Transport Across Membranes — MCQs
Which mechanism is primarily responsible for the transport of glucose in renal tubular cells?
Which membrane channel is mainly affected in Cystic fibrosis?
Which of the following is a genetic disorder that does not primarily affect ion channels?
Which of the following substances can freely permeate the plasma membrane?
Magnesium is not involved in ?
Diffusion of lipid-insoluble substances across the cell membrane depends on all of the following factors except which one?
Which transport process is mediated by carriers and occurs against the concentration gradient?
Which of the following is a major component of the plasma membrane?
Proteoglycan present in the glomerular basement membrane is?
Maximum fluidity of the cell membrane is due to?
Transport Across Membranes Indian Medical PG Practice Questions and MCQs
Question 1: Which mechanism is primarily responsible for the transport of glucose in renal tubular cells?
- A. Facilitated diffusion
- B. Glucose diffusion
- C. Sodium antiport
- D. Sodium-glucose cotransport (Correct Answer)
Explanation: ***Sodium-glucose cotransport*** - Glucose reabsorption in the renal tubules, particularly in the **proximal tubule**, occurs primarily via **secondary active transport** involving **sodium-glucose cotransporters (SGLTs)**. - SGLT proteins use the **sodium concentration gradient** (maintained by the Na+/K+-ATPase on the basolateral membrane) to move glucose against its concentration gradient from the tubular lumen into the cell. *Glucose diffusion* - While passive diffusion may play a minor role, it is insufficient to reabsorb the large amounts of **filtered glucose** - Diffusion would lead to significant **glucose loss in urine**, even at normal blood glucose levels. *Sodium antiport* - Antiport systems move two different ions or molecules in **opposite directions** across a membrane. - While present in renal cells, sodium antiport mechanisms are not the primary means of **glucose reabsorption**; rather, glucose transport is mostly symport. *Facilitated diffusion* - Facilitated diffusion involves carrier proteins (like **GLUT transporters**) that move molecules down their **concentration gradient**. - While GLUT transporters are present on the **basolateral membrane** of tubular cells to move glucose into the interstitium, they are not the primary mechanism for glucose uptake from the tubular lumen, which occurs against a concentration gradient.
Question 2: Which membrane channel is mainly affected in Cystic fibrosis?
- A. Sodium
- B. Chloride (Correct Answer)
- C. Calcium
- D. Potassium
Explanation: ***Chloride*** - Cystic fibrosis is caused by a mutation in the **CFTR (Cystic Fibrosis Transmembrane Conductance Regulator)** gene, which encodes a chloride channel. - Dysfunction of this **chloride channel** leads to impaired transport of chloride ions, mainly affecting epithelial cells in various organs. *Sodium* - While sodium transport is indirectly affected in cystic fibrosis, the primary defect is not in a sodium channel itself but rather in the **chloride channel**, which influences water and sodium movement. - The abnormal **chloride transport** leads to a compensatory but ineffective increase in sodium absorption in some tissues like the airway. *Calcium* - Calcium channels are not primarily implicated in the pathophysiology of **cystic fibrosis**. - **Calcium dysregulation** can occur secondarily in some CF-related processes, but it is not the main affected membrane channel. *Potassium* - **Potassium channels** are not the main membrane channels affected in cystic fibrosis. - While potassium transport is vital for cellular function, it is not the primary defect underlying the disease's respiratory and gastrointestinal manifestations.
Question 3: Which of the following is a genetic disorder that does not primarily affect ion channels?
- A. Cystic fibrosis
- B. Liddle's syndrome
- C. Hypokalemic periodic paralysis
- D. Tay-Sachs disease (Correct Answer)
Explanation: ***Cystic fibrosis*** - Primarily caused by mutations in the **CFTR gene**, which encodes a **chloride channel**, but it is not classified as a classic channelopathy. - The disease mainly affects **mucus production** rather than direct dysfunction of ion channels in the traditional sense. [1] *Hypokalemic periodic paralysis* - This condition is directly related to **ion channel dysfunction**, specifically affecting **sodium channels** in muscle cells. - It causes episodic **muscle weakness** and hypokalemia due to improper ion transport. *Liddle's syndrome* - A genetic disorder resulting from mutations affecting **epithelial sodium channels**, leading to **hypertension**. - It exemplifies classic channelopathy by causing dysregulation of sodium reabsorption in the kidneys. *Tay-sach's disease* - A ***gangliosidosis*** caused by a deficiency in the enzyme **Hexosaminidase A**, rather than ion channel dysfunction. - It results in the accumulation of **GM2 gangliosides** leading to neurological degeneration, not affecting ion channels directly. **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. 120-122.
Question 4: Which of the following substances can freely permeate the plasma membrane?
- A. Glucose
- B. Urea
- C. Glycerol
- D. Alcohol (Correct Answer)
Explanation: ***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.
Question 5: 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 6: 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 7: Which transport process is mediated by carriers and occurs against the concentration gradient?
