Living Things and Air - 7.2 Photosynthesis (OCR)

Living Things and Air Oxygen life and carbon processes respiration and dioxide are both of living things, gas exchange. We involved in three essential including photosynthesis, will further discuss these processes in this Unit. 7fi Photosynthesis . [} Plants make food by photos...

Living Things and Air Oxygen life and carbon processes respiration and dioxide are both of living things, gas exchange. We involved in three essential including photosynthesis, will further discuss these processes in this Unit. 7fi Photosynthesis . [} Plants make food by photosynthesis Plants can make their own food by photosynthesis. This process needs raw materials and an energy source to make food. Oxygen is also produced as a by-product in the process. WCarbon dioxide and water are the raw materials for photosynthesis. Carbon dioxide is obtained from the air. Water is absorbed from the soil. W1 Energy needed for photosynthesis comes from light. The light energy is 4‘ absorbed by chlorophyll, a green pigment in the chloroplasts of green leaves or other green parts of plants. During photosynthesis, light energy is converted into chemical energy in the food produced. Glucose is the food produced. The plants use glucose for growth and respiration, Glucose that is not used immediately is converted into starch for temporary storage. Starch can be converted back into glucose for use later. (Both glucose and starch are carbohydrates. You will learn more in Unit 12.) MOxygen Table 7.1 is produced as a by-product and is given out into the air. Photosynthesis needs raw materials and an energy source to make food. A by-product is also produced. D ypically, leaves are flat, broad . ® and green. How are these features = . related to their functions? . \'\ light energy i » water (from the soil) carbon dioxide (from the air) Fig. 7.5 The process of photosynthesis photosynthesis SKA(ER chlorophyll mfi; glucose T starch T carbohydrate BzKfc2%) '4 M G oxygen (given out into the air) The process of photosynthesis can be summarized by the word equation below: light energy absorbed by chlorophyll carbon dioxide + water glucose + oxygen chemical formulae in Unit 6, Book 1B. In this chemical Besides word equation, we can also write a chemical equation to summarize the process of photosynthesis: equation, the substances are represented by their chemical formulae. light energy absorbed by chlorophyll [substance 6CO, Carbon dioxide Water H,0 Glucose CeHizOg Oxygen o, * You will learn more about how to write chemical equations in Unit 13, Book 3B. + 6H,0O — 2 C¢H{;,0; + 60, From the chemical equation, we can see that: carbon dioxide reacts with water to form glucose and oxygen. the total number of each atom (carbon atom, oxygen atom and hydrogen atom) is the same on both sides of the equation. Change and constancy in photosynthesis In the above chemical equation of photosynthesis, reactants react to give the products (change) while the numbers of different atoms remain the same before and after the reaction (constancy). Energy is also conserved (constancy). It only changes from one form (light energy) to another (chemical energy in glucose) during photosynthesis. word equation XFHBH chemlcal equation {LSH5R NOISN3LX3 / ", You have learned about ' Plants make their own food by Living Things and Air 1. l ) ( Checkpoi’nt 7.2 Chcpl:;szt e-Checkpoint Write ‘T’ for a true statement and ‘F’ for a false statement. (a) [l [ L Chlorophyll is a green pigment present in chloroplasts. (b) Starch produced in plants can be used immediately. (c) 2. Photosynthesis is an energy conversion process. Which of the following statements about photosynthesis are correct? (1) Carbon dioxide is needed for photosynthesis. H3. (2) Oxygen is produced in photosynthesis. (3) It converts light energy into chemical energy. A. (1) and (2) only B. (1)and(3)only C. (2)and (3) only D. (1),(2)and (3) D Write the chemical equation for photosynthesis. ) Test for the products of photosynthesis I can't see q plant carrYiNg out photosynthesis. How can I know if photosynthesis has actually taken place> Fig. 7.6 How canyou show that photosynthesis has taken place? Do you have similar questions? We cannot experiments see a plant can provide carrying out photosynthesis. However, evidence of photosynthesis. If we can show from experiments that the plant has produced starch and oxygen, we can confirm that photosynthesis has taken place. Now, let us do Experiments 7.3 and 7.4 to test for starch and oxygen formed in photosynthesis, respectively. '\ Experiment 7.3 Experiment video a f ! ev07s3 Testing for starch in green leaves Material and apparatus spot plate 1 tripod 1 white tile 1 dropper 1 wire gauze L| distilled water potted green plant 1 gas lighter 1 starch solution beaker (250 cm?) 1 forceps 1 pair iodine solution Bunsen burner heat-proof mat 1 1 test tube holder boiling tube 1 1 alcohol (70%) Procedure Part A: lodine test 1. Add one to two drops of starch solution to one well ' on a spot plate. 