Lesson video

Hello, my name's Mr. Jarvis, and I'm gonna be taking you through today's lesson from the unit living organisms and their environments.

Today, we're going to be looking at the effect of temperature on the rate of decomposition by an enzyme, and we're gonna be looking at a data analysis and evaluation of the practical.

By the end of today's lesson, you should be able to analyse, explain, and evaluate the results of an experiment investigating how temperature affects the rate of decomposition of milk by an enzyme.

There are five keywords in today's lesson.

They are rate, endpoint, hypothesis, active site, and denature.

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

Today's lesson is broken down into two parts.

First of all, we're going to interpret the data from the practical, looking at the effect of temperature on the rate of decomposition by an enzyme, and then we're gonna move on to explaining the effect of temperature on decay.

You might need a calculator for the first part of the lesson, so you can pause the video at this point and get yourself a calculator if you haven't got one to hand, and then press play and we'll carry on.

So if you're ready, let's get started with our first part of today's lesson, which is all about interpreting the data.

Enzymes are biological catalysts.

They speed up the rate of a chemical reaction.

So here we have an enzyme with a substrate.

The substrate enters the active site and it produces products.

The rate of reaction is the measure of how much change takes place per unit of time.

In the reaction catalysed by the enzyme lipase, fats or lipids are converted into fatty acids and glycerol.

On the left hand side of the screen, you can see a diagram of a lipid.

It has one molecule of glycerol and three fatty acids attached to it.

The lipase breaks down the bonds between the glycerol and the fatty acid to release a single molecule of glycerol and three fatty acids.

The rate of reaction should vary depending on the temperature.

We used the decrease in pH, which was caused by the increase in fatty acids, to determine the endpoint of the reaction.

So as we saw, the number of fatty acids that accumulated made the pH turn more acidic, and that was because the fatty acids built up and the fatty acids are acidic.

So we had the end point of the reaction determined by the change of the colour of the indicator cresol red.

So the milk that was in the cresol red at the start of the reaction was a pinky purple, and over time, the fatty acids built up and that changed the colour to yellow and yellow colour was the end point because the pH had reached below 7.

2.

Here's a check.

Which statement defines the term rate of reaction? Is it A, how long a chemical reaction takes? Is it B, how much substrate is converted into product? Or is it C, how fast a chemical reaction occurs? I'll pause for a few seconds and then we'll check your answer.

The correct answer is C.

It's how fast a chemical reaction occurs.

Well done if you got that.

So as the temperature increases, the kinetic energy of enzymes and substrate molecules increase too, and that causes more collisions between the enzyme and the substrate.

This means that the rate of the reaction increases.

There are more collisions that take place, and so the rate of reaction increases.

There are more reactions taking place.

At higher temperatures, the enzyme's active site changes shape and the enzyme is what we call denatured.

The denaturing of the enzyme means that the active site no longer accepts the substrate molecules, and this means that the reaction doesn't then take place.

So here's the image and you can see a denatured active site on our enzyme molecule, and you can see the substrate no longer fits into that active site.

As a result, the rate of reaction decreases as more and more enzymes are denatured.

So here's a check.

What does the term denature mean? Is it A, when an enzyme is killed? Is it B, when the active site of an enzyme is changed? Or is it C, when the shape of a substrate changes and it no longer fits the active site of the enzyme? Again, I'll pause for a few seconds and then we'll check your answer.

The correct answer here is B, when the active site of an enzyme is changed, that's what we mean by the term denature.

Well done if you got that.

Before carrying out the experiment, you made a hypothesis.

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

It should also suggest why it will happen using your scientific knowledge.

So to remind you, here's the hypothesis that I made at the start of the experiment.

Increasing the temperature will increase the rate of reaction because there are more frequent collisions between the molecules of enzyme and the substrate.

However, above the optimum temperature, the enzyme will denature and the rate of reaction will decrease.

We're going to analyse the data from the experiment that you did with the decomposition of milk and decide whether it increases or decreases your confidence in your hypothesis.

Here are my results from the experiment.

You can see the temperature of the milk and the time taken to reach the endpoint, which was the time that it took for the solution to turn yellow.

