Hello, I'm Mr. Jarvis.

Thanks for coming along and joining me today for this lesson from the unit, "Photosynthesis, factors affecting the rate." Today we're gonna look at the start of a practical.

We are looking at the effect of light intensity on the rate of photosynthesis in pondweed.

And today's lesson is all about the plan.

By the end of today's lesson, you should be able to plan 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 rate, light intensity, hypothesis, variable and control variable.

The definitions of these words are all on the screen at the moment and you can pause the video if you want to read through them now, but we will go through them as we go through today's lesson.

Today's lesson is broken down into three parts.

First, we're going to look at the rate of photosynthesis, looking at light intensity.

Then we're gonna move on to look at aims, hypotheses and variables.

And then finally, we're going to plan our method for this investigation.

So if you're ready, let's get started with our first section, which is all about the rate of photosynthesis, looking at light intensity.

The word summary for photosynthesis is water and carbon dioxide giving us glucose and oxygen.

Water and carbon dioxide are both reactants, glucose and oxygen are the products.

Photosynthesis also needs light.

However, light is not a substance, it's not a chemical reactant in photosynthesis and it's not turned into a product during photosynthesis.

Light falls onto the leaves, is absorbed by the chlorophyll in the leaves and gives the chlorophyll the energy to drive the reaction of photosynthesis.

Let's do our first check.

Which of the following is a reactant in photosynthesis? Is it A, light, B, carbon dioxide, C, glucose, D, oxygen? I'll pause for a few seconds and then we'll check your answer.

The correct answer is B, carbon dioxide.

Carbon dioxide is a reactant in photosynthesis.

Glucose and oxygen are both products of photosynthesis.

Light is neither a product or a reactant.

So remember, light is not a substance, it's not a reactant, and it's not a product.

It provides the energy that's needed for the chemical reactions of photosynthesis to take place.

Well done if you got that answer correct.

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

And light is a limiting factor of photosynthesis.

A limiting factor is a condition that when in short supply slows down or limits the rate of a reaction.

When there's little or no light, photosynthesis will stop and that's indicated on the graph on the right hand side of the screen.

At very low light intensity, little or no light, the rate of photosynthesis is very low or stops completely.

The rate of photosynthesis will increase as the light intensity increases.

And we can see here that the graph is now showing us that as the light intensity gets brighter, the rate of photosynthesis increases.

The light intensity is the amount of light that reaches a given surface area in a given period of time.

When the light intensity is high, the rate of photosynthesis is usually limited by other limiting factors such as carbon dioxide concentration, temperature, water availability.

And you can see here the arrow on the graph pointing when other limiting factors slow the rate of photosynthesis down.

Farmers can use artificial light in greenhouses and this increases light intensity.

That's the amount of light that reaches a surface in a given period of time, to help produce crops faster.

They can also increase the temperature, the carbon dioxide concentration in the air and provide plants with the water that they need.

This means that the crops can be grown 24 hours a day and at all times of the year.

Here's a check.

Look at the graph.

As light intensity increases, what happens to the rate of photosynthesis? Does it A, decrease, B, increase, C, increase through a maximum amount and then stays the same, or D, increases to a maximum amount and then decreases? I'll pause for a few seconds and then we'll check your answer.

The correct answer is C.

As the light intensity increases, the rate of photosynthesis increases to a maximum amount and then it stays the same.

It plateaus off and the rate doesn't get any quicker.

And remember, that's because other limiting factors are limiting the rate of photosynthesis.

Let's move to our first task of the lesson.

A farmer grows chrysanthemums in a greenhouse to sell flowers to the public.

I'd like you to first of all write a short paragraph to explain how receiving more light could result in the plants producing more flowers per stalk.

And secondly, how else could the farmer increase the yield of flowers? You'll need to pause the video, write down your answer, then when you're ready, press play and we'll see how well you've done.

Good luck.

So I asked you, first of all to write down a short paragraph to explain how receiving more light could result in the plants producing more flowers per stalk.

