Cellular Physiology Practice Questions

Q: Fixed post-mitotic cells are:

Muscle. ***Muscle*** - **Mature muscle cells** (both skeletal and cardiac myocytes) are **terminally differentiated** and are **fixed post-mitotic cells**. - These cells **cannot undergo mitosis** after reaching maturity and are permanently in the G0 phase of the cell cycle. - They primarily function in contraction and tissue maintenance rather than proliferation. *Spermatocytes* - **Spermatocytes** are germ cells that undergo **meiosis** to produce haploid spermatids. - They are **actively dividing cells** (through meiotic division, not mitosis) and are not in a fixed post-mitotic state. - These cells are derived from spermatogonia (the actual stem cells) and represent an intermediate stage in spermatogenesis. *Fibroblasts* - **Fibroblasts** are connective tissue cells that are capable of **mitotic division**, especially during wound healing and tissue repair. - They can re-enter the cell cycle from G0 phase when stimulated, making them **labile/stable cells**, not fixed post-mitotic cells. *Endothelial cells* - **Endothelial cells** line blood vessels and are typically quiescent but can be stimulated to **proliferate** during processes like angiogenesis and wound healing. - Their ability to divide and re-enter the cell cycle makes them different from fixed post-mitotic cells.

Q: The coupling ratio of the sodium-potassium pump (Na-K ATPase) is:

3:2. ***3:2*** - The **Na-K ATPase** actively transports **three sodium ions (Na+) out** of the cell and **two potassium ions (K+) into** the cell, creating a 3:2 coupling ratio. - This unequal transport of ions maintains the **electrochemical gradient** across the cell membrane, which is crucial for nerve impulse transmission and cell volume regulation. *1:1* - A 1:1 coupling ratio implies the movement of an equal number of ions in opposite directions, which is not characteristic of the **electrogenic action** of the Na-K ATPase. - This ratio would not contribute to the net charge separation required for maintaining the **resting membrane potential**. *1:4* - A 1:4 coupling ratio is not observed in the physiological function of the **Na-K ATPase**, which has a specific stoichiometry. - Such a ratio would significantly alter the **electrochemical gradient** in a way that is inconsistent with normal cellular physiology. *2:3* - A 2:3 coupling ratio would mean two sodium ions are pumped out for every three potassium ions pumped in, which is the **inverse of the actual physiological ratio** of the Na-K ATPase. - This reversed ratio would lead to **depolarization** rather than maintaining the negative resting membrane potential.

Q: Which of the following would cause an immediate reduction in the amount of potassium leaking out of a cell?

Hyperpolarizing the membrane potential. ***Increasing (hyperpolarizing) the membrane potential*** - **Hyperpolarizing** the membrane means making the inside of the cell more negative relative to the outside. - This increased negativity inside the cell will **electrically attract** the positively charged **potassium ions** (K+) preventing them from leaking out. *Reducing the activity of the sodium-potassium pump* - The **sodium-potassium pump** actively transports potassium into the cell, helping to maintain the concentration gradient. - Reducing its activity would lead to an accumulation of potassium outside the cell and subsequent **increase in potassium leakage**. *Decreasing the extracellular potassium concentration* - A **lower extracellular potassium concentration** would steepen the potassium concentration gradient, causing more potassium to leak out of the cell. - This effect is due to the **chemical driving force** for potassium efflux. *Increasing the permeability of the membrane to potassium* - Increasing the **permeability** to potassium, typically through opening more **potassium channels**, would facilitate the movement of potassium ions down their electrochemical gradient. - This would result in a **greater leakage** of potassium out of the cell.

Q: What determines the movement of water-insoluble substances in the body?

Lipid solubility. ***Lipid solubility*** - The movement of water-insoluble (lipophilic) substances across biological membranes is primarily determined by their **lipid solubility**, as these membranes are composed of a **lipid bilayer**. - Highly lipid-soluble substances can readily dissolve in the membrane and pass through via **simple diffusion**, following their concentration gradient. *Hydrated diameter of molecule* - This factor is more relevant for the movement of **water-soluble substances** through aqueous channels or pores. - Large hydrated diameters hinder movement through such channels, but it does not dictate the movement of water-insoluble substances across the lipid bilayer. *Molecular weight* - While molecular weight can generally influence diffusion rates, **lipid solubility** is a more critical determinant for water-insoluble substances moving across lipid membranes. - A substance with a higher molecular weight but significantly greater lipid solubility will often cross a membrane more easily than a substance with a lower molecular weight but poor lipid solubility. *Charge* - The charge of a molecule primarily affects its interaction with other charged molecules and its ability to traverse the **hydrophobic lipid bilayer** of cell membranes. - Charged molecules, even if small, are generally **water-soluble** and have difficulty crossing lipid membranes unless specific transporters or channels are involved.

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

Heterodimer – heterogeneous. ***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

Q: In Na+-K+ ATPase, K+ binds onto

Extracellular binding site on α subunit. ***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.

Q: Sodium potassium pump is a:

Heterodimer. ***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.

Q: What are chalones in relation to cell division?

Inhibitors. ***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.

Q: NO is synthesized by all except -

Platelets. ***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.

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.

Question 1

Fixed post-mitotic cells are:

Accepted Answer

Muscle

Answer

Fibroblasts

Answer

Spermatocytes

Answer

Endothelial cells

Question 2

The coupling ratio of the sodium-potassium pump (Na-K ATPase) is:

Accepted Answer

3:2

Answer

1:1

Answer

1:4

Answer

2:3

Question 3

Which of the following would cause an immediate reduction in the amount of potassium leaking out of a cell?

Accepted Answer

Hyperpolarizing the membrane potential

Answer

Reducing the activity of the sodium-potassium pump

Answer

Decreasing the extracellular potassium concentration

Answer

Increasing the permeability of the membrane to potassium

Question 4

What determines the movement of water-insoluble substances in the body?

Accepted Answer

Lipid solubility

Answer

Hydrated diameter of molecule

Answer

Molecular weight

Answer

Charge

Question 5

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

Accepted Answer

Heterodimer – heterogeneous

Answer

Homodimer – heterogeneous

Answer

Homodimer – homogeneous

Answer

Heterodimer – homogeneous

Question 6

In Na+-K+ ATPase, K+ binds onto

Accepted Answer

Extracellular binding site on α subunit

Answer

Extracellular binding site on β subunit

Answer

Intracellular binding site on α subunit

Answer

Intracellular binding site on β subunit

Question 7

Sodium potassium pump is a:

Accepted Answer

Heterodimer

Answer

Monomer

Answer

Homodimer

Answer

Polymer

Question 8

What are chalones in relation to cell division?

Accepted Answer

Inhibitors

Answer

Regulators

Answer

Promoters

Answer

Initiators

Question 9

NO is synthesized by all except -

Accepted Answer

Platelets

Answer

Neuron

Answer

Endothelium

Answer

Macrophages

Question 10

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

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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