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A typical experiment to investigate permeability of cell membranes involves the use of beetroot. A factor affecting the integrity of the cell membrane needs to be changed, such as different temperatures or different alcohol concentrations. When beetroot cells are exposed to conditions damaging the cell membrane, a pigment (betalain) leaks out through the cell membranes. The more damage occurs to the cell membrane, the more pigment leaks out. This can be measured as absorbance using a colorimeter. A colorimeter gives the absorbance value which is the amount of light that is absorbed by each sample. Higher absorbance values indicate more pigment that has leaked out (greater permeability) and therefore more damage to the cell membrane.
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Cell membrane consists of a phospholipid bilayer which makes the membrane partially permeable. Permeability can be altered by various different variables (e.g. temperature, solvent concentration like ethanol). Cell membrane permeability can be measured by utilising beetroot cells, containing betalain (a purple pigment). At higher permeability of the cell membrane, more pigment leaks out of cells. The permeability can therefore be measured by the amount of pigment leaked from beetroot cells into an aqueous solution using a colorimeter.
Common variables to investigate are the effect of solvents or temperature because both of these factors can change the fluidity of the membrane. An increase in membrane fluidity will cause the pigment to leak out of the cell, and the amount of pigment can be measured simply by using a colorimeter.
The permeability of a cell membrane refers to how easily molecules can flow from inside to outside of a cell, or vice versa. The more permeable a membrane, the more easily cross membrane transport occurs.
There are several ways to investigate the permeability of membranes in a laboratory, however for the biology A level you will need to discuss the classic "Beetroot experiment".
In its most simple form, this experiment involves place slices of raw beetroot into a liquid such as water or ethanol, and then recording how readily the red/purple pigment inside the beetroot cells, moves out of the cells and into the surround liquid. The darker the liquid becomes, the greater the degree of pigment transfer between the cells and the liquid, and so the greater the permeability of the membrane.
In an assessed practical, you will be expected to conduct a semi-quantitative experiment to assess how a named variable impacts the permeability of the beetroot cell membrane. Here a variable can be taken to mean any thing that you can measure during an experiment that can be changed by the person conducting an experiment and can include things like, temperature of incubation, the length of incubation, concentration of ethanol used to suspend the beetroot, etc.
Let us assume that we are interested in how the concentration of ethanol impacts the permeability of beetroot cell membranes, we would design an experiment as follows.
This is a semi-quantitative experiment, meaning that as the researcher, you will be able to make observations about how much the colour changes, but we lack a definitive scale by which to judge the true amount of permeation across the cell membrane. As we are manually classifying colours, it is subjective and can be open to a degree of interpretation. The use of a colourimeter to determine the colour of the solutions removes this subjectivity and is one way in which this experiment could be strengthened. Another way this experiment could be improved is to use replicates of each tested solution concentration, preferably at least 3, to increase our confidence in our collected data.
Permeability of a cell membrane = how easily molecules can flow from inside to outside of a cell, or from outside to inside.
More permeable a membrane = the easier cross membrane transport occurs. For Biology A level you will need to talk about the Beetroot experiment
A simple test is done by using beetroots and a condition damaging the cell membrane (e.g. alcohol or changing temperature).
When the beetroot membrane is exposed to conditions that damage it, the pigment leaks out. The more pigment that leaks out (indicative of more damage) can be measured by absorbance into a colorimeter. This produces an absorbance value which tells us the amount of light absorbed by each sample.
Higher absorbance values tells us that there has been more damage done to the membrane as reflected by more pigment leaking out.
By doing an experiment using beetroot.
Take any plant sample for example onion. Peel the inner layer and place in different solutions of isotonic, hypertonic and hypotonic.
Results will show the permeability of cell membrane.
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Using Beetroot and a colorimeter
Cell-surface membranes are made up of a phospholipid bilayer which makes them selectively permeable. This permeability can be changed by different variables, such as temperature and concentration of solvents, like ethanol.
The permeability of a membrane can be measured by using beetroot cells, which contain a purple pigment called betalain.
When the cell-surface membrane has a higher permeability, more pigment leaks out of cells. The permeability can therefore be measured by the amount of pigment leaked from beetroot cells into an aqueous solution using a colorimeter.
Results will typically be presented as a line graph
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A simple way of investigating the permeability of a cell membrane in a lab is by using beetroots.
Beetroot cells contain betalain - a purple pigment. If the cell membranes have a high permeability, betalain will leak out of the cell, causing the external solution to become coloured. The amount of pigment which leaks out can be determined using a colorimeter.
