Cellular Physiology Practice Questions

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

Vitamin D. ***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.

Q: What is the primary function of hepatic stellate cells?

Vitamin A storage. ***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.

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

It is located on the apical membrane of cell. ***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.

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

Requires specific carrier proteins. ***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.

Q: Cell-to-cell permeability occurs through

Connexins. ***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.

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

Facilitated diffusion. ***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.

Question 1

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

Accepted Answer

Vitamin D

Answer

Folate

Answer

Niacin

Answer

Vitamin C

Question 2

What is the primary function of hepatic stellate cells?

Accepted Answer

Vitamin A storage

Answer

Regulation of liver fibrosis

Answer

Secretion of cytokines

Answer

Formation of extracellular matrix

Question 3

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

Accepted Answer

It permits the passage of ions from one cell to an adjacent cell.

Answer

It extends as a zone around the apical perimeter of adjacent cells.

Answer

Its adhesion is dependent upon calcium ions.

Answer

It possesses dense plaques composed in part of desmoplakins.

Question 4

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

Accepted Answer

It is located on the apical membrane of cell

Answer

It is not directly involved in the generation of action potentials.

Answer

It is electrogenic

Answer

It needs ATP for its functioning

Question 5

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

Accepted Answer

Requires specific carrier proteins

Answer

Does not require energy input

Answer

Transports solute down concentration gradient

Answer

Transports solute against concentration gradient

Question 6

Cell-to-cell permeability occurs through

Accepted Answer

Connexins

Answer

Occludin

Answer

Zona adherens

Answer

Zonulin

Question 7

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

Accepted Answer

Facilitated diffusion

Answer

Transcytosis

Answer

Endocytosis

Answer

Active transport

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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