# Digestion and absorption in humans

> IB Biology SL · Theme B: Form and Function
> Source: https://www.owlsprep.com/study/ib-biology-sl-u2-digestion-and-absorption-in-humans/

This module covers the structure and function of the human digestive system, breakdown of macronutrients by digestive enzymes, and adaptations of the small intestine for absorption of nutrients into blood and lymph.

**Prerequisites:** [Enzyme structure and function](https://www.owlsprep.com/study/ib-biology-sl-u1-enzymes/); [Cell membrane transport](https://www.owlsprep.com/study/ib-biology-sl-u1-cell-membrane-transport/)

## Learning objectives

- Identify organs of the human digestive system and their key functions
- Explain enzyme adaptation to different pH environments in the digestive tract
- Describe digestion of carbohydrates, proteins and lipids
- Explain adaptations of the small intestine for absorption

## Structure of the Human Digestive System

The digestive system consists of a continuous muscular tube called the alimentary canal (or gastrointestinal tract) plus accessory organs that secrete enzymes and chemicals required for digestion. Food travels sequentially from the mouth → esophagus → stomach → small intestine → large intestine → anus for elimination.

**Alimentary Canal** — The continuous digestive tract extending from mouth to anus, where digestion and absorption of nutrients occurs

*Notation:* GI tract

*Example:* Key functional regions include the esophagus, stomach, small and large intestine

**Worked example:** Match three digestive organs (pancreas, gall bladder, small intestine) to their correct positions and basic functions

1. 1. Gall bladder: A small sac located under the liver, its function is to store bile produced by the liver.
2. 2. Pancreas: Located behind the stomach, connected to the first section of the small intestine (duodenum). It secretes digestive enzymes and bicarbonate.
3. 3. Small intestine: Long coiled tube between the stomach and large intestine, it is the site of most digestion and all nutrient absorption.

> **Exam tip:** IB Biology almost always asks you to link structure to function, so always connect structural features to their role in digestion/absorption in your answers.

## Digestion of Macronutrients by Enzymes

Digestion breaks down large, insoluble macromolecules into small, soluble monomers that can be absorbed into the bloodstream. All digestive reactions are hydrolysis reactions catalyzed by enzymes, each adapted to function optimally at a specific pH.

| Macronutrient | Digestion Sites | Key Enzyme | End Product |
| --- | --- | --- | --- |
| Carbohydrate | Mouth, small intestine | Amylase, maltase | Glucose |
| Protein | Stomach, small intestine | Pepsin, trypsin | Amino acids |
| Lipid | Small intestine | Lipase | Fatty acids + glycerol |

**Worked example:** Explain why the stomach protease pepsin does not function in the small intestine

1. 1. Enzymes have an optimal pH that matches the environment they evolved to work in.
2. 2. Pepsin's optimal pH is ~2, which matches the low pH of the stomach created by secreted hydrochloric acid.
3. 3. The small intestine has a neutral pH (~7) because pancreatic bicarbonate neutralizes incoming stomach acid.
4. 4. At pH 7, the charged amino acids in pepsin's active site are altered, causing the enzyme to denature and lose function.
5. Conclusion: Pepsin cannot work in the neutral pH environment of the small intestine.

## Adaptations of the Small Intestine for Absorption

Absorption is the movement of digested monomers from the lumen of the small intestine into epithelial cells, then into blood or lymph. The small intestine has multiple structural adaptations that maximize absorption rate, following the core biological principle of form fits function.

- Large surface area: Folded inner lining, millions of villi, and microvilli on epithelial cells increase surface area ~1000x
- Steep concentration gradient: Dense capillary network in each villus quickly removes absorbed nutrients
- Short diffusion distance: Epithelial layer is only one cell thick, so nutrients diffuse a very short distance
- Specialized vessels: Each villus has a lacteal (lymphatic vessel) that absorbs large lipid products

**Worked example:** Explain how villus structure increases the rate of glucose absorption

1. 1. Villi and microvilli dramatically increase surface area, allowing more glucose transporter proteins to be embedded in the epithelial membrane.
2. 2. The single-cell thick epithelium reduces the diffusion distance from the lumen to the capillary, increasing diffusion rate.
3. 3. Continuous blood flow through the villus capillary removes absorbed glucose, maintaining a high concentration gradient between the lumen and epithelial cells.
4. All three adaptations work together to maximize the rate of glucose absorption into the blood.

## Peristalsis and Absorption Mechanisms

Food is moved along the entire alimentary canal by peristalsis, wave-like contractions of circular and longitudinal smooth muscle layers. This process does not rely on gravity, which is why you can swallow while upside down.

**Check your understanding**

Test your understanding of absorption processes

1. Which process is used to transport glucose from the small intestine lumen into epithelial cells?

   - Simple diffusion
   - Active transport
   - Osmosis
   - Endocytosis

   *Why:* Glucose concentration is often higher in epithelial cells than the lumen, so active transport against the concentration gradient is required.

## Common pitfalls

- **Wrong:** Confusing digestion with absorption
  - Why it fails: Digestion is breakdown of large molecules, while absorption is movement of small molecules into the blood. Questions often test if you can distinguish these.
  - Correct: Read the question carefully: if it asks about digestion, discuss enzyme action; if it asks about absorption, discuss villi adaptations and transport.
- **Wrong:** Claiming bile is produced by the gall bladder
  - Why it fails: This is a common misconception tested frequently in Paper 1 multiple choice.
  - Correct: Bile is produced by the liver, only stored and released by the gall bladder.
- **Wrong:** Stating all lipids are absorbed directly into blood capillaries
  - Why it fails: Large long-chain fatty acids are too big to enter capillaries, so they are absorbed by the lymphatic system instead.
  - Correct: Distinguish between small short-chain fatty acids (enter capillaries) and large long-chain fatty acids (enter lacteals).
- **Wrong:** Thinking peristalsis only occurs in the esophagus
  - Why it fails: Peristalsis moves food through the entire alimentary canal, from esophagus to rectum.
  - Correct: Recognize peristalsis as a continuous process that acts along the whole digestive tract.

## Cheatsheet

| Component | Key Function | Core Fact |
| --- | --- | --- |
| Stomach | Protein digestion | Low pH ~2 for pepsin activity |
| Pancreas | Secrete enzymes + bicarbonate | Neutralizes stomach acid in small intestine |
| Liver | Produces bile | Bile emulsifies lipids, not an enzyme |
| Gall bladder | Stores bile | Releases bile into the duodenum |
| Villi | Absorb nutrients | 1 cell thick, capillaries + lacteal |
| Carbohydrate | Digestion produces glucose | Digestion starts in the mouth |
| Protein | Digestion produces amino acids | Digestion starts in the stomach |
| Lipid | Digestion produces fatty acids | Only digested in the small intestine |

## What's next

Digestion and absorption is a core example of the form follows function theme that runs through all IB Biology. The concepts of enzyme pH dependence and structural adaptation you practiced here are frequently tested in extended response questions, and connect back to foundational topics you learned earlier in the course. This topic also provides the base for understanding how nutrients are used for energy and growth, and how the body maintains homeostasis through coordinated action of different organ systems. Explore the linked topics below to continue building your knowledge of human physiology.

- [Hormones, homeostasis and reproduction](https://www.owlsprep.com/study/ib-biology-sl-u2-hormones-homeostasis-and-reproduction/)
- [Theme C: Interactions and Interdependencies](https://www.owlsprep.com/study/ib-biology-sl-u3-overview/)

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