Membrane Transport Proteins — MCQs
How many transmembrane spanning domains does the Na+-K+-Cl- cotransporter contain?
Assertion: RMP depends on proteins and phosphate ions. Reason: Diffusion potential can be calculated using nernst equation. Choose the best statement regarding the assertion and reason.
Diffusion of lipid-insoluble substances across the cell membrane depends on all of the following factors except which one?
Chloride receptor defect is responsible for:
Which of the following is the primary mechanism that drives sodium reabsorption in the proximal tubule?
Which of the following is a potassium Channelopathy?
Which of the following ion plays a role in exocytosis?
Which of these is true about the highlighted transporter?
Magnesium is not involved in ?
Following occurs in living cells only:
Membrane Transport Proteins Indian Medical PG Practice Questions and MCQs
Question 1: How many transmembrane spanning domains does the Na+-K+-Cl- cotransporter contain?
- A. 5 transmembrane spanning domain
- B. 7 transmembrane spanning domain
- C. 9 transmembrane spanning domain
- D. 12 transmembrane spanning domain (Correct Answer)
Explanation: ***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.
Question 2: Assertion: RMP depends on proteins and phosphate ions. Reason: Diffusion potential can be calculated using nernst equation. Choose the best statement regarding the assertion and reason.
- A. Assertion false, Reason true
- B. Both true, Reason is the explanation of assertion
- C. Assertion true, Reason false
- D. Both true, Reason is not the explanation of assertion (Correct Answer)
Explanation: ***Both true, Reason is not the explanation of assertion*** - The **Assertion is TRUE**: The resting membrane potential (RMP) does depend on intracellular **proteins and phosphate ions**, which are large, non-diffusible anions that remain trapped inside the cell. These molecules contribute significantly to the **net negative charge** of the intracellular compartment and create the **Gibbs-Donnan effect**. At physiological pH, most intracellular proteins are negatively charged, and phosphate ions (HPO₄²⁻, H₂PO₄⁻) are major intracellular anions. While the primary determinants of RMP are the concentration gradients and membrane permeabilities of K⁺, Na⁺, and Cl⁻ ions, the presence of non-diffusible anions (proteins and phosphates) is essential for establishing the baseline negative intracellular environment. - The **Reason is TRUE**: The **Nernst equation** (E = RT/zF × ln[ion]out/[ion]in) is indeed used to calculate the **equilibrium potential** (also called diffusion potential) for a single permeable ion. This equation determines the membrane potential at which the electrical gradient exactly balances the concentration gradient for that specific ion, resulting in no net ion movement. - **However, the Reason does NOT explain the Assertion**: The Nernst equation calculates equilibrium potentials for diffusible ions like K⁺, Na⁺, and Cl⁻. It does NOT explain the contribution of **non-diffusible** anions (proteins and phosphates) to the RMP. The actual RMP, which involves multiple ions with different permeabilities, is calculated using the **Goldman-Hodgkin-Katz (GHK) equation**, not the Nernst equation. The two statements are independently true but address different aspects of membrane potential physiology. *Assertion false, Reason true* - This is **incorrect** because the assertion is actually TRUE. Intracellular proteins and phosphate ions do contribute to the RMP by providing fixed negative charges that influence the distribution of diffusible ions and create the electrochemical environment necessary for RMP establishment. *Both true, Reason is the explanation of assertion* - This is **incorrect** because while both statements are true, the Nernst equation (Reason) does not explain how proteins and phosphate ions contribute to RMP (Assertion). The Nernst equation applies only to permeable ions, whereas proteins and phosphates are impermeant molecules whose role is explained by the Gibbs-Donnan equilibrium and their contribution to fixed negative charges. *Assertion true, Reason false* - This is **incorrect** because the reason is TRUE. The Nernst equation is a fundamental and valid equation in membrane physiology that accurately calculates the equilibrium potential for any permeable ion based on its concentration gradient.
