Photosynthesis
In this lesson, you will be investigating how plants produce their own
food. This process, called photosynthesis, is very important since
animals rely on plants for food. Therefore, the study of plants (botany) is
a very important one for the survival of all living things.
As well as investigating the process of photosynthesis, you'll be learning
how to change the presentation of information from words into drawings.
It is sometimes faster to get information from a picture than by having to
read a lot of words, so this is a good thing to be able to do.
What is photosynthesis?
In the last lesson, you discovered that one of the main differences
between plants and animals is that plants can make their own food and
animals cannot.
What is the name of the
process whereby plants make
their own food?
The process is called
photosynthesis.
Photosynthesis is probably the most important process that occurs on
Earth. All life depends on it. Our present knowledge of photosynthesis
is the outcome of many experiments and theories, built up over at least
300 years.
Scientists are curious people, who like to investigate the world around
them and suggest explanations for why things occur. The following
Lesson 7/20 What are plants and animals?
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investigations demonstrate this curiosity and the way that the scientific
community has gone about answering questions about plants.
van Helmont’s experiment
Johann Baptist van Helmont lived in Belgium and his findings were
reported in 1648. At that time, most people thought that plants grew
because they took materials out of the soil. Dr van Helmont devised a
plan to test this idea.
Dr van Helmont was investigating plant growth. He placed 90.9 kg of
dry soil in a pot. In this he planted a 2.3 kg willow shoot. He watered
the plant. After five years he weighed the plant (now a tree). It weighed
77.0 kg. He also weighed the dry soil. It weighed 90.8 kg.
Let’s examine what this
experiment shows.
Activity: van Helmont’s experiment
Analyse the experiment of van Helmont’s experiment by answering the
following questions.
1
How much weight had the plant gained?
_______________________________________________________
2
How much weight had the soil lost?
_______________________________________________________
3
How long did the experiment go on for?
_______________________________________________________
4
What did van Helmont do to the plant during the experiment?
_______________________________________________________
The plant gained 74.7 kg in five years. ( 77.0 – 2.3 = 74.7 kg). The soil lost
0.1 kg in five years. (90.9 – 90.8 = 0.1 kg). The experiment went on for five
years and only water was added during the experiment.
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Living things
The drawing below was done by a Year 7 student, Daniel. This is one
way to show how van Helmont carried out his experiment and the results
he got.
willow
willow
dry soil
dry soil
5 years
2.3 kg
water only
90.9 kg
90.8 kg
77 kg
van Helmont’s experiment
© State of New South Wales, Department of Education and Training, 2004
What can be learned from the experiment?
Now you can see clearly how van Helmont did his experiment and the
results that he obtained. You need to look at what he learned from his
experiment.
Why do you think that the willow tree increased in weight over five
years?
___________________________________________________________
___________________________________________________________
Van Helmont thought that the increase in weight was due to water alone. He
did not consider the possibility that gases from the air might also be involved.
Did you?
What kinds of skills do you think a scientist should have?
___________________________________________________________
___________________________________________________________
Did you say that a scientist should be able to make careful observations,
measure accurately and make inferences? Did you also think that a scientist
needs to be curious and not just accept what other people think? Maybe you
even said that a scientist should be creative because van Helmont had to think
up an experiment by himself.
Lesson 7/20 What are plants and animals?
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What do we know about plants
so far?
Plants grow without eating food.
Plants need water
Joseph Priestley’s experiments
The next evidence for photosynthesis is from the experiments of Joseph
Priestley. Priestley was an Englishman who had been investigating
gases, particularly oxygen.
Gases, or vapours as they were called by most people of the time, were
little understood. Vapours were thought to cause diseases. People closed
their windows and covered their faces to try to protect themselves from
vapours. Priestley, and other scientists of the 1700s, wanted to logically
investigate what gases were and what they did.
Priestley’s first experiment
Joseph Priestley had observed that a candle burning in a sealed jar soon
went out. If a mouse was then put into the jar it soon suffocated.
This was because burning uses up oxygen in the air. Animals need to
breathe in oxygen.
What are the important points in the passage above? Underline them.
Did you choose these points? Burning uses oxygen. The mouse
suffocates without oxygen in the air. The experiment took place in a
sealed jar.
