a level biology visking tubing experiment

Practical: Investigating the Rate of Diffusion ( OCR A Level Biology )

Revision note.

Biology & Environmental Systems and Societies

Practical: Investigating the Rate of Diffusion

• It is possible to investigate the effect of certain factors on the rate of diffusion
• Different apparatus can be used to do this, such as Visking tubing and cubes of agar

Practical 1: Investigating the rate of diffusion using visking tubing

• Visking tubing (sometimes referred to as dialysis tubing) is a non-living partially permeable membrane made from cellulose
• Pores in this membrane are small enough to prevent the passage of large molecules (such as starch and sucrose ) but allow smaller molecules (such as glucose ) to pass through by diffusion
• Filling a section of Visking tubing with a mixture of starch and glucose solutions
• Suspending the tubing in a boiling tube of water for a set period of time
• Testing the water outside of the visking tubing at regular intervals for the presence of starch and glucose to monitor whether the diffusion of either substance out of the tubing has occurred
• The results should indicate that glucose, but not starch, diffuses out of the tubing

An example of how to set up an experiment to investigate diffusion

• Comparisons of the glucose concentration between the time intervals can be made using a set of colour standards (produced by known glucose concentrations) or a colorimeter to give a more quantitative set of results
• A graph could be drawn showing how the rate of diffusion changes with the concentration gradient between the inside and outside of the tubing

Practical 2: Investigating the rate of diffusion using agar

• The effect of surface area to volume ratio on the rate of diffusion can be investigated by timing the diffusion of ions through different sized cubes of agar
• Purple agar can be created if it is made up with very dilute sodium hydroxide solution and Universal Indicator
• Alternatively, the agar can be made up with Universal Indicator only
• The acid should have a higher molarity than the sodium hydroxide so that its diffusion can be monitored by a change in colour of the indicator in the agar blocks
• The time taken for the acid to completely change the colour of the indicator in the agar blocks
• The distance travelled into the block by the acid (shown by the change in colour of the indicator) in a given time period (eg. 5 minutes)
• These times can be converted to rates (1 ÷ time taken)
• A graph could be drawn showing how the rate of diffusion (rate of colour change) changes with the surface area to volume ratio of the agar cubes

An example of how to set up an experiment to investigate the effect of changing surface area to volume ratio on the rate of diffusion

When an agar cube (or for example a biological cell or organism) increases in size, the volume increases faster than the surface area, because the volume is cubed whereas the surface area is squared. When an agar cube (or biological cell / organism) has more volume but proportionately less surface area, diffusion takes longer and is less effective. In more precise scientific terms, the greater the surface area to volume ratio , the faster the rate of diffusion !

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Author: Alistair

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

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Visking Tubing

This resource describes a visual way of demonstrating diffusion through a semi-permeable membrane. It can be used as a model for the human gut or for investigating the effect of amylase on starch. Two standard tests are used. The first uses iodine to test for starch and the second test uses Benedict’s reagent to test for the presence of reducing sugars, such as glucose.

This resource was produced following National Science & Engineering Week 2013, during which teachers and technicians nominated their favourite demonstrations to be turned into video resources with accompanying written guides.

This resource was funded by the Gatsby Charitable Foundation.

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Investigating Transport Across Membranes (A-level Biology)

Investigating transport across membranes, investigating diffusion.

We can investigate how diffusion occurs in biological cells by using cubes of agar jelly. The basic concept of this experiment is outlined below:

• The agar jelly contains a pH indicator. We can make up agar jelly with an alkaline solution (e.g. sodium hydroxide) and add a few drops of phenolphthalein to it before the jelly sets. Phenolphthalein is a pH indicator which turns pink in the presence of alkaline solutions, thus, the jelly will have a bright pink colour.
• The agar jelly is placed in an acidic solution. Once the jelly has set, we can cut it up into cubes and place it in an acidic solution, such as dilute hydrochloric acid.
• The agar jelly is neutralised by the diffusion of the acid. The acidic solution will slowly diffuse into the agar jelly and neutralise the alkaline solution. As it does, the jelly will lose its pink colour and become colourless, as phenolphthalein turns colourless in non-alkaline environments.

We can alter different parts of this experiment to model how different factors affect the rate of diffusion.

