Cellular Physiology Practice Questions

Q: Which of the following is not a recognized stage of prophase I in meiosis?

Arachytene. ***Arachytene*** - **Arachytene** is not a recognized stage of prophase I in meiosis. - The correct stages are leptotene, zygotene, pachytene, diplotene, and diakinesis. *Diakinesis* - **Diakinesis** is the final stage of prophase I, where homologous chromosomes condense further, and the nuclear envelope begins to break down. - Chiasmata terminalize, and the bivalents are ready for metaphase I. *Leptotene* - **Leptotene** is the first stage of prophase I, characterized by the condensation of chromatin into visible chromosomes. - Chromosomes appear as long, thin threads. *Zygotene* - **Zygotene** is the second stage of prophase I, where homologous chromosomes pair up in a process called **synapsis**, forming bivalents. - The synaptonemal complex begins to form between homologous chromosomes.

Q: How many transmembrane spanning domains does the Na+-K+-Cl- cotransporter contain?

12 transmembrane spanning domain. ***12 transmembrane spanning domain*** - The **Na+-K+-Cl- cotransporter (NKCC)**, specifically NKCC1 and NKCC2 isoforms, is a multipass transmembrane protein. - It is known to contain **12 transmembrane spanning domains**, which are α-helical regions that cross the cell membrane. *5 transmembrane spanning domain* - This number is **too low** for the complex structure of the NKCC cotransporter. - Proteins with 5 transmembrane domains typically have different functional roles and structural characteristics. *7 transmembrane spanning domain* - This number is characteristic of **G protein-coupled receptors (GPCRs)**, a distinct family of membrane proteins. - The NKCC cotransporter functions as an ion transporter and does not share the structural motif of GPCRs. *9 transmembrane spanning domain* - This number is also **insufficient** to describe the known topology of the NKCC cotransporter. - Ion transporters like NKCC generally require a higher number of transmembrane domains to form the necessary pore and binding sites.

Q: What is the process by which water moves from the extracellular space to the intracellular space?

Osmosis. ***Osmosis*** - **Osmosis** is the movement of water across a **semipermeable membrane** from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration). - In the context of fluid shifts, if the **extracellular fluid** becomes hypotonic relative to the **intracellular fluid**, water will move into the cells to equalize the solute concentration. *Diffusion* - **Diffusion** refers to the net movement of particles from an area of higher concentration to an area of lower concentration, down their **concentration gradient**. - While water molecules can diffuse, **osmosis** specifically describes the net movement of water across a membrane due to **solute concentration differences**, which is the precise mechanism for water moving between fluid compartments. *Filtration* - **Filtration** is the process by which water and solutes move across a membrane due to a **pressure gradient**, typically a **hydrostatic pressure gradient**. - This process is crucial in the kidneys for forming filtrate, but it is not the primary mechanism for water movement between the intra- and extracellular spaces based on solute concentration. *Active transport* - **Active transport** involves the movement of molecules across a membrane against their **concentration gradient**, requiring **energy expenditure** (e.g., ATP). - Water movement between fluid compartments is generally a passive process, relying on **osmotic gradients** rather than direct energy input to pump water molecules.

Q: Which of the following factors increases the rate of particle diffusion across the cell membrane?

Maintaining a concentration gradient across the membrane. ***Maintaining a concentration gradient across the membrane*** - **Diffusion** is the net movement of particles from an area of higher concentration to an area of lower concentration, driven by the **concentration gradient**. - A steeper gradient means a larger difference in concentration, leading to a faster rate of net diffusion until equilibrium is reached. - According to **Fick's Law**, the rate of diffusion is directly proportional to the concentration gradient across the membrane. *Decreasing the lipid solubility of the substance* - The cell membrane is primarily composed of a **lipid bilayer**, meaning that substances with **higher lipid solubility** can more easily pass through it via simple diffusion. - Decreasing lipid solubility would **hinder** the substance's ability to cross the membrane, thus slowing down or preventing diffusion. *Increasing the size of the opening in the cell membrane* - While increasing channel or pore diameter can increase diffusion rate for **channel-mediated transport**, this option is less comprehensive than maintaining a concentration gradient. - The concentration gradient is the **primary driving force** for diffusion across all types of membrane transport (simple diffusion through lipid bilayer, channel-mediated, and carrier-mediated). - Channel size is relevant only for specific facilitated diffusion pathways, not for general particle diffusion. *Increasing the size of the particle* - **Smaller particles** generally diffuse faster than larger particles because they have higher diffusion coefficients and can more easily navigate through the membrane. - According to the **Stokes-Einstein equation**, diffusion rate is inversely proportional to particle size. - Increasing particle size would therefore **decrease** the rate of diffusion.

Q: All of the following are true about connexons, except which of the following?

