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Ch 21 Photosynthesis
Practical 21.1 Detection of starch produced from photosynthesis
(the iodine test)
Results (p. 21-3)
Colour of
the leaf
Before adding iodine
solution
After adding iodine
solution
Pale green
Blue-black
Questions (p. 21-3)
1
This destroys the differentially permeable nature of the cell membrane and thus allows the
iodine solution to pass into the cells. This also kills the leaf cells and stops chemical
reactions like photosynthesis.
2
Absolute alcohol dissolves and removes chlorophyll from the leaf, so that any colour
changes can be observed clearly on this decolourized leaf. Besides, boiling alcohol speeds
up the diffusion of chlorophyll out of the cells.
3
This softens the leaf because alcohol makes the leaf hard.
4
Starch is present in the green leaf. Photosynthesis has taken place.
Practical 21.2 Detection of oxygen produced from photosynthesis
Results (p. 21-5)
Yes
Questions (p. 21-5)
1
To supply enough carbon dioxide for photosynthesis.
2
To allow free circulation of solution.
3
The gas released from the Hydrilla plants is oxygen. Photosynthesis has taken place.
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Practical 21.3 Investigation of the need for chlorophyll in
photosynthesis
Results (p. 21-7)
Colour pattern of the leaf
Result of the iodine test
Questions (p. 21-8)
1
To remove starch in the leaves. This ensures that any starch found at the end of this
experiment is newly made.
2
To confirm that starch is absent in the leaves before the experiment starts.
3
The non-green part serves as the control. This is because the non-green part does not
contain chlorophyll.
4
The green part turns blue-black upon the addition of iodine solution. This shows that it
produces starch.
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Conclusion (p. 21-8)
Starch is made only in the presence of chlorophyll. Chlorophyll is needed for photosynthesis.
Practical 21.4 Investigation of the need for carbon dioxide in
photosynthesis
Results (p. 21-10)
Leaf
Presence of starch
A (exposed to normal air)
+
B (exposed to air without carbon dioxide)
–
Questions (p. 21-10)
1
Leaf B is the control because of the absence of carbon dioxide in the flask.
2
Use a plastic bag to enclose leaves A and B. Put potassium hydroxide pellets into the bag
of leaf B. Seal the mouth of each bag by tying it around the leaf stalk.
Conclusion (p. 21-10)
Starch is made only in the presence of carbon dioxide. Carbon dioxide is needed for
photosynthesis.
Practical 21.5 Design an investigation of the need for light in
photosynthesis
Propose a hypothesis (p. 21-12)
Light is needed for photosynthesis.
Design and perform an experiment (p. 21-12)
By masking the leaf with black paper.
A Identifying variables
Independent
Controlled
Dependent variable
variable
variables
(What will you
(What will you
(What will you keep
measure?)
change?)
constant?)
The presence of
The presence of
Carbon dioxide
light.
starch in the leaf.
concentration in the
surroundings,
temperature, water
supply, etc.
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Control
(What is the
control in this
experiment?)
The masked part of
the leaf, i.e. the part
that does not receive
any light.
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B Designing the set-up
(Answer varies with Ss.)
C Collecting data
1
(Answer varies with Ss.)
2
Provide a higher light intensity to the plant.
3
Mask several leaves with black paper, rather than masking only one leaf. Test those leaves
for starch. Repeat the experiment with different green plants.
D Risk assessment and safety precautions
1
The scissors used to cut the paper are very sharp and may cut our fingers. / Alcohol is
inflammable. / Iodine solution is an irritant.
2
Handle the scissors with care. / Do not boil the alcohol directly over a Bunsen flame. /
Wear disposable gloves.
Write an experimental report (p. 21-14)
Objective
To find out if light is needed for photosynthesis.
Hypothesis
Light is needed for photosynthesis.
