This lesson is called "Tests for biological molecules" and is from the unit "Biological molecules and enzymes." Hi there, my name's Mrs. McCready and I'm here to guide you through today's lesson, so thank you very much for joining me.

In our lesson today, we're going to carry out a number of different tests for a range of biological molecules, including sugars, starch, proteins, and lipids.

Now, we're going to come across a few keywords in our lesson today, and they're presented on the screen now.

You might wish to pause the video and make a note of them, but I will introduce them to you as we come across them.

In our lesson today, we're going to firstly test for sugars, then we're going to test for starch, then we'll test for proteins, before finishing up by testing for lipids.

So I hope you're ready to go, there's plenty of practical work in our lesson today, so let's make a start on testing for sugars.

Now, you all know that organisms are made up of many different compounds, including carbohydrates, proteins, and lipids.

And sugars are a type of carbohydrate.

They're a very small type of carbohydrate, in fact, and include sugars such as glucose, such as shown on the screen now, fructose and galactose.

All of these sugars are called monosaccharides.

Now, mono means one and saccharide means sugar, so a monosaccharide is a single sugar, such as glucose.

Now we can test for monosaccharides such as glucose and fructose using a reagent, that's a testing solution, called Benedict's solution.

And if we put Benedict's solution onto what are called reducing sugars like glucose and fructose, there will be a visible chemical reaction take place that we can use to deduce whether those reducing sugars are present in the sample that we have or not.

Now, reducing sugars are a very specific group of sugars, and these include glucose such as found in energy drinks, and fructose that is predominantly found in fruits, so that's sugar that makes fruit very sweet, that's called fructose.

Now, Benedict's solution is blue, that's its starting colour.

And if it stays blue, then we can conclude that there are no reducing sugars present within our sample.

But it can go through a number of different colour changes.

So if it turns a kind of green or yellow colour, then we know that there are only traces of reducing sugars within our sample.

So there's a little bit, but really not very much at all.

If Benedict's solution turns orange, then we know there is a moderate quantity of reducing sugars.

Not loads and loads, but certainly not just a trace.

But if Benedict's solution turns brick red, and that's sort of technical colour term, brick red, then we can say that there is a large quantity of reducing sugars present within our sample.

So you can see that as the colour changes we can say that there's an increasing quantity of reducing sugars, as it goes from blue to yellow/green to orange to finely brick red.

So let's quickly check your understanding.

Which colour does Benedict's solution turn if there is a large quantity of reducing sugar present? Is it A, blue, B, orange, or C, brick red? I'll give you five seconds to think about it.

Okay, so hopefully you have said brick red for a large quantity of reducing sugar.

Now, to test for reducing sugars, you'll need a number of pieces of equipment, including distilled water, a boiling tube, and Benedict's solution.

Now, be careful when using Benedict's solution because it is an irritant.

You must wear safety goggles.

You will also need a water bath.

So have a watch of the video and see the method used for testing reducing sugars before we carry on.

So let's review that method.

Firstly, to test for reducing sugars, we add the sample of the substance that we're testing to a boiling tube.

Then we add a few drops of distilled water to create a suspension.

So a suspension is where tiny particles float in the water but they don't dissolve.

Then add a few drops of Benedict's solution to the boiling tube, mix it gently and place it in a hot water bath of about 80 degrees or higher, and leave for about five minutes until the colour has matured.

Remember, please, that Benedict's solution is an irritant and you must wear safety goggles.

So let's just quickly refresh that.

Which of the following steps is not part of testing for reducing sugars with Benedict's solution? Is it A, to shake vigorously to mix? Is it B, to add water to the sample to create a suspension? Or is it C, to place in a hot water bath and leave for five minutes? I'll give you five seconds to think about it.

So hopefully you've remembered that shaking vigorously to mix is not part of the testing process for testing for reducing sugars, well done.

So what I'd like you to do now is to follow the method to test a sample for reducing sugars.

Then I would like you to write a conclusion for your investigation.

So approximately how much reducing sugar does the sample contain and how can you tell? Then finally, have a look at this.

Sam gives the following description of the method to test for reducing sugars, but they've made a few mistakes.

So Sam says, "Mix the sample with water to create a mixture.

Add Benedict's solution.

Look for the colour change.

The brighter orange, the more sugars are reduced." So they've made a few mistakes.

