Artificial Intestine — Can Digested Food Cross a Membrane?

Breaking food down is only half the story. Its nutrients still have to cross the wall of the intestine.

Imagine eating a slice of bread. We can explain how enzymes break down its starch, but that leaves an important question:

How do the products of digestion get from the gut into the body?

The inside of the digestive tract is a passageway. A nutrient has not entered the bloodstream simply because you have swallowed it. It must cross the intestinal lining.

We can investigate part of that journey with a piece of dialysis tubing, some starch, glucose and a few chemical tests.

This “artificial intestine” brings together digestion, membranes, diffusion and experimental evidence. Add an enzyme, and it becomes a particularly interesting investigation: can breaking down a large molecule change what can cross a membrane?

Suitable for: supervised GCSE and A-level learners, with younger children helping to predict and observe.

Time: 1–2 hours, including preparation and discussion. Pilot the equipment beforehand; visible results depend on the tubing, concentrations and test sensitivity.

Two Different Jobs: Digestion and Absorption

Digestion and absorption are closely connected, but they describe different processes.

Digestion breaks large food molecules into smaller molecules.

Absorption moves nutrients across the intestinal lining into the body’s transport systems.

Carbohydrate digestion involves several enzymes. Amylase breaks starch into smaller carbohydrates, including maltose. Further enzymes, including maltase, complete the production of glucose. OpenStax

A simplified teaching sequence is:

Starch --amylase--> maltose and other smaller carbohydrates

Maltose + water --maltase--> glucose + glucose

The distinction matters. Ordinary amylase alone should not be described as converting all starch directly into glucose.

In our first experiment, we supply glucose ourselves. In the extension, we investigate the smaller reducing sugars produced by starch digestion.

A Membrane That Lets Some Molecules Through

Dialysis tubing, often called Visking tubing in school laboratories, is a non-living membrane that allows some substances to pass through more readily than others.

With suitable tubing, glucose can diffuse across while intact starch is largely retained.

Picture a long necklace and a loose bead. Both contain similar building blocks, but their overall sizes differ. That is a useful starting analogy for starch and glucose, although molecules move randomly and membranes are more complicated than kitchen sieves.

Choose the tubing carefully. Dialysis membranes have different molecular-weight cut-offs. Use educational tubing specified as suitable for starch-and-sugar diffusion, and pilot it rather than assuming every membrane behaves identically.

What You Need

For the basic investigation:

• Suitable dialysis tubing, soaked according to its instructions.
• Laboratory starch solution and glucose solution, approximately 1% each.
• Three small beakers, distilled water, measuring equipment and a timer.
• Thread or tubing clips, pipettes, labelled sample tubes and a spotting tile.
• Iodine solution and either appropriate glucose-specific test strips or Benedict’s reagent.
• Eye protection and, if using Benedict’s reagent, a supervised hot-water bath.

For the extension, add a prepared educational amylase solution and an appropriate near-neutral buffer, following the enzyme supplier’s guidance.

Use prepared enzyme solution rather than asking children to handle enzyme powder. Avoid inhaling enzyme dust or aerosols. Follow the reagent safety information; iodine can irritate and stain, and Benedict’s testing involves alkaline reagent and hot equipment.

Do not taste any material. Keep the activity separate from food preparation and dispose of reagents according to local laboratory guidance.

Practical 1: Which Molecules Can Escape?

1. Make Predictions First

Ask students:

• Will starch appear outside the tubing?
• Will glucose appear outside?
• Will the surrounding water change immediately?
• How could we tell diffusion from a leak?

Writing predictions first helps students separate what they expected from what they actually observed.

2. Set Up Three Bags

Bag	Contents	Purpose
A	5 mL starch solution + 5 mL glucose solution	Investigates both substances together
B	5 mL starch solution + 5 mL distilled water	Checks starch retention and the sugar-test background
C	10 mL distilled water	Provides a blank control

Use equal lengths of tubing and the same total volume inside each bag. Secure both ends, leaving some slack for possible water movement.

