Cellular Physiology Practice Questions

Q: In the sodium-calcium exchanger (NCX), the ratio of Na+ : Ca++ exchange per event is:

3:1. ***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.

Q: All the following mediate their action using cAMP as second messenger except:

Vasopressin. ***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**

Q: 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?

Viscoelastic deformation. ***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.

Q: Glucose is usually transported by:

Facilitated diffusion.. ***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.

Q: Which of the following helps in movement and adhesion?

CD31. ***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.

Q: Neutral molecules are transported by:

Simple diffusion. ***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.

Q: Which is the most diffusible ion across the cell membrane?

K+. ***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+.

Q: Receptor mediated endocytosis is by

Clathrin. ***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.

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

Pumps 3 Na+ out and 2 K+ in. ***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.

Q: Gibbs-Donnan effect is seen on the distribution of:

Diffusible ions. ***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.

Question 1

In the sodium-calcium exchanger (NCX), the ratio of Na+ : Ca++ exchange per event is:

Accepted Answer

3:1

Answer

1:3

Answer

6:1

Answer

1:1

Question 2

All the following mediate their action using cAMP as second messenger except:

Accepted Answer

Vasopressin

Answer

Glucagon

Answer

Dopamine

Answer

Corticotropin

Question 3

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?

Accepted Answer

Viscoelastic deformation

Answer

Elastic deformation

Answer

Plastic deformation

Answer

None of the options

Question 4

Glucose is usually transported by:

Accepted Answer

Facilitated diffusion.

Answer

Secondary active transport.

Answer

Simple diffusion.

Answer

Primary active transport.

Question 5

Which of the following helps in movement and adhesion?

Accepted Answer

CD31

Answer

PGE2

Answer

MCP1

Answer

LTB4

Question 6

Neutral molecules are transported by:

Accepted Answer

Simple diffusion

Answer

Ionophores

Answer

Porin channels

Answer

None of the options

Question 7

Which is the most diffusible ion across the cell membrane?

Accepted Answer

K+

Answer

Ca2+

Answer

Cl-

Answer

Na+

Question 8

Receptor mediated endocytosis is by

Accepted Answer

Clathrin

Answer

Porin

Answer

Oxytocin

Answer

Vimentin

Question 9

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

Accepted Answer

Pumps 3 Na+ out and 2 K+ in

Answer

Pumps 2 Na+ out and 3 K+ in

Answer

Pumps 1 Na+ out and 2 K+ in

Answer

Pumps 2 Na+ out and 1 K+ in

Question 10

Gibbs-Donnan effect is seen on the distribution of:

Accepted Answer

Diffusible ions

Answer

Only proteins

Answer

Non-diffusible ions

Answer

Diffusible + Non-diffusible ions

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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