For this experiment, the variables that can be investigated are the concentration of ethanol solution and the temperature.
The method used is:
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Using a coloured plant tissue, often beetroot, place cut, peeled and washed pieces into water at different temperatures or a range of ethanol concentrations, depending on what you are investigating. Leave for a few minutes and the colour inside the cells will leach out depending on the permeability of the cell membrane. Remove the pieces of tissue and assess the concentration of pigment in the liquid, either by sight, comparison to standards or preferably using a colorimeter. The greater the concentration the more permeable the cell membrane was.
First! we need to understand, 'What is permeability?'. A substance is permeable if it can allow molecules to pass thorough the membrane. This property is called 'Permeability'. And it mainly depends on the substance's electric charge and to some extent on the molecular mass also. So, Now Lets come to Cell membrane permeability, Cell membrane is selective in nature and only allows some molecules to pass through it. The best way to study the 'Permeability of a cell membrane in the lab is 'Beetroot test'. Keeping in mind, factors like optimum temperature, pressure, light exposure is necessary. "Why only Beetroot?', this question may come to our mind, So the answer to this question is, Beetroot is very rich in pigments which helps to understand the mechanism of cell membrane permeability, and thus Beetroot is used as a specimen for the lab investigation. Increase in temperature during the experiment increases the fluidity of the membrane which increases the mobility of the pigment and at a very high temperature, the pigment oozes out of the cell membrane. The quantity of the pigment can be measured with the help of a colorimeter.
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By injecting a coloured dye into the cell of interest and measuring change in colour over time
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Hi, quick practical method by OCR
The effect of temperature on membrane permeability STUDENT
Beetroot cells contain the purple pigment, betalain, as shown in the diagram below:
This experiment investigates the effect of temperature on membrane structure by considering the leakage of betalain from the beetroot cells over a range of temperatures. The quantitative assessment of this is carried out using a colorimeter.
· To investigate the effect of temperature on membrane structure.
· To understand the use of a colorimeter.
Intended class time
· 1 hour
Not applicable for this activity.
Equipment (per class)
· Thermostatically controlled water baths at 30°C, 40°C, 50°C, 60°C and 70°C containing test tube racks
Equipment (per 4-6 students/groups)
· Colorimeter and green filter
Equipment (per student/group)
· 1 beetroot cylinders at least 30 mm long
· Knife / Scalpel
· White tile
· Paper towel
· 6 test tubes
· 6 cuvettes
· Distilled water
· 10 cm3 syringe
· Marker pen / Chinagraph pencil / OHP pen / Stickers
· Test tube rack
Health and Safety
Take care with the knife when trimming the cylinders of beetroot to the correct length. The water in the water baths over 40°C will be hotter than hand-hot so be aware of this.
1. Take the six test tubes and label each with one of the temperatures from 20°C to 60°C.
2. Add 10 cm3 distilled water to each tube using the syringe.
3. Place the correctly labelled test tube in its corresponding water bath for 5 minutes to allow the water to equilibrate to the correct temperature. Record the actual temperature with the thermometer. Leave the tubes in the water baths.
4. Cut the beetroot cylinder to give six 5 mm cylinders using the knife and ruler on the white tile.
5. Rinse the cylinders under a running tap and pat dry using paper towel.
6. Add one 5 mm beetroot cylinder to a tube in each temperature and leave for 15 minutes.
7. Remove the tubes from the water bath, carefully swirl once and use the forceps to remove the cylinders. Throw the cylinders into a waste receptacle.
8. One tube at a time, carefully pour the remaining liquid into a cuvette.
9. Use the colorimeter to measure the absorption for each temperature and record this information in a suitably designed table. The absorption is measured in arbitrary units (AU).
10. Plot a graph of temperature against absorption and draw an appropriate line.
1. Describe the relationship between temperature and the rate of leakage of the pigment from the beetroot cells.
2. Explain your results and the shape of your graph using theory about membrane structure.
3. What effect might a solvent such as ethanol have on membrane permeability? Explain your answer using ideas about membrane structure.
4. State four limitations of this procedure.
What to record
As evidence for the Practical Endorsement, you should have your table and graph of the data as described above. All work should be clearly dated.
In addition you should have considered the above questions as the answers to these questions will aid you in preparation for your written examinations.
Permeability of Cell membrane can be measured by using beetroot cells that contain betalain which is a purple pigment. If permeability of the cell membrane is higher, more pigment'll leak out of cells. This can therefore be measured by using a colorimeter.
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