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Plant nutrition
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Introduction
You and I feed by taking in solid or liquid food substances (vaste of vloeibare voedingsmiddelen).
We eat meat, bread, fruit or vegetables. We drink water, milk or soda (priklimonade). We need
food because it provides us with all the nutrients (voedingsstoffen) necessary for growth. Next
to growth, nutrients give us energy to do things like stay warm, move, think and grow.
Like animals, plants grow as well, although they don’t eat anything. For one, plants take up
water and minerals from the soil, with their roots. Furthermore, it is believed that they live on
gasses present in the air around us.
In this chapter we are going to find out exactly, how plants can survive without eating.
The first thing we are going to do is see if there are any nutrients present inside the plant;
in this case inside the leaves of a plant. We are going to test these leaves on glucose and
starch, two carbohydrates (koolhydraten).
Before we can do that we need to find a way to indicate glucose, we need to show that
glucose is present, chemically. We can do this using an indicator called Benedict’s
solution (Mr. Benedict was the first one to use this test) to the leaf. After adding some
Benedict’s solution to a leaf we need to place the leaf at 70°C for a few minutes (that is
the temperature at which Benedict’s solution and glucose react). There, the leaf will
change colour if glucose is present.
Before we can perform this experiment, we need to test the solution to make sure it
works. We need to perform a positive test and a negative test. In the positive test we add
Benedict’s solution to a glucose solution. In a negative test we add it to plain water. If we
can see a difference here, we know the test works. And, if you do this test the first time,
these tests tell you what colour indicates a positive result (glucose present) or a negative
result (no glucose present).
Make sure not to mix this up with a controlled experiment. In a controlled experiment
we use two groups / set ups which we call the experimental group / set up and the control
group / set up. A positive test and a negative test are only used to see if the indicator test
works. Is the iodine solution in experiment 1 (in exp. 2 Benedict’s solution) still working?
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Experiment 1:
Is there glucose inside a leaf of a Geranium?
demonstration
Materials:
- a test tube with 2 mL of water
- a test tube with 2 mL of glucose solution
- a bottle with Benedict’s solution
Work plan:
- Fill test tube two with 2 mL of water.
- Fill test tube one with 2 mL of a glucose solution.
- Add 2 mL of Benedict’s solution to each test tube
and place them in hot water (70°C).
- After 5 minutes see what the colour changed into.
Figure 1.
A leaf of a geranium
questions:
1. Test tube one (with glucose) was the positive test. It turned ...
2. Test tube two (with water) was the negative test. It turned ...
3. In conclusion, what is the indicator for glucose?
Materials:
- a leaf of a geranium
- a mortar
- a bottle with Benedict’s solution
Work plan:
If all goes well, we can go ahead with the actual experiment: test if glucose in present in a
leaf of a geranium.
- Take a leaf of a Geranium and boil it to open up the cells
The glucose is inside the cells. The Benedict’s solution cannot reach the glucose unless
the cells are opened.
- Grind the leaf in a mortar to get the contents (inhoud) out of the leaf.
- Pour (gieten) the ground leaf in a test tube.
- Add 2 mL of Benedict’s solution to the test tube and place it in hot water (70°C).
- After 5 minutes see what colour it changed into.
questions:
4. What part or parts of the cell must be broken down to let the contents out?
5. Is there any glucose present in the leaves?
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Experiment 2:
Is there starch inside a leaf of a geranium?
demonstration
Materials:
- a test tube with 2 mL of water
- a test tube with 2 mL of starch solution
- a bottle with iodine solution
Work plan:
- Fill test tube one with 2 mL water (negative test).
- Fill test tube two with 2 mL of a starch (zetmeel) solution (positive test).
- Add three drops of iodine to each test tube.
questions:
6. Test tube one (with water - the negative test) turned ...
7. Test tube two (with starch - the positive test) turned ...
Materials:
- a leaf of a geranium
- a mortar
- a bottle with iodine solution
Work plan:
If this went well as well, we can proceed and test the leaf on starch.
- Add a leaf of a Geranium to a beaker and boil it to open up the cells.
- Grind the leaf in a mortar to get the contents of the leaf out.
- Pour the ground leave in a test tube.
