Cellular Physiology — MCQs
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In the sodium-calcium exchanger (NCX), the ratio of Na+ : Ca++ exchange per event is:
All the following mediate their action using cAMP as second messenger except:
Which among the following results in a combination of elastic and viscous behavior, where only the elastic component is recovered when the stress is removed?
Glucose is usually transported by:
Which of the following helps in movement and adhesion?
Neutral molecules are transported by:
Which is the most diffusible ion across the cell membrane?
Receptor mediated endocytosis is by
Which of the following is true regarding the Na+-K+ pump?
Gibbs-Donnan effect is seen on the distribution of:
Cellular Physiology Indian Medical PG Practice Questions and MCQs
Question 271: In the sodium-calcium exchanger (NCX), the ratio of Na+ : Ca++ exchange per event is:
- A. 1:3
- B. 6:1
- C. 1:1
- D. 3:1 (Correct Answer)
Explanation: ***3:1*** - The **sodium-calcium exchanger (NCX)** typically extrudes **one Ca2+ ion** in exchange for the import of **three Na+ ions**. - This 3:1 stoichiometry results in a **net positive charge** moving into the cell, making the NCX electrogenic. *1:3* - This ratio implies one Na+ ion is exchanged for three Ca2+ ions, which is **not the physiological stoichiometry** of the NCX. - Such a ratio would significantly alter the membrane potential in a way that is not characteristic of NCX function. *6:1* - This ratio represents an exchange of six Na+ ions for one Ca2+ ion, which is **not the correct stoichiometry** for the NCX. - This would result in a much larger electrochemical gradient change than typically observed. *1:1* - This ratio indicates an equal exchange of one Na+ for one Ca2+, which would be **electroneutral** and is not the physiological mechanism of the NCX. - The NCX is crucial for maintaining calcium homeostasis, and its electrogenic nature is vital for its function.
Question 272: All the following mediate their action using cAMP as second messenger except:
- A. Glucagon
- B. Dopamine
- C. Corticotropin
- D. Vasopressin (Correct Answer)
Explanation: ***Vasopressin (ADH)*** - Vasopressin has **dual signaling mechanisms** depending on receptor type: - **V2 receptors** (kidney collecting duct): Use **Gs-protein → cAMP pathway** for water reabsorption via aquaporin-2 insertion - **V1 receptors** (vascular smooth muscle): Use **Gq-protein → IP3/DAG pathway** for vasoconstriction - In the context of this question, vasopressin is considered the exception because it has **significant non-cAMP mediated actions** through V1 receptors, unlike the other hormones listed which **predominantly or exclusively** use cAMP - **Note**: This is a teaching point about receptor subtypes; vasopressin DOES use cAMP at V2 receptors *Glucagon* - **Exclusively uses cAMP pathway** in hepatocytes and adipocytes - Binds to **glucagon receptor** (GPCR) → **Gs-protein** → adenylyl cyclase activation → **increased cAMP** → PKA activation - Promotes glycogenolysis, gluconeogenesis, and lipolysis *Dopamine* - **D1 and D5 receptors** are **Gs-coupled** → **stimulate adenylyl cyclase** → **increase cAMP** - Important for neurotransmission (motor control, reward) and renal vasodilation - D2-family receptors (D2, D3, D4) inhibit cAMP but D1-family predominates in many physiological contexts *Corticotropin (ACTH)* - Binds to **melanocortin-2 receptor (MC2R)** on adrenal cortex - **Gs-protein coupled** → adenylyl cyclase activation → **increased cAMP** → PKA activation - Stimulates steroidogenesis and cortisol secretion - **Exclusively cAMP-dependent mechanism**
Question 273: Which among the following results in a combination of elastic and viscous behavior, where only the elastic component is recovered when the stress is removed?
- A. Elastic deformation
- B. Plastic deformation
- C. Viscoelastic deformation (Correct Answer)
- D. None of the options
Explanation: ***Viscoelastic deformation*** - This type of deformation involves both **elastic** and **viscous** components, meaning that biological materials exhibit characteristics of both solids (elastic) and fluids (viscous). - Upon removal of the applied stress, only the **elastic strain** is recovered immediately, while the **viscous component** results in time-dependent behavior such as stress relaxation and creep. - This is the characteristic behavior of biological tissues, cell membranes, and cytoplasm, which demonstrate **viscoelastic properties** essential for physiological function. *Elastic deformation* - Involves only **elastic strain**, meaning the material fully recovers its original shape instantaneously once the applied stress is removed. - There is no time-dependent behavior or permanent deformation, as observed in materials within their elastic limit. *Plastic deformation* - Occurs when a material is stressed beyond its **yield point**, resulting in a permanent change in shape even after the stress is removed. - This is primarily due to **irreversible structural rearrangements** and does not involve time-dependent viscous flow. - This is not characteristic of normal physiological tissue behavior. *None of the options* - This option is incorrect because **viscoelastic deformation** accurately describes the phenomenon where biological materials exhibit a combination of elastic and viscous behavior, with only the elastic component being immediately recoverable.
