Application:
• Use of dialysis tubing to model absorption of digested foods in the intestine
Most food is solid and in the form of large complex molecules which are insolube and chemically inert (not readily usable)
The process of digestion therefore performs two key functions:
- It breaks down insoluble molecules into smaller subunits which can be readily absorbed into body tissues
- It breaks down inert molecules into usable subunits which can be assimilated by cells and reassembled into new products
Modelling Digestion
A core function of the digestive system is to break down large molecules into smaller subunits that can be absorbed by cells
- Cell membranes are impermeable to large molecules (polypeptides, polysaccharides) unless transport is facilitated by proteins
The size-specific permeability of cell membranes can be modelled using dialysis tubing (Visking tubing)
- Dialysis tubing contains pores typically ranging from 1 - 10 nm in diameter and is semi-permeable according to molecular size
- Large molecules such as starch cannot pass through the tubing, however smaller molecules (such as maltose) can cross
- Unlike the membranes of living cells, dialysis tubing is not selectively permeable based on charge (ions can freely cross)
Dialysis Tubing
Digestion Experiments
Digestive enzymes like amylase can break down inpermeable polymers (starch) into permeable subunits (maltose)
- Dialysis tubing is impermeable to amylase and starch, but permeable to maltose (and water)
Experiment 1: Measuring Meniscus Levels
- A length of dialysis tubing is attached to a thistle funnel and filled with starch solution (control condition)
- A second length of tubing is attached to a thistle funnel and filled with starch and amylase solution (experimental condition)
- Both apparatuses are placed in a beaker filled with water
- Over time, water will move into the tubing via osmosis (towards the solute) causing the meniscus level to rise
- The tube with amylase will have less solute (as starch is digested) and hence the meniscus level should not rise as much
Experiment 2: Measuring Maltose Diffusion
- A length of dialysis tubing is filled with starch solution and suspended in a beaker of water (control condition)
- A second length of tubing is filled with starch and amylase solution and suspended in a beaker of water (experimental condition)
- The amylase will digest the starch into maltose, which is small enough to diffuse out of the tubing and into the beaker
- The presence of maltose can be detected using Benedict’s reagent or glucose indicator strips
Using Dialysis Tubing to Model Digestive Processes (Starch Breakdown and Maltose Absorption)