- Add three droplets of iodine.
questions:
8. Is there starch present in the leaves?
9. What colour would the leaf turn if no starch was present in the leaf?
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Anatomy of plants
Plants can take up certain nutrients with their roots. They use the roots to suck up water
and minerals. Plants can also take up substances, probably gasses, with their leaves. In
the next experiment we are going to find a lead as to how they can do this (een aanwijzing
vinden voor ...). There is probably a structure in the leaf through which these gases can
enter the leaf.
For this investigation, we need a microscope and some preparations of a leaf. With the
microscope we can see the individual cells of a leaf. Therefore, if there is such a structure
in the leaf we should be able to find it.
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Experiment 3:
How plants take up nutrients with their leaves.
microscopy
Materials:
- a preparation of a cross section of a leaf (dwarsdoorsnede)
- a preparation of the surface of a leaf (bladoppervlak)
- a microscope
Work plan:
surface of a leaf
- with a microscope, study the preparation of the surface of a leaf and obtain a clear view
using a 100 or 400 times magnification
- find three different kinds of cells (with different shapes) in your preparation
- make a schematic drawing of a group of six cells (among which all three different kinds
of cells) – take no more than two minutes to draw them
questions:
10. Which of these cells is probably involved in letting in gasses?
cross section of a leaf
- with a microscope, study the preparation of the cross section of a leaf and obtain a clear
view using a 100 or 400 times magnification
- the upper part of the leaf is the side where you can find elongated cells (langgerekte
cellen) (the preparation could be upside down …)
- find the cells at the lower surface of the leaf
- try to find the cells that you found in the previous preparation
- make a schematic drawing of four of these cells – no more than two minutes
questions:
11. Indicate in your drawing which of these cells are involved in the uptake of gasses?
These cells are called guard cells and together form stomata (huidmondjes, singular:
stoma).
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The function of the stomata
Plants contain proteins, fats, carbohydrates and vitamins. Since plants can’t take them up
from their environment (uit hun omgeving), they must form these nutrients themselves.
One of these nutrients, starch, has been found in leaves. We have proved that in
experiment 2.
Plants have stomata, little holes in the leaves. Through these stomata, the plants can
take up gases from the air around the plant. Maybe these gases are used by the plants to
substances like starch. If we can prove this, we are one step closer to solving the question
as to where do the nutrients inside the plant come from if they can’t make them
themselves?
question:
12. “Are stomata responsible for the intake of gasses needed to make certain nutrients?”
Describe a way to test the involvement of stomata in the production of starch.
Set up a fair test and mention what you need for this experiment. “I don’t know ...” is
not an option – make something up.
Name the input variables, the outcome variables and the fixed variables.
Below you can find a list of materials you can use in the experiment:
- a bottle with iodine
- two leaves of a Geranium
- wax / Vaseline
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Experiment 4:
Do plants take up gases with their leaves?
demonstration
We believe that the stomata take up gases that are used to make nutrients like starch. I f
we stop the intake of gasses, the plant can’t make nutrients like starch. But how can we
stop the uptake of gasses?
Using Vaseline we can block the stomata located at the bottom side of the leaves so
that no gases can be taken up anymore. And with iodine we can test for the production of
starch.
Materials:
- a Bunsen burner and a tripod
- two Geranium leaves
- Vaseline
- a beaker with water
- a beaker with ethanol / aclohol
Work plan:
- put Vaseline on the bottom side of a Geranium leaf
- put Vaseline on the top side of another Geranium leaf
- put Vaseline on both sides of a Geranium leaf
- leave another Geranium leaf for 24 hours untreated (niets mee doen)
- place the leaves in the light for 24 hours
- after 24 hours remove the Vaseline from the leaf
- boil the leaves for 3 minutes (you do this to open up the cells to let the iodine in)
- place the leaves in a beaker with hot ethanol (you do this to remove the chlorophyll from
the leaves; this way the change in colour due to iodine is clearer)
- remove the leaves from the ethanol and place them on a dish
- add some iodine to the leaves
questions:
13. a) Why should the Vaseline be rubbed on the bottom side of the leaf?
b) Why should the Vaseline be rubbed on the top side of the leaf?
c) Why should the Vaseline be rubbed on both sides of the leaf?
14. Why did we use a leaf without Vaseline?
15. What can be concluded from this experiment?
16. a) What was the input variable in this experiment?
b) What was the outcome variable in this experiment?
c) Name some fixed variables in this experiment.
