Welcome to this lesson from the Oak National Academy.

Today's lesson is called the requirements and products of photosynthesis, and it's a practical lesson.

It is taken from the unit, photosynthesis, requirements and products.

Hiya, I'm Mrs. Wheate, and I'm gonna be your teacher for today's lesson.

By the end of today's lesson, you'll be able to predict and investigate the presence of starch in the leaves of living plants kept in different conditions, and be able to explain your predictions and observations.

Let's have a look at our keywords.

We have five keywords for today's lesson, and I'm gonna read them out now.

Photosynthesis, the process used by producers to make their own food via a series of chemical reactions.

Glucose, a type of sugar, which is a simple carbohydrate.

Starch, a complex carbohydrate made of many glucose molecules joined together to form a polymer.

Iodine solution, a reagent that can be used to test for the presence of starch.

And prediction, a scientific prediction is a testable statement about a possible outcome which could be tested in an experiment.

So if that wasn't enough time and you wanna read them through again, I'll be quiet for about five seconds, but if that still isn't enough time and you wanna read through them multiple times or maybe copy 'em down, you should pause the video and then click play when you're ready to continue with the lesson.

Today's lesson is in two parts.

In the first part of the lesson, we'll talk about how to test leaves for starch.

So we'll talk through the practical, how it works and what to do.

And then the second part of today's lesson, we'll talk about making predictions, explaining those, making observations and explaining those.

But first of all, testing leaves for starch.

Plants make their own food inside their cells using a process called photosynthesis.

The process photosynthesis uses carbon dioxide and water and it makes glucose, which is a type of sugar and a simple carbohydrate.

Glucose is the plant's food, and what I mean by food is that it uses glucose.

Plants use glucose for growth and repair, and they also use glucose as a fuel for cellular respiration.

The reactants and products of photosynthesis can be summarised like this.

Our reactants are carbon monoxide in water and they react to make glucose, the sugar and oxygen gas.

Those are the products.

I just wanna mention here that this is a summary and that photosynthesis is actually a chemical process so it's made up of lots of different chemical reactions.

And this summary as it's as, yeah, what the word summary means, summarises that whole process.

It's not telling you the whole story, but it is a really, really useful tool to help us understand photosynthesis.

You might be wondering how light fits into photosynthesis, as I didn't mention it in the reactants or in the product.

"In a plant, photosynthesis happens in the cells of the leaves and other green plants of the plant that are above the ground.

And this is because photosynthesis requires energy from a light source such as the sun.

There's no point in the roots of a plant carrying out photosynthesis." The underground, there's no light there.

"Leaves are green because they contain chlorophyll and light transfers energy to chlorophyll, to enable the chemical reactions of photosynthesis to take place." So that's light's relationship to the process of photosynthesis.

It's not reactant, it's not a product.

It transfers energy to the chlorophyll so that photosynthesis can take place.

So I said that glucose was the food for the plant.

Let's talk about what the plant can do with that glucose.

So glucose is a type of sugar, which is a simple carbohydrate represented here by this one purple hexagon.

Some of the glucose molecules from that the plant makes in photosynthesis are used to make larger carbohydrates such as starch.

And that's being represented here using these purple hexagons that are all chemically bonded together.

Starch is stored in cells as starch grains, and in this photo here is from a microscope and it's showing the starch grains inside potato cells.

So you have a potato plant, which is, well, the aboveground part looks just like a plant.

It's green, it's got leaves, it's got a stem, and the potatoes grow underground and that's the plant's store of energy.

Starch is a source of glucose for growth and cellular respiration.

So when the plant stores glucose as starch and when it needs that to break down that starch for glucose to use a cellular respiration, it has it.

It's a long-term storage of energy for the plant.

So this is where the practical part of the lesson comes in.

"It is possible to test leaves for the presence of starch.

This test uses a reagent iodine solution." Iodine is usually an orange brown colour, and when starch is present, iodine turns blue black.

Let's check to see if you understood that.

What colour does iodine solution turn in the presence of starch? Is it A, brown and orange? B, blue, black, or C, is there no change? Take five seconds or if that's not enough time, click pause and click play when you're ready to move on with the lesson.

