Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 311: Which of the following is an ionic channel?

A. Nicotinic cholinergic receptors (Correct Answer)
B. Alpha-1 adrenergic receptors
C. Beta-1 adrenergic receptors
D. Muscarinic acetylcholine receptors

Explanation: ***Nicotinic cholinergic receptors*** - **Nicotinic cholinergic receptors** are **ligand-gated ion channels** that open upon binding of acetylcholine, allowing the rapid influx of ions, primarily sodium. - This influx of positive ions leads to **depolarization** of the cell membrane, mediating fast synaptic transmission in the central and peripheral nervous systems, as well as at neuromuscular junctions. *Alpha-1 adrenergic receptors* - **Alpha-1 adrenergic receptors** are **G protein-coupled receptors (GPCRs)** that, when activated, lead to the production of **inositol triphosphate (IP3)** and **diacylglycerol (DAG)**. (These secondary messengers increase intracellular calcium and activate protein kinase C, respectively.) - They are primarily involved in smooth muscle contraction, vasoconstriction, and glandular secretion, processes that rely on slower, metabolic changes rather than direct ion flow. *Beta-1 adrenergic receptors* - **Beta-1 adrenergic receptors** are also **G protein-coupled receptors (GPCRs)** that couple to **Gs proteins**, leading to direct activation of **adenylyl cyclase** and increased production of **cyclic AMP (cAMP)**. - This increase in cAMP mediates effects such as increased heart rate and contractility, which are slower and involve cascades of protein phosphorylation rather than direct ion channel activity. *Muscarinic acetylcholine receptors* - **Muscarinic acetylcholine receptors** are a family of **G protein-coupled receptors (GPCRs)**, not ion channels. - Their activation leads to a variety of intracellular signaling cascades, such as the activation of **Gi/o proteins** (leading to decreased cAMP and opening of potassium channels) or **Gq proteins** (leading to IP3/DAG production), resulting in slower and modulatory effects on cell excitability.

Question 312: Which of the following is an example of active transport?

A. Movement of water across cell membrane
B. Movement of oxygen across cell membrane
C. Co-transport of amino acids with sodium (Correct Answer)
D. Facilitated diffusion of glucose

Explanation: ***Co-transport of amino acids with sodium*** - **Co-transport** (also called symport) involves the movement of two or more substances across a membrane using the same protein carrier, where the movement of at least one substance is **down its electrochemical gradient** (e.g., sodium) to drive the movement of another **against its gradient** (e.g., amino acids or glucose). - This is a form of **secondary active transport** that requires energy indirectly, as the sodium gradient is maintained by the **Na+/K+-ATPase pump**, which directly consumes ATP. - Examples include sodium-glucose cotransporter (SGLT) in intestines and kidneys, and sodium-amino acid cotransporters. *Movement of water across cell membrane* - The movement of water across cell membranes occurs primarily by **osmosis**, a type of **passive diffusion**, often facilitated by **aquaporins**. - **Osmosis** does not directly consume metabolic energy (ATP) but instead relies on the water potential/concentration gradient. - This is a passive process requiring no energy input. *Movement of oxygen across cell membrane* - **Oxygen** is a small, nonpolar molecule that moves across cell membranes by **simple diffusion**, directly passing through the lipid bilayer. - This is a **passive process** that occurs down its concentration gradient and does not require energy or membrane carriers. *Facilitated diffusion of glucose* - **Facilitated diffusion** uses carrier proteins (like GLUT transporters) to move substances down their concentration gradient. - While it requires a protein carrier, it does **not require energy** and is therefore a **passive transport** mechanism, not active transport. - This is often confused with the sodium-glucose cotransporter (SGLT), which IS active transport.

Question 313: Which of the following statements about gap junctions is false?

A. Transmit electric signals
B. Intercellular space 1000 nm (Correct Answer)
C. Seen in cardiac muscle
D. Intercellular space is small

Explanation: ***Intercellular space 1000 nm*** - Gap junctions are characterized by a very narrow intercellular space, typically **2-4 nm**, which allows for direct communication between adjacent cells. - An intercellular space of 1000 nm (1 micrometer) is a large gap, inconsistent with the close apposition required for gap junction function. *Transmit electric signals* - Gap junctions facilitate the **direct passage of ions and small molecules** between cells, enabling the rapid spread of electrical signals. - This property is crucial for coordinated cellular activities, such as in cardiac muscle contraction. *Intercellular space is small* - The narrow intercellular space is a defining feature of gap junctions, bringing the plasma membranes of adjacent cells very close together. - This small distance allows for the formation of **connexons**, which are the channels that bridge the two cells. *Seen in cardiac muscle* - Gap junctions are abundant in cardiac muscle, where they form **intercalated discs** that allow for the rapid and synchronized spread of electrical impulses. - This synchronization is essential for efficient heart contraction.