Think about this experiment and then try the following activity.
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Living things
Activity: Joseph Priestley’s experiment
1
Draw a diagram that shows you how Priestley set up his first
experiment.
Priestley’s first experiment
Compare your answer with the one in the answer pages. Remember that
there is no right or wrong drawing as long as all the information is
presented in a way that most people can understand.
Priestley’s second experiment
In another experiment, Priestley burned a candle in a sealed jar. He then
placed a plant in the jar which contained air with no oxygen. The plant
survived. Then Priestley placed a mouse in the jar that had the plant in it.
The mouse did not suffocate.
2
What are the important points from the passage above?
_______________________________________________________
_______________________________________________________
Lesson 7/20 What are plants and animals?
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3
In the box following, draw a diagram to show the information in the
passage called ‘Experiment 2’.
Priestley’s second experiment
Compare your drawing with the one in the answer pages. Does your
drawing show all the information clearly?
Comparing Priestley's experiments
How were Priestley's experiments similar? How were they different?
Complete the following table to compare them.
4
For each experiment, put a tick beside each statement that is true and
a cross beside each statement that is not true.
Experiment 1
a
A candle was burned in a sealed jar.
b
A mouse was put into the sealed jar.
c
A plant was put into the sealed jar.
d
The mouse died.
Experiment 2
Check your response by going to the suggested answers section.
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Living things
A, B and C are all things that Priestley did in his experiments. They are
steps in his method. D is what happened. It is part of his results.
To learn something new from Experiment 2, you need to compare it with
Experiment 1.
There is only one thing that Priestley did differently in his two
experiments. This is called a fair test, or a controlled experiment. It
means that you can see the effect on the results of changing, or varying,
one thing. Controlled experiments are very important in science.
Priestley's experiments
The results could depend on a change made by the candle so he burned
the candle in both experiments. The results could depend on putting the
mouse into the jar so he did this in both experiments.
Making these variables the same in both experiment is called controlling
them, or making them constant.
The only variable that was different was using the plant. It is the only thing
that changed that could have altered the results. The results did change.
The plant made the difference between the mouse living and dying.
Now answer these questions
about Priestley’s experiments.
5
Together, do Experiment 1 and Experiment 2 make a controlled
experiment? Why or why not?
_______________________________________________________
_______________________________________________________
6
What variable is Priestley testing in his experiments?
_______________________________________________________
_______________________________________________________
7
Can you think of a variable in Priestley's experiments that does not need
to be controlled because it would not alter his results? What is it?
_______________________________________________________
Check your response by going to the suggested answers section.
Lesson 7/20 What are plants and animals?
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What can be learned from Priestley’s experiments?
Other scientists performed experiments that showed that the gas in
Priestley's experiments was oxygen. From Priestley's results we can say
that burning uses oxygen, animals need oxygen and plants give out
oxygen.
What do we know about photosynthesis so far?
• Plants grow without eating.
• Plants need water.
• Plants give out oxygen.
• Animals need the oxygen that plants produce.
Jan Ingen-Housz’s experiment
In the late 1700s, the Dutchman, Jan Ingen-Housz, discovered that plants
only gave out oxygen when they were in light. He also showed that only
the green parts of plants give out oxygen. Do you remember what the
green-coloured matter in plants is called? Well done if you remembered
that it is called chlorophyll.
Jean Senebier’s experiment
At the end of the eighteenth century, a Swiss minister named Jean
Senebier worked out that another gas, carbon dioxide, is involved in
photosynthesis. He also reasoned that the larger the plant the more
carbon there was present. Carbon can be seen when plants are burnt.
The bigger the tree, the more charcoal (carbon) Senebier produced.
Senebier showed that the carbon came from the intake of carbon dioxide.
Carry out the activity to summarise what these experiments tell us about
plants.
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Living things
Activity: Summary
Complete this summary using these words:
controlled
oxygen
sunlight
carbon dioxide
food
variables
make/produce
water
tests
Summary
•
Plants make their own ____________________________________ .
•
Plants need ____________________________________________ .
•
Plants give out __________________________________________ .
•
Animals need the oxygen that plants ________________________ .
•
Photosynthesis occurs when plants are in _____________________ .