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Experiment 5:
Do plants take up carbon dioxide?
demonstration
We proved that plants take up a gas with their leaves. One of the gases believed to be
taken up by plants is carbon dioxide. This may be used to make starch.
For this purpose we created a setup using a special carbon dioxide sensor linked to a
computer using the program Coach. A plant is placed under an air tight bell (luchtdicht
afgesloten) for a day and a night. For the duration of the experiment the amount of carbon
dioxide is measured.
Materials:
- a plant
- an air tight bell
- a carbon dioxide sensor
- a computer with the Coach software
Work plan:
- place a plant under an air tight bell
- link a sensor to a computer
- start the program Coach
- measure the amount of carbon dioxide under the bell for 24 hours
questions:
17. During the day (when it is light) we measure an increase in the amount of carbon
dioxide under the bell.
From this we can conclude that the plant takes up / gives off carbon dioxide (choose).
18. During what process is the carbon dioxide formed / used?
19. Does the plant take up carbon dioxide during the night?
20. During what process is the carbon dioxide formed?
4 Green leaves: light and chlorophyll
In the experiment with the plant under the bell, we noticed that the plant only takes up
carbon dioxide during the daytime. During the night it even produces carbon dioxide. This
could mean that light is essential for making starch. This calls for further investigation.
question:
For a mark (turn it in on a separate paper with your name on it):
21. Think of an experiment to test the involvement of light in this process.
Describe the experiment in detail!! Remember experiment 4...
Name the scientific question, hypothesis, input variables, the outcome variables and
the fixed variables.
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Below you can find a list of materials you can use in the experiment:
- a bottle with iodine
- leaves of a Geranium
- a Bunsen burner and tripod
- a beaker with water
- a beaker with ethanol / alcohol
Leaves are green. The green colour is caused by
chlorophyll, a green pigment located in the
chloroplasts. When you studied the cross section of
a leaf with a microscope, you saw some cells that
Figure. 2. A green leaf with
contained these chloroplasts. Since cells with
silver mottling.
chloroplasts are so close to the stomata, they could
be play a role in the production of starch.
Figure 2 represents a mottled leaf of a yellow dead nettle (een bont blad van een gele
dovenetel). This leaf has both green and white parts. In the white parts, the cells contain no
chloroplasts. This type of leaves could be an interesting subject for studying the
involvement of chlorophyll (chloroplasts) in the production of starch.
question:
For a mark (turn it in on a separate paper with your name on it):
22. How can we use this type of leaf to study the involvement of chloroplasts in the
production of starch? Remember experiment 4...
Set up an experiment in detail!!
Name the scientific question, hypothesis, input variables, the outcome variables and
the fixed variables.
Below you can find a list of materials you can use in the experiment:
- a bottle with iodine
- leaves of a mottled Geranium
- a Bunsen burner and a tripod
- a beaker with water
- a beaker with ethanol / alcohol
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Experiment 6
Do plants need light to make starch?
Materials:
- a bottle with iodine
- leaves of a Geranium
- aluminium foil
- a Bunsen burner and tripod
- a beaker with water
- a beaker with ethanol / alcohol
Work plan:
1 Wrap aluminium foil around one half of a leaf.
2 Leave the leaf with the aluminium foil in the light for 24 hours.
3 After 24 hours take the leaf with the aluminium foil around it and make a schematic
drawing (don't take too long, 30 sec).
4 Place the beaker half filled with water on a tripod.
Use a Bunsen burner to bring the water to the boil.
5 Remove the aluminium foil from the leaf and put the leaf in the boiling water and leave it
there for a few minutes. This will make the leaf soft and easy to penetrate by fluids.
Remove the leaf with a pair of tweezers.
6 Turn out the Bunsen burner!! Pour a little ethanol (just enough to cover the leaf) into a
small beaker. Carefully, place the small beaker inside the larger beaker of hot water. The
ethanol will soon come to boil.
7 Now drop the leaf into the beaker of ethanol. Spread the leaf out in the ethanol. Move the
leaf around every 15 seconds so that it comes in close contact with the ethanol. After a
few minutes the ethanol will have removed the green pigment from the leave, making it go
a whitish colour.
