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1 of 35© Boardworks Ltd 2011 Photosynthesis
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3 of 35© Boardworks Ltd 2011 All living organisms need food to grow and survive. How do plants get the food they need? Plants are known as producers because they provide food for many other organisms. Unlike animals, plants cannot move very much, so how do they get the food that they need? This is because food provides raw materials for growth and energy for chemical reactions. Without plants, other organisms would have no raw materials for growth or energy.
4 of 35© Boardworks Ltd 2011 A scientist called Van Helmont performed an experiment to test this idea. He grew a willow tree and measured the mass of the soil, the tree and the water he added. Do plants eat soil? It used to be thought that plants got their food from the soil. Greek scientists believed that plants increased in mass by taking in minerals from the soil. After five years the plant had gained about 74 kilos but the mass of the soil was the same. He concluded that the tree's weight gain had come from water. Was he right?
5 of 35© Boardworks Ltd 2011 Oxygen from plants Joseph Priestley discovered oxygen, and later showed that it was made by plants. He kept the container closed and relit the candle 27 days later. The candle was able to burn again. Priestley concluded that something must have been made by the plant that changed the air. The gaseous product was oxygen. He placed a mint plant and a candle in an enclosed space, and soon discovered that the candle went out when all of the oxygen was used up in combustion.
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7 of 35© Boardworks Ltd 2011 Plants make their own food by photosynthesis. A few micro-organisms can do this too. What is photosynthesis? light energy The word photosynthesis comes from the Greek language: Photosynthesis is a chemical reaction that uses light energy. ‘photo’ means ‘light’ ‘synthesis’ means ‘putting together’. Photosynthesis just means ‘putting together with light’.
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9 of 35© Boardworks Ltd 2011 Photosynthesis is a chemical reaction that takes place in the chloroplasts of green plant cells, where light energy is used to convert carbon dioxide and water into glucose and oxygen. Photosynthesis: summary How can the process of photosynthesis be summarized in one sentence? What is the word equation for this chemical reaction? light energy chlorophyll carbon dioxide + wateroxygen + glucose
10 of 35© Boardworks Ltd 2011 Photosynthesis: word equation
11 of 35© Boardworks Ltd 2011 The reaction of photosynthesis can be represented by the following equation: CO 2 H2OH2O C 6 H 12 O 2 O2O2 ++ Is this a balanced symbol equation? How would you balance the equation? 66 6 light energy chlorophyll carbon dioxide + wateroxygen + glucose light energy chlorophyll What is the symbol equation for this reaction? Symbol equation for photosynthesis
12 of 35© Boardworks Ltd 2011 Word equation to symbol equation
13 of 35© Boardworks Ltd 2011 Photosynthesis equation quiz
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15 of 35© Boardworks Ltd 2011 How can you test for photosynthesis? The presence of starch in a leaf can be used to show that photosynthesis has taken place. After photosynthesis occurs, the glucose is often converted to starch for storage. The starch test can be used to prove that photosynthesis needs light, carbon dioxide and chlorophyll to take place. Iodine is used to test for starch. It reacts with starch and changes colour from brown to blue-black. How would you set up an experiment to test the conditions needed for photosynthesis? How would you make the experiment fair and reliable?
16 of 35© Boardworks Ltd 2011 Testing leaves for starch
17 of 35© Boardworks Ltd 2011 Only the green areas of the leaf react with the iodine and turn blue-black. Without chlorophyll, the pale areas have been unable to produce starch and do not turn blue-black. Variegated leaves have pale parts, which do not contain chlorophyll. The green parts of the leaf contain chlorophyll and are the control. Chlorophyll cannot be removed from a plant without killing the plant. Instead, variegated leaves can be used to show chlorophyll is essential for photosynthesis. Which areas will react with iodine? Is chlorophyll needed for photosynthesis?
18 of 35© Boardworks Ltd 2011 Testing leaves for starch – activity
19 of 35© Boardworks Ltd 2011 Where does the oxygen come from?
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21 of 35© Boardworks Ltd 2011 Like many reactions, photosynthesis requires enzymes. Photosynthesis is a chemical reaction and so has a rate. What is the rate of photosynthesis? light Which factors do you think affect the rate of photosynthesis? How do these factors affect the rate of photosynthesis? carbon dioxidetemperature Is the rate of photosynthesis always the same?
22 of 35© Boardworks Ltd 2011 How does light affect photosynthesis? Light energy has to be absorbed by chlorophyll for photosynthesis to take place. The brighter the light, the more light energy there is, so will photosynthesis be faster or slower? light energy chlorophyll carbon dioxide + wateroxygen + glucose More light energy means that photosynthesis will be faster. If light intensity is too high plant cells can be damaged. How is photosynthesis affected if this happens?
23 of 35© Boardworks Ltd 2011 Investigating photosynthesis
24 of 35© Boardworks Ltd 2011 Investigation photosynthesis – results
25 of 35© Boardworks Ltd 2011 Carbon dioxide is one of the raw materials used by plants to make their food. The concentration of carbon dioxide in the air is actually quite low (0.03%). light energy chlorophyll carbon dioxide + wateroxygen + glucose More carbon dioxide means more photosynthesis, so plants make more food and grow more quickly. Why is the concentration of carbon dioxide in commercial greenhouses often raised to about 0.1%? Carbon dioxide and the rate of photosynthesis
26 of 35© Boardworks Ltd 2011 Photosynthesis is controlled by enzymes, which usually work best at warmer temperatures. Does increasing the temperature always increase the rate of photosynthesis? If it gets too hot (above 40 °C), the enzymes needed for photosynthesis begin to break down and are destroyed or denatured. The rate of photosynthesis decreases or even stops completely. Does temperature affect photosynthesis?
27 of 35© Boardworks Ltd 2011 What is the ideal combination of factors for the maximum rate of photosynthesis? What is a limiting factor? enough light How does restricting one of these facts affect the rate? If one of the factors is restricted, the rate of photosynthesis will be below the maximum possible rate. The restricted factor controls how quickly photosynthesis occurs and so limits the rate. It is called the limiting factor. enough carbon dioxide ideal temperature (not too hot or cold).
28 of 35© Boardworks Ltd 2011 Light intensity and photosynthesis
29 of 35© Boardworks Ltd 2011 Carbon dioxide and photosynthesis
30 of 35© Boardworks Ltd 2011 Temperature and photosynthesis
31 of 35© Boardworks Ltd 2011 Limiting factors – activity
32 of 35© Boardworks Ltd 2011 Limiting factors in a greenhouse
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34 of 35© Boardworks Ltd 2011 Glossary
35 of 35© Boardworks Ltd 2011 Multiple-choice quiz
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© Boardworks Ltd of 20 © Boardworks Ltd of 37 KS3 Chemistry 9H Using Chemistry.
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