What I'd like you to do is rewrite their explanation and correct it so that it is accurate.

So pause the video and come back to me when you're ready.

Okay, let's see how you've got on with this.

So when you followed the method, you should have followed it accurately and carefully to test the sample provided.

And depending on the quantity of reducing sugar present in the suspension, the Benedict's solution will have turned a different colour.

So it might have stayed blue, it might have turned yellow or green to show that there were traces, orange to show there was a moderate quantity, or brick red to show that there a large quantity.

So just check your results.

Then I asked you to write a conclusion for your investigation.

Approximately how much reducing sugar does your sample contain and how can you tell? So you might have written that "the sample I tested contains large quantities of reducing sugars," where the word "large quantities" you might have changed for something different.

Then you should have said, "It turned Benedict's solution brick red." But again, brick red might be a different colour according to your sample.

Then I asked you to rewrite Sam's explanation and correct it so that it is accurate.

So with this, you should have said that you need to mix the sample with distilled water to create a suspension.

Then you need to add an equal volume of Benedict's solution.

Then you should have added in entirely that you need to place the sample into a hot water bath of approximately 80 degrees centigrade for five minutes.

And then finally, you should have changed the last little bit to say look for a colour change.

The darker red, the more reducing sugars are present.

So just check your answer over and make sure you've corrected Sam's work correctly.

Well done, and I hope you enjoyed that little practical.

It's quite a nice one.

So we're gonna have a look now at testing for starch.

Now starch is a complex carbohydrate made up of many glucose molecules joined together.

It's a huge molecule and it is possible to test for the presence of starch using this time a reagent called iodine.

So iodine solution is usually orange or brown in colour, but it turns a blue/black colour in the presence of starch, and that's a very specific colour name as well.

It isn't blue, it isn't black, it is blue/black.

So make sure you remember that.

So what colour is it that iodine turns in the presence of starch? Is it brown/orange, blue/black, or is there no change? What do you think? I'll give you a few seconds to decide.

Okay, hopefully you've remembered that iodine turns blue/black in the presence of starch, well done.

Now, to test for starch, you will need some equipment.

You will need distilled water, you will need a boiling tube, and you will need iodine solution.

Now please be careful, iodine solution is an irritant, so you must wear safety goggles.

So watch this video clip to see the method for testing for starch.

Okay, so let's just summarise that.

To test for starch, we firstly add the sample to a boiling tube.

Then we add a few drops of distilled water to create a suspension.

Then we add a few drops of iodine solution and note any colour change.

Remember, iodine is an irritant and you must wear safety goggles.

So let's just check that.

Which step in this method is written incorrectly and how would you correct it? So is it statement A, add a sample of the substance to a boiling tube? Is it B, add a few drops of distilled water to create a suspension? Or is it C, add a few drops of blue/black solution and observe any colour change? So which is incorrect and how should it be corrected? I'll give you a few moments to think about it.

So did you spot that statement C is incorrect? Well done if you did.

And have you spotted how to correct it? Well, you should have identified that the words blue/black are incorrect and that they should have been changed to iodine, well done.

So what I'd like you to do now is to firstly follow the method to test for starch.

Then I would like you to write a conclusion for your investigation.

Does the sample contain starch and how do you know? Then finally, I'd like you to consider this.

An experiment is carried out using starch solution and amylase, which is an enzyme which digests starch to maltose, which is a sugar.

So describe how you would for starch and what the result would be at the start and the end of the experiment.

So stop the video and come back to me when you're ready.

Okay, so you should have followed the method accurately and carefully to test the sample that you were provided.

And if there was no starch, then the iodine solution would've stayed orange/brown.

But if starch was present, then it should have turned blue/black.

Then you should have written a conclusion for your investigation.

Does it contain starch, and how do you know? So you might have written that "the sample I tested contains starch, and I know that because it turned the iodine solution from orange/brown to blue/black." And then finally, when I asked you to describe how you would test for starch and what the result would be at the start and end of this experiment with amylase, you should have written an answer including that you would've removed sample of the solution, added a few drops of iodine to the sample, and the sample should contain starch at the start of the experiment, so iodine would turn blue/black.

By the end, however, the amylase enzyme should have digested the starch to maltose, so you would take another sample of the solution and test that with iodine, and that should remain orange/brown because there's now no longer any starch left, it has all been digested to maltose.

