Hello.

How's your day going? Hope it's going okay.

I'm Mr. Jarvis, and I'm here to take you through today's lesson from the unit photosynthesis factors affecting the rate.

Today we're going to be looking at the effect of light intensity on the rate of photosynthesis in pondweed, and we're going to be looking at a practical lesson.

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

There are five key words to today's lesson.

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

You can read through the definitions that are on the screen now, and pause the video if you need a bit more time to do so, but we'll go through the definitions of each of these words as we go through the lesson.

Today's lesson is broken down into two parts.

First of all, we're going to be looking at preparing our practical, and then in the second part of the lesson, we're gonna be actually completing the practical itself.

So if you're ready, let's get started with our first section, which is all about preparing our practical.

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

And light transfers the energy needed for the chemical reactions of photosynthesis to take place.

The reactants and products of photosynthesis are water and carbon dioxide, giving us glucose and oxygen.

Water and carbon dioxide are the reactants, glucose and oxygen, our products, and light transfers the energy for the reactants to be turned into products.

We're going to investigate the effect of light intensity on the rate of photosynthesis in pondweed.

Light intensity is the amount of light that reaches a given surface in a specific period of time.

The higher the light intensity, the more light reaches the surface.

The rate of photosynthesis or any other chemical reaction is how quickly it takes place.

So the higher the rate of photosynthesis, the more photosynthesis is happening and the more products are being formed.

Light is a limiting factor of photosynthesis.

And a limiting factor is something that when it's in short supply, slows down the rate of a reaction.

As the light intensity increases, the rate of photosynthesis increases until the rate is limited by other limiting factors.

And you can see on the graph that the rate increases at a steady rate until such time as other limiting factors slow down the rate of photosynthesis.

To determine the rate of photosynthesis, we will measure the volume of oxygen by pondweed.

And you can see in the picture on the screen there are small bubbles of oxygen gas are attached to the leaves of the pondweed plant.

We would expect that the volume of oxygen will increase as the light intensity increases, and that's because light transfers the energy that's needed for the chemical reactions of photosynthesis to take place.

We're going to change the light intensity and measure the effect this has on the rate of photosynthesis.

And our variables will be the independent variable.

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

The thing that we're going to measure is the amount of oxygen gas produced.

We will also have control variables, and these are the variables that are kept the same during the experiment.

These will include the time that we collect the gas over.

The temperature of the experiment, and the mass or the size of the pondweed that we use.

Here's a check.

Who best describes the investigation that we're completing today? Andeep says, "We will observe the volume of oxygen gas produced by pondweed at different light intensities." Lucas says, "We will measure the amount of oxygen gas produced by pondweed at different light intensities." And Aisha says, "We will measure the volume of gas produced by pondweed in indifferent lighting." I'll pause for a few seconds for you to select your answer, and then we'll check to see whether you got it right.

The correct answer here is Lucas.

He has the best description of the investigation that we're carrying out today.

We're going to measure the amount of oxygen gas that's produced by pondweed at different light intensities.

Well done if you got that right.

So for this practical, you're going to need the following equipment.

You're going to need a measuring cylinder.

You're going to need a beaker filled with water.

You're going to need 10 centimetre length of pondweed.

You're going to need a metre ruler.

A light source.

A stop clock.

Some plasticine and a funnel.

When you're setting up the experiment, we need to take care handling the equipment, and you need to check that the electrical appliances are safe to use, and to keep any electrical appliances away from liquids, such as the water that we're using in this experiment.

We need to set up our equipment and materials neatly and carefully.

So here we have our light source.

We've got 10 centimetres of pondweed.

We've got some plasticine in the bottom of the beaker holding the funnel up.

We've got a stop clock.

You can see we've measured carefully the distance between the pondweed and the light source.

We've got our one metre ruler, we've got an inverted filter funnel, and we've got a beaker of water.

And finally, we've got an inverted measuring cylinder.

Here's another check.

When setting up the practical to investigate the effect of light intensity on photosynthesis in pondweed, what safety precautions do you need to consider? A.

Keep electrical items away from liquids.

B.

Wear goggles to protect your eyes from chemicals.

Or C.

To take care not to scald yourself when using hot liquids.

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

The correct answer is A.

We need to keep electrical items away from liquids.

Well done if you got that.

So now we're going to move to our first task.

You need to now set your equipment up.

So I'd like you to set up your practical equipment for this investigation.

You need to cut a 10 centimetre length of pondweed and place it with the cut edge upwards in a beaker of water.

You need to cover that pondweed with an inverted funnel, which is raised off the bottom of the beaker with plasticine, and it's raised up off the bottom of the beaker so that carbon dioxide can mix throughout the whole of the water within the beaker.

You then need to fill a measuring cylinder with water and carefully place it over the funnel.

And then you need to use a ruler to position the pondweed beaker 100 centimetres from the light source.

While you're setting this equipment up, take care handling the equipment.

If you spill any liquids, make sure you clean them up and prevent anybody from slipping on them, and make sure you check your electrical appliances and keep them well away from liquids.

You'll need to pause the video to set that equipment up.

When you've got it all set up, then you can press play and we'll carry on with the lesson.

So you should have assembled your equipment so that it looks something like this.

You should have a light source that's 100 centimetres from the pondweed.

The pondweed should be with its cut edge upwards inside an inverted funnel, held up off the bottom of the beaker by some plasticine.

You then should have an inverted measuring cylinder that is full with water over the top of the funnel so that it enables you to collect the gas.

And remember, when we carry out the experiment, take care handling the equipment, mop up any spillages, and make sure that the electrical appliances are kept away from liquids.

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

So now we're all set up with the equipment.