Well, you might have included in your answer that the higher light intensity results in an increased rate of photosynthesis and more photosynthesis produces more glucose, which can be used to provide the energy needed to produce more growth.

Secondly, I asked you how else the farmer could increase the yield of the flowers, and you might have included increasing one of the other limiting factors such as increasing carbon dioxide concentrations in the atmosphere, increasing the temperature of the greenhouse, or making sure that the plants had water available to them.

You might even have had all three of those answers.

Well done if you got them.

Now let's move to our second part of the lesson, which is all about the aim, hypothesis and variables of this experiment, the effect of light intensity on the rate of photosynthesis in pondweed.

When we're planning a practical, the first thing that you need to decide is what the practical is aiming to investigate.

The aim of the investigation here must be specific, concise and accurate.

For example, for an investigation into the effect of light intensity on the rate of photosynthesis, the aim might be to measure the effect of light intensity on the road to photosynthesis in pondweed.

Scientists usually state what they think will happen in an experiment and why they think it will happen.

This is called a hypothesis, and a hypothesis is an idea, for example, an idea about an outcome.

And it's based on observations and scientific understanding.

So our aim is to investigate the effect of light intensity on the rate of photosynthesis in pondweed.

Can you make a hypothesis, based on this aim, of this investigation? I'll pause for a few seconds.

You might even want to pause the video at this point to write down a hypothesis for yourself.

When you're ready, press play and we'll carry on.

So for your hypothesis, you might have written something like this.

As the light intensity increases, the rate of photosynthesis in the pondweed will increase as the light provides the energy needed for the chemical reactions of photosynthesis to take place.

Having written your hypothesis, we now need to think about how we can increase our confidence in it by carrying out the investigation.

Here's a check.

Which of the following are features of a hypothesis? A, it must always be right.

B, it should be formed using your scientific understanding.

C, it can always be tested by an investigation, or D, is based on observation.

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

The following features are features of a hypothesis.

They should be formed using your scientific understanding, they should be able to be tested by an investigation and they should be based on observations.

Hypotheses don't have to always be right.

Well done if you've got those answers.

When we plan an experiment, it's important to firstly identify the different variables and variables are factors that can be changed during an experiment.

An independent variable is the one variable that we choose to change.

The dependent variable is the factor that we decide we want to measure.

And the control variables are all of the other factors that we want to keep the same.

The dependent variable is the factor that we measure, and a good factor to measure would be one of the products that are produced because that tells us how quickly photosynthesis is taking place.

And we can see that pondweed plants, looking at the picture here, produce oxygen gas and it's produced as a waste product of photosynthesis.

The more the pondweed photosynthesizes, the more oxygen gas will be produced.

And so oxygen volume or bubbles of gas would be a good dependent variable.

It would be a good factor to measure.

The independent variable is the factor that we will change, and we're investigating the effect of light intensity on the rate of photosynthesis.

And light intensity, remember, is the amount of light reaching a given surface in a period of time, and this could be changed by having brighter bulbs or by moving the light source closer to the pondweed.

The control variables are the factors that we keep the same.

Can you think of any variables that you would want to keep the same for this experiment? I'll pause for a few seconds and give you some time to think.

You might have come up with examples such as the time that you collect the gas over, the mass or the size of the pondweed that you use and the temperature of the experiment.

All of these are examples of control variables that we want to keep the same.

The light source may get hot and warm its surroundings, including the pondweed, if it's close by, and this could affect the results because temperature affects the rate of photosynthesis and we want to control temperature as one of the variables.

So by keeping the pondweed in a large beaker of water helps to shield it from temperature changes.

We could also use an LED light bulb, which doesn't get hot.

How will you control other variables in the experiments, such as the amount of time over which you collect the gas, the mass, or the size of the pondweed? Again, I'll pause for a few seconds for you to think.

So let's consider the variables in this experiment.

Our aim is to investigate the effect of light intensity on the rate of photosynthesis in pondweed.