Question 314: Which of the following equations correctly describes the Nernst equation for equilibrium potential of an ion?

A. EMF = 25 mV * log(Co/Ci)
B. EMF = 41 mV * log(Co/Ci)
C. EMF = 61 mV * log(Co/Ci) (Correct Answer)
D. EMF = 80 mV * log(Co/Ci)

Explanation: ***EMF = 61 mV * log(Co/Ci)*** - The Nernst equation at **37°C (body temperature)** uses approximately 61 mV as the coefficient when using base-10 logarithm (log₁₀). - This value is derived from RT/zF (gas constant × temperature / valence × Faraday constant) ≈ 26 mV for natural logarithm, which becomes **61 mV when converted to log₁₀** by multiplying by 2.303. - The ratio **Co/Ci (outside concentration/inside concentration)** is the standard convention for cations in biological systems to calculate the **equilibrium potential** across a membrane. - For a univalent cation at 37°C: **E = 61 log(Co/Ci)** mV *EMF = 25 mV * log(Co/Ci)* - While 25-26 mV is the correct RT/F value at 37°C, it is used with the **natural logarithm (ln)** form: E = 26 ln(Co/Ci). - This option incorrectly pairs 25 mV with base-10 logarithm, which underestimates the equilibrium potential. *EMF = 41 mV * log(Co/Ci)* - This value is not a standard physiological constant for the Nernst equation. - It does not accurately represent the conversion factor at body temperature. *EMF = 80 mV * log(Co/Ci)* - This value is significantly higher than the standard constant. - It would lead to substantial overestimation of equilibrium potentials in biological systems.

Question 315: Which of the following traits do identical twins not share?

A. DNA copies
B. Iris color
C. Fingerprints (Correct Answer)
D. Blood group

Explanation: ***Fingerprints*** - While identical twins share the same **DNA**, their fingerprints, also known as **dermatoglyphs**, are formed by complex interactions between genetic factors and unique environmental influences during fetal development. - Factors like precise positioning in the womb, umbilical cord blood flow, and the exact timing of finger development lead to unique ridge patterns, meaning every individual, including identical twins, has distinct fingerprints. *DNA copies* - Identical twins originate from a single fertilized egg that splits, resulting in nearly identical **DNA sequences**. - This high degree of genetic similarity is why they are often used in scientific studies to differentiate genetic and environmental influences on traits. *Iris color* - Like other phenotypic traits such as hair color and height, **iris color** is primarily determined by genetic factors and is generally shared between identical twins due to their identical DNA. - While slight variations can occur due to environmental factors, the underlying genetic predisposition for iris color remains the same. *Blood group* - **Blood group** is determined by genetic inheritance, specifically the presence or absence of certain antigens on red blood cells. - Since identical twins share the same genetic code, they will also share the same blood group, as this trait is purely genetically determined.

Question 316: Osteoclasts have all of the following functions except -

A. Receptor for parathormone (Correct Answer)
B. Ruffled border
C. Bone resorption
D. RANK ligand production

Explanation: ***Receptor for parathormone*** - **Osteoclasts** do not directly have receptors for **parathormone (PTH)**; instead, **osteoblasts** have PTH receptors. - When PTH binds to osteoblasts, they release factors (like **RANKL**) that stimulate osteoclast activity, thus indirectly regulating bone resorption. *Bone resorption* - **Osteoclasts** are specialized cells primarily responsible for **resorbing bone matrix**, a critical process in bone remodeling. - They secrete **acids and enzymes** to break down the mineral and organic components of bone. *Ruffled border* - The **ruffled border** is a characteristic morphological feature of active osteoclasts, representing a highly folded plasma membrane. - This specialized structure increases the surface area for the secretion of **protons and lysosomal enzymes** into the bone-resorbing compartment. *RANK ligand production* - **Osteoclasts** do not produce **RANK ligand (RANKL)**; rather, they have **RANK receptors** that bind to RANKL produced by **osteoblasts and stromal cells**. - The binding of RANKL to RANK is essential for the **differentiation, activation, and survival** of osteoclasts.

Question 317: What is the process of passive transport of molecules through protein pores/channels in the cell membrane?

