Senior Science
8.3 Plants
Section 1
How Plants Work
Senior Science
8.3 Plants
Section 1 ::: How Plants Work
8.3.1 An understanding of the ways in which plants function
8.3.1.a
Describe plants as autotrophic organisms
8.3.1.b
Identify mineral nutrients required by plants as
- trace elements and
- macrominerals
8.3.1.i
Gather information from secondary sources to describe van Helmont’s
experiment and discuss the validity of his conclusions
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8.3.1.a
Describe plants as autotrophic organisms
Autotrophs
One way of classifying cells is how they obtain food to live. Plants are autotrophs.
Autotrophs are organisms, which make their own food from simple inorganic substances.
They do this by a process called photosynthesis. The inorganic substances are water and
carbon dioxide. Energy is obtained from sunlight. The product of photosynthesis is a high
energy, organic substance (sugar). Oxygen is a by-product of the process.
Extra sugar made by photosynthesis is stored as food for later use. Other minerals, for
example nitrates, are used to make other materials (proteins) needed by the plant. Since the
stored food in plants can be used by other organisms to feed on, plants are also called
producers.
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8.3.1.b
Identify mineral nutrients required by plants as trace elements and
macrominerals
Minerals (Elements) needed by Plants
Eighteen elements are essential for plant nutrition. There are large differences in the amount
of each required.
 Elements required in relatively large amounts are called macronutrients.
 Elements required in small amounts are called trace elements.
160 grams after drying
12 grams after burning
7200 milligrams of nitrogen
How can a scientist determine if an element is essential for a particular plant? Grow plant in
distilled water with all nutrients except the element under investigation. This can be very
difficult to get clear results. The plant used in the experiment (or even the seed) may have
enough of the element to grow unaffected for a considerable time while under investigation.
Another way to gain some idea of the different levels of these elements is to analyze the
chemical composition of a plant.
Approximate essential element composition of a 2kg cabbage
Component
Water
Oxygen
Carbon
Hydrogen
Nitrogen
Potassium
Calcium
Sulfur
Phosphorus
Amount (mg)
Component
1840 000
66 400
64 200
9 300
7 200
6 400
3 350
1 700
1000
Magnesium
Iron
Manganese
Zinc
Boron
Copper
Molybdenum
Cobalt
Sodium*
Chlorine*
Amount (mg)
580
28
10
6
6
1.1
0.11
0.02
700
470
Much smaller amounts of sodium and chlorine are needed. Plants take up much more
than they need, simply because it is in the soil.
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8.3.1.i
Gather information from secondary sources to describe van Helmont’s
experiment and discuss the validity of his conclusions
Introduction
Writing the method of a first-hand investigation
In order to discuss the validity of a conclusion, certain principles of experimental design need
to be known.
Why do experiments?
When planning a first-hand investigation the following principles
should be considered.
 A first-hand investigation is designed to discover new information.
 Experimenters are interested in the truth.
 The experiment must be able to be repeated by other scientists.
The writing genre To communicate the objectivity of the experiment the tradition is to never
use the words “I” or “WE”. It is normal to write in past tense using complete sentences.
Writing the method
The method is a description of steps to be followed and the
materials used. When writing the method the following topics should be considered.
V
Variables
G
Groups
C
Control
M
Measurements
A
Activity
N
Number
S
Safety
V
Variables
 In an experiment the effect of one variable on another is investigated.
 The independent variable is deliberately changed by the experimenter.
 The dependent variable is the variable that could change because of the change
in the independent variable.
 Controlled variables – The dependent variable and the independent variable can
change. All other variables are kept the same throughout the experiment – these
are called the controlled variables.
G
Groups
 Groups occur because the independent variable is changed for different groups.
 The only difference between the groups at the start of the experiment is the
difference in the independent variable. In all other aspects groups are the same.
C
Control
 There are two types of groups – the Control Group and the Experimental Group.
 The control is the reference against which experimental groups are compared.
 The control allows the experimenter to determine if the changes made to the
independent variable actually cause the dependent variable to change.
