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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.
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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 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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The permeability of a cell membrane could be investigated in a lab using beetroot discs and increasing concentrations of ethanol, or increasing the temperature.
As beetroot cells are plant cells, the pigment Betalain is found in the vacuole of the cell surrounded by the tonoplast membrane. Beetroot discs could be cut to a specific size. These discs could be placed into differing concentrations of ethanol 0.2M, 0.4M. Or placed into distilled water and into a thermostatic water bath at wet temperatures, 30 degrees Celsius, 40 degrees Celsius etc. After a set amount of time small quantities of the liquid surrounding the beetroot disc could be sampled and tested for absorbency. Using a colorimeter and cuvettes, the absorbency can be calculated. The higher the absorbency level the higher the concentration of betalain. A calibration curve could be calculated as well for unknown concentrations. Cell membranes are made of a phospholipid bilayer, these phospholipids are sensitive to changes in temperature and solvents. When the temperature increases the membrane fluidity increases due to increased kinetic energy. This eventually leads to the breakdown of the membrane at high temperatures. With solvents, increasing solvent concentrations cause the membrane permeability to increase leading to the movement of ions and molecules out of the cell through the ‘leaky’ membrane.
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By Beetroot experiment. Leakage of betalains from cell membrane at different temperature or in different alcohol concentration will help to determine the membrane permeability.
Using beetroot cells. Use differing concentrations of ethanol to see if dilution has an effect on pigment. Measure the absorbance of each solution using a colourimeter, to see how permeable it is. Higher pigment concentration, means a more permeable membrane.
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
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.
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.
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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.
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