Modelling the Digestive System

This is a great activity for modelling the digestive system that the students will love.

You will need per group (2-3 students):

  • Potato masher or pestle
  • The leg of a pair of tights (open at both ends)
  • Ziploc bag
  • 3 plastic bowls
  • 2 sponges
  • Vegetable oil
  • Washing-up liquid in a bottle labeled ‘bile’
  • Cereal with milk, bread, biscuits (any leftover food will do)
  • Dilute hydrochloric acid in a beaker labeled ‘stomach acid’
  • Paper towels
  • Plastic gloves
  • Universal indicator solution
  • Five boiling tubes containing the following:
    • Water + blue food colouring (labeled ‘salivary amylase)
    • Water + red food colouring (labeled ‘pepsin’)
    • Water + yellow food colouring (labeled ‘trypsin’)
    • Water + green food colouring (labeled ‘pancreatic amylase’)
    • Water + pink food colouring (labeled ‘lipase’)

Health and safety: Always check for food allergies before you start. Students should wear plastic gloves and goggles when adding the stomach acid.


Even at Key Stage 3 most students have a good knowledge of the different parts of the digestive system so I usually begin with a simple labeling activity and ask one member of the class to record keywords on the board. I emphasise that the digestive system is essentially a long tube running from the mouth to the anus. I have a long piece of rubber tubing which is approximately the length of the alimentary canal (9 metres) which I show the students.

The mouth

I ask the students to put the food into one of the plastic bowls (the mouth). Large pieces of food need to be cut up into smaller bits (by the incisors) and then ground-up using the potato masher or pestle (the molars).


Highlight that this process of physically breaking down the food is mechanical digestion and that it greatly increases the surface area for chemical digestion by enzymes. Add salivary amylase (e.g. the water with blue food colouring) to begin starch digestion.

The esophagus

Tip the slop from the plastic bowl into the top of the tights and ask the students to squeeze the tights in order to push the bolus down the esophagus (this is modelling peristalsis) into the stomach (e.g. the Ziploc bag).


The stomach

Once in the stomach, add the stomach acid (students should test the pH first by adding universal indicator solution) and the pepsin. Seal the bag and churn it to mimic the mechanical digestion of the stomach. Keep going! The food can remain in the stomach for a long time.

Small intestine

Tip the contents of the stomach (the chyme) into the second basin (e.g the small intestine). Add a little vegetable oil to represent oils and fats in the food. Explain that the bile helps emulsify these lipids and neutralises the stomach acid to provide optimum conditions for the pancreatic enzymes. Add some washing-up liquid and give the basin a little shake – the oil should emulsify and settle on top of the liquid as tiny droplets. Add the trypsin, the pancreatic amylase, and the lipase.

Use the sponges to absorb some of the liquid. This models the absorption of nutrients into the blood stream.

Large intestine

Transfer the undigested food into the final basin (e.g. the large intestine) and start absorbing water using the paper towels. Finally, ask the students to model what is left into a stool which will then exit the digestive system via the anus (egestion).

Assessing learning

To follow up the activity and check the students’ understanding of the processes involved, I ask them to illustrate the ‘journey of a cheese sandwich’ as it passes through the alimentary canal. A fun way of doing this is to use large pieces of paper (3 or 4 sheets of A3 stuck together will do) onto which the students first draw their outline, then the parts of the digestive system.

drawing dig

Once complete, add some string so that the paper can be worn around the student’s neck to illustrate the passage of food in-situ. Disposable plastic aprons could also be used.

Google Forms for Flipped Learning

I am a big fan of Google Classroom and although I am not completely paperless quite yet, I am increasingly using Google Docs in class and when setting assignments for homework.

I think one of my greatest discoveries when completing the Google Certified Educator courses (available here and highly recommended) was the fact that you can embed YouTube videos directly into a Google Form then share it with your students as a flipped learning activity. There are a variety of different question types available including text and multiple choice as well as more advanced options such as scales for ordering or sequencing.


The students’ responses are automatically collated in a Google Sheet document allowing you to add comments, apply conditional formatting or review their learning before the lesson.

Conveyor Belt Sushi and the Circulatory System


My students love sushi and there are a proliferation of conveyor belt restaurants in Bangkok. Not only do they serve delicious food but they also make an excellent model of the human circulatory system. Let me explain.

I usually start the lesson by showing the students five minutes of this video ‘Japanology – conveyor belt sushi’. I ask the students if they have ever been to this type of restaurant before and what they like about it. We discuss some of the advantages and disadvantages of conveyor belt restaurants.

