Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 251: Cell that can form all cell types in the body is called?

A. Lineage stem cells
B. Multipotent
C. Totipotent (Correct Answer)
D. Pluripotent

Explanation: ***Totipotent*** - **Totipotent** cells have the ability to differentiate into **all cell types** of the organism, including the **extraembryonic tissues** (like the placenta). - The **zygote** immediately after fertilization is the most well-known example of a totipotent cell, as it can form an entire organism. *Lineage stem cells* - **Lineage stem cells** (or **multipotent** stem cells) are restricted to differentiating into cells within a specific **cell lineage** or germ layer. - For example, **hematopoietic stem cells** can form all types of blood cells but no other tissue types. *Multipotent* - **Multipotent** stem cells can differentiate into a limited number of cell types within a specific tissue or organ, but not all cell types of the body. - Examples include **mesenchymal stem cells** which can form bone, cartilage, and fat cells, or **neural stem cells** which can form neurons and glia. *Pluripotent* - **Pluripotent** cells can differentiate into **all cell types** of the three germ layers (ectoderm, mesoderm, and endoderm) that make up the embryo, but not the extraembryonic tissues. - **Embryonic stem cells** are a prime example of pluripotent cells, as they can form any cell type in the body but cannot form a complete organism on their own.

Question 252: Which of the following ion plays a role in exocytosis?

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

Explanation: ***Calcium*** - **Calcium ions** are crucial for initiating the fusion of **secretory vesicles** with the plasma membrane during **exocytosis**. - An increase in intracellular calcium concentration, often due to an influx from outside the cell, triggers the release of neurotransmitters, hormones, and other substances. *Potassium* - **Potassium ions** are primarily involved in maintaining the **resting membrane potential** and repolarization during action potentials. - While essential for neuronal function, they do not directly trigger the **vesicle fusion** step of exocytosis. *Sodium* - **Sodium ions** are vital for depolarizing the membrane and initiating **action potentials**, as well as for certain co-transport mechanisms. - However, they do not directly bind to proteins involved in **exocytosis** to trigger the release mechanism. *Magnesium* - **Magnesium ions** serve as **cofactors** for many enzymes, including ATPases, and play a role in stabilizing nucleic acids and proteins. - While important for overall cellular function, magnesium does not directly initiate or regulate the **fusion events** of exocytosis.

Question 253: Through which of the following does glucose mediated insulin release occur?

A. ATP sensitive K+ channels (Correct Answer)
B. cAMP
C. Carrier-mediated glucose uptake (GLUT2)
D. Receptor phosphorylation

Explanation: ***ATP sensitive K+ channels*** - Glucose metabolism within pancreatic beta cells leads to an increase in **intracellular ATP**, which then binds to and closes **ATP-sensitive K+ channels**. - Closure of these channels causes **depolarization of the cell membrane**, triggering the opening of voltage-gated calcium channels, leading to insulin release. *cAMP* - While **cAMP** can **potentiate glucose-stimulated insulin secretion**, it is not the primary mechanism by which glucose directly mediates insulin release. - Its effects usually involve the activation of protein kinase A (PKA), which can influence the exocytosis of insulin granules. *Carrier-mediated glucose uptake (GLUT2)* - **GLUT2** is crucial for **glucose entry into pancreatic beta cells**, which is the initial step for glucose sensing. - However, it is the subsequent **metabolism of glucose** and its effect on ATP production, not the uptake itself, that directly mediates insulin release. *Receptor phosphorylation* - **Receptor phosphorylation** is a common mechanism in many signaling pathways, but it is not the direct mechanism by which glucose mediates insulin release in beta cells. - While insulin receptors themselves undergo phosphorylation, this refers to the action of insulin on target cells, not glucose-stimulated insulin secretion from beta cells.

Question 254: Which of the following tissues is most radiosensitive?