2. Clean the dropper. Then, add one to two drops of iodine distilled water to another well. 3. Add well. one to two drops of iodine solution to each record and Observe solution\, any colour change b ——distilled water: e of = iodine solution. lodine solution isaddedto h starch soluti solution 1 | Colour change of iodine solution [D Caution lodine is harmful. Avoid contact with skin distilled water The set-up with distilled water is a control set-up. By comparing the results of the experimental set-up (starch solution) and that of the control (distilled water), we can conclude that the colour change is due to the presence of starch. Part B: Testing for starch in green leaves 4. Remove a leaf from several hours. lodine solution B control set-up ¥{JRYCHE a potted green plant that has been placed in bright light for Living Things and Air [N 5. Put the about leaf into a beaker two o of boiling water This minutes. destroys the — for ) - ! ! cell membranes of the leaf cells. 6. === \Wear safety goggles. b Caution Handle hot water with care. Turn off the Bunsen burner. Use a pair of forceps to transfer the leaf to a boiling tube containing alcohol. Then, put the boiling tube into the beaker of hot water for about 10 minutes. m Caution Alcohol is flammable. Never heat the alcohol directly over the Bunsen flame. What happens to the colours of the alcohol and the leaf? What is the use of alcohol in this experiment? The 7. alcohol becomes _______ in colour and the in colour. Alcohol removesthe ______ Use a pair of forceps to remove the leaf from the alcohol. Wash the leaf in the beaker of hot water for about 30 seconds. This removes the alcohol and makes the leaf soft. leaf becomes from the leaf. b b l N 8. Use a pair of forceps to take the leaf out of the hot water and carefully spread it on a white tile. Add a few drops of iodine solution to it. D Caution lodine is harmful. Avoid contact with skin. iodine solution What is the colour of the leaf now? What does this show? The leafisnow ____ in colour. This shows that 3 * We can use iodine solution to test for starch. lodine solution changes from brown to blue-black in the presence of starch. * Steps of testing for starch in green leaves and the purpose of each step are shown below: 1. Put the leaf in To destroy the cell membrane boiling water 2. 3. 4. Putthe leafin To remove the chlorophyll from hot alcohol the leaf Wash the leaf in To remove the alcohol and hot water make the leaf soft Add iodine solution To test for the presence of starch ‘ to the leaf in the leaf | | ‘ Living Things and Air Experiment \ Q Experiment video Demonstration YW af ! ev07s4 Testing for oxygen produced by photosynthesis Material and apparatus transparent plastic bottle with cap 1 wooden splint 1 spatula table lamp 1 1 gas lighter sodium hydrogencarbonate powder 1 glass rod | green water plants (e.g. Hydrilla) Procedure 1. (a) Fill a water. transparent plastic bottle with Add two spatulas of sodium hydrogencarbonate powder sodium hydrogencarbonate to the water and mix well. Sodium hydrogencarbonate provides carbon dioxide for the plant to water plant carry out photosynthesis. (b) Putsome green water plants (e.g. Hydrilla) into the bottle. 2. Squeeze the bottle gently so that a small amount of solution flows out. Screw the cap tightly and make sure the bottle remains collapsed. 3. Leave the bottle in bright light for at least four hours until the bottle returns to its original shape. What do you observe in the leaves of the green water plant during this period? 4, Unscrew the cap. Immediately put a glowing splint over the mouth of the plastic bottle. £ (a) What happens to the glowing splint? G (b) What does the result show? S ' sodium hydrogencarbonate BtRZESH Hydrilla [R9E Wear safety gocgles. In addition to starch, oxygen is also produced in photosynthesis. We can use a glowing splint to show that the gas given out by the plant is oxygen. If the glowing splint relights, the gas given out by the plant is oxygen, indicating that photosynthesis has taken place. Fig. 7.7 The oxygen given out by the water plant appears as bubbles in water. 