I then ask the students to imagine that they are running a conveyor belt sushi restaurant in Bangkok. The restaurant is really busy and there are lots of hungry customers waiting to be fed. They need to get the food to the customers more quickly. How do they do it? Allow a few minutes for the students to Think, Pair, Share. (e.g. tell the chefs to get a move on and increase the speed of the belt motor).

I then tell the students that empty plates are piling up but there are no waiters to collect them. How could they solve this problem without employing more waiting staff? (e.g. the customers should put their empty plates back onto the belt so that they are returned to the kitchen).


At this point I tell the students that conveyor belt sushi restaurants are similar to the human circulatory system (this tends to be met with lots of ‘ohhs’ and ‘ahhhhs’ as they realise that I haven’t completely lost the plot by talking about sushi in Biology).

The students are asked to extend the analogy by comparing the parts of the restaurant with parts of the circulatory system using a comparison table or bridge maps (bridge maps are used to visualise analogies by quite literally bridging the gap between the familiar and the new. The line of the bridge shows the common relationship that exists between two or more pairs of things).


The students should consider what each of the following represents in the human body and most importantly why (the relating factor):

  • The sushi (e.g. oxygen or nutrients);
  • The empty plates (e.g. deoxygenated blood);
  • The conveyor belt (e.g. blood vessels);
  • The chefs who prepare the food and put it on the plates (e.g. the lungs);
  • The motor which makes the belt go round (e.g. the heart);
  • The hungry customers (e.g. body cells).

Refer back to your opening questions about how best to get sushi to the hungry customers quickly and what to do with the empty plates. How does this relate to the circulatory system?

The activity should be extended by asking the students to evaluate the model. Are there any ways in which the comparison doesn’t quite work? Can the students think of any other ways of modelling the circulatory system?

This is a good lesson to have just before lunchtime because everyone gets very hungry!

Teaching Diffusion

There are lots of fun experiments and demonstrations for showing the movement of particles from a region of their higher concentration to a region of their lower concentration down a concentration gradient (diffusion). Here are some of the methods I use.

Potassium permanganate

Potassium 1potassium2potassium3potassium 4

One demo that is often used is dropping purple potassium permanganate crystals into a basin or beaker of water and observing the slow dissolution and diffusion over time.

Cup of tea

Provide students with a tea bag (fruit tea bags are best because the colour change is more vivid) and a beaker of hot water (or water of different temperatures). Look at the factors affecting the rate of diffusion by telling the students that you are thirsty and want to speed up the time it takes to make your morning cuppa – how can they do this? (e.g. heat the water, put more tea into the tea-bags or teapot, reduce the volume of water).

Agar ‘cells’

Prepare agar plates and then ask the students to carefully cut a 1 cm wide moat around the circumference of the agar using a scalpel. Fill the moat with food colouring. The circle of agar represents a cell and the food colouring, the extracellular fluid. Over the course of the lesson the food colouring will slowly diffuse through the agar into the ‘cell’.

agar1agar 3

If you position a camera phone above the agar plate using a clamp stand and film using the time-lapse function you can capture and speed up the whole process to then show the students in summary at the close of the session. See video below:


The classroom will smell like a changing room but a very simple method of demonstrating diffusion is to spray deodorant in one corner and then ask students to raise their hands when they smell it. This creates a ‘Mexican-wave’ effect as the particles diffuse through the air.

Dialysis tubing


The most effective way of demonstrating diffusion through a semi-permeable membrane is to fill dialysis tubing with starch and place it in iodine solution. The iodine will diffuse through the dialysis tubing (turning the starch blue-black) but the starch particles (being too big) will not diffuse in the opposite direction.

If anyone has any other fun demos of diffusion I would love to hear them!

Not Quite Rocket Science

I’ve just been reading about a brilliant project being run between the UK Space Agency and the RHS in which participating schools can grow seeds that have been sent into space.

Sadly, it’s only available to UK schools. Here in Thailand, Year 7 students have been learning about the conditions necessary for germination and we now have lots of different seedlings growing in planters in the corridor outside my classroom.


Keeping it Simple

I often find that really simple visual aids are all that are needed to make otherwise quite complex concepts spring to life in the students’ minds. For example, a balloon inside a cardboard box to represent the protoplast inside the cell wall of a plant cell (particularly useful when teaching plasmolysis), pipe cleaners as polysaccharide chains or for demonstrating protein structure, and drawing pins stuck in ping-pong balls as viruses or cell-surface antigens.