Hello, how are you doing today? I hope that you're doing well.

I'm Mr. Jarvis, and I'm gonna be taking you through today's lesson, which is a practical lesson on the effect of light intensity on the rate of photosynthesis in algae.

Today's lesson comes from the unit, Photosynthesis, Factors Affecting the Rate.

By the end of today's lesson, you should be able to carry out practical investigation into the effect of light intensity on the rate of photosynthesis in algae.

There are five key words to today's lesson.

They are light intensity, rate, variable, control variable, and accurate.

You can see the definitions on the screen now.

You can pause the video if you want to read through them, but I will go through the definitions as we move through the lesson.

Today's lesson is broken down into two parts.

First, we're going to prepare for our practical, and then in the second part of the lesson, we are going to actually complete the practical.

So if you're ready, let's get started and prepare for our practical.

Photosynthesis happens in the cells of plants and other producers when it is light.

The light transfers the energy that's needed for the chemical reactions of photosynthesis to take place.

The reactants and products of photosynthesis are water and carbon dioxide.

The reactants, the molecules that go into the reaction, and glucose and oxygen, the products, the things that come out.

The light transfers the energy for this set of reactions to take place.

We are going to investigate the effect of light intensity on the rate of photosynthesis.

Light intensity is the amount of light that reaches a given surface in a period of time, and rate is how quickly something happens.

So when we talk about the rate of photosynthesis, it's how much product is being formed in any given period of time.

Light is a limiting factor of photosynthesis, and limiting factors are factors which, when in short supply slow down the rate of photosynthesis.

As light intensity increases, IE, the light gets brighter, the rate of photosynthesis increases.

This is until such time as other limiting factors slow the rate of photosynthesis down.

For example, the availability of carbon dioxide or water.

We are going to determine the rate of photosynthesis by measuring pH.

We'll do this using an indicator.

And an indicator is a substance that changes as pH changes.

We are going to use hydrogen carbonate indicator, and as atmospheric concentrations of carbon dioxide, the indicator is cherry red in colour.

As photosynthesis takes place, carbon dioxide is used by the algae, and this makes the pH slightly more alkaline.

And as a result, the indicator turns purple.

When photosynthesis is slow, carbon dioxide is produced by cellular respiration in the algae, and this makes the pH more acidic, and the indicator turns more yellow.

Hydrogen carbonate indicator can be used to measure carbon dioxide levels and therefore the amount of photosynthesis that's taken place.

And you can see here the range of different colours of the indicator at different pH's.

You can see on the left hand side of the chart, increasing carbon dioxide means that carbon dioxide is being added.

There's reduced photosynthesis because the indicators turning yellow or orange.

And at the other end of the scale in the purple, there's decreasing carbon dioxide within the indicator.

And that's because the carbon dioxide is being used by the algae and it turns the indicator purple.

We are going to change the light intensity and measure the effect that it has on the rate of photosynthesis, and we're going to look at various variables, and variables are factors that we need to take into account when we carry out any investigation.

We are going to look at our independent variable.

The independent variable is the thing that we're going to change, and that's light intensity.

The dependent variable is the thing that we measure, and we're going to measure the change in pH during this experiment.

We also have control variables, and these are the variables that we keep the same during the experiment.

And control variables include the time that's given for the algae to photosynthesize, the number of algal beads that are in each jar, the volume and the starting pH of indicator in each jar, and the temperature of the experiment.

All of these things are kept the same, that all control variables.

Here's a check, who best describes the investigation that we're completing today? Andeep says, "We will observe the pH change caused by algae photosynthesizing at different light intensities." Lucas says, "We will measure the change in pH caused by algae photosynthesizing at different light intensities." And finally, Aisha says, "We will measure the volume of carbon dioxide used up by algae in different lighting." I'll pause for a few seconds and then we'll check your answer.

The correct answer is Lucas.

We'll measure the change in pH caused by the algae photosynthesizing at different light intensities.

Well done if you got that right.

So for this practical, you will need the following equipment.

A clear beaker filled with water, some bijou bottles, hydrogen carbonate indicator, a measuring cylinder, some algal beads, graduated teat pipette, an indicator chart, a spatula, some kitchen foil, a one metre ruler, a stop clock, and a light source.

Remember, when you set up any practical, you need to make sure that you stay safe.

You need to take care handling equipment and report any spillages or breakages immediately, and carry out a visual check of electrical appliances, and keep them away from liquids.

You should also wear safety goggles when you're handling chemicals.

You need to make sure you set up your equipment neatly and tidily so that you are well organised.

Here's the equipment we're going to use.

