Lesson video

This lesson is called "The effects of environmental factors on the rate of water uptake by a plant" and is from the unit, "Transport and exchange surfaces in plants." Hi there, my name's Mrs. McCready and I'm here to guide you through today's lesson.

So thank you very much for joining me today.

In our lesson today, we're going to explain the effects of environmental factors on the rate of water uptake by a leafy shoot.

And in our lesson today, we're going to come across a number of keywords.

They're listed up here on the screen for you now.

You may wish to pause the video and make a note of them, but I will introduce them to you as we come across them.

Now, in our lesson today, we're going to first of all have a look at the effect of light intensity on water uptake before we consider the effects of wind speed and temperature on the rate of water uptake.

So are you ready to go? I certainly am.

Let's get started.

Now, the rate of photosynthesis is dependent on a number of different factors, including temperature and light intensity.

The higher the light intensity, the higher the rate of photosynthesis.

Now, in photosynthesis as the rate increases, so does the requirement for water, because water is an essential substrate for the process of photosynthesis, where it is combined with carbon dioxide to form glucose with waste oxygen.

So as the rate of photosynthesis increases, so does the requirement for water.

Now, when it is dark, photosynthesis does not occur because there's no light and because photosynthesis isn't occurring, the stomata in the leaf can close.

Now, the stomata are the holes in the leaf and they are formed by cells called guard cells.

And you can see them in the diagram there, these sort of sausage-shaped cells surrounding the hole in the middle of the stoma.

One stoma, several stomata.

So the guard cells close and that closes the hole, the stoma, and that occurs when it is dark, when no photosynthesis is taking place because the hole is then closed, no gaseous exchange can occur, no carbon dioxide can enter the leaf, no oxygen can leave the leaf.

And also no water vapour can leave the leaf through the stomata by transpiration either.

Because the holes are closed.

Now, in the light, when the plant is photosynthesizing, the plant requires carbon dioxide from the atmosphere.

And so the stomata open wide to allow carbon dioxide to enter the plant and for waste oxygen to leave.

And this occurs because the guard cells swell up and that forces their central part apart, causing the hole the stoma in the middle to get larger.

So the guard cells swell up causing the hole, and that opens the stomata in the leaves allowing gases exchange to occur.

Now when this occurs, this increases the rate of transpiration as well because as well as gases exchange happening between carbon dioxide and oxygen for photosynthesis, water can also leave by transpiration through the holes in the leaves, the stomata in the leaves as well.

So you can see how increasing the rate of photosynthesis will increase the rate of transpiration and water loss.

So true or false? The rate of photosynthesis influences the rate of transpiration.

So you should have said that that is true.

But why? Okay, so you should have explained this because as the rate of photosynthesis increases, the demand for carbon dioxide increases.

This means that the stomata open wide to allow carbon dioxide in, which allows water out, so the rate of transpiration increases.

Okay, now let's have a look at some data.

So here's some experimental data of the volume of water uptake at different light intensities in a leafy shoot.

The orange line is when the light is only 10 centimetres away from the leaf, the blue line is a distance of 20 centimetres, the green line is a distance of 30 centimetres, and the red line a distance of 40 centimetres.

So what I'd like you to do is take a look at that graph in a bit more detail and consider the trends that are present within that graph as the distances between the lamp and the plant increase.

And therefore, the light intensity changes.

Remember, the light intensity will be greatest when the lamp is closest at 10 centimetres and the light intensity will be dimmest at the greatest distance of 40 centimetres.

So what trends can you identify from this graph? So let's have a look.

How would you finish this statement? From the graph, it can be concluded that as distance from the lamp increases, a, the volume of water uptake increases, b, the volume of water uptake decreases, or c, the volume of water uptake stays the same.

I'll give you five seconds to consider that.

Okay, so you should have said that as the distance from the lamp increases, the volume of water uptake decreases, this is an inversely proportional relationship.

As the distance goes up, the water volume uptake goes down.

And what about this? From the graph, it can be concluded that as distance from the lamp doubles, is it, a, the volume of water uptake doubles, b, the volume of water uptake halves, or c, the volume of water uptake quarters.

So have a good look at that graph and I'll give you a few seconds to decide.

Okay, so you should have concluded that as the distance from the lamp doubles, the volume of water uptake quarters.

Excellent, well done.

And what about this? As the light intensity increases, the rate of water uptake increases because, a, water is used in photosynthesis, b, water is lost through transpiration, or c, water is made in respiration.

I'll give you five seconds to consider.

Okay, so you should have said that as light intensity increases, the rate of water uptake increases because, a, water is used in photosynthesis and b, water is lost through transpiration, c is also correct as a statement but doesn't answer the question.

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

So what I'd like you to do now is to firstly describe the trends in the data, using the graph to support your answer.

By that I mean, you can use the data from the graph within your answer, actually quote from the graph, use the numbers and the values there.

And then I would like you to explain why these trends occur using ideas about photosynthesis, transpiration, and stomata in your answer.

So pause the video and come back to me when you are ready.

Okay, let's see how you got on.

So firstly, I asked you to describe the trends and the data.

So you might have included that as the distance between the lamp and the plant doubles, the volume of water taken up by the plant quarters.

So then you may well have included data from the graph itself, for instance, at 10 centimetres, the volume of water uptaken is nearly eight centimetres cubed in 30 minutes, compared to only two centimetres cubed at 20 centimetres.

This reduces to a quarter again at 40 centimetres to 0.

5 centimetres cubed.

And so you can see that we've got, using the data there, a doubling of the distance and a quartering of the water uptake, using the data from the graph is really valuable in this instance.

Then I asked you to explain why these trends occur.

So you might have included that, when it is light, the plant is photosynthesizing and the stomata in the leaf are wide open.

