Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 351: At concentrations present in the diet, which vitamin is absorbed primarily by diffusion?

A. Vitamin D (Correct Answer)
B. Folate
C. Niacin
D. Vitamin C

Explanation: ***Vitamin D*** - **Fat-soluble vitamins** (A, D, E, K) like Vitamin D are absorbed from the intestine primarily through **passive diffusion** at **dietary/physiological concentrations**. - They are incorporated into **mixed micelles** in the intestinal lumen, which facilitate their transport to the enterocyte membrane. - The **lipophilic nature** of these vitamins allows them to passively diffuse across the lipid bilayer of the enterocyte membrane **without requiring specific transporters**. - Once inside enterocytes, they are packaged into **chylomicrons** for lymphatic transport. *Vitamin C* - **Water-soluble vitamin C** is absorbed via **sodium-dependent vitamin C transporters (SVCTs)** - specifically SVCT1 in the small intestine. - At high (pharmacological) concentrations, passive diffusion can contribute to its absorption, but at dietary concentrations, active transport predominates. *Folate* - Folate, a **water-soluble B vitamin**, is absorbed primarily through **active transport mechanisms**, specifically via the **reduced folate carrier (RFC)** and **proton-coupled folate transporter (PCFT)**. - These transporters are essential for efficient folate uptake even at low concentrations. *Niacin* - Niacin (**Vitamin B3**) is absorbed through **carrier-mediated transport** at physiological (dietary) concentrations. - At pharmacological doses, simple passive diffusion can dominate, but this is not the primary mechanism at normal dietary levels.

Question 352: What is the primary function of hepatic stellate cells?

A. Vitamin A storage (Correct Answer)
B. Regulation of liver fibrosis
C. Secretion of cytokines
D. Formation of extracellular matrix

Explanation: ***Vit-A storage*** - **Hepatic stellate cells** (also known as Ito cells) are primarily responsible for storing **vitamin A** in the form of retinyl palmitate droplets. - In their quiescent state, these cells contain abundant lipid droplets that house the body's largest reserve of **vitamin A**. *Regulation of liver fibrosis* - While **hepatic stellate cells** play a critical role in **liver fibrosis**, this is a function of their activated state in response to injury, not their primary function in a healthy liver. - Upon activation, they transform into myofibroblast-like cells that produce **extracellular matrix**, contributing to fibrosis. *Secretion of cytokines* - **Hepatic stellate cells** do secrete cytokines, particularly upon activation, but this is a secondary response to injury or inflammation. - Their most well-known and constantly active role in a healthy liver is **vitamin A storage**. *Formation of extracellular matrix* - The formation of **extracellular matrix** (ECM) is a characteristic of activated **hepatic stellate cells** during liver injury and fibrosis. - In their quiescent state, these cells maintain a different role, and excessive ECM production is pathological.

Question 353: Which one of the following statements about the gap junction is true?

A. It extends as a zone around the apical perimeter of adjacent cells.
B. It permits the passage of ions from one cell to an adjacent cell. (Correct Answer)
C. Its adhesion is dependent upon calcium ions.
D. It possesses dense plaques composed in part of desmoplakins.

Explanation: ***It permits the passage of ions from one cell to an adjacent cell.*** - **Gap junctions** are specialized intercellular connections that directly connect the cytoplasm of two cells, allowing for the passage of **ions**, small molecules, and electrical impulses. - This direct communication is crucial for coordinating cellular activity, especially in tissues like **cardiac muscle** and **smooth muscle**. *It extends as a zone around the apical perimeter of adjacent cells.* - This description is characteristic of a **zonula occludens** (tight junction) or a **zonula adherens** (adherens junction), not a gap junction. - Gap junctions are more typically scattered across lateral cell membranes rather than forming a continuous apical zone. *Its adhesion is dependent upon calcium ions.* - The adhesion of **adherens junctions** and **desmosomes** is dependent on calcium ions, specifically involving **cadherins**. - Gap junction formation and function are generally not directly dependent on extracellular calcium for their adhesive properties, although calcium can regulate their permeability. *It possesses dense plaques composed in part of desmoplakins.* - This statement describes **desmosomes (maculae adherens)**, which are characterized by dense plaques containing **desmoplakins** anchoring intermediate filaments. - Gap junctions do not contain desmoplakins or similarly dense plaques; they are formed by connexon channels between cells.

Question 354: Which of the following statements about the Na-K pump is false?