A stop clock, a graduated pipette, some indicator, a spatula, some algal beads, an indicator chart, a one metre ruler, a beaker of water, and a light source.

You are also gonna need some bijou bottles, and a measuring cylinder.

Here's another check.

When setting up the practical to investigate the effect of light intensity on photosynthesis in algae, what safety precautions do you need to consider? A, keep electrical items away from liquids.

B, wear goggles to protect your eyes from chemicals.

C, take care not to scold yourself when using hot liquids.

I'll pause for a few seconds and then we'll check to - See whether you got it right.

The correct answer is A and B.

We need to keep electrical items away from the liquids, and we need to wear goggles to protect our eyes when using chemicals.

Well done if you've got both of those answers.

That brings us to our task.

You need to assemble the practical equipment that's required for this investigation.

This includes safety goggles.

Using a spatula, you need to place 15 algal beads into each bijou bottle.

To each bijou bottle, add five cubic centimetres of hydrogen carbonate indicator using a graduated pipette and measuring cylinder, and then put the lids on the bijou bottle.

You need to record the starting pH using the indicator chart.

Place a lamp on the bench, and then place a large clear beaker of water in front of the lamp to act as a heat sink that will stop the bijou bottles from heating up and having an effect on the rate of reaction.

Use a ruler to position the bijou bottles at distances of 20, 30, 40, 50, and 60 centimetres from the light source.

Finally, you need to wrap a final bijou bottle in kitchen foil so that it's trapped in the dark.

Place that next to the 20 centimetre bijou bottle.

And a reminder, when you're setting everything up, take care handling the equipment, check your electrical appliances, and keep them away from the liquids, and wear those safety goggles when you are using chemicals.

You'll need to pause the video, and then when you've got everything set up and ready, press play and we'll carry on and get ourselves ready to carry out the experiment itself.

Hopefully you've got your practical equipment set up something similar to what you can see on the screen.

You should have a light source with a beaker of water in front of it, acting as a heat sink to stop the bijou bottles from increasing in temperature.

Each bijou bottle should contain 15 algal beads, and they should be placed at 20, 30, 40, 50, and 60 centimetres from your light source.

You should have used your indicator chart to record the starting pH, and you can see that our starting pH is 8.

4.

It's cherry red.

And you should have taken care handling the equipment, and making sure that your electrical appliances are safe to use and away from liquid, and wearing safety goggles to make sure that you don't get any chemicals in your eyes.

In addition, you should have had another bijou bottle that you've wrapped in kitchen foil.

But again, before we did that, we should make sure that we've got a starting pH, and you can see 8.

4 is our starting pH.

We can then wrap it tightly in kitchen foil so no light gets in to the bijou bottle.

And then we place it alongside the 20 centimetre bijou bottle.

That brings us to the second part of today's lesson, which is about completing the practical.

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

Before we start the practical, we need to check the distances between the light source and the bijou bottles.

The distances should be accurate, and we can see we've got our first bijou bottle, and the bijou bottle in kitchen foil at 20 centimetres from the light source.

The next is at 30, the third is at 40, our fourth is at 50, and our final bijou bottle is at 60 centimetres from the light source.

Then turn on the light and start the stop clock.

The experiment needs to run through at least 60 minutes.

That brings us to a check.

Why is a clear beaker of cold water placed between the light source and the bijou bottles containing the algal beads? Is it A, it will maintain the room temperature.

B, it will prevent heat from the lamp from heating the bijou bottles.

Is it C, that it will prevent the light from heating the bijou bottles, or is it D, It'll maintain the same levels of light for all bijou bottles? I'll pause for a few seconds and then we'll come back and check your answer.

The correct answer is B.

It' will prevent heat from the lamp from heating the bijou bottles, so it's acting as a heat sink.

Well done if you got that right.

Carbon dioxide is a reactant to photosynthesis, and as the rate of photosynthesis increases, the algae will use more carbon dioxide from the solution.

And this will cause the pH to become slightly alkaline and the indicator will turn purple.

When photosynthesis is slow, carbon dioxide concentrations increase.

This is because carbon dioxide is produced by the process of cellular respiration, and this will cause the pH to become slightly acidic.

And the indicator will turn yellow or orange.

So carbon dioxide concentrations, our starting cherry red colour is pH 8.

4.

As the algae produce carbon dioxide when they respire, that will increase the carbon dioxide concentration, and a yellow colour indicates that carbon dioxide has been produced.

The algae will use up carbon dioxide as they photosynthesize, so photosynthesize will decrease the carbon dioxide concentration and it will cause the indicator to go purple because the purple colour shows that carbon dioxide has been used up.