The first thing that we're going to do is to calculate the mean time to reach the endpoint at each temperature, and you can use the additional materials to help you by using my example results that are in the table here, or you can use your own at this point.

To calculate the mean, what we do is we take the time for the solution to turn yellow and add them together, and then we divide by the number of samples.

So for the 20 degrees milk temperature, we're going to add 365 seconds to 360 seconds, to 345 seconds, and then divide the total by three, and that gives us 356.

7 seconds.

If we do that for the remainder of the temperatures, we get the following results.

And I hope that you can do that yourself as well with your own results or calculate it using the additional materials table.

You'll notice that the time taken for the solution to turn yellow increased after 40 degrees Celsius.

So what do you think might have happened at 50 degrees Celsius and 60 degrees Celsius? I'll pause for a few seconds for you to think about that.

We can plot the results on a graph.

You can see the table of the temperature of the milk and the mean time taken to reach the end point, the time that it took for the solution to turn yellow.

You can use the additional materials to plot your graph.

There's some graph paper there if you need it.

The first thing that we need to do is to draw our axes and label them with what we are plotting on each axis.

Remember, we always put the thing that we control along the x-axis and the thing that we measure along the y-axis, along the side.

So we have put the mean time taken on the y-axis and the temperature of the milk on the x-axis, and you can see that on the screen now.

We also need to remember to go up in equal units as we plot our graph, and you can see that I've gone up in 50s for the mean time taken and tens for the temperature.

So let's plot our individual points.

20 degrees milk temperature had 356.

7 as the mean time taken to reach the endpoint.

The 30 degrees was 63 seconds, the 40 degrees 17.

7, the 50 degrees was 108.

3, and 60 degrees was 310.

So we've now plotted our graph of the results.

What we now need to do is to draw a line of best fit and we need to draw a nice smooth line that matches the points as best as we can.

So here's my line of best fit.

You can do that with your own results when we get to the task in a second.

Scientists convert the time taken into rate and they do this using the calculation rate is equal to one divided by the time taken or t.

And we can do this with the data from this experiment.

You can use your own data or use the example data in the additional materials.

Here is a table of the results, the temperature of the milk, the mean time taken to reach the endpoint in seconds, and we've got a new column, rate of reaction, one divided by t, the time taken.

So we can calculate this.

The first 20 degrees Celsius temperature of milk will be one divided by 356.

7, and that gives us 0.

003 as our rate of reaction.

We can do this for the rest of the results, one divided by the time taken, and you can see the results are on the screen now.

Here's a check.

Which of these graphs is drawn correctly and what is incorrect about the other graph? You may need to pause the video at this point to spot what's incorrect about the wrong graph on this screen, but if you do that, just press play when you're ready and we'll check your answer.

Good luck.

So the correct graph here is B, and the reason why it's the correct graph is we've got the variable that we controlled along the bottom, the temperature, and the variable we measured, the rate of reaction, along the y-axis.

Why was A not a good graph? Well, first of all, we've got our axes the wrong way round.

We've got the thing that we measured, the variable that we measured, along the x-axis and the variable we controlled along the y-axis, which is the wrong way around.

And also if you look at the scale for the rate of reaction, we haven't gone up in equal unit increments.

We've gone from 50 to 100 to 150, then 250 and 300.

It should have been 200 and 250.

So well done if you spotted both of errors with the wrong graph there, which was graph A.

Let's move to a task and using your data or the data provided in the additional materials, calculate the rate of reaction using the equation.

And then I'd like you to plot a graph of the rate of reaction against temperature and to draw a smooth line of best fit.

And remember, to be successful with your graph, you need to make sure that your axes are plotted and labelled correctly, the points are plotted accurately, and there's a smooth line of best fit.

And then the third thing I'd like you to do is to consider your hypothesis or the exemplar hypothesis that's provided in the additional materials.

Do your results increase or decrease your confidence in that hypothesis? I'd like you to explain why and how you could improve your confidence further.

You'll need to pause the video at this point, write down and calculate your answers.

Remember, you can use a calculator if you need to.

And then when you're ready, press play and we'll see how well you've done.

