Hello, my name's Mr. Jarvis, and I'm going to be taking you through today's lesson, which is all about the effect of temperature on the rate of decomposition by an enzyme.

And today is a practical lesson.

The lesson forms part of the unit "Living organisms and their environments." By the end of today's lesson, you should be able to carry out an experiment to investigate how temperature affects the rate of decomposition of milk by an enzyme.

There are five keywords to today's lesson.

They are enzyme, active site, pH, end point and hypothesis.

You can pause the video to read those definitions if you want at this point, but we will go through those definitions as we come to them in the lesson.

Today's lesson is broken down into three parts.

First of all, we're going to look at enzymes and the decomposition of milk.

Then we're going to prepare to carry out our practical.

And then finally, the third and final part of the lesson is when we're going to carry out the practical.

So, if you're ready, let's get started with our first section, which is all about enzymes and the decomposition of milk.

Decomposition is the process where decomposers break down organic matter.

And examples of decomposers are usually microorganisms, for example, bacteria and fungi.

And remember, microorganisms are organisms that we can only see using a microscope.

There are various factors that can affect the rate of decomposition, which we sometimes also call decay.

They are temperature, moisture, the availability of water, and the availability of oxygen.

The decay of organic matter by microorganisms is important in recycling chemicals.

Decomposers return, or cycle, elements back to the surroundings.

And examples of that include carbon and nitrogen.

They're really important because they can then be reused by other organisms. Decomposers use enzymes to break down organic matter, and enzymes are biological catalysts that speed up reactions.

Here's an image of an enzyme.

The enzymes catalyse very specific chemical reactions.

The substrate, which is the substance that's acted upon by the enzyme, must bind to the enzyme's active site for the reaction to occur.

So here on our image of the enzyme is our active site.

Here is our substrate, and you can see that the substrate will bind to the active site.

When the reaction takes place, the products are given out.

Enzymes are only able to catalyse reactions when the substrate's shape matches the shape of the enzyme's active site.

So for example, here's our substrate, and you can see that this substrate's shape fits into the active site.

A reaction will take place.

Here's another enzyme, and it's got the same active site shape.

But here's our substrate.

And you can see that the shape of the substrate doesn't match the shape of the active site.

And so no reaction will occur because the substrate won't be able to bind to the active site.

Let's move to a check.

Look at the images of the enzymes and substrates below.

For each image, I'd like you to decide whether the enzyme would catalyse a reaction if the substrate were able to bind to the active site.

So, we're looking at the shape of the active site and the shape of the substrate and deciding whether the substrate will be able to bind to the active site of the enzyme to allow a reaction to take place.

You'll probably need to pause the video at this point to have a look at the shapes of the substrates and the shapes of the enzymes' active sites more carefully to decide whether a reaction takes place or not.

And then when you're ready, press play and we'll check your answer.

Good luck.

Let's check your answers.

So, first of all, let's look at A.

The enzyme A has a hexagon-shape active site and so does the substrate.

And you can see that the substrate does fit into the active site, it does bind to the enzyme, and as a result, a reaction will take place.

Well done if you got that.

Moving on to B, we've got a star-shape substrate and a star-shape enzyme active site.

However, when the substrate tries to bind or fit into that active site, you can see that it doesn't, and so no reaction would take place.

And then for C, we have this square and circle with a segment out of it as a substrate.

If we have a look at the active site, it sort of looks like it might fit.

And indeed, if we switch and rotate the substrate round, the enzyme does fit into the active site, so a reaction would take place.

Well done if you got all three of those answers correct.

The fats, or lipids, in milk can be broken down by enzymes called lipases.

And enzymes often end in -ase.

Another example of an enzyme that you might have heard of is amylase.

It ends in -ase.

And amylase breaks down carbohydrates into sugars in saliva.

Let's go back to our lipids and lipase.

The lipids, the fats in the milk, are broken down by lipase, the enzyme, and it produces products that we call fatty acids and glycerol.

So, when fats are decomposed, they produce fatty acids and glycerol.

And as more and more lipids are broken down, more and more fatty acids are produced.

And this will change the pH of the milk.

And the pH is how acidic or alkaline a solution is.

So let's do a check.

Lipids, or fats, in milk are broken down by lipases to form fatty acids and glycerol.