Having left our bijou bottles for at least 60 minutes.

Then we can turn off the light.

We can then measure the pH at the end of the experiment using the indicator chart as we've got on the screen.

Here's a check.

Why does the rate of photosynthesis lead to a colour change in the indicator solution? Is it because A, photosynthesis gives out carbon dioxide, which makes solutions more acidic? Is it B, photosynthesis uses carbon dioxide, which increases the pH of the solution, or is it C, the rate of photosynthesis does not result in a change in the colour of the indicator solution.

Again, I'll pause for a few seconds and then we'll check your answer.

The correct answer is B.

Photosynthesis uses carbon dioxide and this increases the pH of the solution, so it gives us an indicator of how much photosynthesis is taking place.

Well done if you got that right.

As well as comparing the pH of all of our bijou bottles that have been in the light, we need also to compare the pH of what we call our control.

That's the bijou bottle that's been wrapped in foil.

It's a control because we've made sure that no light actually gets into the bottle, so the algae will have no light at all to photosynthesize.

So we unwrap the bottle and again, measure the change in pH.

So we need to compare the colours of the bijou bottles with the indicator chart that you can see at the top right hand side of the screen.

We need to record the end pH of each of the bijou bottles.

The 20 centimetre bijou bottle has a pH of 9.

2, and we decide that because the colour of the bijou bottle is the same as our indicator chart that says 9.

2.

Both of them are purple.

At 30 centimetres, the pH is nine, at 40, the pH is 8.

8.

At 50, it's 8.

4.

At 60, it's eight, and the control which has been wrapped in the foil, so as received no light, has a pH of 7.

6.

We then need to work out the change in pH, and we use the following equation to help us do that.

The change in pH is equal to the end pH value minus the start pH value.

So if we look at the first 20 centimetre bijou bottle, our end pH value was 9.

2.

Our bijou bottles all started at the cherry red 8.

4, so it's 9.

2 minus 8.

4, and that gives us a pH change of 0.

8.

You need to record the change in pH in your results table.

Watch the following video clip of the practical being carried out.

Here's a check.

If the indicator changes from cherry red to purple, what does this tell you about the rate of photosynthesis? Is it A, photosynthesis is taking place quickly, B, there is no photosynthesis taking place, or C, photosynthesis is taking place slowly? I'll pause for a few seconds and then we'll check your answer.

The correct answer is A, if it changes from cherry red to purple, it tells us that the pH is increasing, that means that the carbon dioxide is being reduced, and so photosynthesis is taking place quickly.

Well done if you've got that answer.

That brings us to our final task today, which is completing the practical.

You should have your practical set up and ready to go, but before you start, make sure that you've got a results table to record your results in if you haven't done it already.

You also need to make sure that you record the start pH of your bijou bottles.

When you have that done and you're ready, turn on the light, start the stop clock, and leave the bijou bottles in front of the light for at least 60 minutes.

Record the end pH when you've reached that 60 minutes by comparing the indicator solution in each bijou bottle with the pH indicator chart, and record the end pH.

And finally, you need to consider how many times you need to carry out the experiment to give repeatable results.

You might also consider sharing results with other class groups.

You'll need to pause the video to carry out the experiment, and when you've got your results, come back, press play, and we'll finish off today's lesson.

Well, I hope that you got some good results from that experiment.

Your results should be recorded in a table that looks something like this.

You should have the distance from the light source, and you should have the pH at the start of the experiment, and the end of the experiment, and hopefully you've also calculated the change in pH.

You should also made sure that you record the change in pH of your control, and that's the bijou bottle that's had no light enter it at all.

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

We've conducted an experiment to investigate the effect of light intensity on the rate of photosynthesis in algae.

We've considered the variables, the factors that can be changed in this experiment.

We identified the independent variable, the variable that we change, and that's light intensity for this practical.

We've also considered the variable that we measure, which is called the dependent variable, and that's the change in pH.

And we've also considered the control variables for this experiment, those variables that we keep the same.

For example, the temperature, the number of algal beads, and the start pH of each of the bijou bottles.

We've demonstrated the safe and appropriate use of apparatus, and we've accurately measured volumes of indicator, distances from the light source, and time.

We've measured the change in the pH caused by photosynthesis of algae at different light intensities.

I hope that you've enjoyed doing that practical, and I hope that you've got some good results.

Keep hold of the results because you're going to need to do some data analysis, and evaluation of them at a later date.

It's been great, as always, learning with you today, and I look forward to seeing you again soon.

Bye-bye for now.