Membrane Transport Mechanisms — MCQs
Which of the following factors acts in vesicle targeting?
Fructose is transported by which transporter?
What is the physiological condition in which the ratio of potassium permeability to sodium permeability (PK/PNa) is maximized?
Where is the GLUT1 transporter primarily located?
Which of the following statements about the Na-K pump is false?
Which of these is true about the highlighted transporter?
Regarding glutathione all of the following are true EXCEPT
Not true about anterior cruciate ligament -
Baby born with membrane around him at the time of birth. Which of the following conditions is depicted?
Two solutions with equal osmotic pressures are called:
Membrane Transport Mechanisms Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following factors acts in vesicle targeting?
- A. Sec 12
- B. Rab (Correct Answer)
- C. Ras
- D. SNARE
Explanation: ***Rab*** - **Rab GTPases** are small G proteins that regulate many steps of membrane trafficking, including vesicle formation, cargo selection, vesicle budding, uncoating, motility, and fusion. - They act as molecular switches, cycling between an active GTP-bound state and an inactive GDP-bound state, thereby coordinating the proper targeting of vesicles to their destination membranes. *Sec 12* - **Sec12** is a **GEF (guanine nucleotide exchange factor)** for **Sar1**, which is involved in COPII vesicle formation from the ER. - While it initiates a step in vesicle budding, it does not directly act as a targeting molecule to guide the vesicle to its destination. *Ras* - **Ras GTPases** are primarily involved in cell signaling pathways regulating cell proliferation, differentiation, and survival. - They are not directly involved in the process of **vesicle targeting** in membrane trafficking. *SNARE* - **SNARE proteins** (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) are crucial for the **fusion of vesicles** with their target membranes. - While essential for the later stages of trafficking, they mediate membrane fusion rather than initial vesicle targeting.
Question 2: Fructose is transported by which transporter?
- A. GLUT 5 (Correct Answer)
- B. GLUT 4
- C. GLUT 3
- D. GLUT 7
Explanation: ***GLUT 5*** - **GLUT 5** is the primary transporter responsible for **fructose absorption** in the intestine. - It facilitates **fructose diffusion** down its concentration gradient, independent of sodium. *GLUT 4* - **GLUT 4** is an **insulin-dependent** glucose transporter primarily found in **adipose tissue** and **skeletal muscle**. - Its translocation to the cell membrane is stimulated by insulin, allowing glucose uptake in response to fed states. *GLUT 3* - **GLUT 3** is a high-affinity glucose transporter found predominantly in **neurons** and the **brain**. - It ensures a constant supply of glucose to these vital tissues, even at low blood glucose concentrations. *GLUT 7* - **GLUT 7** is primarily expressed in the **endoplasmic reticulum** and is involved in **glucose-6-phosphate transport**. - It plays a crucial role in **gluconeogenesis** and glycogenolysis in the liver.
Question 3: What is the physiological condition in which the ratio of potassium permeability to sodium permeability (PK/PNa) is maximized?
- A. Depolarization
- B. Action Potential
- C. Resting Membrane Potential
- D. Hyperpolarization (Correct Answer)
Explanation: ***Hyperpolarization*** - During **hyperpolarization**, the membrane potential becomes more negative than the **resting membrane potential**, primarily due to the outflow of **potassium (K+)** ions or influx of **chloride (Cl-)** ions. - This increased K+ efflux or Cl- influx signifies a state where potassium permeability is maximal relative to sodium permeability, making the membrane less excitable. *Action Potential* - An **action potential** involves a rapid **depolarization** phase due to a massive influx of **sodium (Na+)** ions, causing the PNa/PK ratio to be high, followed by repolarization where K+ efflux restores the resting potential. - Therefore, during an action potential, the ratio of PK/PNa is at its lowest during the rising phase when sodium channels are open. *Depolarization* - **Depolarization** is characterized by a decrease in the absolute value of the membrane potential, making it less negative or even positive, primarily due to the influx of **sodium (Na+)** ions. - During depolarization, the permeability to sodium is significantly higher than to potassium, thus the PK/PNa ratio is low. *Resting Membrane Potential* - At **resting membrane potential**, potassium permeability is already much higher than sodium permeability due to **leak potassium channels**, but it is not maximized to the extent seen during hyperpolarization. - The resting potential is established by a balance of ion movements, primarily K+ efflux and limited Na+ influx, maintained by the **Na+/K+-ATPase pump**.