So just review your answer, see if you've got all those key points, and well done, that was quite a challenge.

Okay, let's move on to our next practical, which is testing for proteins.

So proteins are really complex polymers which are made by combining many amino acids end to end to end.

Now we can test for the presence of proteins using a reagent.

That's a testing solution called Biuret solution.

Now, Biuret solution is blue, but it turns purple in the presence of protein.

So it goes purple for protein, but it's blue to begin with.

So what I'd like you to do is just complete the three sentences with one of the words below for each gap.

So the statements are, "Proteins are made from something." "To test for proteins, we use something." "If proteins are present, the solution will turn something." I'll give you a few moments to think about it.

Okay, so you should have said that proteins are made from amino acids.

To test for proteins, we use Biuret solution.

And if protein is present, the solution will turn purple, well done.

Now, to test for proteins, you will need some equipment.

You will need distilled water and a boiling tube.

You will also need Biuret solution and sodium hydroxide.

Now please be careful because Biuret solution is an irritant and sodium hydroxide is corrosive, so you must wear safety goggles.

So what I'd like you to do is watch the video clip to show the method for testing for proteins.

So let's just review that.

To test for protein, you firstly add a sample of the substance to a boiling tube.

Then you add a few drops of distilled water to the sample to create a suspension.

Then you'll add a few drops of Biuret solution and then a few drops of sodium hydroxide and then swirl them to mix, be careful.

Then note the colour.

Remember, Biuret solution is an irritant and sodium hydroxide is corrosive, so you must wear safety goggles.

So what I'd like you to do now is to start with the first and put these method steps in the correct order to test for the presence of proteins.

So the steps include add a few drops of distilled water to the sample to create a suspension.

Add a few drops of sodium hydroxide and swirl to mix.

Note the colour.

Add a sample of the substance to a boiling tube.

And add a few drops of Biuret solution.

So starting with the first, put them in order.

I'll give you a few moments to decide.

Okay, so you should have started off with step C, add a sample of the substance to a boiling tube.

You should have then chosen step A, add a few drops of distilled water to the sample to create a suspension.

Then you should have gone to step D, which is add a few drops of Biuret solution.

Before finishing off with step B, add a few drops of sodium hydroxide and swirl to mix.

Note the colour.

Well done.

So what I'd like you to do now is to follow the method to test for proteins.

Then write a conclusion for your investigation.

Does your sample contain protein, and how do you know? Then I'd like you to consider this scenario.

So flystrike is a disease that affects animals including rabbits.

Rabbits with dirty or soiled back ends attract flies, and the flies lay their eggs on the dirty skin.

These eggs then hatch into maggots, which then eat the flesh of the rabbit and cause great harm.

Pretty disgusting, huh? Now a vet wants to check whether a rabbit has soiled fur or has just been rolling in the mud.

So what I'd like you to do is to describe a test which would identify whether the brown substance is protein-rich excrement, that's poo, or protein-poor soil.

So pause the video and come back to me when you're ready.

Okay, so let's see what you did then.

Firstly, you should have followed the method accurately and carefully to test the sample that you were given.

And if protein was present, it would've turned that beautiful purple colour.

Then you should have written up your investigation to draw a conclusion.

You might have written that "the sample I tested contains protein, and I know that because it turned Biuret solution from blue to purple." It's always worth stating the starting colour and then the colour it changes to in these sorts of responses, including in exam question answers.

Then I wanted you to consider this example of flystrike.

So describe how you would test to see where the protein was present in the sample from the rabbit's bottom.

So you might have written that you would've taken a sample of the substance from the rabbit.

Then you would mix it with distilled water to create a suspension.

Then add Biuret solution, then sodium hydroxide to the suspension.

Gently mix it and note the colour.

If it turns purple, protein is present, which would suggest that the substance is excrement rather than soil.

So just review your answer.

How did you get on? Did you get all of those points? Well done if you did, that was quite complicated.

Okay, our final practical for today's lesson is testing for lipids.

Now, lipids are fats and oils and they are, as you probably know, insoluble in water.

They don't dissolve in water.

But it is possible to test for the presence of lipids using ethanol, which is a type of alcohol, instead.

Now, if you test for lipids, so fats or oils, using ethanol, when you mix the two together really, really vigorously, you will see that ethanol turns cloudy if lipids are present.