As more lipids are broken down, what will happen to the pH of the milk? Will it A, decrease because the milk has become more acidic? Will it B, decrease because the milk has become more alkaline? Or C, will the pH just stay the same? I'll give you a few seconds and then we'll check your answer.

The correct answer is A.

When lipids in the milk are broken down by lipases, they form more and more fatty acids and that means that the pH will decrease because the milk is becoming more acidic.

The fatty acids are building up, so there's more and more acid being produced.

Well done if you got that one.

So as the fats in the milk are broken down, more fatty acids are produced and that makes the milk more acidic.

And in reactions where there is a change of pH, an indicator can be used to see how quickly a reaction takes place.

So an indicator is a chemical that undergoes a colour change due to a change in pH.

Here's two examples that you might have come across before.

Universal indicator, which is sometimes a paper and sometimes a solution.

And litmus paper.

They're both examples of indicators.

Cresol red is used as an indicator in this investigation.

And this indicator changes colour when the pH of the solution changes.

At alkaline conditions, where it's above pH 7.

2, the Cresol red indicator turns a pinky purple colour.

The end point of this reaction that we're going to undertake occurs when the fats in milk are digested into fatty acids and glycerol, and that's going to make the indicator go yellow because there's an increase in the acid.

So, yellow is the colour that Cresol red goes when the pH goes below pH 7.

2.

So here's a question to check your understanding.

What's the colour of Cresol red indicator in solutions with a pH below 7.

2? Is it A, green? B, pink or purple.

C, red.

Or D, yellow.

I'll give you a few seconds and then we'll check your answer.

The correct answer here is that it turns yellow.

A pH is below 7.

2, Cresol red turns yellow.

Well done if you got that.

Let's move on to our first task of this lesson.

Lipase is an enzyme that breaks down fats, or lipids.

And milk and cream contain lots of fats.

Answer the following questions relating to lipase and milk.

First of all, state what smaller molecules are formed when lipase breaks down fats.

Secondly, explain how the fats, the substrate, are acted upon by the enzyme.

And then finally, what will happen to the pH of the milk as the fats are broken down? Pause the video, write down your answers, and then when you're ready to check to see how well you've done, then press play and we'll go through what the answers to these questions are.

Good luck.

How did you get on with that? I hope that it was okay for you.

So I asked you three questions.

First of all, I asked you to state what smaller molecules are formed when lipase breaks down fats.

And the answer to this question is, fats are broken down into fatty acids and glycerol.

I then asked you to explain how the fats, the substrate, are acted upon by the enzyme.

And so the enzyme has an active site which is specific to the shape of the substrate, the shape of the fat or the shape of the lipid.

When the substrate binds to the active site, the enzyme breaks down the fat into smaller fatty acids and glycerol.

The active site can then accept another substrate molecule.

And enzymes speed up the rates of reaction without being used up in the reaction themselves.

And then I asked you in question three, what will happen to the pH of the milk as the fats are broken down? Well, the fatty acids will make the milk more acidic, and so the pH will decrease.

Well done if you got all of those.

That brings us to the second part of the lesson today, which is all about preparing the practical.

So if you're ready, let's carry on.

The practical will investigate the effect of temperature on the rate of decay or decomposition of fresh milk, and we'll do this by measuring pH change.

We're going to look first of all over our independent variable, the thing that we're going to change, and that's going to be the temperature of the milk.

The dependent variable, the thing that we're going to measure, is the time taken for the solution to turn yellow, in seconds.

So, we know that Cresol red will turn from a pinky purple in alkaline conditions to a yellow colour when the pH goes below 7.

2.

The practical will also use some control variables, and we're going to control the concentration of the enzyme lipase.

We're going to also control the volume of lipase that we use in the practical.

And we're going to also control the volume of milk that we use in the practical.

Let's do a check.

Who's most accurately describing the investigation that we're completing today? Is it Andeep, who says that, "We will observe how the colour of an indicator changes at different milk temperatures"? Is it Lucas, who says, "We will measure the time taken for lipase to break down fats into fatty acids at different temperatures"? Or is it Aisha, who says, "We will measure the time taken for lipase to change the pH of the milk"? I'll pause for a few seconds and then we'll check your answer.