Rinse the outside thoroughly with clean distilled water.

This small step is crucial. A smear of glucose on the outside could produce a positive result without any glucose having crossed the membrane.

It is an excellent opportunity to ask: “What would make our result misleading?”

3. Prepare the Surrounding Water

Place each bag in a separate labelled beaker containing, for example, 50 mL distilled water. Use beakers small enough for the bags to be submerged.

Keep water volumes and temperatures the same. Room temperature is suitable for the basic investigation.

Take a sample of each beaker’s water before adding the bags. These starting samples should test negative for starch and sugar.

4. Sample Over Time

Collect small, equal samples after approximately 15, 30, 45 and 60 minutes.

Use separate aliquots for the starch and sugar tests. Never return tested samples to the beakers, and avoid transferring drops between containers.

Record the volume removed. Repeated sampling changes the external volume; keep samples small and follow the same procedure for every beaker.

5. Test for Starch and Sugar

Starch: add iodine to a separate sample. A blue-black colour indicates starch; a negative sample retains the iodine’s yellow-brown colour.

Glucose: use glucose-specific strips suitable for aqueous samples, following their instructions.

Reducing sugars: alternatively, mix a measured sample with Benedict’s reagent and heat in a hot-water bath according to your laboratory method. A positive result may range from green through yellow or orange to a brick-red precipitate.

Benedict’s detects reducing sugars, including glucose and maltose. It does not identify glucose specifically.

Check your tests with known positive samples and distilled-water negatives. Otherwise, a negative experimental result could simply mean the test failed.

6. Compare Evidence with Predictions

Expected findings, assuming suitable tubing and sufficiently sensitive tests:

Sample	Starch test	Sugar test
Water outside A	Negative	Becomes positive
Water outside B	Negative	Negative
Water outside C	Negative	Negative
Contents of A at the end	Usually positive	Usually positive

Glucose does not have to disappear from inside the bag for it to appear outside. Molecules move in both directions; initially, there is a net movement down the concentration gradient.

A good conclusion is:

“The results support the explanation that glucose crossed this membrane while intact starch was largely retained.”

If starch appears outside, inspect the knots and membrane for leaks before concluding that intact starch passed through it.

This Is Diffusion — Where Does Osmosis Fit?

Students often call anything involving dialysis tubing “osmosis”.

Here, glucose moving down its concentration gradient is diffusion.

Osmosis concerns the net movement of water across a selectively permeable membrane. It may happen at the same time, potentially changing the bag’s volume or mass.

The two processes can coexist. However, a positive sugar test outside the bag is evidence of solute transfer, not a measurement of osmosis.

My teaching priority here would be to keep asking:

“Which substance moved, and what evidence tells us that?”

A precise answer is more valuable than simply recognising a familiar practical.

Practical 2: Can Digestion Change What Crosses?

Now replace the ready-made glucose with an enzyme.

Prepare two fresh, equal-sized bags:

• D: starch solution, active amylase and suitable buffer.
• E: the same amount of starch and buffer, with water replacing the amylase.

Keep total internal volume, membrane area, external water volume and temperature the same.

A controlled warm-water bath around 35–37 degrees C is a reasonable starting point for many educational amylase preparations; follow the supplier’s instructions for pH, temperature and concentration.

Prepare and immerse the bags promptly, recording when starch and enzyme first meet.

Sample the surrounding water at regular intervals and test with Benedict’s reagent, rather than relying only on glucose-specific strips.

In bag D, amylase should produce smaller reducing sugars. Some can cross suitable dialysis tubing and eventually give a positive external test. Bag E should show little or no comparable change.

Test a sample from inside each bag for starch at the end. Reduced iodine staining in the active-enzyme bag can provide additional evidence of digestion.