Let's carry on.

As photosynthesis takes place, the pondweed will produce oxygen.

And we can see in the image on the side of the screen, here's some pondweed plant with its cut edge pointing upwards, and we can see that as it's photosynthesizing, it's producing bubbles of oxygen.

Oxygen is a waste product of photosynthesis.

Counting the number of bubbles is one method of measuring the volume of oxygen, but it may not give accurate, which means close to the true value results.

Can you think why counting the number of bubbles may not give you accurate results? I'll pause for a few seconds to let you just look at the clip and observe it carefully.

And then we're gonna do a check.

So here is that check.

Why does counting bubbles not give accurate volumes of gas produced by pondweed during photosynthesis? Is it A.

Some of the bubbles might escape.

B.

Bubbles may be of different sizes and volumes.

Or C.

It may be difficult to keep count of the number of bubbles.

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

The correct answer is B.

The bubbles may be of different sizes and volumes.

When you looked at that clip, there were some really small bubbles being produced by the pondweed and some much bigger bubbles.

And in some cases, smaller bubbles combined to make a bigger bubble.

So counting the number of bubbles is one way of measuring the volume of gas that's produced, but it may not be accurate.

Well done if you got that answer.

A more accurate way of measuring the volume of gas produced is to use that inverted measuring cylinder.

And the measuring cylinder reduces the error of counting bubbles of different sizes and allows actual volumes of gas to be measured.

And how that works is that as oxygen is produced, it displaces the water.

And that allows us to use the scale on the measuring cylinder to calculate how much gas has been produced.

So we're now ready to carry out our experiment.

First of all, you need to check the distance between the light and the pondweed.

So we're taking the distance from where the light source is to where the pondweed is.

And in this case, we're going to start with 100 centimetres as our first distance.

We need to record the initial volume of gas in the measuring cylinder by using the scale on the side of the measuring cylinder.

And then when we've got that recorded, we can turn on the light, start the stop clock immediately, and then start counting the number of bubbles produced by the pondweed in three minutes.

When we've reached three minutes, we need to stop counting the bubbles.

And we then also need to measure the final volume of gas in the measuring cylinder.

We can turn our light off at this point and make sure that we've recorded all our results in a results table.

To calculate the volume of gas that's produced in three minutes, we first of all need to take the initial volume, the one that we measured at the start of the experiment, so in this case, 0.

1 cubic centimetres.

And we need to measure the final volume, which in this case was 0.

3 cubic centimetres.

And we work out the volume, and we do this by saying the final volume minus the initial volume gives us the volume of gas.

So 0.

3 minus 0.

1 gives us 0.

2 cubic centimetres.

We need to record the volume of gas produced in our results table.

So here's a short video that shows you how to carry out this practical.

(no audio) (no audio) (no audio) (no audio) (no audio) So here's a check.

True or false? We should record the volume of gas produced at the end of three minutes using a measuring cylinder.

Is that true or is it false? I'll pause for a few seconds for you to work out the answer.

The correct answer is that it's true.

Now, you need to justify your answer.

Is the best answer, A.

This measures the gas produced more accurately.

Or B.

This demonstrates that we can be safe and reliable in the laboratory.

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

The correct answer here is A it's true because it means that we measure the gas that's produced more accurately.

Well done if you got that.

That brings us to our task, and the task is to carry out the practical itself.

You should have your equipment set up and ready to go.

And hopefully, you've watched the video of the practical to give you an idea of how to carry out the practical safely.

You should have a results table to record your results in.

If you haven't, that's your next step.

Design a results table to record those results in.

Then when you're ready, turn on the light and immediately start the stop clock.

Don't forget to record the initial volume of gas in the measuring cylinder.

You need to then count the number of bubbles that are released by the pondweed in three minutes, and record the number of bubbles in your results table.

You also need to record the final volume of gas in the measuring cylinder, so that you can then calculate the volume of gas that's produced in three minutes.

And remember, you do that using the equation final volume minus initial volume is equal to the volume of gas produced.

Then you need to reset your apparatus and move the light source so that it's 80 centimetres from the pondweed, and then repeat steps two to five that are on the screen.

You repeat the process at distances of 60, 40 and 20 centimetres.

And you'll also need to consider how many times you need to carry out the experiment at each light intensity, the distance that you are from your light source, so that you get repeatable results.

You'll need to pause the video, carry out the practical, and then when you're ready, come back and we'll check to see how well you've done.

I hope that you got on okay with that practical.

And I hope that you got some results, both the number of bubbles and the volume of gas that was produced.

Your results table probably looks something similar to this.

The volume and the number of bubbles will obviously be very different, but hopefully, you've recorded the distance from the light source and you've recorded the number of bubbles of gas produced in three minutes, and the volume of gas produced in three minutes.

And hopefully, you've done the experiment more than once so that you've demonstrated that it can be repeatable.

I've got some data here that shows that I've carried out the experiment three times.

That's brought us to the end of today's lesson, but make sure you keep those results safe because you're going to need to do some data analysis and evaluation of your results at a later date.

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 pondweed.

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

We identified that the independent variable, the factor that we changed, was light intensity.

We identified the dependent variable, the factor that we measured, which was the volume of oxygen gas produced.

And we've identified some control variables.

For example, the size of the piece of pondweed that we used and the temperature of the experiment.

We've demonstrated safe and appropriate use of our apparatus, and we've accurately measured the distances, time, and recorded carefully volumes of gas produced by pondweed at different light intensities.

I hope that you've enjoyed doing that practical today, and I hope that you enjoyed the lesson, and I look forward to seeing you again very soon.

Take care for now.

Bye-bye.