Download Biology-Limiting Factors

Photosynthesis — limiting factors
What you should know by the end of this unit:

Photosynthesis is the process by which green plants make their own food.

Light energy is trapped by the green pigment chlorophyll.

Chlorophyll is found in structures called chloroplasts.

Green plants make sugar from carbon dioxide and water.

The energy to synthesise (make) the food is obtained from the sunlight
absorbed by chlorophyll.

Oxygen is a by-product of photosynthesis.

The sugar made by plants during photosynthesis is a high-energy chemical
called a carbohydrate.

A plant uses this sugar as a source of energy to grow.

A plant can store sugar as starch.

Iodine solution turns blue-black when starch is present.

Light, chlorophyll and carbon dioxide are the three requirements for
photosynthesis.

A limiting factor is an environmental factor which slows down the rate of
photosynthesis if it is in short supply.

Light intensity, carbon dioxide concentration and temperature limit the
rate of photosynthesis if they are in short supply.
Skills that you should be able to carry out by the end of this unit:

Design a table with appropriate headings.

Draw a line graph from a table of results.

Interpret information presented as a line graph.

Work as part of a group and take responsibility to support the work of the
group.

Write up experiments with an aim, method, results and conclusion.

Appreciate the importance of variables in an experiment and how to
improve the reliability of results.
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
Photosynthesis
copy this heading
Photosynthesis is the process by which green plants make their own
food.
During photosynthesis green leaves convert light energy to chemical
energy which is contained in sugar.
The green plants make sugar from dioxide and water.
This process involves the trapping of light energy from the sun.
Light energy is trapped by the green pigment chlorophyll. Chlorophyll
is found in structures called chloroplasts.
Photosynthesis goes on mainly in the leaves, though any green part of
the plant can photosynthesise.
Oxygen is given off as a by-product.
Word equation for photosynthesis
Chlorophyll
Carbon dioxide + water
Light
sugar + oxygen
raw materials
products
Essential requirements
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Water and minerals are absorbed from the soil by the roots and
drawn up the stem through tubes to the leaves where the water is used
for photosynthesis.
roots
Carbon dioxide is absorbed from the air through tiny pores mainly on
the underside of the leaf.
Open
Closed
pores

Activity 1
Collect the photosynthesis true/ false/ don’t know cards.
Work in pairs and use the resources available to sort the cards.
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Activity 2 Answer these questions in sentences
1. Name the process by which green plants make food.
2. Where does the green plant get the energy to synthesis food?
3. Name the pigment that traps this energy and where it can be
found.
4. What energy change takes place during photosynthesis?
5. (a)Name the two raw materials green plants use to make
food.
(b) Explain how these raw materials get into the plant.
6. (a)Name the products of photosynthesis.
(b) Which of these is the by-product?

Food storage in plants
copy this heading
The food (sugar) made by plants during photosynthesis is a high-energy
chemical and is a type of carbohydrate. If a plant does not
immediately use this sugar as a source of energy to make new cells and
grow, the plant will store the sugar as starch.
This starch is the plant’s store of food and can be converted back to
sugar for energy when needed.
Some sugar will be stored as starch in the leaves and some will be
transported to other parts of the plant.
Cells in a leaf
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Stored sugar
Fruit is an examples of a part of a plants containing stored sugar e.g.
grapes, plums, oranges.
Other examples of parts of plants containing stored starch are tubers
and seeds such as potatoes, beans, corn, wheat and rice.
Activity 3

copy and complete this note
The sugar made by plants during photosynthesis is a high energy
c______________.
The sugar can be used by the plant immediately as a source of energy
to make n____ c______ and g____ or the plant can store the sugar as
s______.
This starch can be c_______ back into sugar when the plant needs
e___________.
Starch can be stored in the leaves or in s______ or f________.
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Testing leaves for starch

copy this note
We can use the presence of starch in a leaf as proof that photosynthesis
has taken place. We test the leaf with iodine solution.
Iodine solution turns blue-black when starch is present.
It is important that the plants have no starch in them at the start of the
experiment. Potted plants are de-starched by leaving them in a dark
cupboard for two or three days
Activity 4

Collect this instruction sheet and stick it into your jotter
Procedure to test a leaf for starch:
1. Boil the leaf in a beaker of water for one minute (to
burst cell membranes).
2. Switch the Bunsen burner off.
3. Place the leaf into a boiling tube of
ethanol. Place the boiling tube into
the freshly boiled water. Leave until
all the chlorophyll is dissolved out.
3. Remove the leaf from the boiling
tube and rinse it in the beaker of
water. Spread the leaf out over a
white tile and test it for starch using
iodine solution.
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Activity 5


