Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 261: Nernst potential of Na+ is

A. +90 mV
B. +61 mV (Correct Answer)
C. -60 mV
D. -80 mV

Explanation: ***61*** - The **Nernst potential** (or **equilibrium potential**) for an ion is the electrical potential difference across a cell membrane that exactly counteracts the net diffusion of that ion down its concentration gradient. - For **sodium (Na+)**, with typical extracellular concentrations around 145 mM and intracellular concentrations around 15 mM, the calculated Nernst potential is approximately **+61 mV**. *90* - This value is generally too high for the typical **Nernst potential of Na+** in most physiological contexts. - While the exact potential can vary slightly based on specific intracellular and extracellular concentrations, **+90 mV** is not the standard equilibrium potential for Na+. *-60* - **-60 mV** is closer to the **resting membrane potential** of many cells, which is determined by the permeability to multiple ions, particularly potassium. - The **Nernst potential of Na+** is a positive value because Na+ is more concentrated outside the cell, and its electrochemical gradient would drive a net positive charge into the cell if the membrane were only permeable to Na+. *-80* - **-80 mV** is typically the equilibrium potential for **potassium (K+)**, given its high intracellular and low extracellular concentrations. - It is a negative value because K+ is more concentrated inside the cell, and its electrochemical gradient would drive a net positive charge out of the cell.

Question 262: Which of the following is true regarding the Na+-K+ pump?

A. Homodimer – heterogeneous
B. Heterodimer – heterogeneous (Correct Answer)
C. Homodimer – homogeneous
D. Heterodimer – homogeneous

Explanation: ***Heterodimer – heterogeneous*** - The Na+-K+ pump is a **heterodimer**, composed of two different types of protein subunits: an **alpha (α) subunit** and a **beta (β) subunit** - These subunits are functionally and structurally distinct, making the overall structure **heterogeneous** - The α subunit is responsible for **ion binding and ATP hydrolysis**, while the β subunit is important for **membrane insertion and proper folding** *Homodimer – heterogeneous* - A **homodimer** consists of two identical protein subunits, which is not the case for the Na+-K+ pump - While heterogeneous function might be possible, the structural arrangement is not a homodimer *Homodimer – homogeneous* - This option would imply two identical subunits that are also functionally identical or similar - The Na+-K+ pump has two distinctly different subunits with different roles, not a homodimer *Heterodimer – homogeneous* - While the Na+-K+ pump is correctly a **heterodimer**, the term "homogeneous" would suggest that its two different subunits are functionally or structurally very similar - This is **incorrect** as the α and β subunits have distinctly different structures and functions

Question 263: In Na+-K+ ATPase, K+ binds onto

A. Extracellular binding site on β subunit
B. Extracellular binding site on α subunit (Correct Answer)
C. Intracellular binding site on α subunit
D. Intracellular binding site on β subunit

Explanation: ***Extracellular binding site on a subunit*** - The **α subunit** of the **Na+-K+ ATPase** is responsible for both Na+ and K+ binding and the enzymatic hydrolysis of ATP. - **K+ ions** bind to specific sites on the **extracellular face** of the α subunit to be transported into the cell. *Extracellular binding site on b subunit* - The **β subunit** primarily functions in the proper **folding, trafficking, and membrane insertion** of the Na+-K+ ATPase. - It does not directly participate in the **ion binding** or **transport** process. *Intracellular binding site on a subunit* - The **intracellular face** of the **α subunit** binds **Na+ ions** for extrusion from the cell and also contains the **ATP binding site**. - **K+ binding** occurs exclusively on the **extracellular side**. *Intracellular binding site on b subunit* - The **β subunit** is located on the **extracellular side** of the membrane, with a small cytoplasmic tail. - It does not have an **intracellular binding site** for ions.

Question 264: Which of the following tissues is most resistant to electric current entry?