3 Starch and oxygen are produced during photosynthesis. J £ Necessary factors for photosynthesis For photosynthesis to take place, certain factors are necessary. In this section, we are going to investigate the necessary factors for photosynthesis. Before the investigation, we need to destarch the plant by placing it in the dark for at least 48 hours. During this period, all the starch stored in the leaves is converted into glucose which is then used by the plant. This ensures that the leaves do not contain when the investigation begins. Any starch we any starch detect later is formed during the investigation, indicating that photosynthesis has taken place. Fig. 7.8 destarch 354 Destarching aplant Living Things and Air (A9. periment 7.5 Experiment video Investigating the factors necessary for photosynthesis In this experiment, you are going to carry out fair tests to find out if chlorophyll, carbon dioxide and light are necessary for photosynthesis. You will need to design a control set-up in each fair test. Part A: Investigating if chlorophyll is necessary for photosynthesis You are provided with a destarched potted plant with variegated leaves. There are green parts and non-green parts in variegated chlorophyll leaves. The and the non-green green parts parts contain contain little or no chlorophyll. Material and apparatus destarched potted plant with variegated leaves 1 O materials and apparatus used in Part B of Experiment 73 (except the plant) I. Variegated leaves Hypothesis Chlorophyll is necessary for photosynthesis. Il. Variable table Independent variable (the only variable that is changed) Dependent variable (the variable to be measured) Control variables (variables that are kept the same) Put a*v” in the appropriate box. presence of amount of light |:| green parts and temperature l:l non-green parts of the supply of carbon dioxide D supply of water I:' presence of chlorophyll L—_| variegated leaf varlegated leaf BERE mh 1ll. Procedure 1. Remove a variegated leaf from the destarched plant. Follow Steps 5-8 of Experiment 7.3 to test for starch in the leaf. Make sure that the leaf has no starch. 2. Place the destarched plant in bright light for about four hours. 3. Remove a variegated leaf from the plant. Draw a labelled diagram to show the colour pattern of the variegated leaf in the left box below. 4. Test for starch in the variegated leaf. Draw a labelled diagram to show your results in the right box below. IV. Results Colour pattern of the variegated leaf Result of iodine test In the iodine test, the green parts of the variegated leaf become while the non-green parts are _____ is produced in the __ V. in colour. parts This shows that of the variegated leaf. Conclusion Part B: Investigating if carbon dioxide is necessary for photosynthesis Your teacher will give you some soda lime. Soda lime can absorb carbon dioxide. Design a fair test to investigate if carbon dioxide is necessary for photosynthesis, Material and apparatus destarched potted green plant 1 © Soda lime small amount Spatula 1 transparent plastic bag 2 string (about 20 cm long) 2 materlals and apparatus used in Part B of Experiment 7.3 (except the plant) Sodalime fi soda lime MR Living Things and Air |2N I. Hypothesis is necessary for photosynthesis. Il. Variable table Independent variable (the only variable that is changed) | Dependent variable Control variables (the variable to be measured) (variables that are kept the same) 1ll. Procedure 1. Remove a leaf from the destarched plant and test for starch. Make sure that the leaf has no starch. 2. Use the materials and apparatus provided to design an experiment. Draw your experimental set-up in the box on the right. D Caution Soda lime is corrosive. Wear protective gloves and handle it with a spatula. 3. Place the destarched plant in bright light for about four hours. 