This is important because it allows carbon dioxide to move into the leaf, which is then used as a substrate in photosynthesis, and any waste oxygen can leave the leaf.

Now, the wide stomata as well as allowing gaseous exchange for photosynthesis also allows water vapour to move outta the leaf in the process called transpiration.

And as the rate of photosynthesis increases, the rate of water uptake increases, and as more water is used in photosynthesis, it is also lost through transpiration.

So check over your work and make sure you've got all of those salient points and well done.

Okay, let's move on to looking at the effects of wind speed and temperature on the rate of water uptake by a plant.

So transpiration is this process of losing water vapour from leaves mainly by losing it through open stomata.

And we have seen how increasing the light intensity causes an increase in the rate of transpiration, and this is an indirect cause.

But nevertheless, as we increase light intensity, we increase the rate of transpiration.

However, what do you think the effect of increasing wind speed or temperature will have on the rate of transpiration? Well, let's look at that in a little bit more detail.

So what I'd like you to think about when considering your answers to these questions is what happens when you dry your hair with a hair dryer or you dry your clothes on the line on a warm, windy day? So just think about those actual practical scenarios for a moment and then we're gonna apply them to the scenario of a plant.

Okay, so let's go on and have a look at this in more detail.

So what happens when you dry your hair with a hair dryer? Well, it dries faster with a hair dryer than without, for sure.

What about if you dry your clothes on a windy day? Well, it dries faster on a warm, windy day than it would do on a cool, still day.

So you can see that here we are considering the effects of both wind and temperature on the processes of drying hair and drying clothes.

So how does this relate to plants and why do these effects happen in the first place? Let's carry on to find out why.

So in higher temperatures, water evaporates more quickly from clothes or hair because the water has more energy.

Now, in plants, water evaporates from the cells within the leaves and then can diffuse faster through the stomata for the same reason they've got more energy because it's warmer.

And therefore, in higher temperatures, transpiration occurs faster because there is a higher temperature, therefore more energy.

So the rate of transpiration will increase as the temperature increases.

Just like drying your hair with a cool, hair dryer will take longer than drying your hair with a warm, hair dryer.

What about wind speeds then? Well, with wind speeds, water vapour is being removed from the air faster as the wind is passing.

It's taking away the water vapour.

It's particularly in the UK, it's very unusual to have a windy day and also a very humid day because the water vapour in the atmosphere is being moved away really quickly and therefore doesn't build up and cause humidity.

Now, this therefore makes the air drier, which increases the concentration gradient between the wet clothes and the dry air or the wet inside of the plant leaf and the dry air.

And because that concentration gradient between the item which is wet and the air, which is much drier because that concentration gradient increases, so the rate of diffusion increases because there's a great concentration gradient down which water can diffuse.

So water will diffuse away from the clothes drying on the line faster because the air is drier and water will diffuse outta the stomata of the leaves because the air is drier.

So wind speeds dry the air and that increases diffusion rate.

And because it increases diffusion rate, it increases the rate of transpiration.

So as wind speed increases, so will the rate of transpiration.

I hope that's completely relatable given you can compare water loss from the plant to how quickly your hands dry under a hand dryer, your hair dryers with a hair dryer, how quickly clothes dry on the line, whether the pavement dries quickly or slowly after a rain shower, all those sorts of scenarios, they're all based on the same basic theory.

So transpiration will occur fastest in which conditions? a, a cold and windy environment, b, a dark and cold environment, c, a dark and warm environment, d, a warm and windy environment.

I'll give you five seconds to decide.

Okay, so transpiration will occur fastest in a warm and windy environment.

Well done.

So what I'd like you to do is to firstly describe and explain the effects of high temperature and high wind speed on the rate of transpiration in plants.

And then consider the scenario that trees that are growing in dry soil in windy, cool conditions grow slowly.

But can you explain why? So pause the video and come back to me when you are ready.

Okay, so let's see what you've written.

So firstly, I asked you to describe and explain the effects of high temperature and high wind speeds on the rate of transpiration in plants.

So you should have included that as temperature increases, the rate of transpiration increases, and this is because water evaporates and diffuses through the stomata faster.

And that as wind speed increases, the rate of transpiration increases.

And this is because water particles in the air are moved away faster, making the air drier, and this increases the concentration gradient, increasing the rate of diffusion out of the stomata.

So you can see why on a warm, windy day, plants in pots will require more watering than they might otherwise do.

Then I asked you to consider that trees that grow in dry soil in windy, cool conditions grow slowly.

And I asked you to explain why this is.

So you should have included that if there is little water available, then the rate of photosynthesis will be low because water is an essential reactant in photosynthesis.

If there are windy conditions, then this will increase the rate of transpiration, which will reduce the availability of water for photosynthesis.

Cool temperatures lead to a lower rate of photosynthesis because the enzyme rate of reaction will be lower in cooler temperatures, you might have added.

And put all of those factors together, less photosynthesis will occur.

This means that less glucose will be made and this means less growth can occur.

And therefore, growth will be slow.

So again, review your answer, make sure you've got all of those salient points and well done if you've included them all as well.

Good job.

Okay, we've come to the end of our lesson.

So in our lesson today, we've seen how transpiration is the loss of water from the leaves of a plant, mostly through open stomata.

And the rate of transpiration is affected by the rate of photosynthesis.

So when it is light, photosynthesis takes place, the stomata open to allow carbon dioxide to move into the leaf, and this allows water vapour to move out of the leaf.

Therefore, as light intensity increases, so does the rate of transpiration.

Also, as wind speed increases and temperature increases, the rate of transpiration increases also.

And this is because of an increase in the rate of diffusion of water from the leaf via the open stomata.

So I hope you've enjoyed our lesson today.

Thank you very much for joining me and I hope to see you again soon.

Bye!.