Question 4: Where is the GLUT1 transporter primarily located?
- A. Liver
- B. Brain (Correct Answer)
- C. Placenta
- D. Heart
Explanation: ***Brain*** - The **GLUT1 transporter** is primarily located in the **brain**, particularly in the **blood-brain barrier (BBB)** endothelial cells. - It is the major glucose transporter responsible for facilitating glucose transport from blood into the brain, which is essential since the brain depends almost exclusively on glucose for energy. - GLUT1 is constitutively expressed and provides continuous glucose supply to meet the brain's high metabolic demands. - This makes the brain the classic and primary site emphasized in medical textbooks for GLUT1 expression. *Placenta* - The **placenta** also has high expression of **GLUT1**, which facilitates glucose transport from maternal circulation to the fetus. - This is the second major site of GLUT1 expression and is critical for fetal development. - However, when asked about the "primary" location, brain (BBB) is the standard answer in medical education. *Heart* - While **GLUT1** is present in the heart, the heart muscle primarily uses **GLUT4**, which is insulin-dependent and responsible for most glucose uptake. - GLUT1 provides basal glucose uptake but is not the predominant transporter in cardiac tissue. *Liver* - The **liver** primarily uses **GLUT2**, a low-affinity, high-capacity bidirectional transporter that allows both glucose uptake and release depending on blood glucose levels. - GLUT2 is essential for the liver's role in glucose homeostasis, while GLUT1 is minimally expressed in hepatocytes.
Question 5: 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 6: 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 7: Regarding glutathione all of the following are true EXCEPT
- A. It inactivates some enzymes (Correct Answer)
- B. It helps in membrane transport
- C. It helps in conjugation reactions
- D. It helps in absorption of certain amino acids
Explanation: ***It inactivates some enzymes*** - Glutathione is a crucial **antioxidant** that plays a vital role in protecting enzymes and other cellular components from oxidative damage by **reducing disulfide bonds**, thereby preserving or restoring enzyme activity rather than inactivating them. - Its primary function is to **detoxify harmful compounds** and maintain the cellular redox state, which requires enzymes to be active. *It helps in membrane transport* - Glutathione is involved in the active transport of certain substances across cell membranes, particularly in the **GSH-dependent transport systems** in the kidney and liver. - It forms conjugates with toxins, which are then transported out of the cell or body, a process often referred to as **efflux pumps**. *It helps in conjugation reactions* - Glutathione is a key substrate for **glutathione S-transferases (GSTs)**, enzymes that catalyze the conjugation of glutathione with various electrophilic compounds. - This process is vital for the **detoxification** and elimination of xenobiotics and endogenous toxic metabolites. *It helps in absorption of certain amino acids* - The **gamma-glutamyl cycle** (also known as the Meister cycle) in the kidneys utilizes glutathione for the transport of amino acids across cell membranes. - In this cycle, glutathione is broken down to release an amino acid, which is then transported into the cell, thus facilitating **amino acid uptake**.
