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Enrichment resourcesLab Technician support
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Preparing onion tissue video transcript
Preparing onion tissue for microscope work in the lab is a common laboratory procedure. Onions are particularly useful for observing plant cells because the onion’s epidermis is one cell thick, and preparation of onion tissue is relatively simple. In this video we will walk through the process of cutting down an onion to prepare pieces to be used in lab work and Cambridge practical examinations.
Remember that the onions you use should be as fresh as possible, and you shouldn’t use red onions. Instead, make sure the onions you use have white flesh.
The preparation of the onions should be completed at least an hour before they are used, but you can easily prepare the onions up to a day before they are needed.
Equipment screen
Safety screen
Don’t eat or drink anything in the lab.
Always wear eye protection.
In case of food allergies, always wear gloves.
Take care when using a sharp knife.
Start by placing an onion on a suitable chopping surface, and cut both the top and bottom off the onion.
Next, cut across the middle of the onion to produce two halves, putting one half aside for preparation later.
Now cut the onion in half and half again to produce quarters. You can cut the onion down further if it is particularly large.
Choose one of the quarters and discard the inner pieces that are too small to be used in the lab. Pull away the first layer from the remaining onion that is large enough to use and set it aside.
Continue to pull away additional layers and process the other quarters of the onion until you have enough pieces for the lab work or practical exam. Always ensure that you include enough spare pieces should they be required. You should now have a number of onion pieces of uniform size, each with the epidermis still attached.
Next prepare the solutions in which the onion pieces will be stored until they are used. In this case the onion pieces will be stored in solutions of sodium chloride and distilled water.
Take each piece of onion and place them into the beakers. In some cases, the inner epidermis may start to separate or may have become separated from the rest of the onion tissue and may be floating. This is not a problem, though it’s better if each onion piece is presented in a similar way.
Make sure that there is enough solution in the beakers to submerge each piece of onion, and cover each beaker to prevent further evaporation.
Finally, before using the onion pieces in the lab, make sure that the solutions are at room temperature.
We hope you’ve found this video useful. If you’d like more videos showing lab preparations, please contact Cambridge and we’ll be happy to consider any suggestions you have.
Good luck! -
Preparing an iodine solution video transcript
Preparing iodine solution for use in the lab as a test for the presence of starch, is a common laboratory procedure. In this video we will walk through the process of making up a stock solution of 0.1 molar iodine solution, and then diluting a sample down to 0.01 molar for use in a practical exam for example. You should be able to follow the same process to make further dilutions for other uses.
Equipment screen
Safety screen
Wear protective gloves/protective clothing/eye protection/face protection.
Handle iodine solid using forceps/tweezers/spatula etc.
If iodine solid or iodine solution comes into contact with skin, wash with plenty of soap and water.
If skin irritation occurs, get medical advice/attention.
If iodine solid or iodine solution comes into contact with eyes, rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing.
If eye irritation persists, get medical advice/attention
Iodine solid and iodine solution are very toxic to aquatic life
To make the stock solution of 0.1 mol per cubic decimetre iodine solution, first place a suitably sized beaker on a set of weighing scales accurate to two decimal places. Remember to tare the scales.
Next, put 8 grams of potassium iodide into the beaker. Then set the beaker aside while the iodine solid is weighed out. Add a weighing boat to the scales, which should then be tared. Carefully weigh out 2.54 grams of iodine solid on the scales.
Now, moisten the potassium iodide in the beaker with a little distilled water, then add the iodine to the potassium iodide and stir well using a glass rod. Gradually add more distilled water to the solution, stirring as you go.
We’ll be making up 100 centimetres cubed of iodine solution, but to do this accurately for a molar calculation the solution will need to be transferred to a measuring cylinder. So keep adding distilled water until there is approximately 75 centimetres cubed of solution. At this point, remove the glass rod and thoroughly wash it down with distilled water into the beaker.
Transfer the iodine solution from the beaker into a 100 centimetres cubed measuring cylinder. Then using more distilled water, wash the remaining iodine solution from the sides of the beaker, and add it to the measuring cylinder.
There should be now roughly 80 centimetres cubed of iodine solution in the measuring cylinder. Add more distilled water, washing solution from the sides as you go, until there is 100 centimetres cubed of iodine solution in the measuring cylinder. This is the stock 0.1 mol per cubic decimetre iodine solution.