What will your variables be? Well, first of all, our 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 produced.

And the control variables include the time that the gas is collected over, the pondweed mass and the temperature.

Here's a check.

For each factor, identify the correct type of variable in the experiment which investigates the effect of light intensity on photosynthesis.

You'll need to answer A for independent, B for dependent, or C for control.

So let's go through these and I'll pause after each one to give you some time to think.

So here are our factors.

Firstly, the time the gas is collected.

Secondly, the light intensity, and finally, the amount of oxygen gas collected.

So let's do these one by one.

First of all, the time the gas is collected.

Is that A, an independent variable? B, a dependent variable, or C, a control variable? I'll give you five seconds to think of your answer.

The correct answer here is C.

It's a control variable.

We're wanting to keep the time that the gas is collected the same, so it's a control variable.

What about light intensity? Is that independent, dependent or control? Again, five seconds to think about your answer.

Light intensity is A, an independent variable.

It's the thing that we're going to change.

Well done if you got that.

And thirdly, the amount of oxygen collected.

Is that A, independent, B, dependent, or C, control? Again, a few seconds to think about your answer.

The correct answer here is B, it's a dependent variable.

It's the variable that we're going to measure.

Well done if you got those correct.

So let's move to our task.

You're planning the following investigation.

We're investigating the effect of light intensity on the rate of photosynthesis in pondweed.

I'd like you to write an aim for your practical.

I'd like you to identify the variables for this practical and categorise them into independent, dependent and control.

And I'd like you, if you haven't already, write a hypothesis for the practical.

Remember, a hypothesis says what you think will happen and why you think it will happen.

You'll need to pause the video at this point, write down your answers, and then, when you come back we'll check to see how well you've done.

Good luck.

So I asked you to write an aim for the practical.

So the aim would be something like, "To measure the rate of photosynthesis in pondweed at different light intensities." Then I asked you to identify the variables in the practical.

An independent variable, the thing that we change, will be light intensity.

The dependent variable will be the amount of oxygen, O2 gas, produced.

And the control variables will include the temperature of the water that the pondweed's in, the size or mass of the pondweed that we use, the volume of water the pondweed is in, and the time that the oxygen is collected over.

Well done if you got that.

And then thirdly, I asked you to write a hypothesis for the practical.

So here's an example.

As the light intensity increases, the rate of photosynthesis in the pondweed will increase as the light provides the energy for the chemical reactions of photosynthesis to take place.

I hope that you got something similar to that.

That brings us to the third and final part of today's lesson, which is all about planning the method.

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

So we are now going to consider how to carry out the practical and the equipment that we're going to need.

Pondweed produces bubbles of oxygen gas, and this can be collected and measured.

So how will you collect or measure the gas? You're also changing the light intensity in this experiment.

How will you do this? Remember, light intensity is the amount of light reaching a given surface in a period of time.

Have a think about both of those questions.

Here's an exemplar method.

A method is a list of instructions that we follow.

It's a bit like a recipe and it should be written as a numbered list, starting with a verb and being short and succinct.

And you can see I've started my method with a numbered list, one to six.

I've highlighted the verbs in blue and you can see they start my sentences.

So my exemplar method is to cut a 10 centimetre length of pondweed and place it with the cut edge upwards in a beaker of water.

Then, cover the pondweed with an inverted filter funnel.

Raise the funnel off the bottom of the beaker using plasticine.

And that's so that the water mixes so that we're making sure that there's plenty of carbon dioxide in the water available to the pondweed.

Then fill a measuring cylinder with the water and place it over the funnel.

Use a ruler, place the beaker 100 centimetres from a light source.

Turn it on, start the stopwatch and count the number of bubbles released in three minutes and record the volume of gas produced in that same three minutes.

And then repeat the experiment with the light source at 80 centimetres, 60 centimetres, 40 centimetres and 20 centimetre distances.

Here's a check.

How should we write a method? Is it A, listed, started with a verb, B, numbered, starting with a verb or C, numbered, starting with `Then' or `Next?' I'll pause for a few seconds and then we'll check your answer.