Question 274: Glucose is usually transported by:
- A. Secondary active transport.
- B. Facilitated diffusion. (Correct Answer)
- C. Simple diffusion.
- D. Primary active transport.
Explanation: ***Facilitated diffusion*** - **Facilitated diffusion** is the primary mechanism for glucose uptake into most cells, especially down its concentration gradient, via specific **carrier proteins** (e.g., GLUT transporters). - This process does not require direct energy expenditure, as glucose moves from an area of higher concentration to lower concentration, but it still needs the help of a **membrane protein**. *Secondary active transport* - **Secondary active transport** of glucose (e.g., SGLT1 in the intestine and kidneys) involves the co-transport of glucose with Na+ ions, using the electrochemical gradient of Na+ as an energy source. - While important in specific locations for glucose absorption against a concentration gradient, it is not the general or "usual" transport mechanism for glucose into most other cells. *Simple diffusion* - **Simple diffusion** involves the passive movement of substances across a membrane directly, without the help of membrane proteins or energy. - Glucose molecules are too large and polar to cross the lipid bilayer directly via simple diffusion at physiologically significant rates. *Primary active transport* - **Primary active transport** directly uses energy from ATP hydrolysis to move substances against their concentration gradient, for example, the Na+/K+ ATPase. - Glucose transport itself does not typically involve direct ATP hydrolysis for movement across the cell membrane under normal physiological conditions in most cells.
Question 275: Which of the following helps in movement and adhesion?
- A. PGE2
- B. MCP1
- C. LTB4
- D. CD31 (Correct Answer)
Explanation: ***CD31*** - **CD31 (Platelet Endothelial Cell Adhesion Molecule-1, PECAM-1)** plays a critical role in **leukocyte transendothelial migration**, which is essential for leukocyte movement and adherence to endothelial cells during inflammation. - It mediates both **homophilic and heterophilic binding** at intercellular junctions, facilitating the passage of leukocytes through vessel walls. *PGE2* - **Prostaglandin E2 (PGE2)** is involved in inflammation, primarily causing **vasodilation**, fever, and pain sensitization. - While it modulates immune responses, its primary function is not direct cell movement or adhesion. *MCP1* - **MCP-1 (Monocyte Chemoattractant Protein-1), also known as CCL2**, is a chemokine that primarily functions in **chemoattraction** of monocytes, macrophages, and T cells. - It guides these cells to sites of inflammation but does not directly mediate adhesion to endothelial surfaces. *LTB4* - **Leukotriene B4 (LTB4)** is a potent **chemoattractant for neutrophils**, guiding their migration to inflammatory sites. - Its main function is **chemotaxis**, promoting the directed movement of neutrophils, rather than direct cell adhesion.
Question 276: Neutral molecules are transported by:
- A. Ionophores
- B. Porin channels
- C. Simple diffusion (Correct Answer)
- D. None of the options
Explanation: ***Simple diffusion*** - **Neutral molecules** are typically **lipid-soluble** and can directly pass through the lipid bilayer of the cell membrane. - This process does not require **energy** or specialized **transporters** and occurs down a concentration gradient. *Ionophores* - **Ionophores** are organic molecules that increase the permeability of membranes to specific **ions**, not neutral molecules. - They act by either forming **channels** or by directly binding and ferrying ions across the membrane. *Porin channels* - **Porin channels** are primarily found in the outer membranes of **gram-negative bacteria** and **mitochondria**. - They facilitate the passage of small **hydrophilic molecules**, which can be charged or uncharged, but their primary role is not for the transport of neutral molecules via simple diffusion. *None of the options* - This option is incorrect because **simple diffusion** is a well-established mechanism for the transport of neutral molecules.
Question 277: Which is the most diffusible ion across the cell membrane?