4. Test for starch in the leaves. destarched plant IV. Results V. Conclusion =N Part C: Investigating if light is necessary for photosynthesis Material and apparatus destarched potted green plant 1 materials and apparatus used in Part B of Experiment 7.3 (except the plant) I. black paper paper clip some some Hypothesis Il. Variable table Independent variable Dependent variable Control variables (the only variable that is changed) (the variable (o be measured) (variables that are kept the same) IIl. Procedure Draw a labelled diagram to show your experimental set-up. Also, write the procedure of the experiment. IV. Results V. ¢ Conclusion Living Things and Air shaills up yevel” Water is also necessary for photosynthesis. = However, there is no simple experiment to show that water is a necessary factor. This is Understanding because a plant will die if we remove all the water from it. control set-ups and 7.2 Rate of photosynthesis in different coloured lights Photosynthesis takes place at different rates under different coloured lights. Do you know in which coloured light photosynthesis takes place the fastest? Explore more on our Web. fi A4 ( Checkp 2. 3. e-Checkpoint Write ‘T’ for a true statement and ‘F’ for a false statement. (a) lodine solution can be used to test for the presence of starch. (b) Green plants can carry out photosynthesis in the dark. (c) Carbon dioxide is a necessary factor for photosynthesis. ([ 1. ‘ int7.3 ex07s2 Arrange the following steps of testing for starch in a green leaf in the correct order. (a) Put the leaf in hot alcohol. (b) Put the leafin boiling water. (c) Add iodine solution to the leaf. (d) Wash the leaf in hot water. (a) How can you destarch a plant? (b) Why is it important to destarch > the plant before > > carrying out the investigations about the factors necessary for photosynthesis? This ensures that the plant does not have any )\ of the investigation. Any starch detected (ii). at the beginning [ significance of photosynthesis to other living things What would happen if all plants died? Four explorers went to an island that is far away. No humans lived on the island and there were only wild plants and animals. The explorers discussed what would happen if all plants on the island died. animals would be alive All of the animals would be glive eventually as they, eventually. Could eat other food. Only the most fierce The animals that feed on plants would eventually die. but the animals that eat other animals would not. All of the animals Would eventually die, Peter Daniel Which explorer do you agree with the most? N Photosynthesis is an essential life process. It allows plants to make their own food. On the other hand, animals cannot make their own food. Some animals feed animals. Therefore, on plants and other animals feed photosynthesis is important on in providing food directly or indirectly for living things. Photosynthesis and carbon also dioxide helps in maintain Nature. We the will balance further of discuss oxygen this Section 7.5. The following are the significance of photosynthesis: e providing food for living things * maintaining the balance of oxygen Nature and carbon dioxide in in Living Things and Air Food chains Plants can make their own food by photosynthesis. They are called producers. Animals cannot produce food on their own. They feed on plants or other animals. They are called consumers. The feeding NOISN3LX3 relationship among living things can be shown by a food chain. S Grasshopper (consumer) Grass (producer) Fig. 7.9 Bird (consumer) Anexample of afood chain In the above example, grass converts the light energy from the Sun into chemical energy which is stored in the food produced. This energy is then transferred to the grasshopper through feeding. ecosystem. The In arrows this way, in the energy food chain can and then flow show to the bird through the direction the of the flow of energy. Some other examples of food chains are shown below. Grass (producer) Rabbit (consumer) Snake (consumer) Hawk (consumer) Water plant (producer) Small fish (consumer) Big fish (consumer) Human (consumer) Fig. 7.10 24 Some examples of food chains producer 75 consumer ;i feeding relationship Ji¥1 B4R food chaln £#7t flow of energy HZEIL Since plants make use of the light energy from the Sun to carry out photosynthesis, the Sun is the ultimate source of energy for all food chains. ?Whal would happen to the plants Fig. 7.11 Food chains start with the energy from the Sun. and animals If the Sun stopped - _ shows the feeding relationship and the flow of energy among living things. ° In a food chain, plantsare ______ and animals are W ( Checkpo’nt 7.4 The photos below show four living things. Wild dog Rabbit Grass Lion (a) Construct a food chain involving these four living things. (b) Which of the above living things is/are producer(s)? (c) Which of the above living things is/are consumer(s)? (d) What is the ultimate source of energy for this food chain? (e) State ONE importance of the producer in a food chain. @ _J/