Edexcel IGCSE Biology: Double Science 复习笔记 2.5.11 Absorption

Edexcel IGCSE Biology: Double Science 复习笔记 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



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.