So the correct answer is Lucas.

We're going to be measuring the time taken for lipase to break down fats into fatty acids at different temperatures.

Well done if you got that right.

For this practical, you'll need the following chemicals.

You'll need some full-fat milk or cream.

You'll need 5% lipase solution.

You'll need some sodium carbonate solution.

And you'll need some Cresol red indicator.

You also need a pair of safety goggles because sodium bicarbonate and lipase can be irritants.

So you should always wear safety goggles when carrying out this practical.

You're also going to need some equipment.

You're going to need some boiling tubes and a boiling tube rack.

A stirring thermometer.

Graduated teat pipettes.

A 250-cubic-centimeter beaker and thermometer.

You're going to need some ice.

A kettle.

A stop clock.

Those safety goggles.

Remember to wear those during the practical.

And a marker pen.

You need to set up your equipment and materials neatly.

You need to make sure that you're organised so that you can carry out the practical really well.

So, when you get started, you'll need to set up your boiling tube rack and fill it with the boiling tubes for your milk solution and your lipase solution.

And you'll also need to set up a water bath in a 250-cubic-centimeter beaker and thermometer.

There are two things that we need to do before we start this practical activity.

And it's worth having a look at the additional materials that are available with this lesson as you move on through this part of the lesson.

First of all, you need to decide the range of temperatures that you'll investigate the decay of milk over.

What will your lowest and highest temperatures be? Think about how you're going to maintain the temperature of your water bath, as an example.

And will you increase your temperatures in a systematic way? I.

e.

, will you have a system to increasing temperature? Will you increase every 10 degrees centigrade or will you actually just randomly increase your temperatures of your water baths? And secondly, what effect will changing the temperature have on the rate of milk decay? We can use this to write a hypothesis.

And a hypothesis is an idea based on observations about how something works.

And it should also suggest what will happen using your scientific knowledge and understanding.

So a hypothesis can always be tested using an experiment.

And in this experiment, to make a hypothesis, we need to think about how temperature affects the rate of reaction, for example, the decomposition of milk.

What will your hypothesis be? It may be worth pausing the video at this point and writing down a hypothesis for yourself.

When you've written a hypothesis, remember, it should be an idea based on observations about how something works but also have some sort of scientific knowledge to back up why you think that will happen.

And when you're ready, press play and we'll talk about what a good hypothesis might look like.

So if you came up with something like increasing the temperature will increase the rate of decomposition, you're not wrong because that is probably what we would expect to happen.

However, you've written a prediction.

Predictions are just simple statements that can be tested by an experiment.

What that prediction doesn't do is explain why we think it might happen.

So, a hypothesis needs to explain why we might expect a prediction to happen.

So, a better sentence to write would be, increasing the temperature will increase the rate of decomposition because there are more frequent collisions between the molecules of an enzyme and substrate.

So what we're saying is we'd expect the reaction to go more quickly because there's more frequent collisions between the enzyme and the substrate molecules.

Here's a check.

Which of the following are features of a hypothesis? A, it must link to scientific knowledge.

B, it must be correct.

C, it can always be tested by an experiment.

Or D, it's based on observation.

I'll give you a few seconds to think and then we'll check the answers.

The correct answers are A, C, and D.

A hypothesis must link to scientific knowledge, it can always be tested by an experiment, and it's based on observation.

Well done if you got all of those answers correct.

So, we're now going to go to your practise task and you need to assemble the practical equipment that's required for this investigation.

I'd like you to label a boiling tube "lipase" and add five cubic centimetres of lipase solution using a graduated teat pipette.

Label a second boiling tube "milk" and add five drops of Cresol red indicator.

Add five cubic centimetres of milk to the "milk" boiling tube using a graduated teat pipette.

Using another teat pipette, add seven cubic centimetres of sodium carbonate solution to the "milk" boiling tube, and place a thermometer into this "milk" boiling tube.

And then finally, set up a water bath using a 250-cubic-centimeter beaker at your first temperature.

And remember, wear those goggles because sodium bicarbonate and lipase can be irritants.

You can find those details on your additional material sheet if you want them written down in front of you.

At this point, you need to then pause the video, go through steps one, two, three, four, and five, and then when you're ready, press play and we'll talk about how to complete this practical.

So what you should have set up is the following.