Good luck.

So how did you find that? I hope it wasn't too tricky because there was quite a lot there for you to do.

The first task was to calculate the rate of reaction using the equation rate is equal to one divided by the time taken or one divided by t, and your results should have been calculated something like this.

If you used the exemplar data, then these should be your answers.

Then I asked you to plot a graph of rate of reaction against temperature and to draw a smooth line of best fit.

So your graph should have had the axes correctly plotted and labelled.

So temperature along the bottom because that's the variable that we controlled, and rate of reaction along the y-axis along the side because that's the variable that we've measured.

And you can see I've put rate of reaction one divided by t.

You should have also made sure that your axes go up in equal steps.

So you can see the temperature is going up in 10 degrees C at the time and the rate of reaction in 0.

02 increments.

Well done if you did that.

Then you needed to plot your points accurately.

Here are my results that I plotted on the graph.

And then finally, drawing a smooth line of best fit, and here's my line of best fit that I've drawn through the points on my graph.

Well done if yours looks something like that.

And then thirdly, I asked you to consider your hypothesis.

Do your results increase or decrease your confidence in that hypothesis and explain why and how you could improve your confidence further.

So your answer might have included that your results support your hypothesis and they increase your confidence in your hypothesis because the rate is fastest at the optimum temperature.

And as the temperature increases, the number of collisions between the substrate and the active site increase too.

However, as temperature moves away from the optimum, the rate decreases.

At temperatures above the optimum, the enzyme denatures.

So to increase your confidence further, you could repeat the experiment using different temperatures.

For example, increasing the number of intervals between the range of 30 degrees and 50 degrees to determine what the optimum temperature is.

Well done if you've got some or all of those answers or got some additional points for yourself.

That brings us to the second part of the lesson, which is all about explaining the effect of temperature on the rate of decay.

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

The experiment shows how temperature affects the rate of decomposition of milk by an enzyme, and enzymes are formed by a chain of amino acids.

They're proteins and they're folded into a specific shape.

And you can see the amino acids in a chain folded in a specific shape as we zoom in on a small part of the enzyme and that gives the enzyme its particular shape and the shape of the active site.

There are bonds between the amino acids and that holds the protein chains into a shape that keeps the active site in a really stable way.

Maintaining the shape of the enzyme is really, really important because the active site of the enzyme is where the chemical reactions are catalysed.

Only one substrate often will fit the active site because it has that specific shape that fits into the active site as we can see on the screen here.

The enzyme will then break down the substrate and make products.

Many of the bonds that maintain the enzyme's shape are affected by temperature.

When temperatures are low, there are less collisions.

The enzyme works really slowly as a result, and we can see this by our graph and by the slow nature of the rate of reaction.

When all of the enzyme active sites are full, it's working as fast as it can.

The active site is still the same shape.

And as soon as a product is formed and released from the active site, another substrate collides with the active site and another reaction takes place.

It's going as quickly as it can, and that's our optimum temperature, and that's the point which the rate of the reaction is fastest, and that's seen by the arrow that's pointing to the fastest rate of the reaction, which is the peak of our graph.

At high temperatures, the enzyme shape starts to change and we can see that the protein starts to change its shape as the bonds are broken.

The enzyme is what we call denatured, and that means that it reduces the enzyme's ability to catalyse the reaction, and so the rate of reaction starts to decrease.

And you can see the active site here in the green circle is not now the same shape as it was at the optimum, so that enzyme is not able to catalyse the reaction.

Here's a check.

Whose explanation of the effect of temperature on enzyme activity is correct and who is incorrect? Laura says, "The enzyme's active site changes at high temperature because bonds break." Is that correct or incorrect? Izzy says, "At the optimum temperature, the enzyme works really fast because the active site is always full of substrate molecules." Is Izzy correct or incorrect? And Jun says, "The rate is slow at low temperatures because there are not as many collisions between the enzyme and the substrate." Is that correct or is it incorrect? If you need some time to think about it, you can pause the video and then press play when you're ready to go.

So Laura, is she correct or incorrect? The enzyme's active site changes shape at high temperatures because bonds break.