Has five subunits. ***Has five subunits*** - A **connexon** is formed from **six protein subunits** called **connexins**, not five. - These six connexins assemble to form a single functional connexon channel. *It is a protein* - A **connexon** is indeed a **protein complex** that forms the channels within **gap junctions**. - It consists of multiple protein subunits called connexins. *Gap junction unit* - A **connexon** is the fundamental **transmembrane channel unit** of a **gap junction**. - Two opposing connexons (one from each adjacent cell) align to form a complete gap junction channel. *Charcot marie tooth disease* - Mutations in certain **connexin genes** (e.g., **connexin 32**) are associated with neurodegenerative diseases like **Charcot-Marie-Tooth disease type 1X**. - These mutations impair the proper function of connexons in myelinated nerve fibers.

Q: What is the primary action of the α-subunit of G protein in the signaling pathway?

Activation of downstream effectors. ***Activation of downstream effectors*** - Upon activation, the **α-subunit** dissociates from the βγ subunit and binds to and **activates specific downstream effector proteins**, such as adenylyl cyclase or phospholipase C. - This activation initiates a cascade of intracellular events leading to the cellular response. *Conversion of GDP to GTP* - The **α-subunit** binds **GTP** in its active state and **hydrolyzes GTP to GDP** to become inactive; however, its primary action is not the conversion itself but rather the subsequent signaling that occurs while GTP-bound. - This exchange is controlled by the **receptor**, which acts as a guanine nucleotide exchange factor (GEF), facilitating the release of GDP and binding of GTP. *Internalization of receptors* - **Receptor internalization** is a process by which cells take up receptors from the cell surface, often for degradation or recycling. - This process is primarily mediated by **clathrin-coated pits** and is distinct from the immediate signaling function of the G protein α-subunit. *Binding of agonist* - The **agonist binds to the G protein-coupled receptor (GPCR)**, not directly to the α-subunit. - Agonist binding to the GPCR induces a conformational change in the receptor, which then activates the associated G protein, leading to the GDP-GTP exchange on the α-subunit.

Q: Which of the following substances can freely permeate the plasma membrane?

Alcohol. ***Alcohol*** - **Small, lipophilic (fat-soluble), uncharged molecules** like alcohol can **freely and rapidly diffuse** through the lipid bilayer of the plasma membrane without any carrier or channel. - Among the given options, alcohol has the **highest membrane permeability** due to its optimal lipid solubility and small size. - This rapid permeability explains the quick systemic effects of alcohol consumption. *Glucose* - **Glucose** is a relatively large, highly polar molecule and **cannot freely permeate** the lipid bilayer. - It requires **specific carrier proteins (GLUT transporters)** for facilitated diffusion across the plasma membrane. - Its transport is often regulated and can be **insulin-dependent** in certain cells (e.g., GLUT4 in muscle and adipose tissue). *Urea* - While **small and uncharged**, urea is a **polar molecule** with significant hydrogen bonding capability, which **reduces its lipid solubility**. - Urea can cross membranes by simple diffusion but at a **much slower rate** compared to lipophilic molecules like alcohol. - In physiologically relevant contexts (e.g., renal tubules, RBCs), **specific urea transporters (UTs)** are required for efficient movement to meet cellular demands. *Glycerol* - **Glycerol** is a small molecule but contains **three hydroxyl groups**, making it relatively polar and **limiting its free permeability** through lipid bilayers. - While it can passively diffuse, the rate is **significantly slower than lipophilic molecules** like alcohol. - In many cells, particularly **adipocytes and renal tubules**, glycerol transport is facilitated by **aquaglyceroporins (AQP3, AQP7, AQP9)** to achieve physiologically adequate flux rates.

Q: According to Fick's law of diffusion, particle flux is directly proportional to which of the following?

Particle's concentration difference across the membrane. ***Particle's concentration difference across the membrane*** - Fick's first law states that the **rate of diffusion** (flux) is **directly proportional** to the **concentration gradient** (ΔC). - The mathematical expression: J = -D × (ΔC/Δx), where J is flux, D is diffusion coefficient, ΔC is concentration difference, and Δx is membrane thickness. - A greater concentration difference drives **higher net movement of particles**, directly increasing the flux. - This is the **primary driving force** for passive diffusion across membranes. *Area of the membrane* - While flux is also **proportional to membrane area** in Fick's law, this is a separate variable. - The question specifically asks about direct proportionality, and among the options, concentration gradient is the **classical factor** emphasized in Fick's first law. - Increasing surface area increases total flux but doesn't change the concentration gradient. *Temperature of the solution* - Temperature affects the **diffusion coefficient (D)** by increasing particle kinetic energy. - However, temperature is **not explicitly included** in Fick's first law formula as a directly proportional factor. - It indirectly affects diffusion rate through changes in D, making it an **external modulator** rather than a direct proportional factor. *Thickness of the Membrane* - Membrane thickness (Δx) has an **inverse relationship** with flux in Fick's law. - Greater thickness **decreases** flux as particles must travel a longer distance. - This demonstrates **inverse proportionality**, not direct proportionality as asked in the question.