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M
Measurements
 There are two types of measurement
 Qualitative: This measurement does not involve numbers. It uses words to
compare – eg green, greener; more, less; slower, faster
 Quantitative:
This measurement involves numbers. They should be
carefully measured.
Errors should be recorded and considered in the
Discussion section of the Laboratory Report.
 It can be very important to clearly describe how and what to measure in the
method.
A
Activity
 A scientific investigation can occur using a number of different activities. Types of
activities include:
 An experimental investigation
 Survey
 Model building
 Literature search, or other second hand investigation
 The method should be written so that
 Steps are outlined in a logical order.
 There is enough detail so that the investigation can be repeated.
 It is expected that another scientist following the method described can obtain
similar results.
N
Number
 Reliability depends on the number of results obtained. Data is reliable if it is
consistent. In some investigations only three results may be necessary. In other
investigations many hundreds of results are necessary before reliability is certain.
 It is possible that a single unusual event could occur. A large number of results
reduce the influence of an unusual event on the overall results and the conclusion.
S
Safety
 Identifying and minimising hazards must be considered for all investigations
 Identifying and using safe work practices is essential.
Questions
1. Consider the following experimental aim
To observe the level of learning in a classroom for different levels of noise.
Analyse this experiment using VGCMANS & then complete table below (teacher guided).
V = variables
Independent – noise levels
Dependent Controlled -
G = Groups
2. Repeat question for another aim or hypothesis (possible homework)
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Van Helmont’s Experiment
What to do
1. Read the following FOUR articles of Van Helmont’s experiment.
2. Answer the questions that accompany each of the three articles.
3. Then complete the cloze passage that discusses the validity of van Helmont’s
conclusions.
Article 1
Van Helmont’s Method
1.
2.
3.
4.
Select a small willow tree sapling
Weigh the sapling.
Measure 90 kg of dry soil.
Plant the sapling in the soil in a pot
Van Helmont’s Results
Original weight of sapling
Original weight of soil
Final weight of tree
Final weight of soil
5. Water the sapling regularly.
6. Wait 5 years
7. Weigh the tree
8. Dry the soil and then weigh
Questions
1. What was the decrease in the weight of the soil over the 5
years?
2. What was the increase in weight of the tree over 5 years?
3. How much water was added to the soil?
4. Could all the weight of the tree have come from the soil?
Give a reason.
5. Name one substance that could have caused the tree to
gain weight.
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=
=
=
=
2.2kg
90kg
76.1kg
89.8kg
Article 2
The gathering of scientific information can be a slow process. For a particular investigation, a
conclusion be drawn if enough data is obtained. Conclusions are not accepted as correct
until they have been supported by several tests. It can be easy to draw wrong conclusions.
Designing valid experiments can be quite difficult. It took humans over 300 years of effort to
understand the basic process of photosynthesis (Photosynthesis is the process by which
plants make their own food).
As an example of how easy it is to draw the wrong conclusion consider a famous experiment
performed by van Helmont (1577 – 1644). Until the 17th century, it was believed that all the
material for plants came from the earth (one of the four elements – earth, water, air and fire;
this view was put forward by Aristotle, nearly 2000 years earlier). Van Helmont believed that
plants obtained all their material from water, not the earth. He designed an experiment to
prove his idea - that plants obtained their material from the element water.
Van Helmont’s error (ie the error of his conclusion) shows two important points about
experiments.
 Experiments are carried out to test ideas.
They provide facts that are either
relevant or irrelevant to the idea being tested. Van Helmont’s experiment was not well
designed to be a valid test of his idea. It was an excellent experiment to test Aristotle’s
idea.
 Experiments cannot prove that an idea is correct; the most it can do is show that an
idea is not wrong.
If the results (facts) agree with an idea, we cannot say that the
idea is proved correct – only the experiment supports the idea.
Questions
1. According to Aristotle, how many elements were there?
2. Name the four elements suggested by Aristotle.
3. Suggest an aim for van Helmont’s experiment.
4. What two facts were provided by van Helmont’s experiment? (Use article 1)
5. What did Aristotle suggest about the material required by plants?
6. Did van Helmont’s experimental results support Aristotle’s view about the requirements of
plants?