8 When the leaf has turned white, take the leaf out of the ethanol with tweezers, and wash it
in tab water (kraanwater).
9 Place the leaf in a dish and pipette dilute iodine solution over it. Watch what happens.
10 Make a drawing of the leaf and compare it with the first drawing. Label with words which
parts turned black and which part yellow.
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Write down the following for this experiment:
Turn in the answers to these questions; one per group.
Turn in the answers to questions 19 and 20 you did as homework, individually!!
questions:
1. (2p) what is the scientific question?
2. (2p) what is the hypothesis?
Describe briefly (omschrijf kort):
3. (1p) what is the input variable in this experiment?
4. (1p) what is the outcome variable in this experiment?
5. (1p) name some fixed variables present in this experiment
6. (2p) results:
- drawings (see steps 1 and 10 above)
- for each drawing, in one or two sentences describe what you see
7. (1p) what is the conclusion in this experiment?
question 8
question 9
question 10
question 11
question 12
question 13
(1p) Why was a part of the leaf covered with aluminium foil?
(1p) What is the function of boiling the leaf?
(1p) Why should the Bunsen burner be turned out when the beaker with
ethanol was placed in the hot water?
(1p) What is the "green substance" that makes the ethanol turn green?
(1p) What is iodine used for in this experiment?
(2p) Does the whole leaf turn blue-black?
How can you explain this? Give an extensive answer (geef een uitgebreid
antwoord).
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Experiment 7
Do plants need chlorophyll to make starch?
Leaves are green. The green colour is caused by chlorophyll, a green pigment located in
the chloroplasts. When you studied the cross section of a leaf with a microscope, you saw
some cells that contained these chloroplasts. Since cells with chloroplasts are so close to
the stomata, they could be play a role in the production of starch.
A mottled leaf has both green and white parts. In the white parts, the cells contain no
chloroplasts. This type of leaves could be an interesting subject for studying the
involvement of chlorophyll (chloroplasts) in the production of starch.
Materials:
- a bottle with iodine
- leaves of a mottled Geranium
- a Bunsen burner and tripod
- a beaker with water
- a beaker with ethanol / alcohol
Work plan:
1 Leave the mottled leaf for 24 hours in the light.
2 After 24 hours take the leaf and make a schematic drawing indicating the green and white
parts carefully phone (don't take too long, 30 sec) or take a picture with your cell.
3 Place the beaker half filled with water on a tripod.
Use a Bunsen burner to bring the water to the boil.
4 Put the leaf in the boiling water and leave it there for a few minutes. This will make the
leaf soft and easy to penetrate by fluids. Remove the leaf with a pair of tweezers.
5 Turn out the Bunsen burner!! Pour a little ethanol (just enough to cover the leaf) into a
small beaker. Carefully, place the small beaker inside the larger beaker of hot water. The
ethanol will soon come to boil.
6 Now drop the leaf into the beaker of ethanol. Spread the leaf out in the ethanol. Move the
leaf around every 15 seconds so that it comes in close contact with the ethanol. After a
few minutes the ethanol will have removed the green pigment from the leave, making it go
a whitish colour.
7 When the leaf has turned white, take the leaf out of the ethanol with tweezers, and wash it
in tab water (kraanwater).
8 Place the leaf in a dish and pipette dilute iodine solution over it.
Watch what happens.
9 Make a drawing of the leaf (or another picture) and compare it with the first drawing. Label
with words which parts turned black and which part yellow.
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Write down the following for this experiment:
Turn in the answers to these questions; one per group.
Turn in the answers to questions 19 and 20 you did as homework, individually!!
questions:
1. (2p) what is the scientific question?
2. (2p) what is the hypothesis?
Describe briefly (omschrijf kort):
3. (1p) what is the input variable in this experiment?
4. (1p) what is the outcome variable in this experiment?
5. (1p) name some fixed variables present in this experiment
6. (2p) results:
- drawings (see steps 1 and 9 above)
- for each drawing, in one or two sentences describe what you see
7. (1p) what is the conclusion in this experiment?
question 8
question 9
question 10
question 11
question 12
question 13
(1p) Why did we use a mottled leaf and not a normal leaf?
(1p) What is the function of boiling the leaf?
(1p) Why should the Bunsen burner be turned out when the beaker with
ethanol was placed in the hot water?
