Small Intestine: Structure & Adaptations (Edexcel IGCSE Biology)

Absorption of Food & Water

Spec point — 2.32 · understand how the small intestine is adapted for absorption, including the structure of a villus

• Absorption is the **movement of small digested food molecules from the digestive system into the blood (glucose and amino acids) and lymph **(fatty acids and glycerol)
• Absorption of small soluble molecules occurs through diffusion and sometimes active transport
• Water is absorbed (by osmosis) primarily in the small intestine,**** but also in the large intestine

Adaptations of the Small Intestine

Spec point — 2.32 · understand how the small intestine is adapted for absorption, including the structure of a villus

• The small intestine is adapted for absorption as it is very long and has a highly folded surface with millions of villi (tiny, finger-like projections)

  • These adaptations massively increase the surface area of the small intestine, allowing absorption to take place faster and more efficiently

• Peristalsis helps by mixing together food and enzymes and by keeping things moving along the alimentary canal

The role of villi in the small intestine

• Villi have several specific adaptations which allow for the rapid absorption of substances, such as:

  • A large surface area

    • Microvilli on the surface of the villus further increase the surface available for absorption

  • A short diffusion distance

    • The wall of a villus is only one cell thick

  • A steep concentration gradient

    • The villi are well supplied with a network of blood capillaries that transport glucose and amino acids away from the small intestine in the blood
    • A lacteal(lymph vessel) runs through the centre of the villus to transport fatty acids and glycerol away from the small intestine in the lymph
    • Enzymes produced in the walls of the villi assist with chemical digestion
    • The movement of villi helps to move food along and mix it with the enzymes present

Adaptations of the small intestine

Exam Hint The way in which the structure of a villus is related to its function comes up frequently in exam questions so it is worth ensuring you have learned these adaptations and how they influence the rate of absorption.