Q: Which phase of the cell cycle does not have a fixed duration?

G1 phase (cell growth). ***G1*** - The **G1 phase** of the cell cycle is variable in length and can differ significantly between cell types and conditions, unlike S, M, and G2 phases [1][2]. - Cells can spend an **indeterminate amount of time** in G1, depending on factors like nutrients and signals for division [2]. *S* - The **S phase** is characterized by a fixed duration where **DNA replication** occurs, and is critical for cell division [1]. - It typically has a well-defined time frame in the cell cycle that is consistent across different cells [1]. *M* - The **M phase** (mitosis) requires a set duration to ensure that the **cell divides** accurately and equally into two daughter cells [2]. - Fluctuations in this phase can result in aberrant cell division and aneuploidy. *G2* - The **G2 phase** also has a consistent timeframe dedicated to preparing the cell for mitosis, focusing on DNA repair and organelle duplication [2]. - The cell ensures readiness for division during this phase, which is critical for genomic integrity. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 78-79. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 37-38.

Q: Diffusion of lipid-insoluble substances across the cell membrane depends on all of the following factors except which one?

Lipid solubility. ***Lipid solubility*** - This property is crucial for substances that **readily diffuse directly through the lipid bilayer**. - Lipid-insoluble substances, by definition, **cannot diffuse through the lipid bilayer based on their lipid solubility**, requiring other mechanisms or factors like channels or carriers. *Hydrated radius* - The **size of a hydrated ion or molecule** is a critical determinant for its ability to pass through specific protein channels or pores in the cell membrane. - A larger hydrated radius impedes passage through narrow channels, directly affecting the diffusion of lipid-insoluble substances. *Electrical charge* - For **charged lipid-insoluble substances** (ions), their movement across the membrane is significantly influenced by the **transmembrane electrical potential difference**. - The electrical gradient can either facilitate or hinder the diffusion of these substances through channels or transporters. *Shape* - The **three-dimensional configuration** of a lipid-insoluble substance can affect its ability to bind to and pass through specific protein channels or carrier proteins. - A substance's shape must complement the architecture of the transport mechanism for efficient diffusion.

Question 1

Which of the following is not a recognized stage of prophase I in meiosis?

Accepted Answer

Arachytene

Answer

Diakinesis

Answer

Leptotene

Answer

Zygotene

Question 2

How many transmembrane spanning domains does the Na+-K+-Cl- cotransporter contain?

Accepted Answer

12 transmembrane spanning domain

Answer

5 transmembrane spanning domain

Answer

7 transmembrane spanning domain

Answer

9 transmembrane spanning domain

Question 3

What is the process by which water moves from the extracellular space to the intracellular space?

Accepted Answer

Osmosis

Answer

Diffusion

Answer

Filtration

Answer

Active transport

Question 4

Which of the following factors increases the rate of particle diffusion across the cell membrane?

Accepted Answer

Maintaining a concentration gradient across the membrane

Answer

Decreasing the lipid solubility of the substance

Answer

Increasing the size of the opening in the cell membrane

Answer

Increasing the size of the particle

Question 5

All of the following are true about connexons, except which of the following?

Accepted Answer

Has five subunits

Answer

Gap junction unit

Answer

It is a protein

Answer

Charcot marie tooth disease

Question 6

What is the primary action of the α-subunit of G protein in the signaling pathway?

Accepted Answer

Activation of downstream effectors

Answer

Conversion of GDP to GTP

Answer

Internalization of receptors

Answer

Binding of agonist

Question 7

Which of the following substances can freely permeate the plasma membrane?

Accepted Answer

Alcohol

Answer

Glucose

Answer

Urea

Answer

Glycerol

Question 8

According to Fick's law of diffusion, particle flux is directly proportional to which of the following?

Accepted Answer

Particle's concentration difference across the membrane

Answer

Area of the membrane

Answer

Temperature of the solution

Answer

Thickness of the Membrane

Question 9

Which phase of the cell cycle does not have a fixed duration?

Accepted Answer

G1 phase (cell growth)

Answer

S phase (DNA synthesis)

Answer

M phase (mitosis)

Answer

G2 phase (preparation for mitosis)

Question 10

Diffusion of lipid-insoluble substances across the cell membrane depends on all of the following factors except which one?

Accepted Answer

Lipid solubility

Answer

Hydrated radius

Answer

Electrical charge

Answer

Shape

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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