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UNIT 12 : PLANT AND ANIMAL NUTRITION YEAR 8, 2013 12.1 – PHOTOSYNTHESIS At the end of the lesson, you should be able to: define photosynthesis as the process by which plants manufacture food using carbon dioxide and water in the presence of sunlight and chlorophyll. state the word equation for photosynthesis. All organisms require the energy from the Sun. I use the energy from the Sun to Sunbath my body to give me a nice brown colour and my mother dries the clothes under the sun everyday. However, there are some living organisms such as green plants that make use of this solar energy to make their own food. Green plants are at the beginning of all food chains. Plants are known as 'producers' because only plants make their own food by a process called “photosynthesis” [Photo means light and synthesis means to combine] Animals depend directly or indirectly on green plants for food. Green plants need sunlight to produce food. They 'capture' the sun's light energy using the chlorophyll in their leaves and use it to make a sugar called glucose. Glucose is a simple sugar and is needed to provide energy for the plant. PHOTOSYNTHESIS is the process in which green plants make food (glucose) using raw materials such carbon dioxide and water with the help of sunlight and chlorophyll. MSPSBS/LSS/Unit 12/2013 Page 1 of 6 UNIT 12 : PLANT AND ANIMAL NUTRITION YEAR 8, 2013 How photosynthesis works Plants can photosynthesize to make their own food because they have chloroplasts (a disc-like structure) in the green parts of their leaves. Inside these chloroplasts are green pigments called the chlorophyll. These chlorophylls are the ones which absorb the light energy from the sun and use it to change carbon dioxide and water into glucose and releases oxygen. The glucose produced in the photosynthesis reaction is carried to other parts of the plant through the phloem vessels. Glucose cannot be stored as it is very soluble, therefore it is converted to starch and stored in the leaves. Oxygen is a 'waste' product of photosynthesis which is released into the air through tiny pores under the leaves called stomata. Exchange of gases such as carbon dioxide, water vapour and oxygen takes place through the stomata of the leaves. Photosynthesis is a complex process (chemical reaction) which occurs in a series of steps. However, it can be summarized into a simple word equation like this: Light energy from the sun Carbon dioxide + water ----------------------------------------> glucose + oxygen Absorbed by chlorophyll Conditions needed for photosynthesis Plants need 4 important factors in order to photosynthesize: 1. 2. light energy (from the sun) chlorophyll (the green 3. 4. substances in plant) carbon dioxide (from the air) water (from the soil) Photosynthesis produces: 1. 2. glucose (converted to starch) oxygen (a waste product or byproduct of photosynthesis) MSPSBS/LSS/Unit 12/2013 Page 2 of 6 UNIT 12 : PLANT AND ANIMAL NUTRITION YEAR 8, 2013 The following practical activities will investigate some of these factors listed above. (1) LIGHT & PHOTOSYNTHESIS Aim: To show that light is necessary for photosynthesis. Conclusion: Starch is produced only in areas that received light, showing that light is necessary for photosynthesis. (2) CHLOROPHYLL & PHOTOSYNTHESIS Aim: To show that chlorophyll is necessary for photosynthesis. A variegated leaf (a leaf with green and non-green parts) Conclusion: Starch is made only in areas of the leaf with chlorophyll, showing that chlorophyll is necessary for photosynthesis. Only the green parts of the leaf make starch because the non-green parts have no chloroplast hence no chlorophyll. MSPSBS/LSS/Unit 12/2013 Page 3 of 6 UNIT 12 : PLANT AND ANIMAL NUTRITION YEAR 8, 2013 3) CARBON DIOXIDE (GASES) & PHOTOSYNTHESIS Both soda lime and sodium hydroxide solution absorbs carbon dioxide from the air. The presence of starch can be tested using ‘iodine solution’ which will turn from yellow to blue-black in colour when starch is present. When testing the leaves from plant A & B, it is found that: In plant A The leaves do not turn blue-black upon testing with iodine solution (the iodine solution remains yellow). It is concluded that STARCH IS ABSENT here. In plant B The leaves turn blue-black upon testing with iodine solution. It is concluded that STARCH IS PRESENT. Therefore CARBON DIOXIDE must be present for photosynthesis to take place. STARCH TEST IN A GREEN LEAF To find out that food (starch) is really synthesized by green plants during photosynthesis, we can do a test on a green leaf for starch. Here are the steps necessary to carry out when testing a green leaf for starch: (1) Pick a small leaf. Boil the leaf in a beaker of water to kill and soften it. (2) Heat the leaf again but in a test tube of alcohol. The alcohol decolourizes the leaf. Caution: Turn off the flame as alcohol burns easily. (3) Remove the decolourised leaf and put it in a beaker of hot water to soften it. (4) Place the soften leaf on a white tile and add a few drops of iodine solution to it. Conclusion: Starch is present if the leaf turns from yellow to dark blue-black. MSPSBS/LSS/Unit 12/2013 Page 4 of 6 UNIT 12 : PLANT AND ANIMAL NUTRITION YEAR 8, 2013 IMPORTANCE OF PHOTOSYNTHESIS Photosynthesis is the only natural process that: can convert light energy into chemical energy. enables green plants to make food. releases oxygen into the atmosphere and removes carbon dioxide from the atmosphere. In this way, photosynthesis keeps the proportion of oxygen and carbon dioxide in the atmosphere balanced. Glucose to all parts of the plant Mineral requirements of plants Green plants make food by photosynthesis. However, for healthy growth, plants also need minerals such as nitrates, calcium, phosphates and potassium. These nutrients (absorbed as minerals, dissolved in water) which plants take through their roots in the soil. The experiment below shows the effect of minerals on plant growth. [Yellow leaves] MSPSBS/LSS/Unit 12/2013 Page 5 of 6 UNIT 12 : PLANT AND ANIMAL NUTRITION YEAR 8, 2013 It is found that: 1. Seedlings are grown in the test tube containing different minerals solutions. 2. After a few weeks, the seedlings look quite different. Only in the first tube, with all minerals present, the seedling grows well. 3. This experiment demonstrates the importance of certain elements in healthy plant growth. 4. Plants grow well in soil which contains minerals. Only small quantities of the mineral are needed for healthy growth. A healthy tomato plant. Tomato plant suffering from potassium deficiency. In smaller quantities, plants also need minerals such as: Magnesium Iron Calcium Sulphur MSPSBS/LSS/Unit 12/2013 Page 6 of 6