Question 3: Diffusion of lipid-insoluble substances across the cell membrane depends on all of the following factors except which one?
- A. Hydrated radius
- B. Electrical charge
- C. Lipid solubility (Correct Answer)
- D. Shape
Explanation: ***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 4: Chloride receptor defect is responsible for:
- A. Cystic fibrosis (Correct Answer)
- B. Alpha-1 antitrypsin deficiency
- C. Wilson's disease
- D. Hemochromatosis
Explanation: Cystic fibrosis - Cystic fibrosis (CF) is caused by mutations in the **CFTR gene**, which encodes the **cystic fibrosis transmembrane conductance regulator protein**, a **chloride channel** [1]. - A defective CFTR protein leads to abnormal chloride transport across epithelial cells, resulting in thick, viscous secretions in various organs like the lungs, pancreas, and sweat glands [1]. *Alpha-1 antitrypsin deficiency* - This condition is due to a genetic defect in the production of **alpha-1 antitrypsin**, a protective enzyme. - It primarily affects the lungs (emphysema) and liver (cirrhosis) and is not related to chloride receptor function. *Wilson's disease* - Wilson's disease is an autosomal recessive disorder characterized by accumulation of **copper** in various tissues, especially the liver, brain, and eyes. - It is caused by mutations in the **ATP7B gene**, which codes for a copper-transporting ATPase, not a chloride channel. *Hemochromatosis* - Hemochromatosis is a disorder of **iron overload** in the body, primarily due to excessive absorption of dietary iron. - It is often caused by mutations in the **HFE gene** and does not involve chloride receptor defects.
Question 5: Which of the following is the primary mechanism that drives sodium reabsorption in the proximal tubule?
- A. Sodium reabsorption through cotransport with amino acids at the luminal membrane.
- B. Sodium reabsorption through cotransport with glucose at the luminal membrane.
- C. Sodium reabsorption through countertransport with hydrogen ions at the luminal membrane.
- D. Active sodium transport via the Na+-K+-ATPase pump at the basolateral membrane. (Correct Answer)
Explanation: ***Active sodium transport via the Na+-K+-ATPase pump at the basolateral membrane.*** - This pump **actively transports sodium out of the cell** into the interstitial fluid, creating a low intracellular sodium concentration. - The **Na+-K+-ATPase** is the primary driver of sodium reabsorption throughout the nephron, creating the electrochemical gradient for other sodium transporters. *Sodium reabsorption through cotransport with amino acids at the luminal membrane.* - While **sodium-amino acid cotransport** does occur in the proximal tubule, it accounts for only a fraction of total sodium reabsorption. - The primary driving force for this cotransport is the **low intracellular sodium concentration** maintained by the Na+-K+-ATPase. *Sodium reabsorption through cotransport with glucose at the luminal membrane.* - **Sodium-glucose cotransporters (SGLTs)** are crucial for glucose reabsorption in the proximal tubule, moving glucose into the cell along with sodium. - However, glucose cotransport represents a specific mechanism for glucose handling, not the overarching mechanism for sodium reabsorption. *Sodium reabsorption through countertransport with hydrogen ions at the luminal membrane.* - The **Na+-H+ exchanger (NHE3)** is significant for exchanging sodium for hydrogen ions at the luminal membrane in the proximal tubule. - This mechanism is important for **acid-base balance** and some sodium reabsorption, but it is secondary to the Na+-K+-ATPase in driving the overall sodium gradient.
Question 6: Which of the following is a potassium Channelopathy?