Apparatus and materials
1 beaker (250 cm3)
1 boiling tube
1 pair of forceps
1 dropper
1 Bunsen burner
1 tripod with wire gauze
1 heat insulating mat
1 test tube holder
1 white tile
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1 pair of scissors
1 pair of safety goggles
1 pair of disposable gloves
1 paper hole-punch
black paper
paper clips
absolute alcohol
iodine solution
1 green dicotyledonous potted plant
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Procedure
1
Destarch a plant by putting it in the dark for at least 24 hours. Pick a leaf and test it for
starch (see Practical 21.1).
2
Cut out several identical black paper masks and punch a hole at the centre.
3
Attach a paper mask to both sides of some of the leaves with paper clips.
4
Expose the whole plant to bright light for about 5 hours.
5
Remove the paper masks and test the leaves for starch (see Practical 21.1).
Results
Analysis and discussion
1
The unmasked parts of the leaf turn blue-black after adding iodine solution. This shows
that the unmasked parts produce starch because they are illuminated by light for
photosynthesis. The masked parts of the leaf remain brown after adding iodine solution.
This shows that the masked parts cannot produce starch because they do not receive any
light for photosynthesis.
2
(Answer varies with the design.)
Conclusion
Starch is made only in the presence of light. Light is needed for photosynthesis.
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Practical 21.6 Examination of the structure of leaves
Results (p. 21-17)
External structures of a dicotyledonous leaf
Transverse section of a dicotyledonous leaf
Questions (p. 21-18)
1
Palisade mesophyll cells, spongy mesophyll cells and guard cells.
2
a
The broad and flat leaf blade provides a large surface area for absorbing light. The
thin leaf allows light to reach the photosynthetic cells easily. The cuticle is
transparent to allow light to pass through. The upper epidermis is one-cell thick and
colourless to allow light to reach the chloroplasts below. The palisade mesophyll cells
are tightly packed and contain many chloroplasts. This allows effective absorption of
light.
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b
The network of veins ensures the materials are transported to all cells quickly. The
midrib contains a vascular bundle made up of xylem and phloem. This provides
efficient transport of materials. Xylem transports water and minerals to the leaf.
Phloem transports food away from the leaf.
c
The thin leaf allows gases to diffuse rapidly. The spongy mesophyll cells are loosely
packed with many air spaces. This allows gases to diffuse freely inside the leaf. The
upper and lower epidermis contains numerous stomata. This allows gases to pass into
and out of the leaves freely. Guard cells are present to control the opening and closing
of stomata. Therefore, the rate of gas exchange can be regulated.
The cuticle covering the epidermis is impermeable to water. Only a few or no stomata are
present on the upper epidermis to reduce water loss by evaporation. Guard cells close the
stomata when the conditions are not favourable for photosynthesis.
Practical 21.7 Examination of the structure of chloroplasts
Results (p. 21-21)
Questions (p. 21-21)
1
Chloroplasts can be found in all photosynthetic cells, such as palisade mesophyll cells,
spongy mesophyll cells and guard cells.
2
a
A large number of thylakoids provide a large surface area to pack more chlorophyll
for absorbing light.
b
The stroma provides a site to hold starch grains which are temporary stores of the
photosynthetic products.
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The interconnected grana allow efficient transport of photosynthetic products within
the chloroplast.
Practical 21.8 Investigation of the effect of light intensity on the
rate of photosynthesis
Results (p. 21-24)
1
Distance
(m)
Light intensity
(1 / distance2)
0.1
100
0.2
25
0.3
11
0.4
6
0.5
4
2
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Rate of photosynthesis (cm3 / min)
Reading 1
(Results vary
with Ss.)
Reading 2
Reading 3
Average
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Questions (p. 21-25)
1
Factors other than the one being investigated (light intensity in this practical) must be kept
constant or conclusion cannot be drawn.
2
To allow the rate of photosynthesis to become steady.
3
Taking the average of three readings gives a more accurate result.