So what I'd like you to do is to match the reagent on the left to the positive result if the substance is present on the right.

So I'll give you five seconds to think about this.

Okay, so you should have matched Benedict's solution to brick red, ethanol to cloudy, iodine solution to blue/black and Biuret solution to purple.

Well done if you've got them all right.

Do be clear which reagent tests for which substance, what its starting colour is, and what colour it turns to if there is that substance present.

It's really important that you know them all.

Now, all of the tests we've done today, so that's testing for starch, for proteins, for reducing sugars, and for lipids, all of these are what are called qualitative tests and therefore give qualitative results.

So a qualitative result is a result based on an observation, in this case a colour change, but it could be other things like a sound produced.

And therefore the test is not producing a number output that we could plot on a graph.

It's simply giving us an observation.

Now that's different from quantitative tests where measurements are taken which do include numbers, such as the distance travelled or the change in mass or the temperature change.

Now all of these are quantitative tests because the measurement is a number, a numerical value that can be plotted on a graph, for instance, or a mean can be calculated from them.

And this is where the word quantity, quantitative, that's how those two words are linked.

So Benedict's solution is a qualitative test.

Is that true or false? I'll give you five seconds to think about that.

Okay, so you should have said true, but why? Can you justify your answer? Is it because the results are numerical values, or is it because the results are based on observations, not measurements? I'll give you five seconds to decide.

Okay, so you should have decided that the results are based on observations, not measurements, which is why Benedict's solution is a qualitative test.

Well done if you got both of those points right.

Now, to test for lipids, you will need some equipment.

You will need a boiling tube with a bung, and you will need some ethanol.

Now, ethanol is highly flammable, so no naked flames in the room and you must use a ventilated room.

Please, also wear safety goggles.

So watch the video clip to show the method for testing for lipids.

And now let's review that method.

So first of all, you need to add a sample of the substance to a boiling tube.

Then you need to add an equal volume of ethanol.

Then you need to shake them vigorously until they are thoroughly mixed together.

Then observe the colour.

Now please remember that ethanol is highly flammable, so you must not have naked flames like a Bunsen burner or a match, and you must use a ventilated room.

You must also wear safety goggles.

So what I'd like you to do now is to follow the method to test for lipids, then write a conclusion for your investigation.

So does your sample contain lipids, and how do you know? Then I want you to consider this.

The cloudy suspension is formed because the lipids and the ethanol do not mix.

So have a think about that and then state what you think might happen to these two liquids if they are left undisturbed for 30 minutes.

Just on the side, for instance.

What do you think might happen to them? So pause the video and come back to me when you're ready.

Okay, so you should have followed the method to accurately and carefully test your sample.

And after a vigorous shake, if the ethanol turned cloudy, then you can conclude that lipids are present.

So you should have written up your conclusion and said that "the sample that I tested contains lipids, and I know that because the ethanol turned cloudy when I mixed it vigorously together." And then I asked you to consider this, that the cloudy suspension is formed because lipids and ethanol do not mix.

So what do you think might happen to these two liquids if they're left undisturbed for 30 minutes? Well, your answer might have included that the two solutions, so that's the lipid and the ethanol, will slowly separate.

And the least dense solution will form the top layer, the most dense solution will form the bottom layer.

And if you leave your sample in its test tube on the side for a little bit, you will see that happen.

It really doesn't take very long at all.

Well done indeed.

Well, I hope you enjoyed our lesson today, I certainly did.

All those practicals are great fun and they are really short and snappy.

They're very, very interesting to do as well.

So we've seen in our lesson today that Benedict's solution can be used to test for the presence of reducing sugars, and it turns from blue to green/yellow, orange and then brick red as the concentration of reducing sugars increases.

We then saw that we used the reagent iodine to test for the presence of starch, and it turns blue/black if starch is present.

Next, we tested for the presence of proteins and we saw that Biuret solution is used to do this, and that turns from blue to purple if proteins are present.

Then finally, we tested for lipids using ethanol, and after a really vigorous mix we can say that ethanol turns cloudy if lipids are present.

And we also learned that all of these tests are qualitative tests, which means that the results we are getting are based on observations and no numerical values are given at all.

So thank you very much for joining me, I really hope you enjoyed our lesson today and enjoyed doing all of those practicals together.

Thank you very much and I hope to see you again soon, bye.