Investigating the effects of surface area on diffusion

• Cut the agar jelly into different sized cubes to investigate the effects of surface area . Cut the jelly into cubes of different sizes and work out each cube’s surface area to volume ratio . For example, a cube with 2cm edges will have a surface area to volume ratio of 3:1.
• Place the cubes in the same volume and concentration of acid. Put the cubes into containers which hold the same volume and concentration of hydrochloric acid. Then measure the time it takes for the different cubes to go colourless.
• The cube with the largest surface area: volume ratio will go colourless the quickest. The cube with the largest surface area: volume ratio has the greatest amount of space available for the hydrochloric acid to diffuse into the jelly so it will be neutralised the fastest.

Investigating the effects of concentration on diffusion

• Place the agar jelly cubes in different concentrations of acid. Cut the agar jelly into equal sized cubes and put them in different containers, each with a different concentration of hydrochloric acid. Measure the time it takes for the different cubes to go colourless.
• The cube placed in the highest concentration of acid will go colourless the quickest. The cube placed in the container with the highest concentration will have the greatest concentration of acid being diffused into the jelly per minute. As such, it will go colourless the quickest.

Investigating the effects of temperature on diffusion

• Place the agar jelly cubes in different temperatures. Cut the agar jelly into equal sized cubes and put them in different containers, each with the same concentration of hydrochloric acid. Put the containers in water baths heated to different temperatures. Be careful not to heat the water baths over 65° as the agar jelly will melt.
• The cube placed in the highest temperature of acid will go colourless the quickest. As high temperatures speed up the rate of diffusion, the cube in the hottest container will be neutralised the quickest.

Investigating Osmosis

Osmosis is the movement of water molecules from an area of high water potential to an of low water potential by osmosis. Water potential is determined by the concentration of solutes in the solutions on either side of the cell membrane.

Investigations using plant tissue

This experiment involves placing plant tissue, e.g. potato cylinders, in varying concentrations of sucrose solutions to determine the water potential of the plant tissues.

• Prepare the different concentrations of sucrose solutions . Using distilled water and 1M sucrose solution, prepare a series of dilutions such that you now have 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0M sucrose. Place 5cm 3 of each dilution into separate beakers.
• Prepare equal sized pieces of potato chips. Using a cork borer, cut out 18 pieces of potato chips, all of equal sizes.
• Weigh the mass of the potato chips. Dry the potato chips gently with a paper towel. Divide them into groups of three and weigh each group.
• Place each group of potato chips in each solution . The potato chips should be left in the solutions for a minimum of 20 minutes.   All groups should be left in the solution for the same amount of time.

• Weigh the mass of the potato chips again. Once your desired amount of time has passed, remove the chips from the solutions, and dry them gently using a paper towel. Reweigh each group again.
• Calculate % change in mass. Using the mass of the potato chips before and after being placed in the solution, calculate the % change in its mass.
• Plot the % change in mass on a calibration curve. The calibration curve helps us determine the water potential in the potato sample. Plot the % change in mass against concentration of sucrose solution.   The point at which the curve crosses the x axis is when the sucrose solution is isotonic with the potato samples i.e. the water potential of the sucrose solution is the same as the water potential of the potatoes. At this point, there is no movement of water in or out of the potato. Overall:
• The potato samples in the dilute solutions will have a net increase in mass – the water potential is greater in the potato than in the sucrose solution, so water moves into the potato samples via osmosis.
• The potato samples in the concentrated solutions will have a net decrease in mass – the water potential is lower in the potato than in the sucrose solution, so water moves out of the potato samples via osmosis.

Investigations using Visking tubing

Visking tubing is an artificial membrane that is selectively permeable as it has many microscopic pores. This allows smaller molecules such as water and glucose to pass through it, while larger molecules such as starch and sucrose are unable to cross the membrane.

• Prepare three equal-sized pieces of Visking tubings. Run the tubing under tap water to soften it and knot each tubing on one end to create a bag.
• Place a rubber bung at the open end of the Visking tubing. Find rubber bungs with an opening in the centre that will fit the open end of the Visking tubing. Then seal the tubing using the bung and fix it in place using a rubber band.
• Prepare sucrose solutions with concentrations of 0.5M and 1.0M. You may wish to add a food dye to the 0.5M solution so that it is easier to see later on.
• Pipette in the 0.5M sucrose solution. Using a pipette or a syringe, fill each tubing through the opening of the rubber bung with the 0.5M sucrose solution. Make sure it is filled completely to the brim with no air bubbles.
• Insert capillary tubes into each of the tubings . Insert a capillary tube through the rubber bung’s opening. Mark the level at which the sucrose solution has risen to in the capillary tube.
• Place each Visking tubing into containers of different solutions. Prepare three beakers, each containing distilled water, 0.5M sucrose, and 1.0M sucrose. Place each Visking tubing into each of the beakers and leave them in for the same amount of time.
• Measure the change in liquid level. Mark the new liquid level on the capillary tube before removing the Visking tubing from its beaker. Measure the change in the liquid level. Overall:
• The liquid level of the Visking tubing placed in distilled water will have risen as the sucrose solution in the tubing is hypertonic to the water i.e. the sucrose is more concentrated. Thus, there is net movement of water into the Visking tubing via osmosis.
• The liquid level of the Visking tubing placed in 0.5M sucrose will remain the same as the solution inside the tubing and outside the tubing are isotonic i.e. the solutions are the same concentration.
• The liquid level of the Visking tubing placed in 1.0M sucrose will have decreased as the solution inside the tubing is hypotonic to the solution outside the tubing i.e. the solution inside the tubing is less concentrated.

