Visking tubing model gut sim

What the sim shows

A length of Visking (dialysis) tubing is knotted at the bottom and filled with 10 cm³ of a mixture of 1 % starch and 1 % glucose, which looks cloudy white. It hangs in a boiling tube of 30 cm³ distilled water, kept at 37 °C in a water bath. Every 5 minutes a sample of the water outside the tubing is tested: a drop on a spotting tile of iodine solution tests for starch, and a sample heated with Benedict's solution in a hot water bath tests for reducing sugar such as glucose. The tubing is a partially permeable membrane. Its tiny pores let small molecules, such as water and glucose, pass through, but not large molecules such as starch. So glucose diffuses out of the tubing, down its concentration gradient, and the Benedict's test on the outside water turns from blue to green, yellow, orange and brick red. Starch stays inside, so the iodine test on the outside water stays orange-brown however long you wait, while a sample from inside turns it blue-black. The model gut: the tubing stands for the wall of the small intestine, the mixture inside for the food in the gut, and the water outside for the blood. Starch must be digested into glucose before it can be absorbed, because only small molecules can pass through. The limits of the model: the real gut wall is living tissue, folded into villi with a huge surface area and a rich blood supply that carries the glucose away; it also absorbs glucose by active transport, using energy, which the tubing cannot do. Model: glucose leaves at a rate proportional to the difference in concentration, so the outside level rises quickly at first, then levels off at 25 au, where both sides are equal. Glucose is in arbitrary units (au): 100 au is the glucose in the mixture at the start. The time constant (30 min) and the Benedict's colour thresholds are illustrative, chosen to match the usual class result. The colour thresholds are scaled so that all five colours appear. With a real 1 % mixture the outside water usually reaches only green or yellow, and the mixture inside turns about orange. Water also moves into the tubing by osmosis, because the mixture has a lower water potential than distilled water, so the column of mixture rises a little; that small change in volume is not fed back into the glucose numbers. Time moves only when you press Wait 5 min. The heating time for each Benedict's test is not added to the clock: each result belongs to the moment its sample was taken.

• Cloudy white: the starch and glucose mixture; the tubing is a clear grey membrane
• Iodine: orange-brown, blue-black with starch. Benedict's: blue, green, yellow, orange, brick red
• Close-up: hexagons are glucose, chains of hexagons are starch; water is not drawn

Use it in your lessons

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