Edexcel IGCSE Biology

Revision Notes

2.5.11 Absorption

Absorption of Food & Water

The Stages of Food Breakdown

  • 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 colon
  • 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:
  • 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, IGCSE & GCSE Chemistry revision notes

Adaptations of the small intestine

Exam Tip

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.

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Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. With 10 years of teaching experience across the 3 key science disciplines, Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.
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