considering the higher extracellular concentration of sodium ions compared to their intracellular concentration. - At this potential, the **electrical force** driving sodium out of the cell is equal and opposite to the **chemical force** driving sodium into the cell, resulting in no net movement of sodium ions across the membrane. *-70 mV* - This value typically represents the **resting membrane potential** of a neuron, which is primarily determined by the high permeability to potassium ions. - At -70 mV, there is a strong electrochemical gradient for **sodium influx**, rather than equilibrium. *-90 mV* - This value is close to the **equilibrium potential for potassium** (E_K), due to the high intracellular concentration of potassium ions and its significant membrane permeability at rest. - At -90 mV, potassium ions are close to equilibrium, but sodium ions are far from their equilibrium. *0 mV* - This represents the potential where there is **no electrical gradient** across the membrane, but it is not the equilibrium potential for sodium. - At 0 mV, the chemical gradient would still drive sodium ions into the cell, as sodium concentration remains higher outside the cell.

Question 334: What is the primary action of the α-subunit of G protein in the signaling pathway?

A. Conversion of GDP to GTP
B. Internalization of receptors
C. Binding of agonist
D. Activation of downstream effectors (Correct Answer)

Explanation: ***Activation of downstream effectors*** - Upon activation, the **α-subunit** dissociates from the βγ subunit and binds to and **activates specific downstream effector proteins**, such as adenylyl cyclase or phospholipase C. - This activation initiates a cascade of intracellular events leading to the cellular response. *Conversion of GDP to GTP* - The **α-subunit** binds **GTP** in its active state and **hydrolyzes GTP to GDP** to become inactive; however, its primary action is not the conversion itself but rather the subsequent signaling that occurs while GTP-bound. - This exchange is controlled by the **receptor**, which acts as a guanine nucleotide exchange factor (GEF), facilitating the release of GDP and binding of GTP. *Internalization of receptors* - **Receptor internalization** is a process by which cells take up receptors from the cell surface, often for degradation or recycling. - This process is primarily mediated by **clathrin-coated pits** and is distinct from the immediate signaling function of the G protein α-subunit. *Binding of agonist* - The **agonist binds to the G protein-coupled receptor (GPCR)**, not directly to the α-subunit. - Agonist binding to the GPCR induces a conformational change in the receptor, which then activates the associated G protein, leading to the GDP-GTP exchange on the α-subunit.

Question 335: Which of the following substances can freely permeate the plasma membrane?

A. Glucose
B. Urea
C. Glycerol
D. Alcohol (Correct Answer)

Explanation: ***Alcohol*** - **Small, lipophilic (fat-soluble), uncharged molecules** like alcohol can **freely and rapidly diffuse** through the lipid bilayer of the plasma membrane without any carrier or channel. - Among the given options, alcohol has the **highest membrane permeability** due to its optimal lipid solubility and small size. - This rapid permeability explains the quick systemic effects of alcohol consumption. *Glucose* - **Glucose** is a relatively large, highly polar molecule and **cannot freely permeate** the lipid bilayer. - It requires **specific carrier proteins (GLUT transporters)** for facilitated diffusion across the plasma membrane. - Its transport is often regulated and can be **insulin-dependent** in certain cells (e.g., GLUT4 in muscle and adipose tissue). *Urea* - While **small and uncharged**, urea is a **polar molecule** with significant hydrogen bonding capability, which **reduces its lipid solubility**. - Urea can cross membranes by simple diffusion but at a **much slower rate** compared to lipophilic molecules like alcohol. - In physiologically relevant contexts (e.g., renal tubules, RBCs), **specific urea transporters (UTs)** are required for efficient movement to meet cellular demands. *Glycerol* - **Glycerol** is a small molecule but contains **three hydroxyl groups**, making it relatively polar and **limiting its free permeability** through lipid bilayers. - While it can passively diffuse, the rate is **significantly slower than lipophilic molecules** like alcohol. - In many cells, particularly **adipocytes and renal tubules**, glycerol transport is facilitated by **aquaglyceroporins (AQP3, AQP7, AQP9)** to achieve physiologically adequate flux rates.

Question 336: According to Fick's law of diffusion, particle flux is directly proportional to which of the following?

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

Explanation: ***Particle's concentration difference across the membrane*** - Fick's first law states that the **rate of diffusion** (flux) is **directly proportional** to the **concentration gradient** (ΔC). - The mathematical expression: J = -D × (ΔC/Δx), where J is flux, D is diffusion coefficient, ΔC is concentration difference, and Δx is membrane thickness. - A greater concentration difference drives **higher net movement of particles**, directly increasing the flux. - This is the **primary driving force** for passive diffusion across membranes. *Area of the membrane* - While flux is also **proportional to membrane area** in Fick's law, this is a separate variable. - The question specifically asks about direct proportionality, and among the options, concentration gradient is the **classical factor** emphasized in Fick's first law. - Increasing surface area increases total flux but doesn't change the concentration gradient. *Temperature of the solution* - Temperature affects the **diffusion coefficient (D)** by increasing particle kinetic energy. - However, temperature is **not explicitly included** in Fick's first law formula as a directly proportional factor. - It indirectly affects diffusion rate through changes in D, making it an **external modulator** rather than a direct proportional factor. *Thickness of the Membrane* - Membrane thickness (Δx) has an **inverse relationship** with flux in Fick's law. - Greater thickness **decreases** flux as particles must travel a longer distance. - This demonstrates **inverse proportionality**, not direct proportionality as asked in the question.