That is correct.

What about Izzy? At the optimum temperature, the enzyme works really fast because the active site is always full of substrate molecules.

Izzy is correct too.

And Jun, the rate is slow at low temperatures because there are not many collisions between the enzyme and the substrate.

Jun is also correct.

Well done if you've got all three of those.

Decomposition is the breakdown of organic matter by decomposers.

And sometimes we call decomposition decay.

Decomposers such as bacteria and fungi secrete enzymes to break down organic matter and they then absorb the smaller products to use as food.

As this process continues, the organic matter starts to decay or decompose.

And often we can see that with food.

Here's some bread that's got some mould growing on it.

That mould, that fungi and bacteria that we can't see are secreting enzymes onto the bread and then absorbing the products of digestion into their bodies to help them grow and reproduce.

Decomposers are really important because they're the cleaners of the natural world.

When organisms die or produce waste, it's the decomposers that break down all of the organic matter and recycle elements for other organisms to use.

Just think about what the planet would look like without decomposers.

Humans try to prevent food from decomposing, and we do this by keeping it cold or frozen, by eating dry foods or storing foods in dry places, or from preventing oxygen from getting into the food.

This is because decomposition increases as we increase the temperature.

Decomposition also increases as moisture is available, and decomposition often needs oxygen.

In the natural world, organisms decay at different rates, and this depends on the conditions.

In the Arctic, it can take years for the body of an organism to decompose, whereas in a rainforest, decomposition takes place really rapidly.

Can you think of reasons why that might be? In which of the following ecosystems do you think the rate of decomposition will be fastest? Is it the desert, which is very hot in the day and very cold at night but dry? The tundra, cold with very little precipitation, so very little rain? Or is it woodland, which has cool but damp conditions? I'll pause for a few seconds and we'll check your answer.

The correct answer is C, the woodland.

And the reason for this is that the temperature isn't too high, but it's also damp, so there's plenty of moisture that's available.

And remember, moisture and availability of moisture is one of the factors that help decomposition to take place quickly.

Well done if you got that.

Here's our second and final task for today.

I'd like you to think about the decomposition of organic matter, that's dead organisms and their waste, in different ecosystems. In what type of ecosystems will the rate of decomposition be fast? I'd like you to explain why, and I'd like you to link your answer to enzyme activity.

You'll need to pause the video at this point, write down an answer, and then when you're ready, press play and we'll see how well you've done.

Good luck.

How did you get on? Well, I asked you, in what ecosystems would the rate of decomposition be fast? And I'd ask you to explain why that was the case.

You also needed to link your answer to enzyme activity.

So your answer might have included, for example, an ecosystem such as a tropical rainforest or another ecosystem that's warm and damp, i.

e.

the temperature is relatively high and there's plenty of moisture and oxygen available.

Decomposition will take place rapidly as the temperature is warm, and that means that enzymes and substrates will move more quickly and there'll be more collisions taking place.

That will mean that the rate of reaction of breaking down the organic matter will take place more quickly.

Water is important as enzymes are secreted by decomposers onto organic matter, and the absorption of products also requires water.

Damp ecosystems help this to happen quickly.

And finally, organisms tend to reproduce more quickly when the temperature is warmer.

Well done if you got that.

That brings us to the summary of today's lesson.

We've seen that the rate of reaction can be calculated by dividing one by the time taken to reach the endpoint of the reaction, and that rate of reaction can be plotted on a graph against temperature, and that can be used to consider whether the data increases or decreases our confidence in our original hypothesis.

We've seen that enzyme reactions take place more quickly at what's called the optimum temperature, and that's because all of the active sites are full.

And at temperatures above the optimum temperature, the active site becomes denatured as the enzyme bonds are broken.

Decomposition in the natural world helps to recycle elements and to keep the environment clean.

In warmer damper ecosystems, decomposition takes place at a faster rate.

And remember, decomposition is really important as it helps to recycle those elements, and it helps to break down all that organic waste that might build up otherwise in our ecosystem.

It's been great learning with you once again.

I hope that you've enjoyed today's lesson and I look forward to seeing you again soon.

Bye-bye.