Lesson video

Hello, my name's Dr.

Warren.

I'm so pleased that you can join me for today's lesson on the effect of light on the growth of seedlings practical.

It's part of the Plant Growth and Development Unit.

I'm here to work with you today and support you all the way through this lesson, as especially the tricky parts.

Here are our keywords for today's lesson, quantitative data.

This is data that is collected through taking measurements.

Qualitative data, data that is collected through making observations.

Auxins, plant hormones that affect the growth of plants.

Phototropism, growth response stimulated by light.

Conclusion, a summary of the results of an investigation with data and scientific explanations.

Our learning outcome for today's lesson is, I can measure, record and explain the growth response of seedlings to different light intensities.

In today's lesson, we have three learning cycles.

In the first learning cycle, we're going to look at the investigation and quantitative data.

Then we're going to look at the investigation, qualitative data.

And then finally, we're going to try and draw a conclusion from all the results.

So, let's get started with our first learning cycle on the investigation where we are looking at quantitative data.

Plants gain nutrition through photosynthesis, and this will be a chemical reaction that you are very familiar with by now.

It is when we have carbon dioxide and water stimulated by sunlight to produce glucose and oxygen.

And you'll see from the images that we have on the screen that plants often grow towards the sunlight.

Well, this is not unusual, because plants grow in a way that increase their chances of taking enough light and water for photosynthesis.

These responses are called tropisms. So for example, phototropism, it's growth stimulated by light and that's what we're going to be investigating in today's lesson.

We get positive tropism and this is when the growth goes towards a stimulus and negative tropism is when growth is away from the stimulus.

So, what actually happens when a chute grows? Well, light is detected by cells in the chute tip.

Auxin is then released from the meristems cells in the tip and diffuses evenly throughout the plant chute.

This has the effect that cells elongate and the chute grows up towards the light.

Now, sometimes the light isn't coming directly, down on top of the chute and it causes it to bend towards it.

So, let's have a look and see what happens.

So again, the light this time is coming from the side and it is detected by the cells in the chute tip.

Auxin is released from the meristem cells in the tip and diffuses down the shaded side of the root.

This has results that the cells elongate on the shaded side, causing the chute to grow and bend towards the light.

And that is why we get our plants growing towards the light.

So, in today's lesson, you are going to investigate the effect of light intensity on the growth of newly germinated seedlings.

And to do this, we're gonna set up three Petri dishes.

Like you can see in the image in the Petri dish, we need to put some cotton wool and equal volumes of water.

When we've got our three Petri dishes set up, the next thing we're going to do is place the same number of either mustard or crest seeds into the Petri dish and leave them in a warm place to germinate.

So, once they've germinated, we know that they've germinated, because we'll be able to see a little chute, coming outta those seedlings.

What we'd like you to do is to remove any ungerminated seeds and also ensure that there is the same number of germinated seeds in each Petri dish.

So, look for those seeds that have not got a chute coming outta them and take them out and then distribute them.

So, we've got the same number in each of the dishes.

Then we're gonna place the three Petri dishes in three different lights intensities.

Somewhere in full sunlight.

So, that might be on the window ledge where it's nice and sunny.

And partial sunlight and in darkness.

And for the water and darkness, you may need to put that into a cupboard.

So, in this investigation, the data collected can be both quantitative, such as the height of the seedlings and qualitative.

That is an indication of the overall health or curvature of the seedlings.

So, we will get that by observing them, after they've had a chance to grow.

So, to collect quantitative data, you need to measure the height of the seedlings in millimetres.

And to do this, you'll need to use a ruler.

So, if it's growing up quite straight, you can place the ruler in the Petri dish and measure the height in millimetres.

However, some may be a bit bendy, in which case you might need to use a thread to trace along the stem and then use a ruler to measure the length of the thread instead.

But either way, we need to measure how much they have grown and that is quantitative data.

So, quick check for understanding.

Which of the following is quantitative data, A, the colour of the seedlings.

B, the height of the seedlings, or C, the curvature of the seedlings? Well done if you chose B, the height of the seedlings.

And we can measure that in millimetres.

In investigation, you need to always identify the three main variables.

So, just recap what they are.

First of all, the independent variable.

That's a variable you change or select values for.

The dependent variable, that's a variable that you need to measure or observe to get your results.

And the control variables, the variables that must remain the same, throughout the investigation.

So, you might just want to pause for a moment and note down for this investigation what the independent variable is, what the dependent variable is and what the control variables are.

Okay, so what is the independent variable in this investigation? What have you written down? Was it A, the light intensity, B, the number of days, or C, the height of the seedlings? Well done if you chose A, it's the light intensity.

That is a thing that we are changing.

And we're going to be looking at the strong intensity, the partial intensity and when it is dark and very low intensity.