(1p) What is the "green substance" that makes the ethanol turn green?
(1p) What is iodine used for in this experiment?
(2p) Does the whole leaf turn blue-black?
How can you explain this? Give an extensive answer (geef een uitgebreid
antwoord).
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5 The final word equation
Until now, we know quiet a lot about the way plants make their own nutrients. That is, we
know a lot about the production of starch. We found out that the plant needs carbon
dioxide and light and that it can make starch in cells that contain chlorophyll (located in
chloroplasts). We saw hat the carbon dioxide enters the leaves through stomata,
structures located at the bottom side of the leaves.
There is one more thing we already know about plants, a very important feature of plants.
They produce oxygen we need to live. Maybe the oxygen the plant makes is formed
during the production of starch. Just a thought. Could we test this?
If oxygen is formed during this process, what kind of an experiment could we perform to
test it? During this lesson you will be given 5-10 minutes to work this out in groups.
question:
23. Set up an experiment to test if oxygen is formed during the process of starch
production.
Name the input variables, the outcome variables and the fixed variables.
You can use the following materials (or more)
- some pond weed (waterpest)
- a beaker with water
- aluminium foil
- a funnel (trechter)
- a test tube
- a candle
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Experiment 8
Do plants make oxygen while making starch?
Materials:
- pond weed (waterpest)
- two beakers
- water,
- aluminium foil
- two funnels (trechters) and test tubes
- a splint (a wooden stick) and matches
Work plan:
- Fill two beakers with water
- In both beakers, place a handful of Canadian
pondweed under a funnel (trechter) and place a
test tube over the opening of the funnel as shown in Fig. 3.
- Leave the other beaker uncovered (set up 1): control set up.
- Cover one beaker with aluminium foil (set up 2): experimental set up.
- Leave both setups in a well-lit place for a week.
- After a week measure the amount of gas caught in the test tubes in both set ups.
- Place a glowing splint in the test tube from setup 1 after taking it out of the water gently.
questions:
24. What was the scientific question in this experiment?
25. Name the input and the outcome variable in this experiment?
26. Name the three most important fixed variables.
27. Explain that the splint will start to burn brightly after it is placed in the test tube.
28. The experiment could have been performed by only using setup 1.
In a fair test we need a second setup to compare the results if the first setup to.
Why is it important put aluminium around the second setup?
29. What conclusion can be drawn from this experiment?
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Experiment 9
Do plants need carbon dioxide for making oxygen?
Materials:
- pond weed (waterpest)
- three beakers
- tap water,
- boiled water: boiling removes gases (carbon dioxide) from the water,
- water to which extra carbon dioxide has been added
- three funnels (trechters) and test tubes
- a splint (a wooden stick) and matches
Work plan:
- Fill one beaker with normal tap water (beaker 1): control set up.
- Fill one beaker with water that has boiled for 30 minutes (beaker 3): experimental setup.
- Fill one beaker with water to which extra carbon dioxide has been added (beaker 4):
experimental set up.
- In each beaker, place a handful of Canadian pondweed under a funnel (trechter) and set
it up as shown in Fig. 3.
- Leave these setups in a well-lit place for a week.
- After a week measure the amount of gas caught in the test tubes in each set up.
questions:
30. What was the scientific question in this experiment?
31. Name the input and the outcome variable in this experiment?
32. What conclusion can be drawn from each experiment?
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The results of all experiments carried out in the last few weeks lead to the following
summary:
light and chlorophyll
carbon dioxide  starch and oxygen
This word equation seems complete. However, recent experiments in other laboratories
have shown that there is more to it than this. Others have found that not only carbon
dioxide but water is necessary as well.
Furthermore, starch is formed during this process but only after glucose was formed
first. Once glucose is formed, it is immediately turned into starch. This is done so fast that
our experiments (experiment 1) couldn’t measure it. A plant needs glucose to grow or as a
source of energy. If glucose isn’t used, it is stored for later. Starch is how it is stored.
Therefore, the final word equation of the process that we will call from now on
“photosynthesis” (photo means 'light' and synthesis means 'making' or 'manufacturing') is:
light and chlorophyll
carbon dioxide and water  glucose and oxygen
(raw materials)
(products)
The substances in the first part of the word equation, the substances that are changed, are
called the raw materials. The substances that are formed by this process are called the
products and can be found in the second part of the word equation.