A. Transcytosis
B. Diffusion (Correct Answer)
C. Endocytosis
D. Active transport

Explanation: ***Diffusion*** - **Diffusion** is the net movement of particles from an area of higher concentration to an area of lower concentration without requiring energy. - When diffusion occurs through **protein channels or pores** in the cell membrane, it is specifically termed **facilitated diffusion** or **channel-mediated diffusion**. - This remains a form of **passive transport** as it moves substances down their concentration gradient without ATP expenditure. - Examples include ion channels (Na⁺, K⁺, Ca²⁺) and aquaporins for water transport. *Active transport* - **Active transport** requires energy (typically ATP) to move substances **against** their concentration gradient. - It involves carrier proteins (pumps) like Na⁺-K⁺ ATPase that undergo conformational changes. - This is fundamentally different from passive transport through pores. *Transcytosis* - **Transcytosis** is a vesicular transport mechanism for moving substances across an entire cell. - It combines **endocytosis** on one side and **exocytosis** on the other side. - This is not passive transport through pores but rather bulk transport. *Endocytosis* - **Endocytosis** involves engulfing extracellular substances by forming membrane-bound vesicles. - Types include phagocytosis, pinocytosis, and receptor-mediated endocytosis. - This requires energy and does not involve transport through pores.

Question 318: Fastest receptor mediated action is through?

A. Intrinsic ion channels (Correct Answer)
B. Intracellular receptors
C. Cell surface receptors
D. Receptor tyrosine kinases

Explanation: ***Intrinsic ion channels*** - Receptors that are also **ion channels** (ligand-gated ion channels) allow direct and rapid ion flow across the membrane upon ligand binding, leading to immediate changes in membrane potential. - This direct mechanism bypasses complex intracellular signaling cascades, resulting in the **fastest cellular response** compared to other receptor types. *Cell surface receptors* - This is a broad category that includes **G protein-coupled receptors** and **receptor tyrosine kinases**, which typically involve more complex and slower signaling pathways. - While located on the cell surface, not all receptors in this category mediate action as quickly as intrinsic ion channels. *Receptor tyrosine kinases* - These receptors initiate signaling by **phosphorylating tyrosine residues** on target proteins, triggering a cascade of intracellular events that take time to manifest. - Their action involves **multiple phosphorylation steps** and protein interactions, making their response slower compared to direct ion channels. *Intracellular receptors* - These receptors, such as **steroid hormone receptors**, are located in the cytoplasm or nucleus and require their ligands to diffuse across the cell membrane. - The activated receptor then typically translocates to the nucleus to regulate gene transcription, a process that is much **slower** due to gene expression and protein synthesis.

Question 319: What is the composition of epithelial sodium channels?

A. 2α, 1β, 1γ
B. 1α, 1β, 1γ (Correct Answer)
C. 2α, 1β
D. 2α, 1β, 2γ

Explanation: ***1α, 1β, 1γ*** - Epithelial sodium channels (**ENaCs**) are heterotrimeric complexes composed of one **alpha (α)**, one **beta (β)**, and one **gamma (γ) subunit**. - This specific subunit composition is essential for the channel's proper function in **sodium reabsorption** across epithelial tissues. *2α, 1β* - This composition is incomplete as it lacks the **gamma (γ) subunit**, which is a crucial component of the functional ENaC. - While alpha and beta subunits are present, the absence of the gamma subunit would impair the channel's ability to efficiently transport sodium. *2α, 1β, 2γ* - This composition is incorrect because a functional ENaC typically includes only **one gamma (γ) subunit**, not two. - An imbalance in subunit stoichiometry can lead to misfolding or improper assembly, affecting channel function. *2α, 1β, 1γ* - This combination correctly includes all three types of subunits (alpha, beta, gamma) but incorrectly states there are **two alpha (α) subunits**. - A functional ENaC has a single alpha subunit, making this option incorrect.

Question 320: Which transport process is mediated by carriers and occurs against the concentration gradient?

A. Facilitated diffusion
B. Osmosis
C. Active transport (Correct Answer)
D. Endocytosis

Explanation: ***Active transport*** - **Active transport** systems use carrier proteins to move molecules across a membrane **against their concentration gradient**, requiring **metabolic energy** (e.g., from ATP hydrolysis). - This process is crucial for maintaining cellular homeostasis, accumulating specific substances, and establishing ion gradients. *Facilitated diffusion* - **Facilitated diffusion** also uses **carrier proteins**, but it moves substances **down their concentration gradient**, thus **not requiring metabolic energy**. - It increases the rate of diffusion for molecules that cannot easily cross the lipid bilayer, like glucose. *Osmosis* - **Osmosis** is the movement of **water molecules** across a selectively permeable membrane **down their water potential gradient**, driven by solute concentration differences, and does **not involve carrier proteins**. - This process equalizes solute concentrations on both sides of the membrane. *Endocytosis* - **Endocytosis** is a bulk transport mechanism where cells **engulf substances** from outside by forming vesicles from the plasma membrane; it's a form of active transport but **does not typically involve specific carrier proteins** embedded in the membrane for individual molecules. - This process is used for taking in larger molecules, particles, or even other cells.

Practice by Chapter

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Cellular Aging

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Apoptosis and Cell Death

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