One Essential Extra Control

Check the amylase preparation itself for reducing sugars. Some commercial enzyme preparations contain ingredients that can interfere with the investigation. Practical Biology specifically highlights this potential problem. practicalbiology.org

An additional bag containing amylase and buffer, but no starch, helps reveal whether sugar outside came from the enzyme preparation.

The most convincing result combines:

• Evidence that starch was broken down.
• Evidence that reducing sugars appeared outside.
• Controls that make leakage and pre-existing sugar less plausible explanations.

Avoid claiming that this proves glucose production. For that extension, use a validated enzyme system that completes starch digestion to glucose and confirm the product with a glucose-specific assay.

Turn a Demonstration into an Investigation

Once the basic method works, students can change one factor.

Does Concentration Matter?

Does a larger starting concentration of glucose produce a faster detectable rise outside?

Keep the membrane area, volumes, temperature and sampling method constant.

Does Membrane Area Matter?

Does greater exposed area increase transfer?

Design this carefully because changing bag dimensions can also change volume and shape. This is a useful introduction to the difficulty of changing just one variable.

Does Mixing Matter?

Does gentle, standardised stirring reduce the build-up of sugar near the membrane?

This provides a starting point for discussing why movement of fluids can affect exchange.

Does Temperature Matter?

Investigate glucose diffusion without enzymes first.

In the amylase version, temperature affects both enzyme activity and diffusion, making interpretation more complicated.

Can We Draw a Graph?

Use time on the horizontal axis and measured external glucose concentration on the vertical axis, if your assay provides valid quantitative readings.

Benedict’s colour categories can be recorded as observations, but they should not be treated as exact concentrations.

For stronger measurements, use a suitable calibrated assay and repeat the experiment.

How Good Is Our Artificial Intestine?

A scientific model is valuable partly because we can identify its limits.

Feature	Dialysis-tubing model	Real small intestine
Barrier	Non-living membrane	Living epithelial cells
Sugar transfer	Passive diffusion through suitable tubing	Specific membrane transport proteins
Surface area	Relatively simple bag	Folds, villi and microvilli
Transport away	Sugar accumulates in surrounding water	Blood carries absorbed nutrients away
Digestion	Only the enzymes supplied	Several enzymes acting in different locations

In people, glucose uptake into intestinal cells involves sodium-linked transport. Glucose then exits towards the blood through facilitated diffusion. The tubing does not reproduce these mechanisms.

Likewise, maltose crossing dialysis tubing does not mean the human intestine normally absorbs maltose intact. Human carbohydrate absorption principally involves monosaccharides. OpenStax

The model illustrates a relationship between molecular size, digestion and transfer. It does not establish that the intestinal wall works like a perforated plastic bag.

Understanding where a model stops being accurate is part of understanding the science.

Why I Would Use This in Tuition

In my home laboratory, I want practical work to give students something to explain, rather than simply a result to remember.

This activity creates several useful teaching moments:

Why rinse the bag? Why take a starting sample? Why might a test remain negative? Why is Benedict’s not a glucose-specific test?

For GCSE learners, it connects enzymes, digestion, diffusion and food tests. For A-level learners, it opens discussion of co-transport, experimental controls and model evaluation.

I would ask students to finish with three sentences:

1. What did we observe?
2. What does that support?
3. What does our experiment leave unresolved?

That habit helps students move from describing a colour change to making a defensible scientific argument.

The Membrane Is Only the Beginning

A bag in a beaker looks modest beside the complexity of a living intestine. Yet it lets us investigate an important sequence:

Break down a large molecule. Produce smaller molecules. Investigate whether those molecules can cross a barrier.

The strongest lesson is not simply that “small molecules get through”. It is that biological processes have distinct stages, and each stage needs evidence.

At Hemel Private Tuition, this is the kind of practical work that can turn a memorised sentence into understanding: students predict, test, question and explain.

Digestion makes nutrients available. Absorption brings them into the body. This experiment helps students see why both matter.