For this process, sunlight and chlorophyll are needed. Since sunlight is not a
substance it is not allowed in the word equation. Chlorophyll does not disappear in this
process and is therefore left out of the word equation as well.
The formation of glucose out of carbon dioxide and water isn’t an easy process. This
process requires a lot of energy. This is where chlorophyll comes in. It absorbs the energy
in sunlight and uses it to make glucose.
The arrow in the middle indicates that during the chemical reaction raw materials are
turned into products. The carbon dioxide and water disappear and glucose and oxygen are
formed. The gas, oxygen, is actually a by product of photosynthesis. Glucose is what the
plant needs.
The oxygen that is formed during photosynthesis is released through the stomata out of
the leaves. This is the oxygen that we use for respiration.
questions:
33. What does a plant need to take up from its environment?
34. Why is chlorophyll not called a raw material?
35. What is the function of starch in a plant?
36. What will happen to the amount of starch in a leaf during the night?
37. It is said that oxygen is a by-product. What does this mean?
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6
Light and energy
Green plants grow only in places where there is light. The plants need light for making
glucose. Like anything that needs to be put together, making glucose costs energy. Sunlight
contains a lot of energy.
The human body, however, can’t use the energy in sunlight, unless it is to warm up when
we are feeling cold on a sunny spring day. Human beings need technical equipment
(technische apparatuur) to turn solar energy into a useful form. Solar panels on roofs of
houses generate electricity that can be used for all the electrical equipment in our houses.
Architects are building houses that provide (verzorgen) their own energy in a way that is good
for the environment. This is called “green building”.
questions:
38. In Mediterranean countries, solar heat is used in a different, simpler way.
What way?
39. How do some reptiles use solar heat?
40. How is solar energy good for the environment?
41. How many electrical apparatuses do you have in your home (lighting not included)?
42. What other sources of environmental friendly energy can architects use for their houses?
43. As homework for next lesson find as much information on green building as you can.
Bring pictures, information, books, print whatever you can use. On a poster, you are
going to make a sketch of a house that does as little damages to nature as possible. It
doesn’t have to be a complete house as long as you have thought about it enough. Think
of more than solar panels…
44. Above an entry on renewable energy from Wikipedia is presented. In a diagram present
these numbers (read carefully). Add a text to the diagram in which you explain what is
presented. Next lesson include this diagram in your poster.
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7 What happens to the products of photosynthesis?
The sugar, a plant makes in its leaves is dissolved (opgelost) in water. In other words it is
soluble (oplosbaar). In this state it can be transported from the leaves to other parts of the
plant since not all parts of the plant can make their own glucose. Structures, such as
flowers and roots can’t do photosynthesis. Here, glucose can be used as a source of
energy or as building block (bouwstof) or it may be turned into starch and stored for later
use.
The product of photosynthesis can also be turned into other substances. These include
cellulose, fats and oils, vitamins and proteins. For the production of proteins and vitamins,
plants need additional minerals that are taken up by the roots of the plant. Carbohydrates,
fats, oils and proteins are used by the plant as building blocks (bouwstoffen) for growth.
questions:
45. ‘Plants are like factories, churning out (produceren) useful things that can be used by
humans'. Explain this statement, giving examples to show what it means.
46. In which part of the plant do we find cellulose?
47. Name two functions of proteins in a plant.
48. Photosynthesis is a plant's way of making food.
a) List four things which a leaf must have for photosynthesis to take place.
b) Which of these things are 'raw materials'?
c) Describe four experiments to show that a green plant cannot make starch if one of
the four things which you have listed is missing.
49. Your friend was away when you were shown how to test a leaf for starch. Write out
some instructions, stressing the precautions to be taken.
50. In the 17th century a scientist called van Helmont did the following experiment. He
planted a willow (een wilg) in a pot of soil, after he had weighed the willow and the soil.
He then left the willow for five years, making sure that the soil was kept well watered.
Five years later he dug up the willow and weighed it. He also weighed the soil again.
His results are shown in Table 1.
How can you explain that the soil doesn’t decrease in mass whereas the tree does
increase in mass?