Transport across membranes is the movement of substances such as ions, molecules, and fluids from one side of a biological membrane to the other. This process is crucial for maintaining cellular homeostasis and allowing cells to exchange materials with their environment.

Investigating transport across membranes is important because it helps us understand the mechanisms by which cells regulate the flow of substances in and out of the cell. This is essential for understanding cellular processes such as metabolic reactions, waste removal, and communication between cells.

There are several methods used to investigate transport across membranes, including: Diffusion experiments to study the movement of substances through the lipid bilayer Osmosis experiments to study the movement of water across a semi-permeable membrane Active transport experiments to study the movement of substances against a concentration gradient with the use of energy Electrochemical experiments to study the movement of ions across the membrane

Factors that can affect transport across membranes include the size of the substance being transported, the charge of the substance, the concentration gradient, and the presence of specific transport proteins.

Transport across membranes can be measured in a variety of ways, including measuring changes in substance concentration, changes in electrical potential, and changes in fluid movement.

The limitations of investigating transport across membranes include the difficulty of obtaining pure and intact biological membranes, the potential for damage to the membrane during experimentation, and the limitations of experimental techniques.

In A-Level Biology, knowledge of transport across membranes can be applied to understand cellular processes such as the movement of nutrients and waste, the regulation of cell volume, and the communication between cells. This knowledge is also important for understanding diseases and disorders related to the malfunction of transport processes, such as cystic fibrosis and diabetes.

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Practical Biology

A collection of experiments that demonstrate biological concepts and processes.

Observing earthworm locomotion

Practical Work for Learning

Published experiments

Investigating the effect of amylase on a starchy foodstuff, class practical or demonstration.

Place rice in a Visking tubing bag to model food in the gut . Investigate amylase action by adding water, amylase, or boiled amylase to the rice. Leave for 10-15 minutes in a water bath at around 37 °C then test for the presence of a reducing sugar in the water surrounding the Visking tubing bag.

Lesson organisation

This experiment could be done as a demonstration or in groups. Each group needs to set up three Visking tubing bags, so a group of 3 students is ideal.

Apparatus and Chemicals

For each group of students:.

3 x 15 cm lengths of Visking tubing

Syringe barrels, sawn off, 3

Boiling tube, 3

Test tubes, 6

Test tube racks to accommodate 6 test tubes and 3 boiling tubes per group

Teat pipettes, 6

White dimple (spotting) tile

Beaker, 250 cm 3

Kettle for boiling water for Benedict’s test

Eye protection for each student

For the class – set up by technician/ teacher:

Length of Visking tubing, knotted at one end, 15 cm, 3 per group ( Note 1 )

Syringe barrel, sawn off, 3 per group ( Note 2 )

Elastic bands, 3 per group

Electric water baths set at 35-40 °C, with thermometer to show temperature accurately

Cooked rice

Iodine solution ( Note 3 )

Benedict’s reagent ( Note 4 )

Amylase solution, 5 cm 3 per group ( Notes 5 and 6 )

Boiled amylase, 5 cm 3 per group

Clinistix (as an alternative to Benedict’s reagent) ( Note 7 )

Health & Safety and Technical notes

Students should wear eye protection when handling chemicals. Electrical apparatus should be maintained and checked according to your employer’s instructions. Ensure students know how to deal with breakages of glass or thermometers

Read our standard health & safety guidance

1 Soak the Visking tubing in warm water beforehand so it is ready to use.

2 The end of an old syringe makes a convenient support for the Visking tubing, and makes it easier to take samples of the contents with a teat pipette.