A. Skin
B. Bone (Correct Answer)
C. Blood
D. Muscle

Explanation: ***Bone*** - **Bone tissue** has a very high electrical resistance due to its low water content and dense, compact structure, making it a poor conductor of electricity. - This high resistance means that a significant amount of energy is required for electric current to pass through bone, making it the most resistant tissue listed. *Skin* - The **epidermis**, especially dry skin, offers significant resistance to electric current, acting as the primary barrier to entry. - However, once the skin is breached or wet, its resistance drops considerably, allowing current to pass more easily into deeper tissues. *Blood* - **Blood** is primarily composed of water and electrolytes, making it an excellent conductor of electricity. - Its low resistance allows electric current to flow through it with relative ease, contributing to systemic effects in electrical injuries. *Muscle* - **Muscle tissue** contains a high percentage of water and electrolytes, which makes it a good conductor of electricity. - Consequently, electric current can readily pass through muscle, leading to muscle spasms, contractions, and significant tissue damage.

Question 265: Sodium potassium pump is a:

A. Monomer
B. Homodimer
C. Heterodimer (Correct Answer)
D. Polymer

Explanation: ***Heterodimer*** - The **sodium-potassium pump (Na+/K+-ATPase)** is composed of two distinct subunits: an **alpha (α) subunit** and a **beta (β) subunit**. - This arrangement of two different protein subunits forms a **heterodimer**, which is essential for its function as an ion transporter. *Monomer* - A monomer refers to a **single protein molecule** that functions independently. - The sodium-potassium pump requires at least two different subunits to be assembled to be functional, thus it is not a **monomer**. *Homodimer* - A homodimer is formed when **two identical protein subunits** associate. - The Na+/K+-ATPase consists of two *different* subunits (α and β), not identical ones, therefore it is not a homodimer. *Polymer* - A polymer is a large molecule made up of **many repeating smaller units (monomers)** linked together. - While composed of multiple subunits, the Na+/K+-ATPase is specifically a two-subunit complex (dimer), not a larger-scale polymer of many repeating units.

Question 266: Nernst equilibrium potential for Cl– is

A. –90mV
B. –70mV (Correct Answer)
C. +80mV
D. +60mV

Explanation: ***–70mV*** - The **Nernst equilibrium potential** for an ion represents the membrane potential at which there is no net movement of that ion across the membrane, as the **electrical gradient** exactly balances the **chemical (concentration) gradient**. - For Cl⁻, the extracellular concentration is **higher than intracellular**, creating a **chemical gradient that drives Cl⁻ inward**. - At **–70mV** (negative inside), the **electrical gradient repels Cl⁻ outward** (negative ions repelled by negative charge), balancing the inward chemical drive and achieving equilibrium. - This is the typical value in **most neurons**. *–90mV* - A potential of **–90mV** is typically the **equilibrium potential for potassium (K⁺)** in neurons, reflecting K⁺'s high intracellular concentration. - In **skeletal muscle**, ECl can approach –90mV, but in neurons it is typically less negative (around –70mV). *+80mV* - At a **positive membrane potential**, the electrical gradient would **attract Cl⁻ into the cell** (negative ion attracted to positive charge). - This would **reinforce** the inward chemical gradient, causing net Cl⁻ influx, not equilibrium. - This value does not represent the equilibrium potential for any major physiological ion. *+60mV* - At **+60mV**, Cl⁻ would experience both electrical attraction inward and chemical drive inward, causing **strong net influx**, not equilibrium. - This value approximates the **equilibrium potential for sodium (Na⁺)**, which has high extracellular concentration and reaches equilibrium at positive potentials.

Question 267: The second most abundant intracellular cation is

A. Calcium
B. Magnesium (Correct Answer)
C. Iron
D. Sodium

Explanation: ***Magnesium*** - **Magnesium** is the **second most abundant intracellular cation** after potassium. - It plays a crucial role in over 300 enzymatic reactions, including **ATP metabolism**, protein synthesis, and nucleic acid synthesis. *Calcium* - **Calcium** is primarily concentrated **extracellularly** and in intracellular stores like the endoplasmic reticulum, rather than being a highly abundant free intracellular cation. - Its main roles are in **bone mineralization**, muscle contraction, and neurotransmitter release. *Iron* - While **iron** is essential for cellular functions like **oxygen transport** (hemoglobin) and enzyme activity, it is not considered a bulk intracellular cation. - Its intracellular concentration is carefully regulated due to its potential toxicity. *Sodium* - **Sodium** is the **most abundant extracellular cation**, with a significantly lower concentration inside cells. - The **sodium-potassium pump** actively maintains this gradient, which is vital for nerve impulse transmission and osmotic balance.