Table 1
51. In the 18th century Joseph Priestley carried out the following experiment. He burned a
candle in a sealed chamber until the flame went out. He then divided the air into two
separate glass containers. In one container he placed a green plant; no plant was
placed in the other container. Both containers were put in a sunny place. Ten days
later he found a lighted candle would burn in the first container, but not in the second.
How would you explain this?
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8
Structures inside a leaf
In the leaf photosynthesis takes place in the chloroplasts of the palisade cells and spongy
cells (see Fig. 4, 7) These cells are located on the inside of the leaf. For photosynthesis these
cells need carbon dioxide, water and sunlight. Since leaves are very thin, these cells get
Figure 4. The internal parts of a leaf
enough sunlight. By making an enormous number of leaves, the plant has formed a large
surface area for absorbing sunlight.
Carbon dioxide needs to be obtained through the stomata since the palisade cells and the
spongy cells are located on the inside of the leaf. Covering the leaves we find epidermal cells
(lower and upper epidermis, see Fig. 4, 6, 7). These cells
do not have chloroplasts and can therefore not perform
photosynthesis.
A plant consists of several organs: leaves, stem,
flowers and roots. All these parts need nutrients, but
only the leaves are able to make them since they only
have chlorophyll (chloroplasts). Therefore a plant
needs to bring nutrients to the other parts of the plant.
They do this through a system of hollow tubes that run
throughout the plant. After the leaves make glucose
from carbon dioxide and water, some of the glucose is
Figure 5. The external structure
of a leaf
sent to other parts of the plant through these tubes.
These particular tubes are called phloem vessels.
Not only glucose in transported through the plant. From the roots, minerals are
transported to the leaves. Some of these minerals are used by the leaves to turn glucose
into proteins and vitamins (section 7). The vessels that transport minerals to the leaves are
called xylem vessels. In the leaves, the bundles of phloem and xylem tubes are called
veins (bladnerven) (see Fig 5, 6).
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Figure 6a. a cross section
of a leaf
Figure 6b. a cross section
of a leaf
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Experiment 10: Looking at the structure of a leaf
Examine the preparation (cross section) of a Liguster leaf under the microscope.
In this preparation (as in Fig. 4) four layers of cells can be seen: lower and upper
epidermis, spongy cells and palisade cells. In the lower epidermis you can find guard cells.
Make a drawing of this cross section in which you draw four cells of each layer and at least
one set of guard cells. Remember the drawing rules.
When you are done and have some time left, ask your teacher for the bottom view of the
Liguster leaf and make a drawing of these cells as well (see Fig. 7).
questions:
52. What makes the palisade and spongy cells so suitable for
photosynthesis?
53. What are air spaces for?
54. The air spaces contain carbon dioxide at night but not
during the day. Explain?
55. As well as transporting substances, the veins strengthen
the leaves and help to hold them out flat. What is this
important for?
56. Explain that no photosynthesis can take place in
epidermal cells.
Figure 7. Stomata in the
epidermis of a leaf.
9
Transporting water through the plant
In the surface of the leaves we find stomata (see fig. 7) to let in carbon dioxide. The
oxygen produced during photosynthesis is released through these stomata.
The third substance to pass through the stomata is water. W ater is absorbed in the
roots and is transported upwards through the xylem vessels towards the leaves. The water
needs to flow (stromen) to get the minerals to the leaves. By evaporating the water from
the leaves through the stomata, the plant keeps the water flowing.
This means that the plant can regulate the water flow by opening and closing the
stomata. When there is a lot of water in the soil, the plants can keep the water flowing. If
there is a shortage of water, it is better for the plant to keep the water inside by closing the
stomata. Loss of water will lead to dehydration (uitdroging). If they keep the stomata
closed for too long, the plants can’t do photosynthesis…
Some of the water is used for growth, since cells consist for the larger part of water. Only a
very small fraction of water transported to the leaves is used for photosynthesis. Having
heard this, one should conclude that the most important role of water to a plant is to
transport minerals to the leaves. About 90% of the water taken up by the roots is
evaporated through the leaves. Only ten percent is used for growth and respiration.
Study figure 8, to see if you understand.
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Figure 8. transport and
photosynthesis
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Experiment 11: Is water evaporated from the leaves.
Susan reads that the total surface area of the leaves is
important to plants. She thinks that the leaves are involved in
the evaporation of water.