The solution will now need to be thoroughly stirred, so pour the stock solution into a suitable beaker and add a magnetic stirrer. Place the solution on a stirrer and set it to stir. Set this aside to stir for 15 minutes while you prepare the diluted solution.
In a 100 centimetres cubed measuring cylinder, add approximately 80 centimetres cubed of distilled water. We’ll be adding 10 centimetres cubed of the stock iodine solution and then making this up to 100 centimetres cubed, so the distilled water measurement just has to be less than 90 centimetres cubed to allow room for washing out.
Once the stock solution is stirred thoroughly, pour 10 centimetres cubed of it into a suitable measuring cylinder. Then pour the solution into the measuring cylinder containing the distilled water.
Wash out the remaining iodine solution in the small measuring cylinder and add the liquid to the larger measuring cylinder. Then make it up to 100 centimetres cubed with more distilled water.
This makes the 0.01 mol per decimetre cubed dilution which can be decanted into suitable containers for use in the lab. If the iodine solution is being prepared for a Cambridge practical exam, it must be made up immediately before the start of the examination and kept out of sunlight.
We hope you’ve found this video useful. If you’d like more videos showing lab preparations, please contact Cambridge and we’ll be happy to consider any suggestions you have.
Good luck! -
Preparing yeast suspension video transcript
Preparing a yeast suspension in a solution of sucrose is a common laboratory procedure. The suspension can be used in experiments testing metabolic reactions in Cambridge practical exams for example. In this video we’ll work through the process of how to prepare 100 centimetres cubed of 7% yeast suspension in a solution of 2% sucrose.
We’ll be using dried yeast like that that can be bought in shops for breadmaking in this preparation. However, as some yeast will require more time to become active than other yeasts, it is always worth checking in advance of the examination or using the suspension in the lab, that the yeast you’ll be using will become active. That means it will produce a froth on top when mixed with the sucrose solution. Try this out well before you need to make up the yeast suspension for lab use, so you can be certain that you will have successful results.
Also, remember that you shouldn’t use brewer’s yeast, as this doesn’t always work actively enough in time.
Equipment screen
Safety screen
Don’t eat or drink anything in the lab.
Always wear eye protection.
In case of food allergies, always wear gloves.
Begin by measuring out 100 centimetres cubed of distilled water in a measuring cylinder, then pour approximately half the water into a suitably sized beaker. Put the remainder of the water aside.
Now measure out 2 grams of sucrose, remembering to tare the weighing scales first.
Sprinkle the sucrose into the beaker of water and stir until it is dissolved using a glass rod.
Now add the remaining distilled water to the beaker to make up the 2% sucrose solution and swirl to mix. The sucrose solution can be prepared the day before being used, and kept in covered containers in a refrigerator until it is heated up in preparation for adding the yeast.
To do this, place the beaker of sucrose solution on a heating plate. Set the temperature on the heating plate to between 35 and 40 degrees centigrade, and leave the solution to warm for approximately 30 minutes.
You should start preparing the 7% yeast in 2% sucrose solution 30 minutes before the suspension is required in the lab. Start by weighing out 7 grams of dried yeast on some weighing scales.
The pour the yeast into a suitably sized beaker, then add some of the warmed sucrose solution and stir with a glass rod to mix the solution and yeast.
Wash off the glass rod with some more of the warm sucrose solution, before topping up the solution in the beaker to 100 centimetres cubed with most of the remainder of the warm sucrose solution.
You’ve now made up the yeast suspension, but it needs to be given time to activate. To do this, place the beaker containing the suspension back on the heating plate and set the temperature to between 35 and 40 degrees centigrade again. Leave the suspension to warm through for approximately 30 minutes.
The yeast in the suspension will begin to activate, forming a froth on top of the liquid. When decanting the suspension into smaller beakers for use in the lab or by candidates in practical exams, it is helpful if the yeast suspension is poured into a second beaker first, leaving the froth behind.
The 7% yeast in 2% sucrose solution should be prepared 30 minutes before it is required in the lab. It will have cooled down before it is used, but it may still be warm.
We hope you’ve found this video useful. If you’d like more videos showing lab preparations, please contact Cambridge and we’ll be happy to consider any suggestions you have.
Good luck!