A. It is not directly involved in the generation of action potentials.
B. It is electrogenic
C. It needs ATP for its functioning
D. It is located on the apical membrane of cell (Correct Answer)

Explanation: ***It is located on the apical membrane of cell*** - The **Na-K pump**, or **Na+/K+-ATPase**, is primarily located on the **basolateral membrane** of epithelial cells, not **apical membrane**. - Its strategic placement on the basolateral membrane is crucial for maintaining cellular polarity and driving transepithelial transport processes, such as reabsorption in the kidneys. *It is electrogenic* - The Na-K pump is indeed **electrogenic** because it transports three **Na+ ions** out of the cell for every two **K+ ions** pumped in. - This unequal charge distribution creates a net movement of one positive charge out of the cell, contributing to the **resting membrane potential**. *It is not directly involved in the generation of action potentials.* - While the Na-K pump is essential for **maintaining the ion gradients** necessary for **action potentials**, it is not directly involved in their rapid depolarization or repolarization phases. - Action potentials are primarily generated by the rapid opening and closing of **voltage-gated ion channels** (e.g., Na+ and K+ channels). *It needs ATP for its functioning* - The Na-K pump is an **active transport mechanism** that moves ions against their concentration gradients, requiring **energy in the form of ATP hydrolysis**. - This **ATP-dependent process** ensures the continuous maintenance of the Na+ and K+ gradients, crucial for various cellular functions, including nerve impulse transmission and muscle contraction.

Question 355: Cell-to-cell permeability occurs through

A. Connexins (Correct Answer)
B. Occludin
C. Zona adherens
D. Zonulin

Explanation: ***Connexins*** - **Connexins** are the proteins that form **gap junctions**, which are specialized intercellular channels that allow direct passage of ions and small molecules between adjacent cells. - These channels facilitate **cell-to-cell communication** and regulate permeability by enabling the rapid exchange of electrical and chemical signals. *Occludin* - **Occludin** is a key protein component of **tight junctions** (zona occludens), which are primarily responsible for sealing the space between cells and preventing paracellular leakage. - Tight junctions **restrict cell-to-cell permeability** rather than promoting it, defining tissue polarity. *Zona adherens* - The **zona adherens** (adherens junctions) are cell junctions that provide strong **mechanical attachment** between cells through the binding of adjacent cell membranes. - They are involved in maintaining tissue integrity and cell shape but do not directly regulate **cell-to-cell permeability** of substances. *Zonulin* - **Zonulin** is a protein that modulates **intestinal tight junction permeability**, acting as a regulator of the paracellular pathway. - While it affects permeability, it does so by *loosening tight junctions*, not by forming direct cell-to-cell channels that allow substance passage.

Question 356: Which of the following characteristics is shared by both active transport and facilitated diffusion?

A. Does not require energy input
B. Transports solute down concentration gradient
C. Transports solute against concentration gradient
D. Requires specific carrier proteins (Correct Answer)

Explanation: ***Requires specific carrier proteins*** - Both **active transport** and **facilitated diffusion** rely on specific **transmembrane proteins** to move substances across the cell membrane. - These carrier proteins bind to the specific molecule they transport, undergoing conformational changes that facilitate its movement. *Does not require energy input* - This statement is only true for **facilitated diffusion**, which is a form of passive transport. - **Active transport** requires an input of **metabolic energy**, typically in the form of ATP, to move substances. *Transports solute down concentration gradient* - This is characteristic of **facilitated diffusion**, where molecules move from an area of higher concentration to an area of lower concentration. - **Active transport** moves solutes **against** their concentration gradient, requiring energy. *Transports solute against concentration gradient* - This is a defining feature of **active transport**, which allows cells to accumulate substances even when their external concentration is lower. - **Facilitated diffusion** moves solutes **down** their concentration gradient and cannot transport against a gradient.

Question 357: What is the process of translocation through pores in cell membranes?

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

Explanation: ***Facilitated diffusion*** - This process involves the movement of molecules across the cell membrane **down their concentration gradient** with the help of **specific membrane proteins**, without the expenditure of cellular energy. - When the question refers to "pores," it specifically means **channel proteins** (also called pore proteins), which form aqueous pathways through the membrane for ions and small polar molecules. - **Facilitated diffusion** includes both channel-mediated transport (through pores) and carrier-mediated transport (through conformational changes), making it the correct answer. - This allows substances that cannot easily pass through the lipid bilayer to traverse the membrane passively. *Transcytosis* - This is a type of **vesicular transport** that involves the transport of substances across an entire cell, typically from one extracellular space to another, using **endocytosis** and **exocytosis**. - It does not involve direct translocation through membrane pores but rather the formation and movement of vesicles. *Endocytosis* - This is a process by which cells **engulf extracellular material** by forming vesicles from the plasma membrane. - It involves the invagination of the cell membrane to internalize substances, not their passage through pre-existing pores or channels. *Active transport* - This process moves molecules **against their concentration gradient**, requiring direct or indirect **expenditure of cellular energy** (ATP). - While it often uses protein carriers or pumps, it is distinguished by its energy requirement and ability to move substances uphill, unlike facilitated diffusion which is passive movement down the gradient.

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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