- A. Hyperkalemic periodic paralysis
- B. Long QT-syndrome
- C. Episodic ataxia I (Correct Answer)
- D. Hypokalemic periodic paralysis
Explanation: ***Episodic ataxia I*** - This condition is caused by mutations in the **KCNA1 gene**, which encodes the **Kv1.1 voltage-gated potassium channel**. - It represents a **classic neuromuscular potassium channelopathy** with episodes of **ataxia**, **dysarthria**, and myokymia. - This is a pure potassium channelopathy affecting the nervous system. *Hyperkalemic periodic paralysis* - This condition is caused by mutations in the **SCN4A gene**, which encodes a **sodium channel** subunit in skeletal muscle. - Despite the name suggesting potassium involvement, it is a **sodium channelopathy**, not a potassium channelopathy. - Episodes are triggered by elevated serum potassium levels. *Long QT syndrome* - Several subtypes (LQT1, LQT2, LQT5) are indeed caused by mutations in **potassium channel genes** (KCNQ1, KCNH2, KCNE1). - However, Long QT syndrome is a **heterogeneous group** that also includes sodium (LQT3) and calcium channelopathies. - It is classified as a **cardiac channelopathy** affecting ventricular repolarization. - In the context of this question, **Episodic ataxia I** is the most specific example of a pure potassium channelopathy. *Hypokalemic periodic paralysis* - This condition is most commonly caused by mutations in the **CACNA1S gene** (encoding a **calcium channel**) or **SCN4A gene** (encoding a **sodium channel**). - It is not a potassium channelopathy despite the hypokalemia that triggers episodes.
Question 7: 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 8: Which of these is true about the highlighted transporter?
- A. Both molecules go in
- B. One molecule goes in, other molecule goes out (Correct Answer)
- C. Both molecules go out
- D. One molecule goes in and two exit
Explanation: ***One molecule goes in, other molecule goes out*** - The highlighted transporter is the **Na+/K+ ATPase**, which actively pumps 3 **Na+ ions out** of the cell and 2 **K+ ions into** the cell, maintaining an electrochemical gradient. - This counter-transport (one molecule type going in and another going out) is characteristic of an **antiporter** pump. *Both molecules go in* - This option would describe a **symporter** mechanism where two different molecules move in the **same direction** across the membrane. - The Na+/K+ ATPase explicitly shows Na+ moving out and K+ moving in, which contradicts simultaneous inward movement. *Both molecules go out* - This would mean two molecules are expelled from the cell. The Na+/K+ ATPase, however, has K+ entering the cell. - While Na+ is pumped out by this transporter, K+ is actively transported inward. *One molecule goes in and two exit* - The Na+/K+ ATPase transports three Na+ ions out of the cell and two K+ ions into the cell, which is a 3:2 ratio and not one in and two out. - This option incorrectly describes the stoichiometry and directionality of ions for the Na+/K+ ATPase.
Question 9: 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 10: Following occurs in living cells only:
- A. Simple diffusion
- B. Facilitated diffusion
- C. Osmosis
- D. Active transport (Correct Answer)
Explanation: ***Active transport*** - **Active transport** requires energy (ATP) to move substances against their concentration gradient, a process only possible in **living cells** that can produce ATP. - This process is crucial for maintaining cellular homeostasis, accumulating nutrients, and removing waste, all of which are vital functions of **living organisms**. *Simple diffusion* - **Simple diffusion** is the passive movement of substances across a membrane from an area of higher concentration to lower concentration, without the need for energy or membrane proteins. - This process can occur in **both living and non-living systems**, as it is driven by random molecular motion and concentration gradients. *Facilitated diffusion* - **Facilitated diffusion** involves the passive movement of molecules across a membrane with the help of **transport proteins** (channels or carriers) but still moves down the concentration gradient without direct energy expenditure. - While it uses proteins, these proteins can sometimes function in **isolated membrane systems** even if the cell is not metabolically active (e.g., in a cell lysate). *Osmosis* - **Osmosis** is the specific type of diffusion involving the net movement of **water molecules** across a selectively permeable membrane, driven by differences in solute concentration. - Similar to simple diffusion, osmosis is a **physical process** based on water potential gradients and can occur in both **living and non-living membranes** given the right conditions.
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