It is blue, black.

Well done if you got that right.

Brown, orange is what the colour of iodine solution usually is, and there is definitely a change.

So A is wrong for that reason, and C is wrong for that reason.

It wouldn't be a very good test if it didn't change at all.

To test a leaf for starch, you will need a kettle, a large beaker, a heat-proof mat, boiling tube, a leaf, forceps, glass rod, ethanol, a Petri dish, a white tile, a dropper, and some iodine solution.

Let's take a look at the method.

So here is a method for testing a leaf for starch.

You need to wear safety goggles for this practical.

So first thing you need to do is you need to collect a leaf to test.

Then you need to fill a large beaker with hot water, which should be around 90 degrees Celsius from a kettle.

Use the forceps, which are a bit like tweezers to hold the leaf in the hot water for approximately one minute.

This is to stop all the chemical reactions taking place in the leaf 'cause all the enzymes will be denatured at this temperature.

The next thing you need to do, step four, is use the forceps to remove the leaf from the hot water and place it into a boiling tube.

Use the glass rod to carefully push the leaf to the bottom of the tube.

Then the next thing you're gonna do, step five, is to add the ethanol to the tube to cover the leaf.

So really, really important to remember that ethanol is very flammable.

So do not use this in the same room as a naked flame.

So if someone in your room is using a Bunsen burner or whatever, well, they just shouldn't be.

We shouldn't be using any naked flames, any matches, any Bunsen burner anywhere near this practical 'cause Ethanol is so flammable.

So step six, stand the boiling tube in the beaker of hot water for approximately five minutes or until the ethanol has removed the chlorophyll from the leaf.

Step seven, use the forceps to remove the leaf from the ethanol and rinse in cold tap water.

Step eight, place the leaf in a Petri dish on the white tile.

And finally, use the dropper to add ironing solution to cover the leaf.

The white tile is used so that you can see the colour contrast.

There we go, I got it.

In the end easily between what the leaf started as and what the leaf's gonna end up.

So yeah, so it's easier to see the colour of the final test result.

Okay, let's check to see if you understood that.

"Put these steps in the correct order to test a leaf for starch.

A, add ethanol to cover the leaf in a boiling tube.

B, add iodine solution to cover the leaf.

C, hold the leaf in hot water for approximately one minute.

D, stand the boiling tube in a beaker of hot water for approximately five minutes." Take five seconds now to put those in the correct order, or if that's not enough time, click pause and then click play when you're ready to see the answers.

Let's look at the answers.

So first, you should, C, hold the leaf in hot water for approximately one minute and then you need to, A, add ethanol to cover the leaf that's in a boiling tube.

Then you need to, D, stand the boiling tube in a beaker of hot water for approximately five minutes.

And finally you need to, B, add iodine solution to cover the leaf.

Great job if you got that right.

This is the first practise task of today's lesson.

"This plant was allowed to photosynthesize for two days, then one of its leaves was tested with iodine solution.

Number one, which result would you expect to see? A or B? And explain your answer.

Number two suggest why ethanol is used to remove the chlorophyll from the leaf before it's tested with iodine solution.

Number three, describe two safety precautions that should be taken when testing a leaf with iodine solution." You'll need to pause the video now to give yourself enough time to think about your answers and to write it down and then click play when you're ready to see the answers and move on with the lesson.

Good luck.

Let's have a look at the answers.

So in question one, you need to say which result you'd expect to see and explain your answer.

So result B would be the most likely, and this is because photosynthesis produces glucose, which is stored in the leaf cells as starch grains.

Iodine solution turns blue black when starch is present.

Number two, suggest why ethanol is used to remove the chlorophyll from the leaf before it's tested with the iodine solution.

So chlorophyll is a green pigment.

Removing the green colour allows the result of the iodine test to be clearly seen.

Question three, describe two safety precautions that should be taken when testing a leaf with iodine solution.

So you may have said, "Wear safety goggles." You could have also said, "Do not use ethanol in the same room as a naked flame." Great job if you got those right.

We completed the first part of today's lesson.

We've carried out the practical, which is to test a leaf for starch.

Now we're gonna talk about making predictions, observations, and explaining those predictions and observations.

