Certificate Biology - New Mastering Basic Concepts Suggested answers to Activity Books Ch 7
Chapter 7 Photosynthesis
Practical 7.1 Detection of starch produced from photosynthesis (the starch test)
(Bk 1, p.102)
Results
From pale green to blue-black.
Questions
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.
(Bk 1, p.103)
3 This softens the leaf because alcohol makes the leaf hard.
4 Starch is present in the green leaf. Photosynthesis has taken place.
Practical 7.2 Detection of oxygen produced from photosynthesis
(Bk 1, p.104)
Results
Yes.
Questions
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.
Practical 7.3 Investigation of the need for chlorophyll in photosynthesis
(Bk 1, p.105)
Results
Colour pattern of the leaf / Result of the starch test(Bk 1, p.106)
Questions
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 yellow part serves as the control. This is because the yellow part does not contain chlorophyll.
4 The green part turns blue-black upon the addition of iodine solution. This shows that it
produces starch.
Conclusion
Starch is made only in the presence of chlorophyll. Chlorophyll is needed for photosynthesis.
Practical 7.4 Investigation of the need for carbon dioxide in photosynthesis
(Bk 1, p.107)
Results
Leaf / Presence of starchA (exposed to normal air) / Yes
B (exposed to air without carbon dioxide) / No
(Bk 1, p.108)
Questions
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
Starch is made only in the presence of carbon dioxide. Carbon dioxide is needed for photosynthesis.
Practical 7.5 Design an investigation of the need for light in photosynthesis
(Bk 1, p.109)
Design and perform an experiment
1 By masking the leaf with black paper.
2 Carbon dioxide concentration in the surrounding, temperature, water supply, etc.
3 The presence of light.
4 The presence of starch in the leaf.
5 The masked part of the leaf, i.e. the part that does not receive any light.
6 (Answer varies with Ss.)
7 Provide a higher light intensity to the plant.
8 Mask several leaves with black paper, rather than masking only one leaf. Test those leaves for starch. Repeat the experiment with different green plants.
9 Before the experiment, destarch the plant and test a leaf for starch.
(Bk 1, p.110)
Write an experimental report
Objective
To find out if light is needed for photosynthesis.
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 7.1).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 7.1).
Results
Analysis and discussion
1 The unmasked parts.
2 (Answer varies with the design.)
Conclusion
Starch is made only in the presence of light. Light is needed for photosynthesis.
Practical 7.6 Investigation of the effect of light intensity on the rate of photosynthesis
(Bk 1, p.112)
Results
1
Distance (m) / Light intensity (1 / distance2) / Rate of photosynthesis (cm3 / min)Reading 1 / Reading 2 / Reading 3 / Average
0.1 / 100
0.2 / 25
0.3 / 11 / (Results vary / with Ss.)
0.4 / 6
0.5 / 4
2
(Bk 1, p.113)
Questions
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 are other light sources.
Improvement: Carry out the practical in a dark room.
6 Record the number of bubbles given off per unit time.
Conclusion
The rate of photosynthesis increases with increasing light intensity. The increase stops when a certain level of light intensity is reached.
Practical 7.7 Design an investigation of the effect of carbon dioxide concentration on the rate of photosynthesis
(Bk 1, p.114)
Design and perform an experiment
1 Temperature, light intensity, time allowed for the release of oxygen, etc.
2 Carbon dioxide concentration.
3 By using sodium hydrogencarbonate solution of different concentrations.
4 The volume of oxygen released per unit time.
5 An apparatus that has sodium hydrogencarbonate solution replaced with distilled water.
6 (Answer varies with Ss.)
7 Provide a higher light intensity.
8 Allow the plants to equilibrate before taking any readings.
Take the average of three readings at each concentration of solution.
9 Include a heat shield and check its temperature at intervals.
Make sure the apparatus is air-tight by closing the clip completely.
(Bk 1, p.115)
Write an experimental report
Objective
To investigate the effect of carbon dioxide concentration on the rate of photosynthesis.
Apparatus and materials
1 pipette1 boiling tube
1 beaker
1 thermometer
1 retort stand and clamp
1 rubber tubing
1 clip / 1 100-watt bench lamp
1 razor blade
1 meter ruler
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.111 (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 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.
Results
(Bk 1, p.116)
Analysis and discussion
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.
Conclusion
The rate of photosynthesis increases with increasing carbon dioxide concentration. It slows down gradually when other factors become limiting.
Practical 7.8 Test for the different types of food in plants
(Bk 1, p.119)
Results
(Results vary with the plant materials studied)
Questions
1 No.
2 Starch is used for storage.
Glucose is used for generating energy to drive metabolic reactions.
