Correct Answer: C. Secondary active transport
Glucose absorption in the small intestine occurs via secondary active transport (also called co-transport or symport), mediated by the SGLT1 transporter (sodium-glucose linked transporter 1) located on the apical membrane of enterocytes. The mechanism is coupled to the Na+ gradient established by the Na+/K+-ATPase pump on the basolateral membrane. For every glucose molecule transported into the cell, one sodium ion is co-transported down its electrochemical gradient (established by primary active transport). This energy is derived indirectly from ATP—not directly, which distinguishes it from primary active transport. Once inside the enterocyte, glucose exits via GLUT2 transporter (facilitated diffusion) on the basolateral membrane into the bloodstream. This two-step process (SGLT1 apical entry + GLUT2 basolateral exit) is the physiological mechanism for glucose absorption in the Indian population and globally. The SGLT1-mediated transport is saturable, active, and requires both sodium and glucose to be present simultaneously—hallmarks of secondary active transport. This is the DOC mechanism taught in Guyton and Harrison, and forms the basis of oral rehydration therapy (ORT) used extensively in India for diarrheal disease management.
Why the other options are wrong
A. Facilitated diffusion — Facilitated diffusion is passive and does not require energy coupling to an ion gradient. While GLUT2 (basolateral exit) uses facilitated diffusion, the rate-limiting step for glucose absorption is the apical entry via SGLT1, which is active and sodium-coupled. Facilitated diffusion alone cannot explain the active, saturable, and energy-dependent nature of intestinal glucose uptake. This is a common trap—students confuse the basolateral exit mechanism with the overall absorption process. B. Simple diffusion — Simple diffusion is non-saturable, non-selective, and passive—glucose absorption is the opposite. Glucose does not cross the intestinal epithelium by simple diffusion under physiological conditions; the concentration gradient alone is insufficient. Simple diffusion would be extremely slow and inefficient for nutrient absorption. This option tests whether students understand that glucose is a polar molecule requiring active transport mechanisms, not passive movement. D. Primary active transport — Primary active transport directly hydrolyzes ATP to move substances against their gradient (e.g., Na+/K+-ATPase, Ca2+-ATPase). Glucose absorption does not directly use ATP; instead, it harnesses the Na+ gradient created by the Na+/K+-ATPase. This indirect coupling defines secondary active transport. The NBE trap here is that students may conflate 'active' with 'primary active'—both require energy, but the source and mechanism differ fundamentally.
High-Yield Facts
- SGLT1 is the apical transporter for glucose in enterocytes; it is sodium-coupled and defines secondary active transport.
- GLUT2 is the basolateral transporter for glucose exit; it operates via facilitated diffusion (passive).
- The Na+/K+-ATPase on the basolateral membrane establishes the Na+ gradient that powers SGLT1—this is the energy source for glucose absorption.
- Glucose absorption is saturable (Km ~5 mM), selective, and active—properties that exclude simple and facilitated diffusion.
- Oral rehydration therapy (ORT) in India exploits the SGLT1-Na+ coupling: glucose enhances sodium absorption, which drives water absorption in diarrheal disease.
- Phlorizin (a competitive SGLT1 inhibitor) blocks glucose absorption and causes glycosuria—a classic pharmacology proof of SGLT1-mediated transport.
Mnemonics
SGLT = Sodium-Glucose Linked Transporter Sodium goes in → Glucose goes in (coupled, active). Remember: sodium is the 'engine,' glucose is the 'passenger.' When to use: any question on glucose absorption mechanism or why glucose absorption is active. Two-Step Glucose Absorption Apical (SGLT1) = Active (secondary, Na+-coupled) → Basolateral (GLUT2) = Passive (facilitated diffusion). The apical step is rate-limiting and defines the overall process as active. When to use: distinguishing between apical and basolateral mechanisms.
NBE Trap
NBE pairs "active transport" with "primary active transport" to trap students who conflate energy-dependent mechanisms. The discriminator is the direct vs. indirect ATP use: secondary active transport harnesses an ion gradient (created by primary active transport elsewhere), not direct ATP hydrolysis. Students must recognize that glucose absorption is active but not primary.
Clinical Pearl
In Indian children with acute diarrhea, ORT works because glucose and sodium are co-absorbed via SGLT1—glucose enhances sodium absorption, which osmotically drives water reabsorption. This is why WHO-recommended ORT (1:1 glucose-to-sodium ratio) is more effective than plain water, and why it remains the gold standard in rural India where IV fluids are inaccessible.
_Reference: Guyton & Hall Textbook of Medical Physiology, Ch. 66 (Digestion and Absorption in the GI Tract); Harrison's Principles of Internal Medicine, Ch. 297 (Approach to the Patient with Diarrhea)_