This plant in the picture here has been kept in the dark for two days.

Why is that significant? I want you to take a moment and I want you to think about how being kept in the dark will have affected the plant.

I'll be quiet for five seconds or if that's not enough time for you to think about it, you can pause the video and then you can click play when you're ready to continue with the video.

Okay, let's talk about this plant.

During this time for the two days where there's been in the dark, there's been no light, no photosynthesis will have taken place because there was no light to provide the energy required for photosynthesis to take place.

So no new starch will have been made as no glucose was made by photosynthesis.

The plant will have used up its stores of starch to provide glucose for cellular respiration.

We could say now that the plant has been destarched.

How did you do? Did you get close to that? We can predict the outcome of testing a leaf from this plant for the presence of starch.

A scientific prediction is a testable statement about a possible outcome which can be tested in an experiment.

So let's make a prediction about this plant.

A leaf from a plant kept in the dark for two days will not contain starch.

And this is a testable statement because we can test this by adding iodine to the leaf, and seeing if the iodine remains orange brown or if it turns blue black, which would indicate that there is starch present.

It's good practise to explain our prediction.

So our explanation for this is, "No photosynthesis has taken place so no new starches were made.

Stores of starches been used up to provide glucose for cellular respiration." We can test our prediction using the method for testing a leaf for starch using iron solution and observing the outcome.

So our observation would be, "When tested with iodine solution, the leaf appeared orange brown.

This observation supports our prediction that this leaf would not contain starch." The explanation for that is, "Iodine solution remains an orange brown colour when there is no starch present." Okay, let's check to see if you understood that.

"Who makes a testable scientific prediction? Is it A, Jacob? 'The leaves will not contain any starch.

' B, Sam.

'The iodine stayed an orange brown colour.

' C, Sophia.

'No photosynthesis has taken place.

'" Take five seconds or if that's not enough time, click pause and click play when you're ready to see the answer.

Let's look at the answer.

Jacob is correct.

Our prediction would be the leaves will not contain any starch.

Let's look at the other answers.

Let's look at Sam's.

Sam's sentence is an observation, something that we can see, something we can observe.

The iodine stayed an orange brown colour.

Sophia's sentence is an explanation.

This sentence explains why the leaves weren't continually starched and why the iodine will stay in that orange brown colour.

Well done if you got that right.

Let's talk about another experiment.

We can make some predictions, some observations and explain them.

Three plants are kept in the dark for two days to destarch them.

Then they're kept in the light for two days in different conditions.

So this is destarched plant number one, which has foil strips on the leaves to block out the light.

This is destarched plant number two.

Destarched plant number two is in a jar which has got soda lime in it and soda lime removes the carbon dioxide from the air in the jar.

This is destarched plant number three.

This is a jar as well, but it's not soda lime in it.

It's marble chips in acid and marble chips and acid reacts to make carbon dioxide, and so in this jar, there is more carbon dioxide in the air.

We're going to test leaves from each of the three plants for starch using iodine solution.

So this is a leaf from plant number one.

This is the one that had the foil strip on it.

This is a leaf for plant number two.

This is the one that was in the gel with the soda lime.

And this is a leaf from plant number three.

This is the one that had marble chips in acid in it.

Make sure you don't get the leaf from part number two and the leaf from part number three model 'cause those two will probably look the same.

So these leaves are from a geranium plant and geraniums can come in variegated varieties in a variegated leaf.

Some parts of the leaf have more chlorophyll in others.

You can see that in the picture here.

We've got a darker green part, which has got chlorophyll in it, and then around the edges of the leaf, it's this pale white colour, and so that part doesn't have any chlorophyll in it.

So that's another dimension that we can think about.

Okay, well, what's gonna happen with the iodine test in the parts that have more chlorophyll versus the parts that don't have very much chlorophyll in them at all? So if you are watching this lesson in a group with me, you can pause video now, have a discussion of what kinds of things do you expect to see from our experiment here.

What is the purpose of the soda lime? A, to block out the light.

B, to provide carbon dioxide.

C, to remove carbon dioxide.

Take five seconds.

So if that's not enough time, click pause and click play when you're ready to move on to the lesson and see the answers.

