Small Intestine: Structure & Adaptations (Edexcel IGCSE Biology (Modular))

Absorption of Food & Water

• Food taken into the body goes through several different stages during its passage through the alimentary canal (the gut):

  • Ingestion

  • Mechanical digestion

  • Chemical digestion

  • Absorption

  • Assimilation

  • Egestion

• 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

• After absorption, assimilation and egestion occurs

• Assimilation is the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells

• Egestion is the passing out of food that has not been digested or absorbed (as faeces) through the anus

Adaptations of the Small Intestine

• 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

Villi of 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