What are the two control variables in this investigation? Is it A, the light intensity? B, the time interval between height measurements, C, the number of seedlings in each Petri dish, or D, the height of the seedlings.

So, what can we control? Well done if you chose B and C, they are the correct answers.

Now, we come to our first task where you are going to carry out your own investigation.

So, you need to follow the instructions to carry out the practical, complete the results table and draw a graph to show how light intensity affects the height of the seedlings.

So first of all, we carry out the practical, then we complete the results table for each of the Petri dishes.

And finally, we draw a graph.

So, we're going to have a look at some model data together and you can compare this with your own results.

So, here's a sample mean data for the graph plotting.

So, this investigation does take time.

So, over five days, we've been able to measure the mean height of seedlings in each of the light intensity, for full sunlight, for partial sunlight and for darkness.

And you can see some sample results there and you might like to compare that to your own.

And here is an example graph, showing the growth of seedlings in the different light intensities.

And when we're doing this, we can draw each graph on the same set of axes.

On our x-axis, we need to put the number of days, so knot to five.

And then on the y-axis, we're gonna plot the mean height of the seedlings in the millimetres going up from knot to 25.

Remember, always try and fill the whole of the graph paper so we can actually see the graph really well.

Then on the same axes we can plot each set of results.

So for darkness, for partial light and also for sunlight.

And it's always good to do them in slightly different colour or with crosses or dotters just to then give a key as well and label them so we know very clearly which results are which.

So, very well done if your results look like this.

So, we've collected our quantitative data and now we're gonna move on and have a look at our qualitative data.

So, in this investigation, the data collected can both be quantitative, which we've talked about in the previous learning cycle, where we've looked at the height of the seedlings, but it can also be qualitative.

And this is gives us an idea of the overall health or curvature of the seedlings.

And we've got an example here of a scientific line drawing of muscle cells, which shows the qualitative data.

So, in order to record qualitative data, we need to make observations and then draw scientific line drawings.

And it's important when we're drawing scientific line drawings that we produce good line drawings.

And there are some rules for doing that.

And we're just gonna quickly go through the rules now, before you move on to have a look at your own.

So first of all, we want to have a large size.

So as large as we can, the lines are smooth and joined up.

We don't want to see sketchy lines.

They are continuous lines.

We also have very clear labels, so we know each feature of the diagram and we don't have shading.

So, no colouring in parts apart from stippling.

So, here are two examples of drawings that have been done of this cress seedling.

So, here we've got a photo of the cress seedling grown in partial sunlight and just have a look at those two drawings.

One of them is a good example of a line drawing and the other one isn't.

So that top one, well, that's really not a good example.

Why not? Well, first of all, the lines are sketched.

You can see lots of little lines drawn there, so it's very sketchy.

We've got shading and parts coloured in and there are no labels.

So, your diagrams when you do them, should not look like this.

But on the lower example, this is good, because we've first of all got a smooth continuous line.

I'm showing the stem and the leaves.

We've got stippling.

We do not have shading, and it is labelled.

We've got a label of the leaf and the stem.

So, when you come to draw your diagrams, this is the example that we need to follow and use as a model diagram.

So, just a quick check for understanding, scientific line drawings should be a realistic 3D image.

True or false? Well done if you picked false.

Why? They need to be a clear and simple representation.

So, even the worst artists like me who aren't very good at drawing, should be able to do a scientific line drawing, because all we want to do is make a clear representation of what we are observing.

So, for our second task, what we'd like you to do is produce a scientific line drawing for one of the seedlings in each of your light conditions.

So, you could use the examples now shown on the screen or you can use your own, but take one from the full light, one from the partial sunlight and one from the darkness samples and produce your own line diagrams. So, pause the video and have a go at making those sketches and then we will have a look at some examples together.

So, these ones could be used.

So, when we come to draw our own diagrams, hopefully you've drawn something that has smooth continuous lines, like the ones we can see here that we don't have any shading, just stippling and they are labelled.

So, if you remember to do that, very, very well done.

That's really excellent work.

Okay, so that brings us to the end of our second learning cycle.

And now we're gonna have a look at the conclusions to this investigation.

A conclusion is a summary of what has been found out by the end of the investigation.

And to make this conclusion, what we need to do is make careful observations of what we see.

So, we've got our three seedlings here, one from the dark, one from the partial sunlight, and one from the full sunlight.

What could you conclude from these observations of the seedlings? You might want to just note down some ideas of your own, before we look at them together.

Well, writing down a simple statement or sentence about each one.

First of all, I would say for the darkness, the seedling in the dark, turned yellow.

We could also say the seedling in the partial sunlight has grown straight upwards and has small leaves.

The seedling in the full sunlight has larger leaves and has grown towards the direction of the light.

So, when we're making a conclusion, we want to write down some simple sentences just saying or describing what we see or observe.