- A. Facilitated diffusion
- B. Osmosis
- C. Active transport (Correct Answer)
- D. Endocytosis
Explanation: ***Active transport*** - **Active transport** systems use carrier proteins to move molecules across a membrane **against their concentration gradient**, requiring **metabolic energy** (e.g., from ATP hydrolysis). - This process is crucial for maintaining cellular homeostasis, accumulating specific substances, and establishing ion gradients. *Facilitated diffusion* - **Facilitated diffusion** also uses **carrier proteins**, but it moves substances **down their concentration gradient**, thus **not requiring metabolic energy**. - It increases the rate of diffusion for molecules that cannot easily cross the lipid bilayer, like glucose. *Osmosis* - **Osmosis** is the movement of **water molecules** across a selectively permeable membrane **down their water potential gradient**, driven by solute concentration differences, and does **not involve carrier proteins**. - This process equalizes solute concentrations on both sides of the membrane. *Endocytosis* - **Endocytosis** is a bulk transport mechanism where cells **engulf substances** from outside by forming vesicles from the plasma membrane; it's a form of active transport but **does not typically involve specific carrier proteins** embedded in the membrane for individual molecules. - This process is used for taking in larger molecules, particles, or even other cells.
Question 8: Which of the following is a major component of the plasma membrane?
- A. Protein
- B. Carbohydrate
- C. Cholesterol
- D. Phospholipid (Correct Answer)
Explanation: ***Phospholipid*** - **Phospholipids** form the fundamental **phospholipid bilayer** structure of the plasma membrane, acting as a barrier. - Their **amphipathic nature** (hydrophilic head and hydrophobic tails) allows them to spontaneously form this bilayer in an aqueous environment. *Carbohydrate* - **Carbohydrates** are present on the outer surface of the plasma membrane, forming the **glycocalyx**, but are not a major structural component of the bilayer itself. - They primarily function in **cell recognition** and adhesion. *Protein* - **Proteins** are embedded within or associated with the phospholipid bilayer, facilitating various functions like **transport**, signaling, and adhesion. - While crucial for function, they do not form the basic structural framework of the membrane. *Cholesterol* - **Cholesterol** is a type of lipid that helps regulate the **fluidity** and stability of the plasma membrane. - It is interspersed within the phospholipid bilayer but is not the primary structural component.
Question 9: Proteoglycan present in the glomerular basement membrane is?
- A. Keratan sulfate 1
- B. Keratan sulfate 2
- C. Heparan sulfate (Correct Answer)
- D. Chondroitin sulfate
Explanation: ***Heparan sulphate*** - Heparan sulphate is a key component of the **glomerular basement membrane** (GBM), crucial for its **negative charge and filtration function** [1][2]. - It plays a significant role in **filtration barrier** properties and affects the permeability of the GBM to proteins [1]. *Keratan sulphate 1* - Predominantly found in **cartilage** and **corneal tissue**, not associated with the glomerular basement membrane. - It contributes to **mechanical support** but lacks the essential role in renal filtration. *Keratan sulphate 2* - Similar to Keratan sulphate 1, this variant is involved in **cartilage** but not in the structure of the glomerular basement membrane. - Has distinct functions related to **tissue hydration** rather than the filtration dynamics of the GBM. *Chondroitin sulphate* - Commonly located in **cartilage** and connective tissues, it does not play a significant role in the structure of the glomerular basement membrane. - While it assists in **cell signaling** and regeneration, it does not influence the GBM permeability like heparan sulphate. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Kidney, pp. 905-907. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 34-35.
Question 10: Maximum fluidity of the cell membrane is due to?
- A. palmitic acid
- B. alpha-linolenic acid
- C. arachidonic acid (Correct Answer)
- D. linoleic acid (18:2)
Explanation: ***arachidonic acid*** - **Arachidonic acid** is a polyunsaturated fatty acid (PUFA) with 20 carbons and 4 double bonds, denoted as C20:4. The presence of multiple **double bonds** creates kinks in the fatty acid tails, preventing tight packing of phospholipids in the membrane and thus increasing fluidity. - Cell membrane fluidity is enhanced by **unsaturated fatty acids** due to the presence of C=C double bonds. The more double bonds a fatty acid has (i.e., higher degree of unsaturation), the greater its contribution to membrane fluidity. Since arachidonic acid has the most double bonds among the options, it confers the greatest fluidity. *alpha-linolenic acid* - **Alpha-linolenic acid** is an omega-3 fatty acid (C18:3). While it is a **polyunsaturated fatty acid** and contributes to fluidity, it has fewer double bonds (3) than arachidonic acid (4), making it less effective in maximizing fluidity compared to arachidonic acid. - Its presence increases membrane fluidity, but not to the same extent as fatty acids with a higher degree of unsaturation. *linoleic acid (18:2)* - **Linoleic acid** (C18:2) is an omega-6 fatty acid with two double bonds. It contributes to membrane fluidity because it is unsaturated, but **less so than fatty acids with more double bonds** such as alpha-linolenic acid or arachidonic acid. - The fewer double bonds mean the fatty acid tails can pack a bit more closely, offering less fluidity compared to highly unsaturated fatty acids. *palmitic acid* - **Palmitic acid** is a **saturated fatty acid** (C16:0), meaning it has no double bonds. Saturated fatty acids have straight hydrocarbon chains that can pack tightly together in the cell membrane. - This tight packing **reduces membrane fluidity** and makes the membrane more rigid, which is the opposite of what maximizes fluidity.
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