You should have your "milk" boiling tube with the thermometer inside it.

You should have your "lipase" boiling tube with the lipase solution in it.

You should have a water bath set up at your first temperature.

And you should have used the kettle or ice to make that water bath at the temperature that you need.

Really important not to mix up your graduated pipettes, and in particular, the pipette that you used the lipase solution for.

Because if you add any lipase into any of the milk, then a reaction will start straight away.

So, really take care not to mix up your pipettes.

So now you've got your practical set up, all the equipment's ready, we can now move on to the third part of today's lesson, which is all about completing the practical.

So if you're ready, let's move on.

So wearing your safety goggles, you need to put your "milk" and your "lipase" boiling tubes into the water bath.

And the temperature of the water bath should be shown by your thermometer that's within the water of the water bath.

Over time, the temperature of the "milk" tube should go up.

Once the "milk" temperature is the same as the temperature of the water bath, you need to then add one cubic centimetre of lipase to the "milk" boiling tube.

So taking it from the "lipase" tube in the water bath, you add one cubic centimetre of lipase into the "milk" tube.

Immediately start the stop clock.

Stir the contents of the "milk" boiling tube until the end point is reached.

And remember that end point is when the indicator turns from a purply pink colour to a yellow colour.

When this change has happened, then you need to stop the stop clock.

So the lipase will have digested the lipids, the fats, into glycerol and fatty acids.

So, here's a lipid.

It's made up of a glycerol and fatty acids.

The lipase breaks down the bonds between the glycerol and the fatty acids and produces glycerol and fatty-acid molecules.

And remember, the production of fatty acids lowers the pH.

And the end point is reached when the indicator turns yellow.

That's when the pH goes below 7.

2.

Once you have the results at that temperature, change the temperature of your water bath to your next temperature and repeat the process.

Here's a video clip which shows how to carry out the practical, which is worth watching before you actually start it for yourself.

Let's do a check.

True or false? It doesn't matter which pipette is used to measure the lipase solution during this experiment.

And I'd also like you to justify your answer.

Is it A? Because using the same pipette to measure and transfer the lipase solution ensures that it's a fair test.

Or is it B? By not using different pipettes, you may transfer lipase into the milk and the lipase will immediately start to break down fats.

I'll give you a few seconds and then we'll check your answer.

The correct answer is false.

It does matter which pipette's used to measure the lipase solution.

And the reason for that is B, because if you don't use different pipettes, you may transfer lipase into the milk and the lipase will immediately start to break down the fats.

Well done if you got that.

The final task of today is completing that practical.

Here are the steps that you need to follow that I've outlined during this part of the lesson.

The additional materials that are available to you also give you a step-by-step process on how to carry out this practical.

Remember, you need to take care to ensure that you're using the lipase pipette only for the lipase solution.

And you need to stir the contents of your milk using your stirring thermometer until the solution reaches the end point.

That's when the indicator turns yellow.

You need to record the time that's taken in seconds for that end point to happen.

And you should repeat the investigation at different temperatures of water bath.

You should also ideally repeat each experiment at each temperature to make sure your results are reliable and valid.

You could do that by sharing your results with another group.

You'll need to pause the video at this point to carry out the practical.

Once you've got some results, then you can press play and we'll finish today's lesson.

So hopefully, you got some results that look similar to these.

Of course, it will depend on the temperature that you undertook your experiment at.

But hopefully, you've got a table where you've got the temperature of milk, the time taken for the solution to reach its end point when it turns yellow, and hopefully, you've got some repeated results.

So today, we've investigated the effect of changing temperature on the rate of decomposition of milk using the enzyme lipase.

We've seen that enzymes have active sites, which is where reactions take place.

Lipase decomposes milk fats into fatty acids and glycerol.

And as more fatty acids are produced, the pH gets lower, i.

e.

, more acidic.

We've made a hypothesis that's based on our knowledge.

And remember, a hypothesis is a prediction with some additional information about why we know this should happen.

And finally, we've collected data by measuring the end point of a reaction.

And that's based on the colour change of an indicator going from that pink purple to the end point, which is yellow.

I hope that you've enjoyed today's lesson and I hope that you've enjoyed doing the practical.

And I look forward to seeing you again soon.

Bye-bye.