What’s the Door Policy? Modelling Cell Transport.

doorman

When explaining how substances enter and leave cells, I use the different door policies of (fictional) exclusive restaurants and nightclubs to model the processes of simple diffusion, facilitated diffusion, active transport and osmosis. I often ask the students to act out each scenario (using various props) before asking them to identify the process, explain their reasoning, and discuss the limitations of each model.

Explanations are given in italics beneath each description.

Luigi’s

Luigi’s is a simple Italian restaurant which allows anyone in; so long as they are not being too noisy (really boisterous people tend to go to Jack’s instead). There are no doormen and it is free to enter. However, it is quite small inside and so it quickly fills up and then, once full, no one else can get in. There are usually an equal number of people waiting outside as there are inside but as one person leaves, another can enter so the actual number of diners never changes.

Luigi’s represents simple diffusion. Small, non-polar molecules can diffuse across living cell membranes (i.e. do not require transport proteins or ‘doormen’) but large, polar molecules (‘noisy people’) can not. Diffusion is passive (‘free to enter’) and net movement continues until equilibrium is reached (‘the actual number of diners never changes.’)

Havanna’s

rooftop

Havanna’s is an exclusive rooftop restaurant. It is free to enter but there are doormen who are notoriously fussy about who they let in; usually only the big names in town (who would never dream of going to Luigi’s or Jack’s). In fact, Havanna’s is so strict that the doormen actually accompany you up in the elevator all the way from the ground floor to the restaurant. As with Luigi’s it is only small and once full, it is a one out, one in policy even if there are lots of people waiting downstairs.

Havanna’s models the facilitated diffusion of large, polar molecules (‘big names’) via carrier proteins (‘the doormen actually accompany you up in the elevator’). Again, facilitated diffusion is passive (‘free to enter’) and net movement continues until equilibrium is reached (‘once full, it is a one out, one in policy’).

The Oxford Club

doormen

You have to pay to get into The Oxford Club, an exclusive members club downtown. There are doormen and they are extremely fussy about who they let in. It is a very strange place though as it is always busy inside but you rarely see people outside waiting to get in.

The Oxford Club represents active transport. It requires energy (‘you have to pay to get into the Oxford Club’) and carrier proteins (‘doormen’). Active transport involves the accumulation of ions against a concentration gradient (‘it is always busy inside but you rarely see people outside waiting to get in’). 

Jack’s

Jack’s is free and tends to be full of the particularly lively people who were turned away from Luigi’s. There is a doorman but he just politely holds the door open and in you go. It does tend to fill up quickly though and once full, the policy is strictly one out, one in only.

Jack’s models facilitated diffusion through channel proteins (‘just politely holds the door open and in you go’). Facilitated diffusion is passive (‘free’) and net movement continues until dynamic equilibrium is reached (‘once full, the policy is strictly one out, one in only’).

The Penalty Spot

The Penalty Spot is free but only open to supporters of the local football team after a match. There are lots of entrances but you can only get through if you present your season ticket (it is very selective). Away fans certainly can not get in. By the way, did you know that the nickname of the local team is the H2Os because they play in blue?

The Penalty Spot represents the net movement of water molecules by osmosis (‘only open to supporters of the local football team’). Water moves from a region of higher water potential to a region of lower water potential (‘open to supporters of the local football team after a match’), through a partially permeable membrane (‘you can only get through if you present your season ticket’). 

 

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:

Deodorant

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

dialysis

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!