Activity 6
Complete the starch test sort cards
Collect the results sheet shown below.
You are going to test a leaf from plant A and plant B for starch.
Record your result and your conclusion on the sheet.
Is light necessary for photosynthesis?
Set up
starch present?
Plant
A
Conclusion
Green plant
that has been
destarched
Black
plastic
bag
Plant
B
Green plant
that has been
destarched
Clear
plastic
bag
Tidy away all your equipment and get your teacher to check
your results and conclusion.
.
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
Activity 7
Collect the results sheet shown below.
You are going to test a leaf from plant A and plant B for starch.
Record your result and your conclusion on the sheet.
Is carbon dioxide necessary for photosynthesis?
Set up
Result
Conclusion
Plant
A
Green plant
that has been
destarched
Clear
plastic
bag
Sodium
hydroxide
solution
Plant
B
Green plant
that has been
destarched
Clear
plastic
bag
Water
Tidy away all your equipment and get your teacher to check
your results and conclusion.
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
Activity 8
Collect the results sheet shown below.
You are going to test a leaf from a variegated plant for starch.
Green part contains
chlorophyll
White part does not
contain chlorophyll
Record your result and your conclusion on the sheet.
Is chlorophyll necessary?
Set up
Result
Conclusion
Variegated plant
that has been in
the dark for 2
days to destarch
Clear
plastic
bag
Green
(chlorophyll)
White
(no chlorophyll)
Tidy away all your equipment and get your teacher to check
your results and conclusion.
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Activity 9

Answer these questions in sentences
1. Name the chemical reagent used to test for starch.
2. Describe the colour change shown by this reagent if starch is
present.
3. Copy and complete
Photosynthesis will not take place in the absence of
 c_________ d__________
 l_____________
 c__________________

Limiting factors
copy this note
A limiting factor is an environmental factor which if in short
supply slows down the rate at which a plant can carry out
photosynthesis
Activity 10 Group activity
THINK/ PAIR/ SHARE
Can you think of an environmental factor which will affect the rate of
photosynthesis? Share and discuss with your partner and the other
groups in the class.
Make a list in your jotter.
Use the help cards if you find this difficult.
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We can measure the rate of
photosynthesis using aquatic plants
like Elodea and Cabomba.
During photosynthesis, these
plants give off bubbles of oxygen.
The rate of photosynthesis can be
measured by counting the number
of bubbles produced or by
collecting the oxygen and
measuring its volume.
Activity 11
The effect of light intensity
on the rate of photosynthesis

Copy this heading
Look at the equipment shown in the diagram below.
This equipment can be used to find the effect of changing light
intensity on the rate of photosynthesis.
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 Discuss in your groups how you could change the light intensity
the plant is receiving.
 Discuss what variables have to be kept the same in order to make
it a fair experiment.
 Think about how your group would measure the rate of
photosynthesis.

Use the information gathered during your group
discussion to Copy and complete this table.
What is the aim of this
experiment?
What are you changing?
How are you changing it?
What will you keep the same?
How will you measure the
rate of photosynthesis?
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Use the webpage “water weed”
 Set the carbon dioxide levels to 2.0
 Set the light intensity to 1 and note the rate of photosynthesis in
your jotter.
 Repeat for a light intensity of 2 to a light intensity of 10,
remember to note down your results.


Copy and complete the table below into your notes.
Light
intensity
Rate of photosynthesis
1
2
3
4
5
6
7
8
9
10
 Collect graph paper and draw a line graph of your results.
Answer the following questions in sentences
1. What happens to the rate of photosynthesis as the light
intensity increases?
2. Why does the graph level off?
3. What could be limiting the rate of photosynthesis at this point?
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
Copy this heading
Activity 12
The effect of carbon dioxide concentration
on the rate of photosynthesis
 Look at the equipment shown in the diagram below.
This equipment can be used to find the effect of changing Carbon
dioxide concentration on the rate of photosynthesis.
 Discuss in your groups how you could change the carbon dioxide
concentration the plant is receiving.
 Discuss what variables have to be kept the same in order to make
it a fair experiment.
 Think about how your group would measure the rate of
photosynthesis.

Copy and complete this table
What is the aim of this
experiment?
What are you changing?
How are you changing it?
What will you keep the same?
How will you measure the
rate of photosynthesis?
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Use the webpage “water weed”
 Set the light intensity to 7
 Set the carbon dioxide levels to 2.0 and note the rate of
photosynthesis in your jotter.
 Repeat for a carbon dioxide level of 2 to a level of 10; use the
results to copy and complete the table below
Carbon dioxide
concentration
Rate of
photosynthesis
2
3
4
5
6
7
8
9
10
 Collect graph paper and draw a line graph of your results.
Answer the following questions in sentences
1. What happens to the rate of photosynthesis as the carbon dioxide
concentration increases?
2. Why does the graph level off?
3. What could be limiting the rate of photosynthesis at this point?
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Activity 13
The effect of temperature on the rate
of photosynthesis
 Design an experiment to find the effect of changing temperature
on the rate of photosynthesis.
 Discuss in your groups how you could change the temperature the
experiment is at.
 Discuss what variables have to be kept the same in order to make
it a fair experiment.
 Think about how your group would measure the rate of
photosynthesis.

Copy and complete this table
What is the aim of this
experiment?
What are you changing?
How are you changing it?
What will you keep the same?
How will you measure the
rate of photosynthesis?
Activity 12

Copy this table of results into your notes.
Temperature (OC)
Rate of
photosynthesis
5
10
20
30
40
50
60
70
1
4
9
26
35
30
0
0
 Collect graph paper and draw a line graph of these results.
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Answer the following questions in sentences
1. What happens to the rate of photosynthesis as the temperature
increases?
2. What is the optimum temperature?
3. Why does the rate of photosynthesis fall above the optimum
temperature?
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