What is the difference between villi and microvilli?

Villi are finger-like projections of the intestinal lining composed of many cells, whereas microvilli are microscopic projections of the cell membrane on individual epithelial cells. Both serve to increase surface area.

How does the transport pathway of fatty acids differ from that of glucose?

Glucose is absorbed directly into the blood capillaries within the villus. Fatty acids are absorbed into the central lacteal (lymph vessel) because they are too large to enter capillaries directly.

What is the difference between diffusion and active transport in the context of absorption?

Diffusion is the passive movement of nutrients down a concentration gradient (high to low) without energy. Active transport requires energy to move nutrients against the concentration gradient (low to high) when luminal concentrations are low.

Why is it incorrect to say food is absorbed in the stomach?

The stomach is primarily for mechanical churning and initial protein digestion. The specialized structures for absorption (villi/microvilli) and the completion of digestion occur in the small intestine.

What common mistake do students make regarding the 'thickness' of the small intestine wall?

Students often describe the wall as 'thin' without quantification. The correct biological description is that the epithelium is 'one cell thick,' which specifically highlights the minimized diffusion distance.

Why is 'large surface area' alone an insufficient explanation for rapid absorption?

While large surface area is crucial, rapid absorption also depends on a short diffusion distance (thin walls) and a steep concentration gradient. You must mention how the structure facilitates these physical principles.

A lacteal is a lymphatic vessel located in the center of each intestinal villus. Its primary function is to absorb dietary fats (fatty acids and glycerol) which cannot enter the blood capillaries.

Define 'absorption' in the context of the alimentary canal.

Absorption is the movement of small, soluble digested food molecules (like glucose and amino acids) from the lumen of the intestine into the blood or lymph circulation.

What is the function of the capillary network in a villus?

The capillary network absorbs water-soluble nutrients (glucose and amino acids) and transports them away from the intestine, maintaining a steep concentration gradient for further diffusion.

Why is the small intestine wall only one cell thick?

A single-cell layer minimizes the physical distance that molecules must travel during diffusion. According to diffusion principles, a shorter distance results in a significantly faster rate of transport.

Small Intestine: Structure & Adaptations | Pearson Edexcel IGCSE Science

Double Award Modular / Biology Unit 1

2. Structure & Functions in Living Organisms: Part 1

Small Intestine: Structure & Adaptations

Summary

The small intestine is the primary site for nutrient absorption in the alimentary canal. Its efficiency is derived from specialized structural adaptations that maximize surface area, minimize diffusion distances, and maintain steep concentration gradients, facilitating the transport of glucose, amino acids, fatty acids, and glycerol into the body's transport systems.

1. Definition & Core Function

Absorption: The physiological process where small, soluble digested food molecules move from the lumen of the gut into the blood or lymph systems.

Transport Mechanisms: Nutrients move across the intestinal wall via diffusion (passive movement down a gradient), active transport (movement against a gradient requiring energy), and osmosis (movement of water).

Primary Site: While some water absorption occurs in the colon, the small intestine is the main location for the absorption of nutrients and the majority of water.

2. Structural Adaptations for Efficiency

The small intestine is designed to maximize the rate of diffusion according to Fick's Law principles:

Length: The organ is extremely long, providing a substantial amount of time and space for absorption to occur as food passes through.
Folding: The inner surface is highly folded, not smooth, which drastically increases the surface area relative to a simple tube.
Villi: The lining contains millions of tiny, finger-like projections called villi (singular: villus).
Microvilli: The epithelial cells on the surface of each villus possess their own microscopic projections called microvilli, further increasing the surface area by orders of magnitude.

Diagram of a single intestinal villus showing the central lacteal, surrounding capillary network, and thin epithelial layer.

3. The Villus Unit: Internal Structure

Each villus acts as a functional absorption unit with specific internal structures:

Epithelium: The outer wall is only one cell thick. This minimizes the diffusion distance for nutrients to travel from the intestine lumen into the blood.
Capillary Network: A dense network of blood capillaries surrounds the lacteal. This transports glucose and amino acids directly into the bloodstream.
Lacteal: A central lymph vessel runs through the middle of the villus. This absorbs fatty acids and glycerol, which are too large to enter capillaries directly.
Goblet Cells: Produce mucus to lubricate the passage of food and protect the lining.

4. Maintenance of Concentration Gradients

5. Key Distinctions: Transport Pathways

6. Exam Strategy & Tips