Below a technique is described to measure the amount of
water evaporated through the leaves. Use this technique to
test Susan’s hypothesis.
Materials:
- Some measuring cylinders with a capacity of 25 cm 3.
- Several branches from a Liguster
Work plan (see Fig. 9):
- Place the three branches in three test tubes. Then fill
the measuring cylinder with water to about two
centimetres under the top.
- Fill one test tube with only water.
Figure 9. Measuring
water transport.
- Make sure that one branch has twice as many leaves as
the next one.
- With a marker draw lines on the test tubes to mark the
water levels. Label the test tubes with your initials or a special sign.
- Add three millimetres of oil on top of the water in each test tube.
questions:
57. What was your control experiment?
58. What were the input, outcome and fixed variables?
59. Why did you add oil to the test tubes?
60. What conclusion can you draw on basis of the results of this experiment?
61. Water does not always travel through a plant at the same speed; sometimes it travels
very slowly but at other times it may travel much faster. The speed depends on how
quickly water evaporates from the leaves. What conditions in the plant's environment
are likely to speed up the rate at which water evaporates from the leaves?
62. In a similar experiment done with Hedera, pupils found that a plant placed at a
temperature of 15°C evaporated 10 mL, at a temperature of 25°C, 15 mL and at a
temperature of 40°C 30 mL.
Present these results on a diagram.
63. Predict how much water will evaporate at a temperature of 32°C.
How did you come to this value?
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64. If a plant does not get enough water, it may droop (Fig 10). This is called wilting
(verleppen). It happens if water evaporates from the leaves faster than it is absorbed by
the roots.
Figure 10. A normal and a wilting plant
a When is this most likely to happen to garden plants?
b What does it tell you about one function of water in plants?
c In very dry weather the stomata close. The guard cells move together rather like a
pair of doors. Why is this useful to the plant?
d What does closing stomata do with photosynthesis?
e The surfaces of leaves are covered by a waterproof cuticle (see Fig. 4).
What is the cuticle useful for?
As a summary, watch this clip:
http://www.youtube.com/watch?v=C1_uez5WX1o
25
Glossary with plants
nutrients
a root
a leaf
to indicate
an indicator
a positive test
a negative test
a controlled experiment
Benedict’s solution
a mortar
iodine
glucose
starch
guard cells
stoma - stomata
input variable
outcome variable
26
fixed variables
carbon dioxide
an air tight bell
a Bunsen burner
ethanol
chlorophyll,
chloroplasts.
a mottled leaf
a scientific question
a hypothesis
a mottled Geranium
oxygen
Canadian pondweed
a funnel
a word equation
photosynthesis
raw materials
products
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an arrow
a chemical reaction
a by product
a green building
to dissolve
soluble
a source of energy
a building block
to store
cellulose
fats and oils
vitamins
proteins
carbohydrates
palisade cells
spongy cells
epidermis / epidermal cells
phloem vessels
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xylem vessels
a vein (in a plant)
total surface area
dehydration
to wilt
a cuticle
What must you know?
- Tell how to use the indicators for starch and glucose in an experiment
- Name the cell types found in a leaf
- What substances can be formed from glucose after photosynthesis
- Name the six nutrients
- Name examples of carbohydrates.
- The word equation of photosynthesis.
- The word equation of respiration.
- What is transported through the xylem and phloem vessels
What do you need to understand / be able to explain?
- What do plants and animals need nutrients for?
- Explain that plants don’t eat and animals do.
- Explain what the function is of stomata in the leaves of plants.
- Explain that the carbon dioxide concentration around a plant decreases in the light and
increases in the dark.
- Explain what a chemical reaction is.
- Explain the function of an indicator in a scientific experiment.
- Explain that glucose is not present much in a leaf while it is formed during photosynthesis.
- Explain that plants can’t use sunlight as a source of energy in the cell
- Explain what plants do with the glucose they make during photosynthesis
- What is the advantage to a plant of being able to open stomata and to keep them closed
sometimes?
- Explain what substances come into and go out of the leaf during the day
- Explain what substances come into and go out of the leaf at night
- Describe the functions of xylem vessels and that of phloem vessels in a plant.
- You need to know how to set up an experiment (input, outcome, fixed variables)
- You need to know how to make a table and a diagram
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