Question 337: Barr bodies are not present in which individuals?

A. XX
B. XO (Correct Answer)
C. XXY
D. XXX

Explanation: ***XO*** - Individuals with an **XO karyotype** (Turner syndrome) have only one X chromosome and therefore lack an additional X chromosome to be inactivated and form a Barr body. - A Barr body is formed from the **inactivated X chromosome**, and since only one X chromosome is present, no inactivation occurs. *XXY* - Individuals with an **XXY karyotype** (Klinefelter syndrome) will have **one Barr body** because one of their two X chromosomes will be inactivated. - The number of Barr bodies is typically **N-1**, where N is the number of X chromosomes. *XX* - Individuals with a normal female **XX karyotype** will have **one Barr body**, as one of the two X chromosomes is randomly inactivated during development. - X-inactivation ensures proper gene dosage compensation between males and females. *XXX* - Individuals with an **XXX karyotype** (Triple X syndrome) will have **two Barr bodies**, as two of their three X chromosomes will be inactivated. - This follows the N-1 rule, where N=3, so 3-1=2 Barr bodies.

Question 338: Which of the following binds to intracellular receptors?

A. Growth hormone
B. Vitamin E
C. Estrogen (Correct Answer)
D. Insulin

Explanation: ***Estrogen*** - **Estrogen** is a **steroid hormone** that, due to its **lipophilic nature**, can easily pass through the cell membrane to bind to **intracellular receptors** in the cytoplasm or nucleus. - This binding leads to the formation of a **hormone-receptor complex** that acts as a transcription factor, regulating **gene expression**. *Growth hormone* - **Growth hormone** is a **peptide hormone** and therefore **hydrophilic**, meaning it cannot freely cross the cell membrane. - It binds to **transmembrane receptors** on the cell surface, initiating intracellular signaling cascades through pathways like the **JAK/STAT pathway**. *Vitamin E* - **Vitamin E** is a **lipid-soluble vitamin** and an important **antioxidant**, but it does not function as a signaling molecule that binds to intracellular receptors to regulate gene expression in the same manner as steroid hormones. - While it diffuses across membranes due to its lipophilicity, its primary role is to protect cell membranes from **oxidative damage**. *Insulin* - **Insulin** is a **protein hormone** that is **hydrophilic** and cannot pass through the cell membrane. - It binds to **tyrosine kinase receptors** on the cell surface, triggering a cascade of intracellular events like the **PI3K/Akt pathway** to regulate glucose metabolism.

Question 339: The net diffusion of water from one solution to another through a semipermeable membrane, where the concentration of water is lower in the receiving solution, is termed?

A. filtration
B. diffusion
C. osmosis (Correct Answer)
D. brownian motion

Explanation: ***osmosis*** - **Osmosis** specifically describes the **net movement of water** across a **selectively permeable membrane** from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration). - The driving force for osmosis is the difference in **water potential** between the two solutions. *filtration* - **Filtration** is a process by which fluid and small solutes are forced through a membrane by **hydrostatic pressure**, often from a higher pressure to a lower pressure area. - This process typically involves the separation of particles based on size, as seen in the **kidneys' glomeruli**, rather than water concentration gradients. *diffusion* - **Diffusion** refers to the general movement of any substance (solutes or solvent) from an area of **higher concentration to a lower concentration**, down its concentration gradient. - While osmosis is a type of diffusion, it is specific to **water movement** across a **semipermeable membrane**, which is a more precise description for the scenario presented. *brownian motion* - **Brownian motion** is the random movement of particles suspended in a fluid (a liquid or a gas) resulting from their collision with the fast-moving atoms or molecules in the fluid. - It describes the **random jiggling** of molecules, which contributes to diffusion, but it is not the term for the net diffusion of water across a membrane.

Question 340: Diffusion of lipid-insoluble substances across the cell membrane depends on all of the following factors except which one?

A. Hydrated radius
B. Electrical charge
C. Lipid solubility (Correct Answer)
D. Shape

Explanation: ***Lipid solubility*** - This property is crucial for substances that **readily diffuse directly through the lipid bilayer**. - Lipid-insoluble substances, by definition, **cannot diffuse through the lipid bilayer based on their lipid solubility**, requiring other mechanisms or factors like channels or carriers. *Hydrated radius* - The **size of a hydrated ion or molecule** is a critical determinant for its ability to pass through specific protein channels or pores in the cell membrane. - A larger hydrated radius impedes passage through narrow channels, directly affecting the diffusion of lipid-insoluble substances. *Electrical charge* - For **charged lipid-insoluble substances** (ions), their movement across the membrane is significantly influenced by the **transmembrane electrical potential difference**. - The electrical gradient can either facilitate or hinder the diffusion of these substances through channels or transporters. *Shape* - The **three-dimensional configuration** of a lipid-insoluble substance can affect its ability to bind to and pass through specific protein channels or carrier proteins. - A substance's shape must complement the architecture of the transport mechanism for efficient diffusion.

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