But that's not just it.

It's a little bit more complicated, because the conclusion describes the relationship, between the independent and the dependent variables, and we need to use the data for it.

So, that's where we can go back to our graph that we drew in our first part of the lesson and see what statements we can make about the growth of the seedlings in different sunlight intensities.

So, here we have our graphs.

So, let's see what we can actually say.

You might again want to try and make some of your own sentences first.

Right, so, one thing we can say, the seedling in the darkness grew the most from two millimetres to 22 millimetres.

And that's following the green line, green dotted line on the graph.

I could also say there was little difference in the growth between the seedlings grown in full or partial sunlight.

And our evidence there is from the red and blue graphs.

So again, we're looking at the data and we're writing statements or sentences describing what that data tells us.

So, a conclusion also explains the findings of the investigation using scientific knowledge.

So, far we've just described what we saw from the data and what we actually physically observed.

Now, we need to try and explain what has happened to find the rest of our conclusion using scientific knowledge.

So, for example, the seedling in the partial sunlight grew straight upwards and had small leaves.

It increased its height steadily from two to 12 millimetres in five days.

That's our description that describes what happens, but why? And the seedling grew upwards towards the light.

That's the final part of the description.

Now we're going to explain it.

Why did that happen? Use your scientific knowledge a bit like this.

Auxin was released in the chute tip and diffused down the stem evenly.

This caused the cells to elongate and the ceiling to grow upwards to absorb the light it needs for photosynthesis.

Which of the following do you not include in a conclusion? A, data, B, scientific explanation, C, method or D, trends.

Well done if you chose C.

When we're doing our conclusion, we don't talk about the method because that's the plan.

What we actually do in the experiment, all the other bits are needed.

Data, scientific explanation, and trends.

So, well done if you got that correct.

So, our final task is around the conclusion.

What we're going to ask you to do is to use the example for partial sunlight.

Write an explanation for the seedling growth in full sunlight and you can use model results.

So, in the minute, I am going to give you an example for partial sunlight.

We'll read through that together.

And then you need to go on, use the model answer and write your own explanation for the seedling growth in full sunlight.

So, when you're writing a conclusion, it's always a good idea to either have a photograph or the actual seedling in front of you, so you can refer to that as well.

So, let's have a look at the model results first for partial sunlight.

The seedling in the partial sunlight grew straight upwards and had small leaves.

It increased its height steadily from two to 12 millimetres in five days.

The seedling grew upwards towards the light.

Auxin was released in the shoot tip and diffused down the stem evenly.

This course cells to elongate and the seedling to grow upwards to absorb the light it needs for photosynthesis.

Okay, now you have a go at writing an answer for the full sunlight seedling.

So remember, when you are writing your model answer, you need to have some results in front of you to do this and suggest you have a picture or the actual seedling so you know what it looks like, but also the data that you've plotted out in the graph, you can use the model results here or your own results.

So, let's have a look at a possible answer.

So, for sunlight, full sunlight, here is a model conclusion.

The seedling in the full sunlight has increased in height steadily from two to 13 millimetres.

It has large leaves and has grown towards the direction of the light.

Auxin was released in the chute tip and diffused down the shaded side of the stem.

This caused the cells to elongate on the shaded side, causing the stem to bend as the seedling grew towards the light needed for photosynthesis.

So, really well done if you've got all of those parts into your answer.

And if not, just correct your answers, so you've got everything included.

Let's move on to question two.

Suggests reasons for the growth of the seedlings grown in the darkness.

So again, look at your seedling grown in the darkness or the photograph and the graph of the results.

Pause the video while you have a go at this question.

Okay, so your suggestions might have included the following.

Seedlings grown in the dark grow taller to try to reach the light needed for photosynthesis.

Without light seedlings produce more auxin which makes their stems grow longer.

Seedlings are yellow because without light, they can't make enough chlorophyll, which gives them their green colour.

The leaves are small, because the seedlings use most of their energy to grow taller instead of making bigger leaves.

So, really well done if you've got all those points included in your answer.

So, this brings us to the end of our lesson on the effect of light, on the growth of seedlings practical.

So, we're just gonna have a little recap of the key learning points.

Cress seeds can be used to investigate the response of newly germinated seedlings to different light intensities.

Plant responses to stimuli, such as light are known as tropisms. After germination the growth of seedlings can be measured in millimetres, providing quantitative data, which could be plotted on a line graph.

Scientific line drawings can be used to record qualitative data, about the overall health or curvature of the seedlings in different light conditions.

Quantitative and qualitative data are used to write a conclusion on the effect of light intensities, on seedling growth.

And this can be explained using our knowledge of auxin action and the process of phototropism.

I hope that you have enjoyed today's lesson and I look forward to learning with you again very soon.