It is C, to remove carbon dioxide.

Great job if you did that right.

Let's look at the other answers.

A to block out the light, that was the foil strip.

That was the purpose of the foil strip from destarched plant number one.

And B, to provide carbon dioxide.

Those were the marble chips that were reacting with acid.

This is the final practise task for today's lesson.

Collect a leaf from each plant.

Number one, predict the outcomes of testing each leaf with iodine solution.

Explain your predictions.

Number two, follow the method on the worksheet to test each leaf with iodine solution and observe what happens.

Number three, do your observations support your predictions? Explain why.

You'll need to pause the video now so you can have a go at those questions and tasks, and then click play when you're ready to see the answers and move on with the lesson.

Good luck.

Let's take a look at some answers.

Number one, predict the outcome of testing each leaf with iodine solution.

Explain your predictions.

So we're gonna talk about leaf.

The leaf from plant one first, which was the plant that had foil strips over part of the leaf.

Prediction? The parts covered with foil will not contain starch and the uncovered parts will contain starch.

An explanation for that could be no photosynthesis will have occurred in the covered part as there was no light so no starch was made.

Leaf from plant two, which is from the jar which had soda lime in it.

Prediction? No parts of leaf will contain any starch.

The explanation for that is no photosynthesis will have occurred because soda lime removed all the carbon dioxide from the air in the jar so no starch was made.

Okay, leaf from plant number three, which was from the jar with the marble chips in the acid.

Prediction.

The white parts will not contain starch and the green parts will contain starch.

No photosynthesis will have occurred in the white part because there is no chlorophyll there so no starch was made.

There is chlorophyll in the green parts and the marble chips in the acid provided a carbon dioxide, so photosynthesis will have occurred and starch was made.

Okay, so you should have tested each leaf with ironing solution and observed what happens.

So this is what it could look like before testing, and this is what you should have seen afterwards.

So this is what leaf from plant number one should have looked like.

And you can see in that strip that was underneath the foil, there was no starch in that specific part, and you can see that the iodine has remained in that orange brown colour.

Okay, and this is the leaf from plant number two.

So in this one, the whole leaf is that orange brown colour, so there was no starch in that leaf.

And this is the leaf from plant number three.

So in the pale part of the leaf, there was no starch in it because we can see that that part of the leaf remained orange brown colour.

All right, number three.

Do your observations support your predictions? Explain why.

So for leaf from plant one, which is the one that had the foil strip in it, the parts covered with foil will not contain starch and the uncovered parts will contain starch.

The observation does support this prediction and the explanation for that is, "No photosynthesis will have occurred in the covered part as there was no light so no starch was made and the ironing solution stayed orange brown.

Photosynthesis occurred in the uncovered parts where there was light so starch was made and the iodine solution turned blue black.

Okay, leaf from plant two.

The prediction, no parts of leaf will contain any starch.

The observation also supports this prediction because no photosynthesis will have occurred because soda lime removed all the carbon dioxide from the air in the jar, so no starch was made and the ironing solution stayed orange brown.

Okay, number three.

Leaf from plant three, which was the leaf that the plant that was in the jar with the marble chips in the acid.

The prediction was, "The white parts will not contain starch and the green parts will contain starch.

This observation also supports prediction." Explanation for that is, "No photosynthesis will have occurred in the white parts because there is no chlorophyll there.

So no starch was made and the iodine solution stayed orange brown.

There is chlorophyll in the green parts, and the marble chips in the acid provided carbon dioxide.

So photosynthesis will have occurred, starch was made and the iodine solution turned blue black." Amazing work if you got that right.

Great job on today's lesson.

Let's summarise what we've learned to help us remember it.

"Some of the glucose made by photosynthesis is converted into starch and stored in starch grains in leaf cells.

The reagent iodine solution can be used to test plant leaf tissue for the presence of starch.

A scientific prediction is a testable statement about a possible outcome which can be tested in an experiment.

We can predict the iodine test result for destarched leaves kept in various conditions.

It is good practise to explain the predictions.

After observing the iodine test results for these leaves, it is good practise to explain the observations." Again, really great work.

I hope you enjoyed the lesson and I hope to see you again soon for our next lesson.