Question 268: What are chalones in relation to cell division?

A. Regulators
B. Promoters
C. Inhibitors (Correct Answer)
D. Initiators

Explanation: ***Inhibitors*** - Chalones are **tissue-specific glycoproteins** that were theorized to act as **reversible inhibitors** of cell proliferation. - According to the **chalone theory**, they participate in **negative feedback loops** to maintain **tissue homeostasis** by stopping cells from dividing once a certain density is reached. - This concept, while **historically important**, has limited modern evidence, but the term remains in medical literature describing **mitotic inhibitors**. *Regulators* - While chalones do regulate cell division, this term is **too broad** to specifically describe their inhibitory function. - Many molecules regulate cell division, including both activators and inhibitors, but chalones specifically **block** it. *Promoters* - Chalones have the opposite effect of promoters; they **decrease** the rate of cell division rather than increasing it. - **Growth factors** and **mitogens** are examples of cell division promoters. *Initiators* - Chalones do not initiate any process in cell division; instead, they act to **halt** or **prevent** further division. - Cell cycle cyclins and CDKs are initiators of different phases of the cell cycle.

Question 269: NO is synthesized by all except -

A. Platelets (Correct Answer)
B. Neuron
C. Endothelium
D. Macrophages

Explanation: ***Platelets*** - **Platelets** do not synthesize nitric oxide (NO); instead, they are a primary target for NO's **anti-aggregatory effects** in the vasculature, preventing spontaneous clotting. - While platelets play roles in hemostasis and inflammation, their physiological function is regulated by NO produced by other cell types, primarily **endothelial cells**. *Neuron* - **Neurons** synthesize **neuronal nitric oxide synthase** (nNOS), which produces NO as a **neurotransmitter** involved in synaptic plasticity, learning, and memory. - **NO** also plays a role in regulating cerebral blood flow and neuronal excitability in the central and peripheral nervous systems. *Endothelium* - **Endothelial cells** synthesize **endothelial nitric oxide synthase** (eNOS), producing NO that diffuses into vascular smooth muscle cells, causing **vasodilation** and regulating blood pressure. - **Endothelial NO** is crucial for maintaining vascular tone, inhibiting platelet aggregation, and preventing leukocyte adhesion to the vessel wall. *Macrophages* - **Macrophages** express **inducible nitric oxide synthase** (iNOS) following activation by inflammatory stimuli (e.g., cytokines **TNF-α, IFN-γ**), producing large quantities of NO. - This **NO** acts as a cytotoxic molecule, involved in the host defense against pathogens and in inducing oxidative stress in chronic inflammatory conditions.

Question 270: Endogenous electric signal that is also seen in bone that is not stressed:

A. Streaming Potential (Correct Answer)
B. Bioelectric Potential
C. Capillary potential
D. None of the options

Explanation: ***Streaming Potential*** - This is the **correct answer** - streaming potentials are endogenous electrical signals present in bone even without mechanical stress - Generated by the **flow of interstitial fluid** through the charged porous structure of bone (canalicular system) - Exists continuously due to **normal physiological processes** like blood pressure fluctuations, osmotic gradients, and metabolic activity - Increases with mechanical loading but is **always present as a baseline signal** in living bone - Plays an important role in bone remodeling and mechano-transduction *Bioelectric Potential* - This is a **generic term** that encompasses all electrical potentials in biological tissues - Too broad and non-specific - it could refer to action potentials, membrane potentials, or any bioelectric phenomenon - Not a specific, well-defined electrical signal characteristic of bone tissue - While technically present in bone, it's not the specific answer for endogenous bone electrical signals *Capillary potential* - This term relates to **capillary action** and surface tension phenomena in porous materials - More relevant to fluid mechanics in soil science and plant physiology - Not a recognized term for endogenous electrical signals in bone physiology *None of the options* - Incorrect because **Streaming Potential** is the accurate description of endogenous electrical signals in unstressed bone

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