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Article 3
Van Helmont’s famous “willow tree” experiment is sometimes regarded as the origin of
experimental plant physiology. Van Helmont planted a 5 lb willow sapling in 200 lb of soil; 5
years later he weighed the two again – the tree had grown to weigh 164 lb, while nearly all the
soil remained (198 lb).
He concluded that the bulk of the mass of the tree had come from the water (the basic
element) that had been added to the pot.
At the time it was believed that there were 4 elements – earth, water, fire and air. It was
commonly believed that a tree was a mixture of these four elements. Van Helmont designed
his experiment to show that a tree was made from only one basic element – water. That is, a
tree was not a mixture of earth, water and fire. The results of his experiment showed that
very little soil was used (by the tree) and the tree needed water to grow. Common sense said
that plants needed just water to grow. His conclusion was consistent with his observations
and “common sense” obviously supported this.
Obviously, science students today can see many problems with van Helmont’s experiment.
Certainly, there are issues that affect the validity of his conclusions. The issues include:
1. Reliability – Only one tree was used rather than several; the result could have been an
unusual single event. Repeating the experiment would give more reliable results.
2. Uncontrolled variables – There are several variables that could affect the growth of the
plant. Van Helmont did not take these into account in designing his experiment.
 Air - The tree should have been grown in the absence of air.
 Soil minerals – The tree could have been grown hydroponically using distilled water.
Too much criticism of van Helmont’s experiment would be a little harsh. The concept of the
scientific method – a controlled experiment was unknown. Van Helmont’s work shows an
experiment that was designed and interpreted at a particular time. In its day it was valuable
experiment, even though today it is recognised that the conclusion is invalid. The experiment
was useful because it caused others to think about the problem, in some cases repeat the
experiment and in others it acted as a stimulus for further research.
It is important to remember that Van Helmont knew nothing of plant physiology. He knew
nothing about photosynthesis; nothing about the absorption of soil minerals through the roots
or carbon dioxide through the leaves. Van Helmont had no concept of distilled water. Also,
he would have known that plants do not grow outside soil – therefore he buried the pot in the
earth. Finally in weighing the soil before and after, he could clearly show that the soil (earth)
was not important in the tree gaining weight.
Questions
1.
2.
3.
4.
5.
6.
7.
8.
Write an aim for Van Helmont’s experiment?
[1 mark]
According to van Helmont there were only four elements – name them.
[1 mark]
Name the element, according to Van Helmont, that caused the tree to grow? [1 mark]
Identify Van Helmont’s conclusion? [1 mark]
Identify one piece of evidence that supports Van Helmont’s conclusion.
[1 mark]
Identify two issues that affect the validity of van Helmont’s conclusion. [1 mark]
Suggest one reason why the experiment was useful.
[1 mark]
Identify at 3 facts about plant physiology, about which Van Helmont had no knowledge.
[3 marks]
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Article 4
Writing the method of a first-hand investigation
Consider van Helmont’s experiment
Answer the following questions and then complete the table below
V = variables
Independent –
Dependent Controlled -
G = Groups
C = Control
1. Outline the information that was discovered in van Helmont’s experiment.
2. Identify TWO reasons van Helmont’s experiment could not be repeated.
3. In van Helmont’s experiment:
a] Identify the independent variable
[1 mark]
b] Identify the dependent variable
[1 mark]
c] Name 2 variables that should have been controlled. [2 marks]
4. How many groups did van Helmont have in his investigation?
5. Outline the purpose of a control.
6. Identify why it is obvious there was not a control in van Helmont’s experiment.
7. Identify the type of measurement van Helmont used?
8. Identify what van Helmont measured.
9. Identify other measurements that could have been recorded by van Helmont.
10. How many results were recorded by van Helmont.
11. Would you consider van Helmont’s method reliable? Explain.
12. Explain one piece of data that could have effected van Helmont’s conclusion if he had
planted more trees?
13. Identify TWO hazards van Helmont should have considered before he began his
investigation.