•
Plants take in _________________________ during photosynthesis.
•
______________________________ are things that can be changed
in an experiment.
•
An experiment should be planned so that it ____________________
one variable only.
•
In a __________________________ experiment, only one variable
is changed; the other variables are kept the same.
Check your response by going to the suggested answers section.
Lesson 7/20 What are plants and animals?
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Now use the information you have
summarised to complete
the exercises.
Preparing for the Exercise
In the Exercise, you'll test your understanding of controlled experiments
and variables, and your skill at presenting information as drawings.
Go to the exercises section and complete Exercises – Discovering
photosynthesis.
What did you achieve?
Tick what you can do.
list the things needed by plants for photosynthesis
complete a summary about photosynthesis and its importance
identify variables in an experiment
present information as drawings.
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Living things
Suggested answers
Check your responses against these suggested answers.
Joseph Priestley’s experiments
1
The drawing below is one way that the information about Priestley’s
experiment on oxygen could be presented.
© State of New South Wales, Department of Education and Training, 2004
2
3
Here are some points that are important in the description of Priestley’s
second experiment. Your points may not be exactly the same. As long as
what you have written means the same then you are doing well.
•
The plant survived in air without oxygen.
•
The plant replaced the oxygen in the air.
•
The mouse did not suffocate because the plant replaced the oxygen –
at least until the mouse used up all the oxygen.
The drawing below is one way of presenting the information about
Priestley’s second experiment with oxygen.
© State of New South Wales, Department of Education and Training, 2004
Your diagram may not be exactly the same but this only means that you
have chosen a different way to show all the important points. As long as
the important points are shown and it is clear for the reader then you are
doing well.
Lesson 7/20 What are plants and animals?
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4
Experiment 1
a
A candle was burned in a sealed jar.
b
A mouse was put into the sealed jar.
c
A plant was put into the sealed jar.
d
The mouse died.
Experiment 2
5
Experiment 1 and Experiment 2 make a controlled experiment because
only one thing changes from Experiment 1 to Experiment 2. Experiment 2
uses a plant but Experiment 1 doesn't.
6
The variable that is being tested is the one thing that Priestley chose to
change – whether or not there was a plant in the test. (The variable that
Priestley observes tells him whether the plant affects the results – he
observes whether the mouse lives or dies.)
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Some variables that would not affect the results are: the kind of jar that was
used (although the size of the jar might not be); the clothes Priestley was
wearing; the weather; the colour of the mice. (All these things are still
variables because they can be changed. It is just that changing them does
not affect the experiment.)
Summary
12
•
Plants grow without food.
•
Plants need water.
•
Plants give out oxygen.
•
Animals need the oxygen that plants make or produce.
•
Photosynthesis occurs when plants are in sunlight.
•
Plants take in carbon dioxide during photosynthesis.
•
Variables are things that can be changed in an experiment.
•
An experiment should be planned so that it tests one variable only.
•
In a controlled experiment, only one variable is changed.
Living things
Exercise
Living things
Exercise
1
Name
___________________________
Teacher
___________________________
Discovering photosynthesis
Complete this summary of photosynthesis using the words below.
that
is
in
to
the
and
for
their
from
as
Plants and animals are living things. They are similar ____________
many ways. Both plants and animals grow, reproduce, respond ____
stimuli and exchange materials with the environment.
Plants and animals obtain their food in different ways. Green plants can
make their own food, whereas animals rely on plants or other animals
_______________________ their food. Animals cannot make their own
food.
The process by which plants make _____________ own food is called
photosynthesis. For photosynthesis to occur, plants need energy raw
materials.
Plants take in carbon dioxide through their leaves _ the air. Water is
absorbed from the soil through the roots. These are the raw materials
__________________________ plants need to make food.
Energy is needed so that plants can make their own food. The source of
this energy ___________________ sunlight. Green plants contain
chlorophyll. Chlorophyll is the green pigment that can trap energy from
sunlight.
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Photosynthesis produces oxygen and water as well ____________ food
in plants. Oxygen and water are given out by plants through the leaves
during ___________________ day, since photosynthesis only occurs
when there is light.
Animals rely on plants for their food and their oxygen.
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Living things