- A. Ca2+
- B. Cl-
- C. K+ (Correct Answer)
- D. Na+
Explanation: ***K+*** - The cell membrane is significantly more **permeable to potassium ions (K+)** than to other ions due to the presence of numerous **leak potassium channels**. - This higher permeability allows K+ ions to move more freely across the membrane, contributing significantly to the **resting membrane potential**. *Ca2+* - **Calcium ion (Ca2+)** permeability is generally very low in resting cells, with tightly regulated transport systems to maintain steep concentration gradients. - While essential for signaling, its diffusibility across the membrane is not the highest. *Cl-* - **Chloride ions (Cl-)** can diffuse across the membrane, but their permeability is typically lower than that of K+ in most resting cells. - Cl- movement often contributes to **hyperpolarization** or stabilization of the membrane potential. *Na+* - The cell membrane has relatively **low permeability to sodium ions (Na+)** in a resting state, as reflected by the action of the **Na+/K+ ATPase pump** actively pumping Na+ out of the cell. - While Na+ influx is crucial for **depolarization** during action potentials, its basal diffusibility is less than K+.
Question 278: Receptor mediated endocytosis is by
- A. Clathrin (Correct Answer)
- B. Porin
- C. Oxytocin
- D. Vimentin
Explanation: ***Clathrin*** - **Clathrin-mediated endocytosis** is the primary mechanism for **receptor-mediated endocytosis**. - **Clathrin** forms a characteristic **triskelion structure** that assembles into a polyhedral cage around the forming vesicle, facilitating the uptake of specific molecules. *Porin* - **Porins** are **beta-barrel proteins** that form pores in the outer membranes of gram-negative bacteria, mitochondria, and chloroplasts. - They are involved in the **passive diffusion** of small molecules and ions, not receptor-mediated endocytosis. *Oxytocin* - **Oxytocin** is a **peptide hormone** produced in the hypothalamus and released by the posterior pituitary. - It plays roles in **social bonding**, childbirth, and lactation, and is not involved in endocytosis. *Vimentin* - **Vimentin** is an **intermediate filament protein** found in various cell types, especially mesenchymal cells. - It provides structural support to the cell and is involved in cell migration and signaling, not the process of endocytosis.
Question 279: Which of the following is true regarding the Na+-K+ pump?
- A. Pumps 3 Na+ out and 2 K+ in (Correct Answer)
- B. Pumps 2 Na+ out and 3 K+ in
- C. Pumps 1 Na+ out and 2 K+ in
- D. Pumps 2 Na+ out and 1 K+ in
Explanation: ***Pumps 3 Na+ out and 2 K+ in*** - The **Na+-K+ pump (Na+/K+-ATPase)** is an active transporter that maintains the electrochemical gradients across the cell membrane. - For every molecule of **ATP** hydrolyzed, it expels **three sodium ions (Na+)** out of the cell and brings **two potassium ions (K+)** into the cell. *Pumps 2 Na+ out and 3 K+ in* - This option incorrectly reverses the stoichiometry of Na+ and K+ ions. - The pump's function is to create a net change in charge, bringing in fewer positive ions than it expels. *Pumps 1 Na+ out and 2 K+ in* - This option incorrectly states the number of both sodium and potassium ions transported. - The pump's characteristic ratio for Na+ and K+ is 3:2. *Pumps 2 Na+ out and 1 K+ in* - This option also presents an incorrect stoichiometry for the ions pumped. - The physiological ratio of ions moved by the Na+-K+ pump is consistently 3 Na+ out and 2 K+ in.
Question 280: Gibbs-Donnan effect is seen on the distribution of:
- A. Only proteins
- B. Non-diffusible ions
- C. Diffusible ions (Correct Answer)
- D. Diffusible + Non-diffusible ions
Explanation: ***Diffusible ions*** - The **Gibbs-Donnan effect** describes the unequal distribution of **diffusible ions** across a semi-permeable membrane in the presence of a non-diffusible charged macromolecule. - This effect is crucial for maintaining **osmotic balance** and electrical neutrality, as the diffusible ions adjust their concentrations to balance the charge imbalance caused by the non-diffusible species. *Only proteins* - While **proteins** are often the **non-diffusible macromolecules** that cause the Gibbs-Donnan effect, the effect itself is observed in the distribution of the *diffusible* ions. - Proteins themselves do not *distribute* in response to the effect; they *cause* the effect on other ions. *Non-diffusible ions* - **Non-diffusible ions** (like large protein anions) are the *cause* of the Gibbs-Donnan effect, by creating an **osmotic and electrical gradient**. - The effect is not seen *on* their distribution because they cannot cross the membrane in the first place. *Diffusible + Non-diffusible ions* - The Gibbs-Donnan effect specifically concerns the **unequal distribution of *diffusible* ions**. - Non-diffusible ions do not distribute across the membrane, so their distribution is not influenced by this effect.
Practice by Chapter
Cell Membrane Structure and Function
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Apoptosis and Cell Death
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