Proteins are used for growth and repair.
Fats and oil are used for forming cell membranes and storage.
Practical 7.9 Examination of the structure of leaves
(Bk 1, p.120)
Results
External structures of a dicotyledonous leaf
Transverse section of a dicotyledonous leaf
(Bk 1, p.121)
Questions
1 Palisade mesophyll cells, spongy mesophyll cells and guard cells.
2 a The leaf blade is broad to provide a large surface area for light absorption.
The leaf is thin to allow light to reach the mesophyll 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 chlorophyll-containing palisade mesophyll cells are closely-packed and in the upper region of the leaf to receive more light.
b It has an extensive network of veins to make sure that materials are transported to all cells quickly. Veins are actually made up of vascular bundles which consist of the xylem and phloem. The xylem transports water into the leaf. The phloem transports food out of the leaf.
c The leaf is thin to allow rapid diffusion of gases.
A large number of air spaces in the spongy mesophyll allow gases to move freely inside the leaf.
The leaf has stomata which allow gases to pass into and out of the leaf.
3 Each leaf surface is covered by a layer of cuticle which is impermeable to water.
The upper epidermis, which exposes to the sun, has a thicker layer of cuticle.
More stomata are found on the lower epidermis, rather than the upper epidermis.
Stomata close up when conditions are not favourable for photosynthesis.
STS connection 7.1 Enhanced plant growth in a greenhouse
(Bk 1, p.124)
Task 1
1 It allows sunlight to reach the plants inside the greenhouse so that they can carry out photosynthesis.
2 Photosynthesis is controlled by enzymes. Their activity increases with temperature, until the optimum temperature is reached. Therefore, warm air enhances plant growth by increasing the rate of photosynthesis.
3 It helps control the temperature by allowing hot air to escape when the temperature becomes too hot. / It helps control the humidity by allowing moisture to escape when the humidity becomes too high. / It increases the rate of photosynthesis by allowing carbon dioxide to go into the greenhouse on a sunny day, when carbon dioxide concentration is a limiting factor.
4 Plants are better protected from bad weather, pests, diseases and pollution.
Carbon dioxide given out by the plants and heat are trapped inside the greenhouse.
The increases in carbon dioxide concentration and temperature lead to a higher rate of photosynthesis.
Exercise 7
(Bk 1, p.125)
Multiple-choice questions
1 A 2 C 3 D 4 D 5 C 6 D
7 B
(Bk 1, p.126)
Structured questions
1 a Light (0.5m) and water (0.5m)
b Starch made by the plant before the experiment may interfere with the result. (1m)
As a result, starch must be removed (or destarched) first by putting it in the dark. (1m)
c Leaf B would show a positive result (i.e. blue black) (0.5m), while leaf A would show a negative result (i.e. brown) (0.5m).
It was because the potassium hydroxide pellets absorbed the carbon dioxide in the flask of leaf A (0.5m), which is essential to photosynthesis (0.5m). Therefore, leaf A could not carry out photosynthesis to produce starch (0.5m). Leaf B could carry out photosynthesis (0.5m) with all essential conditions present (0.5m) to produce starch (0.5m).
d Leaf A was the control (1m). It was used to show that carbon dioxide is necessary for photosynthesis (1m).
(Bk 1, p.127)
2 a Chloroplast (1m)
b Stage 1: light reactions (1m); Stage 2: dark reactions (1m)
Stage 1 requires light. (1m)
c P: oxygen; Q: carbon dioxide; X: water; Y: glucose / carbohydrates (0.5m each)
d Rate of photosynthesis increases with increasing light intensity because more energy is supplied to stage 1 (1m). More hydrogen will be produced for stage 2 (1m). However, the rate will stop increasing (1m) because other factors (e.g. carbon dioxide concentration) limit the rate of stage 2 (1m).
e It maintains the oxygen and carbon dioxide balance. (1m)
It provides food to other organisms directly or indirectly. (1m)
(Bk 1, p.128)
3 a Water, light, chlorophyll and carbon dioxide. (0.5m each)
b The gas heater can increase the rate of photosynthesis by increasing the temperature (1m). Increase in temperature can increase the rate of enzymatic reactions in photosynthesis (1m). Moreover, the heater can supply more carbon dioxide for photosynthesis (1m). The more the raw materials provided to the plants, the higher the photosynthetic rate can be reached (1m).
c The plant will eventually die (1m).
It is because green glasses only allow green light to pass through (1m). However, green leaves reflect most green light and thus no food can be produced by photosynthesis (1m).
d The plants are better protected against pests / drastic climatic changes. (1m)
ã Oxford University Press 2005 Page 6 / 10