The correct answer is B.

A method should be numbered and start sentences with a verb.

Well done if you got that.

The final aspect to consider, when planning an experiment, is the hazards, risks and how to limit them.

A hazard is something that may cause somebody harm, and a risk is the harm that the hazard might cause.

A limitation is an action that's taken to reduce the risk of a hazard.

And it's important to identify the risks of hazards in an experiment and put measures in place to reduce these risks.

For example, if we did an experiment to measure the rate of photosynthesis at different light intensities, the following hazards might be identified and methods put in place to limit their risk.

So a hazard is electricity.

The risk is that we could get an electrical shock.

And so we limit that risk by checking that there's no damage to the cables before using them, and to ensure that we keep electrical items and outlets away from beakers of water.

Glassware is another hazard.

The risk is that we could break it and that could cause us cuts.

So if we are going to limit that risk, we would warn other people that there's a breakage that's occurred and we would either clear up the broken glassware carefully ourselves or get some help to do that as quickly as we possibly could.

Here's another check.

It's importance to minimise the hazards of risk.

Is that true or false? I'm gonna pause for a few seconds.

The correct answer here is false.

Now I'd like you to justify that answer.

Is it because A, a hazard is what can cause harm and a limitation is what reduces the likelihood of risks occurring.

Or is it B, that a risk is what can cause the harm, and a limitation is how much harm that risk can cause.

I'll pause again, let you choose your answer and then we'll check to see whether you got it right.

The correct answer here is A.

A hazard is what can cause harm and a limitation is what reduces the likelihood of that risk occurring.

Well done if you got that.

That brings us to our third task.

You're planning the following investigation.

You're investigating the effect of light intensity on the rate of photosynthesis in pondweed.

I'd like you to do the following.

First of all, write a method for this experiment.

You should write your method as a numbered list.

Start your sentences with a verb and use short succinct sentences.

Then I'd like you to list the apparatus that you'll use.

And then third and finally, I'd like you to identify the hazards and their risks and then plan some appropriate limitations to manage and reduce those risks.

And a table such as the one on the screen, may be something that will help you to write those down.

You'll need to pause the video at this point, write down your answers and then when you're ready, press play and we'll run through what you should have.

So first of all, I asked you to write a method for the experiment.

It remember, should be a numbered list with sentences starting with a verb and short, succinct sentences that are really clear and easy to follow.

So here's an exemplar method for this experiment.

You can also find this in the additional materials if you need some more additional help.

Then I asked you to list the apparatus that you'll use.

What I've planned to use a beaker, a filter funnel, plasticine, a measuring cylinder, a 10 centimetre piece of pondweed, a light source, for example, a bench lamp, a metre rule and a stopwatch.

And then thirdly, I asked you to identify the hazards and their risks and then plan some appropriate limitations to manage and reduce the risks.

You may have got more than me, but I've picked out three hazards, three sets of risks and some limitations.

They include electricity, with the risk being electric shocks.

Glassware causing breakages and warning others and clearing up with the glassware as needed.

And spilling liquids 'cause that could make us slip or fall.

And so again, warning other people that there's a spillage and clearing up those spillages as quickly as we can.

So we've now come to the summary of today's lesson.

We've seen that photosynthesis needs light.

Light is not a substance, product or reactant in photosynthesis.

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

We've seen as light intensity increases, the rate of photosynthesis increases, and we can investigate the effect of light intensity on the rate of photosynthesis in pondweed by planning an experiment.

We can write a hypothesis that we can test.

A hypothesis is an idea about the outcome, which describes how it will change and why it will change.

And then finally, when planning an experiment, we need to identify the variable that we will change, that's the independent variable.

The variable that we will measure, the dependent variable.

And the control variables, those that we keep the same.

I hope that you've enjoyed today's lesson.

I hope that you got a good plan for your experiment and I look forward to seeing you again sometime soon.

Bye-Bye for now.