Question
How are different nutrients absorbed in the small intestine? Distinguish between active transport and passive transport during absorption. Which nutrients are absorbed by each mechanism? What special pathway do fats follow?
(NEET + CBSE Board — mechanism + specifics)
Solution — Step by Step
| Feature | Active Transport | Passive Transport |
|---|---|---|
| Energy required? | Yes (ATP) | No |
| Direction | Against concentration gradient | Along concentration gradient |
| Carrier protein? | Yes | Diffusion — may or may not need carrier |
| Examples | Amino acids, glucose (in some regions), Na⁺ | Fructose, fatty acids, water, some minerals |
| Nutrient | Mechanism | Site | Enters |
|---|---|---|---|
| Glucose | Active transport (co-transport with Na⁺) | Small intestine (jejunum) | Blood capillaries → portal vein → liver |
| Fructose | Facilitated diffusion (passive) | Small intestine | Blood capillaries |
| Amino acids | Active transport (Na⁺ co-transport) | Small intestine (ileum) | Blood capillaries → portal vein → liver |
| Fatty acids + glycerol | Passive diffusion into enterocytes | Small intestine (jejunum) | Reassembled as fats → chylomicrons → lacteals (lymph) |
| Water | Osmosis (passive) | Small and large intestine | Blood capillaries |
| Vitamins (water-soluble) | Diffusion | Small intestine | Blood capillaries |
| Vitamins (fat-soluble: A, D, E, K) | Absorbed with fats | Small intestine | Lacteals (lymph) |
| Iron | Active transport | Duodenum | Blood (bound to transferrin) |
| Vitamin B₁₂ | Active transport (requires intrinsic factor) | Ileum | Blood |
Fats take a unique route. After digestion into fatty acids and glycerol:
- They diffuse into intestinal epithelial cells (enterocytes)
- Inside the cells, they are re-esterified back into triglycerides
- Triglycerides are packaged with proteins into chylomicrons
- Chylomicrons enter lacteals (lymphatic capillaries in villi), NOT blood capillaries
- From lacteals, they travel through the lymphatic system and eventually enter the blood via the thoracic duct
This is why fats bypass the liver initially — unlike glucose and amino acids, which go directly to the liver via the portal vein.
graph TD
A["Digested Nutrients"] --> B["Glucose, Amino acids"]
A --> C["Fats"]
A --> D["Water, Fructose"]
B -->|"Active transport"| E["Blood Capillaries"]
C -->|"Passive → Chylomicrons"| F["Lacteals - Lymph"]
D -->|"Passive diffusion"| E
E --> G["Portal Vein → Liver"]
F --> H["Lymphatic System → Blood"]
style A fill:#fbbf24,stroke:#000,stroke-width:2px
style E fill:#86efac,stroke:#000
style F fill:#f9a8d4,stroke:#000Why This Works
The intestine uses different absorption strategies because nutrients have different properties. Small, water-soluble molecules (glucose, amino acids) can be actively pumped against their gradient — the intestine invests ATP to ensure maximum absorption even when concentrations are low. Fats, being hydrophobic, cannot dissolve in blood directly — they must be packaged into lipoprotein particles (chylomicrons) and transported via the lymphatic system.
The portal vein route (for glucose, amino acids) sends nutrients to the liver first for processing, detoxification, and storage. The lymphatic route (for fats) bypasses the liver initially.
Common Mistake
Students commonly write that “all nutrients are absorbed by active transport.” This is incorrect. Fructose is absorbed by facilitated diffusion (passive), water by osmosis (passive), and fatty acids by simple diffusion. Only glucose and amino acids use active transport as their primary mechanism. NEET specifically tests which nutrients use which mechanism.
The most tested NEET fact here: fats are absorbed into lacteals (lymph), NOT blood capillaries. This is unique to fats — all other nutrients enter blood capillaries directly. If you see “lacteal” or “chylomicron” in the options, it almost always relates to fat absorption.