Question 8: Not true about anterior cruciate ligament -
- A. It provides proprioceptive inputs to knee
- B. It is taut in extension
- C. It is extrasynovial
- D. It begins just behind the anterior horn of lateral meniscus on tibia (Correct Answer)
Explanation: ***It begins just behind the anterior horn of lateral meniscus on tibia*** - This statement is incorrect. The **anterior cruciate ligament (ACL)** originates from the **posteromedial aspect of the lateral femoral condyle** and inserts into the **anterior intercondylar area** of the tibia, anterior to the tibial spine, and medial to the anterior horn of the lateral meniscus. - Its tibial attachment is **not just behind the anterior horn of the lateral meniscus**, but rather anterior to the intercondylar eminence and medial to the lateral meniscus. *It provides proprioceptive inputs to knee* - The **ACL** contains numerous **mechanoreceptors** (e.g., Ruffini endings, Pacinian corpuscles, free nerve endings) that provide **proprioceptive feedback** to the central nervous system. - This feedback is crucial for **joint position sense** and neuromuscular control of the knee, helping to prevent injury. *It is taut in extension* - The **ACL** is under increasing tension as the knee moves from flexion to **full extension**, particularly in the last 10-20 degrees. - This tautness in extension is critical for its role in preventing **hyperextension** and anterior translation of the tibia relative to the femur. *It is extrasynovial* - The **ACL** is located **intracapsularly** (within the joint capsule) but **extrasynovially** (outside the synovial membrane). - This means it is bathed in synovial fluid but does not have a synovial lining itself, which is a characteristic feature of all cruciate ligaments.
Question 9: Baby born with membrane around him at the time of birth. Which of the following conditions is depicted?
- A. X-linked ichthyosis (steroid sulfatase deficiency)
- B. Generalized hyperkeratosis (thickened skin)
- C. Ichthyosis vulgaris (dry, scaly skin)
- D. Lamellar ichthyosis (collodion membrane at birth) (Correct Answer)
Explanation: ***Lamellar ichthyosis (collodion membrane at birth)*** - This condition is characterized by a "collodion membrane" at birth, which is a **tight, shiny, parchment-like membrane** that covers the entire body. - The membrane typically **sheds within weeks**, revealing underlying scaling and erythema characteristic of lamellar ichthyosis. *X-linked ichthyosis (steroid sulfatase deficiency)* - Marked by **dark brown, adherent scales**, primarily affecting the neck, trunk, and extensor surfaces. - It usually becomes apparent **several weeks or months after birth** and is not typically associated with a collodion membrane. *Generalized hyperkeratosis (thickened skin)* - This is a general term for **thickening of the outermost layer of the epidermis** and is a feature of many ichthyoses, not a specific condition with a "membrane at birth." - It describes a **symptom** rather than a primary diagnosis presenting with a specific birth membrane. *Ichthyosis vulgaris (dry, scaly skin)* - Presents with **fine, white scaling**, most prominent on the extensor surfaces of the limbs, but it **rarely appears at birth**. - It is typically **mild** and often worsens in dry, cold weather, lacking the characteristic "membrane around him" at birth.
Question 10: Two solutions with equal osmotic pressures are called:
- A. Normal solution
- B. Hypertonic solution
- C. Isotonic solution (Correct Answer)
- D. Hypotonic solution
Explanation: ***Isotonic solution*** - **Isotonic solutions** have the same solute concentration, and therefore the same **osmotic pressure**, as another solution. - In biological systems, an isotonic solution has the same osmotic pressure as the **cytosol** inside cells, preventing net water movement. *Normal solution* - "Normal solution" is a general term often referring to a solution at standard conditions or a commonly used concentration, but it does not specifically mean equal osmotic pressure. - While **normal saline** (0.9% NaCl) is isotonic to human plasma, the term "normal solution" itself is not a direct definition of equal osmotic pressure. *Hypertonic solution* - A **hypertonic solution** has a higher solute concentration and thus a higher **osmotic pressure** compared to another solution. - When a cell is placed in a hypertonic solution, water moves out of the cell, causing it to **crenate** or shrink. *Hypotonic solution* - A **hypotonic solution** has a lower solute concentration and thus a lower **osmotic pressure** compared to another solution. - When a cell is placed in a hypotonic solution, water moves into the cell, causing it to **swell** and potentially lyse.
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