IB Biology SL

Revision Notes

6.1.2 Villi & Absorption

Villi: Increasing the Surface Area

  • The ileum is adapted for absorption as it is very long and has a highly folded surface with millions of villi (tiny, finger like projections)
  • The folds are formed in the epithelium of the mucosa tissue layer on the inside of the intestine
  • The presence of villi and microvilli (found on the surface of each villus) significantly increase the surface area of the ileum, allowing absorption to take place faster and more efficiently

Villi: Absorption

  • Absorption takes place in the ileum (the second section of the small intestine)
  • It is the movement of digested food molecules, vitamins and mineral ions from the digestive system into the blood and lymph
  • This includes the following products of digestion:
    • Simple monosaccharides e.g. glucose, fructose, galactose
    • Amino acids
    • Fatty acids, monoglycerides and glycerol
    • Nucleotide bases
  • And the following additional substances:
    • Mineral ions e.g. calcium, potassium, sodium
    • Vitamins e.g. vitamin C
  • Water is absorbed in both the small intestine and the colon, but most absorption of water occurs in the small intestine

Absorption of unwanted substances

  • The epithelium provides a barrier to prevent absorption of some harmful substances
  • However, some unwanted substances can still pass into the blood. This includes:
    • Some harmless chemicals found in food colourings and flavours – these are removed by the kidney and excreted in the urine
    • Small numbers of bacteria – these are engulfed and digested by phagocytes in the blood
    • Some other harmful substances – these are removed from the blood and broken down by the liver

Dialysis Tubing Experiment

Investigating the absorption of the products of digestion using dialysis tubing

  • Dialysis tubing (sometimes referred to as Visking tubing) is a non-living partially permeable membrane made from cellulose
  • It is sometimes used to model the process of digestion and 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, downloadable AS & A Level Biology revision notes

Image showing how a visking tubing membrane can be used to mimic the walls of the small intestine


  • Fill a section of dialysis tubing (Tube 1) with a mixture of:
    • 1ml of 1% amylase solution
    • 10ml of 1% starch solution
  • Tie up the tubing
  • Suspend the tubing in a beaker of water for a set period of time at 40°C
  • Take samples from the liquid outside of the dialysis 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. An orange-red precipitate is formed in the presence of glucose
  • Repeat the same method in a second dialysis tube (Tube 2) with a mixture of:
    • 1ml of distilled water
    • 10ml of 1% starch solution


  • Tube 1:
    • The amylase present inside the dialysis 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 dialysis tube should increase as more starch (substrate) has been digested
      • As a result, the amount of precipitate produced from the Benedict’s reagent test will increase over time
  • Tube 2:
    • Without amylase present, the starch is not broken down into glucose
    • Starch molecules are too large to diffuse across the partially permeable membrane
    • The glucose tests done on the water outside the dialysis tube show no glucose is present as no precipitate is formed


  • The rate of digestion or absorption is not very accurate when measured qualitatively
    • It can be investigated more quantitatively by using the semi-quantitative Benedict’s test
      • Comparisons between the time intervals can be made with a set of colour standards (known glucose concentrations) or a colorimeter to give a more quantitative set of results
      • A graph could be drawn showing how the rate of absorption changes with the concentration gradient between the inside and outside of the tubing

Other investigations using dialysis tubing

  • Dialysis tubing can also be used to investigate other features such as
    • The effects of different factors on the rate of digestive enzyme activity
      • Investigating the effect of pH
        • Eg. multiple dialysis tubings are set up containing solutions of starch and amylase kept at different pH levels using buffer solutions
      • Investigating the effect of temperature
        • Eg. multiple dialysis tubings are set up in water baths of different temperatures
    • The effect of membrane permeability on absorption
      • E.g. using cola to show how some smaller particles (glucose) can diffuse through the partially permeable membrane whilst larger molecules (food colouring) cannot

Using Models to Represent Real Life

NOS: Use models as representations of the real world

  • Models are often used to study the behaviour of living systems which may be too complicated to observe and achieve useful results
  • Scientists may have access to specialised equipment such as computer based models, which allow a complex comparison to be made with real life systems e.g. the Dynamic Gastric Model used to analyse factors influencing digestion
  • It is also possible to model some systems using much simpler equipment, such as the dialysis tubing model from the experiment above
  • This experiment mimics absorption in the small intestine in the following ways:
    • The dialysis tubing membrane is used to represent the membrane of the small intestine:
      • It is an accurate model because both are selectively/partially permeable so smaller particles can pass through the membrane whilst larger particles cannot
      • The membrane allows the passive movement of particles in diffusion and water particles in osmosis
      • However, the small intestine has a much larger surface area due to the presence of villi
      • Additionally, dialysis tubing is limited in the processes it can mirror and cannot show active transport 
    • Distilled water is used to represent blood:
      • This is a good model because both have an initially low solute concentration
      • However, the distilled water does not flow in the same way as blood and so does not maintain the concentration gradient the same way blood does

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