3 Iodine solution (See CLEAPSS Hazcard and Recipe card): a 0.01 M solution is suitable for starch testing. Make this by 10-fold dilution of 0.1 M solution. Once made, the solution is a low hazard but may stain skin or clothing if spilled.

4 Benedict’s (qualitative) reagent. (See CLEAPSS Recipe card) No hazard warning is required on the bottle, as the concentrations of each of the constituents are low. Take care making up the reagent; sodium carbonate is an irritant to the eyes and copper(II) sulfate(VI) is harmful if swallowed. See CLEAPSS Hazcards.

5 Amylase solution: Check your amylase supply as many contain starch or reducing sugars, which would interfere with the results of this test. Alpha amylase is bacterial amylase with high activity, and does not give a positive reducing sugar test or starch test. You can use lower concentrations of this enzyme. Some bacterial amylases may survive boiling!

Using saliva: the CLEAPSS Laboratory Handbook provides guidance on precautions, including hygiene precautions, for safe use of saliva as a source of amylase. This has the advantage of being cheaper and technicians do not need to make up fresh solutions each lesson. It is directly interesting to students, and salivary amylase is reliable. It also provides an opportunity to teach good hygiene precautions, including ensuring that students use only their own saliva samples.  Provide small beakers to spit into. Students must be responsible for rinsing their own equipment. All contaminated glassware is placed in a bowl or bucket of sodium chlorate(I) for technicians wash up.

6 Working with enzymes: It is wise to test, well in advance, the activity of stored enzymes at their usual working concentrations to check that substrates are broken down at an appropriate rate. Enzymes may degrade in storage, and this allows time to adjust concentrations or to obtain fresh stocks.

7 Clinistix are quick and easy to use. Each stick can be cut into two or three pieces.

Ethical issues

There are no ethical issues associated with this protocol.

Preparation

a Prepare boiled rice, enzyme solution, boiled enzyme solution, iodine solution, and Benedict’s reagent.

b Set up a water bath at 37 °C.

c Soak Visking tubing, cut 15 cm lengths (3 per group) and set up model guts with syringe barrels, or leave for students to assemble.

Investigation

d Label 3 boiling tubes 1, 2, 3.

e Label 3 test tubes 1, 2, 3.

f Set up 3 model guts: take a wet piece of Visking tubing, tie a knot in one end, place the sawn off syringe barrel in the other end and secure with an elastic band. These may have been set up for you (see diagram).

g Use the spatula to add rice to each of the model guts until they are half full.

h Rinse the outside of each piece of Visking tubing under a running tap.

i Place the rice-filled model gut in a labelled boiling tube. Add warm water to boiling tube outside the Visking tubing until it reaches about 2 cm higher than the level of the liquid inside the Visking tubing (see diagram).

j Immediately withdraw one drop of the water you have added and test it with iodine on a dimple tile.

k Add 5 cm 3 of water to model gut 1.

l Add 5 cm 3 of amylase to model gut 2.

m Add 5 cm 3 of boiled amylase to model gut 3.

n Place all the boiling tubes containing the model guts in the water bath at approximately 37 °C.

o Leave for at least 15 minutes.

p While you are waiting:

• Place a grain of rice in a well on the white tile and add a drop of iodine.
• Put some rice in a test tube. Add 2 cm 3 of water and 2 cm 3 of Benedict’s reagent, and place into a large beaker of boiling water. Check the colour after 2-3 minutes.
• Record your results in a suitable table.

q After 15 minutes, use a teat pipette to remove 2-3 cm 3 of the water surrounding the model gut in boiling tube 1.

r Place one drop of this water in a well on the white tile and add a drop of iodine. Record the result.

s Place the rest (around 2 cm 3 ) of the water from boiling tube 1 into test tube 1. Add an equal volume of Benedict’s reagent and place test tube 1 into a large beaker of boiling water. Check the colour after 2-3 minutes. Record the result.

t Repeat steps q , r , s with water from boiling tubes 2 and 3. Record the results.

Teaching notes

The sawn off syringe barrel acts as a model mouth to the gut. It is a good idea to use cooked rice, as this is real food and can be seen in the (model) gut.

Many students will need help to understand this activity. When interpreting the results, students have to think in terms of two types of model: the model gut with Visking tubing representing the selectively permeable membranes lining the gut wall, and a simplified chemical model of large and small molecules. A further complication is that the movement of chemicals is unseen and only inferred from the results of chemical tests. An additional model could be used, with chicken wire or mesh, fruit or satsuma bags to represent the membrane, and poppet beads in chains to represent starch and singly to represent glucose.