4
At low to moderate level of light intensity, the rate of photosynthesis increases
proportionally with increasing light intensity. This is because more energy is provided to
the plants to carry out photosynthesis.
When light intensity has reached a certain level, the rate of photosynthesis does not
increase with further increase in light intensity. This is because other factors such as
carbon dioxide concentration become limiting.
5
Error: There may be other light sources.
Improvement: We can improve this by carrying out the practical in a dark room.
6
Record the number of bubbles given off per unit time.
Conclusion (p. 21-26)
The rate of photosynthesis increases with increasing light intensity. The increase stops when a
certain level of light intensity is reached.
Practical 21.9 Design an investigation of the effect of carbon
dioxide concentration on the rate of photosynthesis
Design and perform an experiment (p. 21-28)
1
By using sodium hydrogencarbonate solution of different concentrations.
2
Measure the volume of oxygen released per unit time.
A Identifying variables
Independent
Controlled
Dependent variable
variable
variables
(What will you
(What will you
(What will you keep
measure?)
change?)
constant?)
Carbon dioxide
The volume of
Temperature, light
concentration.
oxygen released per intensity, time
unit time.
allowed for the
release of oxygen,
etc.
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Control
(What is the
control in this
experiment?)
An apparatus that has
sodium
hydrogencarbonate
solution replaced
with distilled water.
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B Designing the set-up
(Answer varies with Ss.)
C Collecting data
1
(Answer varies with Ss.)
2
Provide a higher light intensity.
3
Allow the plants to equilibrate before taking any readings.
Take the average of three readings at each concentration of solution.
D Risk assessment and safety precautions
1
The razor blade used to cut the plant is very sharp and may cut our fingers.
2
Handle the razor blade with care.
Write an experimental report (p. 21-30)
Objective
To investigate the effect of carbon dioxide concentration on the rate of photosynthesis.
Apparatus and materials
1 pipette
1 boiling tube
1 beaker (500 cm3)
1 thermometer
1 stand and clamp
1 clip
1 100-watt bench lamp
1 razor blade
1 meter ruler
rubber tubing
0.1%, 0.15%, 0.2%, 0.25%, 0.3% and 0.4%
sodium hydrogencarbonate solution
distilled water
Hydrilla plants
Procedure
1
Cut the stem of a Hydrilla plant to about 10 cm long.
2
Set up the apparatus on p. 24 (but replace sodium hydrogencarbonate solution with
distilled water). Check the thermometer at intervals to ensure a constant temperature.
Renew the water if necessary.
3
Turn on the lamp and allow the plant to equilibrate for 5 minutes.
4
Suck up the distilled water from the boiling tube. Close the clip completely. Record the
starting position of the meniscus in the pipette.
5
After 5 minutes, record the final position of the meniscus in the pipette. Calculate the rate
of photosynthesis (volume of oxygen released per minute). Record two more readings.
6
Repeat steps 2 to 5 with 0.1%, 0.15%, 0.2%, 0.25%, 0.3% and 0.4% sodium
hydrogencarbonate solution instead of distilled water.
7
Draw a graph of rate of photosynthesis against concentration of sodium
hydrogencarbonate solution.
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Results
Analysis and discussion
1
From 0 to 0.25% sodium hydrogencarbonate solution, the rate of photosynthesis increases
rapidly with increasing carbon dioxide concentration. This is because more carbon dioxide
is provided to the plants as raw material to carry out photosynthesis.
From 0.25% to 0.4% sodium hydrogencarbonate solution, the rate of photosynthesis slows
down with increasing carbon dioxide concentration. This is because other factors such as
temperature become limiting.
2
Error: Carbon dioxide in air may dissolve into the sodium hydrogencarbonate solution.
Improvement: Cover the opening of the boiling tube with plastic food wrap.
Conclusion
The rate of photosynthesis increases with increasing carbon dioxide concentration.
The increase stops when a certain level of carbon dioxide is reached.
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