Absorption of Substances in the Small Intestine (WJEC GCSE Biology: Combined Science)

• The small intestine is the location of the absorption of soluble food substances, such as glucose, amino acids, glycerol and fatty acids, into the blood
• These soluble substances diffuse through the epithelial cells that line the small intestine 
• The lining of the small intestine is made up of specially adapted structures called villi, which increase the rate of absorption of soluble food substances

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 diagram

Adaptations of the small intestine including the large surface area caused by the structure of the villi and the steep diffusion gradient caused by the close proximity to the blood vessels

Investigating the absorption of the products of digestion using Visking tubing

• Visking tubing (sometimes referred to as dialysis tubing) is a non-living partially permeable membrane made from cellulose
• It is sometimes used to model the process of absorption that occurs in the small intestine
• Pores in the membrane are small enough to prevent the passage of large molecules (such as starch and sucrose) but allow smaller molecules (such as glucose) to pass through by diffusion

Visking tubing diagram

How a Visking tubing membrane can be used to mimic the walls of the small intestine

Method

• Fill a section of Visking tubing with a mixture of starch and amylase solutions
• Suspend the tubing in a beaker of water for a set period of time
• Take samples from the liquid outside of the Visking tubing at regular intervals and test for the presence of starch and glucose
  • Starch is tested for using iodine. A blue-black colour is produced in the presence of starch
  • Glucose is tested for using Benedict's reagent. A brick red colour forms in the presence of glucose
• The amylase present inside the Visking tube digests and breaks down starch into glucose
  • Glucose is small enough to diffuse across the partially permeable membrane
• Over time the concentration of glucose in the liquid outside the Visking tube should increase as more starch (substrate) has been digested and the glucose diffuses out of the tubing into the surrounding solution

Limitations

• Using Visking tubing membrane as a substitute for the membrane of the small intestine:
  • Both are selectively/partially permeable
  • The small intestine has a much larger surface area due to the presence of villi
• Using distilled water as a substitute for blood:
  • Both have an initially low solute concentration
  • The distilled water does not flow and so does not maintain the diffusion concentration gradient the way blood does