Health and safety checked, September 2008

Related experiments

Evaluating Visking tubing as a model for a gut

Investigating the effect of pH on amylase activity

Digestion of starch by microbes

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Modelling digestion using visking tubing

This is the second Biology film we’ve made as part of the “ Get, Set, Demonstrate ” project. One of the films we were asked to look at was “Making Poo: The Digestive System” but we felt that this was not what we would strictly call a “demonstration” of digestion (since no actual digestion takes place), but rather an illustration of the process. Instead, we chose to make a film about using Visking tubing to model digestion and use it to explore the reasons why you might choose to carry out a demonstration of an activity which can be (and often is) done as a class practical.

3 thoughts on “Modelling digestion using visking tubing”

Visking tubing is an excellent model for a partially permeable membrane. Great for osmosis demonstrations too, measuring the mass of the tube before and after placing it in concentrations of salt water.

Rather than a solution of starch and glucose, I use a sample of saliva (just chew a disinfected elastic band) to add to the starch to show how it is broken down by the enzyme to produce smaller sugars. Maltose is small enough to get through the visking tubing and is a reducing sugar so reacts with the Benedict’s test.

What are the advantages and disadvantages of doing the Visking Tubing experiment?

what does the water represent

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Experiment to find the effect of different concentrations of sucrose solution on osmosis using Visking tubing.

Biology                 

Introduction

Osmosis is a special type of diffusion, where water diffuses across a selectively permeable membrane. This would then stop as the concentration of water is equal on both sides of the membrane. The movement of water is from a lower solute concentration to an area of higher solute concentration.

This leads to this experiment in which the Visking tubing will act as a membrane between the solutions. Knowing the above information, a prediction can be made such that a Visking tube containing a very low concentration of sucrose solution will have a less effect than that which has a higher concentration. This is due to the movement of water from the test tube into the visking tube.

Distilled water

0.0mol dm -3  sucrose solution – distilled water

0.5mol dm -3  sucrose solution

1.0mol dm -3  sucrose solution

2.0mol dm -3  sucrose solution

Boiling test tubes x 4

Test tube rack x 1

Visking tubes x 4

Paper towels

Measuring scale

Methodology

This is a preview of the whole essay

The test tubes were prepared first. This was done by labelling each one with 0.0mol dm -3  sucrose solution, 0.5mol dm -3 , and so forth with all four concentrations. This is so that the test tubes correspond to the correct Visking tubing. Each were then filled to three quarters with distilled water, and placed in a test tube rack in the order of least concentration to most.

The visking tubes were then filled with the sucrose solutions. To carry this out, each tube were wet under a tap, so that they open. One end was tied with a knot, to close it up from losing any solution. They were each then filled. The first was half filled with distilled water, being the 0.0mol dm -3  concentration of sucrose solution. The next three were filled with each of the other concentrations by syringe. To keep the experiment consistent they were all half filled. They were tied with another knot on the opposite end, keeping a small amount of space within the tube for the flow of water.

The outside of the visking tubes were washed under a running tap and dried with paper towels, the knots were also squeezed to take out any water absorbed. Each was then weighed using the digital weighing scale and this mass was recorded in the results table. Once all four were measured, they were simultaneously placed into the test tubes on the rack, and the timing had begun.

At every 10 minute intervals the visking tubes were taken out of the test tubes, placed on paper towels and dried, again squeezing out any solution in knots. These were weighed and noted in the table and then placed back into the test tubes. This was repeated for 40 minutes.

Table 1: Weight change of visking tubes with different concentrations at 10min intervals

Table 2: Percentage change in mass of visking tubes with different concentrations.

The above information was calculated using the following method.

Percentage change in mass = mass – initial mass     x 100

                                Initial mass

Table 2 is shown in graph 1 on the following page.

This experiment was carried out to find the effect that different concentrations of sucrose solution have on osmosis using Visking tubes. The graph shows that there was a very low rate of diffusion within the visking tube containing 0.0mol dm -3 sucrose solution, and therefore the rate of diffusion gradually increases with increasing concentrations of sucrose solution. These results accept the prediction made.

Diffusion should not occur when a visking tube containing distilled water is placed within a test tube containing distilled water, as the concentrations are equal. The tube containing 0.0mol dm -3 sucrose solution is distilled water, though in the graph a percentage change of 0.974% is shown. This could be due to human error of weighing the visking tube. As mentioned the knots absorbed some solution, if they were not dried, the weight would be more.

Document Details

• Word Count 785
• Page Count 3
• Level AS and A Level
• Subject Science

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