14. Identify TWO safe working practices required for van Helmont’s investigation.
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Questions
1. Describe van Helmonts experiment.
[3 marks]
2. Discuss the validity of van Helmont’s conclusions
[5 marks]
Discuss (and evaluate) questions are unstructured and require an extended answer
In order to answer such a question, you must provide some structure.
 STEP 1 Identify (& highlight) the important words in the question
 Conclusion –
 Van Helmont  STEP 2 Recall definitions of these important words (if necessary)
 Discuss – identify issues and provide points for and against
 STEP 3 Develop your own answer that reflects the depth required.
 Complete the cloze passage (to answer the question)
 A useful structure to answer a discuss question is shown below.
 A marking criteria for this question is also shown
CLOZE Passage
Question
Discuss the validity of Van Helmont’s conclusions.
What to do
Copy and complete passage below using the words from the list.
Right, consistent, experimental group, chance event, controlled, growth,
quality of soil, control, different conclusion, deliberate, did not consider,
compared, independent, several, meant to measure, confirmed, amount of
water, repeated, VGCMANS, conclusion, appropriate device, one sapling,
Validity refers to ___ method. In a scientific investigation, the effect on one variable
(dependent) of a ___ change in a second variable (independent) is measured. An
investigation is valid only if the changes to the ___ variable cause the changes in the
dependent variable.
Van Helmont concluded that the growth in the tree was due only to the absorption of the
element water. For a scientific investigation to be valid a number of issues need to be
addressed. The mnemonic ___ provides a useful checklist of issues
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Issue - Variables
A scientific investigation must have an independent and dependent variable to be
valid. In van Helmont’s investigation the independent variable was the___. The dependent
variable was the ___ of the willow sapling. As such, van Helmont was investigating what
would happen to growth if the sapling was watered.
Other variables must be controlled for a scientific investigation to be valid. That is any
variable (other than the dependent variable) that could cause changes to the growth of the
tree should be___. These controlled variables include the ___ and sunlight. Van Helmont
___ these variables. Again, since only one sapling was used there is no way of measuring
the effect of these variables – therefore the ___ was not valid.
Issue – Groups and Control
A scientific investigation needs a ___ to be valid. That is, there should be at least two
groups – a control group and an___. This allows comparison – the effect of a change in the
independent variable can be measured and ___. Since van Helmont used only ___ he did
not have a control – therefore the conclusion could not be valid.
Issue
Measurement
An investigation is valid when an ___ is used to accurately measure the dependent
variable – ie what it is ___. Van Helmont did not measure how much water was used.
Therefore, he could not say actually where the water went. He simply decided (probably
before he started the experiment) that all the growth in the plant was due to the water.
Issue
Number (Reliability)
An investigation is valid if the results are reliable. Results are reliable if data is ___. Van
Helmont used only one sapling. Therefore, the particular result could be a ___. More
saplings would have given him ___ varying results. These different results would need to be
interpreted. The need to interpret several different results could have led to a ___.
It is important that a scientific investigation can be ___ (by other scientists) to produce
similar results. While other scientists could plant a sapling and water it, they could not
repeat the investigation in the same way as van Helmont did. Therefore the validity of the
conclusion could not be ___.
Issue Knowledge
Van Helmont knew nothing of plant physiology. He did not know how plants absorbed water.
He did not know that plants absorbed materials from the air. The problem was he could not
control variables he did not know existed.
 eg a plant grown hydroponically will not survive, if only distilled water is used. It would be
obvious that something else besides water is needed for a plant to grow.
 eg a plant grown in sand will not survive. Again, it could be determined that something in
the soil besides water is needed for a plant to grow.
Both examples show that van Helmont’s conclusion is not valid. He did not because he did
not control certain variables.
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Structure and Marking Criteria for a discuss question
Issue 1
Points for
Issue 2
Points for
Points against
Points against
Marking Criteria
Marks
Define validity
Identify at least two issues that relate to validity
Provide points for and/or against each issue
5
Identify two issues that relate to validity
Provide points for and/or against each issue
3-4
Identify at least one issue that relates to validity
Provide points for and/or against this issue
1-2
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