A. Growing skin (Correct Answer)
B. CNS
C. Adult bone
D. Pancreas

Explanation: ***Growing skin*** - Tissues with actively dividing cells, such as **growing skin**, are highly sensitive to radioactivity due to the disruption of DNA replication and cell division. - This vulnerability also applies to other rapidly proliferating tissues like the **bone marrow** and the **lining of the gastrointestinal tract**. *CNS* - The **central nervous system (CNS)** is generally considered less radiosensitive than rapidly dividing tissues. - While high doses can cause damage, its mature, non-dividing cells are more resistant to the immediate effects of radiation. *Adult bone* - Like the CNS, **adult bone** generally has a lower radiosensitivity because its cells divide much less frequently than those in growing tissues. - However, the bone marrow within the bone is highly radiosensitive due to its active cellular proliferation. *Pancreas* - The **pancreas** is also relatively radioresistant compared to rapidly growing tissues. - While it can be affected by high doses of radiation, chronic or acute pancreatitis due to radiation exposure is less common than damage to highly proliferative organs.

Question 255: Magnesium is not involved in ?

A. Cellular oxidation
B. Hemoglobin synthesis (Correct Answer)
C. Membrane transport
D. Glucose tolerance

Explanation: ***Hemoglobin synthesis*** - **Magnesium** is not directly involved in the synthesis of **hemoglobin**; **iron** is the crucial mineral for this process. - While magnesium is vital for many enzymatic reactions, it does not play a direct role in forming the heme structure or globin chains. *Cellular oxidation* - **Magnesium** acts as a **cofactor** for numerous enzymes involved in **cellular respiration** and **oxidative phosphorylation**, which are key processes in cellular oxidation. - These enzymatic reactions are critical for energy production within the cell. *Membrane transport* - **Magnesium** ions are essential for the proper functioning of various **ion channels** and **pumps**, such as the **Na+/K+ ATPase**, which are fundamental for maintaining **membrane potential** and **active transport**. - It influences the permeability of cell membranes and the movement of substances across them. *Glucose tolerance* - **Magnesium** plays a significant role in **glucose metabolism** and **insulin signaling**, affecting **glucose uptake** and utilization by cells, thereby influencing **glucose tolerance**. - Deficiency in magnesium has been linked to **insulin resistance** and an increased risk of **type 2 diabetes**.

Question 256: Fixed post-mitotic cells are:

A. Fibroblasts
B. Spermatocytes
C. Endothelial cells
D. Muscle (Correct Answer)

Explanation: ***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.

Question 257: The coupling ratio of the sodium-potassium pump (Na-K ATPase) is:

A. 1:1
B. 1:4
C. 2:3
D. 3:2 (Correct Answer)

Explanation: ***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.

Question 258: Which of the following would cause an immediate reduction in the amount of potassium leaking out of a cell?

A. Hyperpolarizing the membrane potential (Correct Answer)
B. Reducing the activity of the sodium-potassium pump
C. Decreasing the extracellular potassium concentration
D. Increasing the permeability of the membrane to potassium

Explanation: ***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.

Question 259: Nutrient and oxygen reach the chondrocytes across perichondrium by

A. Along neurons
B. Active transport
C. Diffusion (Correct Answer)
D. Capillaries

Explanation: ***Diffusion*** - **Cartilage** is an **avascular tissue**, meaning it lacks its own direct blood supply. - Nutrients and oxygen from the capillaries in the surrounding **perichondrium** move into chondrocytes via **simple diffusion** through the extracellular matrix. *Along neurons* - **Neurons** are responsible for transmitting electrical signals and are not involved in the transport of nutrients and oxygen to chondrocytes. - Cartilage itself is also an **aneural tissue**, meaning it lacks nerve innervation. *Active transport* - While active transport is a mechanism for moving substances across cell membranes, it requires energy and specifically transports substances against their **concentration gradient**. - The primary mechanism for bulk nutrient and oxygen delivery from the perichondrium to chondrocytes due to the concentration gradient is **diffusion**. *Capillaries* - **Capillaries** are indeed the source of nutrients and oxygen, but they are located within the **perichondrium**, not directly within the **avascular cartilage** itself. - Nutrients must leave the capillaries and then **diffuse** through the perichondrium and cartilage matrix to reach the chondrocytes.

Question 260: What determines the movement of water-insoluble substances in the body?

A. Hydrated diameter of molecule
B. Molecular weight
C. Lipid solubility (Correct Answer)
D. Charge

Explanation: ***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.

Practice by Chapter

Cell Membrane Structure and Function

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Membrane Transport Proteins

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Cellular Energetics and Metabolism

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Mitochondrial Function

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Cell Volume Regulation

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Cellular Responses to Stress

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Calcium Signaling

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Cell Cycle and Regulation

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Cellular Aging

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Apoptosis and Cell Death

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