IGCSE Biology 0610 Photosynthesis Notes PDF

IGCSE BIOLOGY 0610 Digitized Notes Photosynthesis and carbon cycle Resources/References: - Lower secondary science & IGCSE BIOLOGY MACKEAN Learner’s Book 1 1.1 Photosynthesis In this topic you will: p...

IGCSE BIOLOGY 0610 Digitized Notes Photosynthesis and carbon cycle Resources/References: - Lower secondary science & IGCSE BIOLOGY MACKEAN Learner’s Book 1 1.1 Photosynthesis In this topic you will: practice using the word equation for photosynthesis learn what photosynthesis is, how it happens and why it is important. Key words Chlorophyll light intensity photosynthesis How plants make food Plants make food by photosynthesis.. In photosynthesis, plants use: water, which they get from the soil carbon dioxide, which they get from the air energy, which they get from sunlight. The energy in sunlight is captured by a green pigment, called chlorophyll, which is inside some of the cells in the leaves of plants. The plants use the energy to make the water and carbon dioxide combine together in a chemical reaction. Two new substances are made in the reaction. These are glucose and oxygen. You can write the word equation for photosynthesis like this: water + carbon dioxide glucose + oxygen Photosynthesis happens in the leaves of a plant. Questions 1 What are the reactants in photosynthesis? 2 What are the products in photosynthesis? 2 Collecting the gas produced in photosynthesis USES OF GLUCOSE PRODUCED IN PHOTOSYNTHESIS I. Some of the glucose is used for respiration to release energy for the growth of the plant. II. Some is converted into cellulose which is used to make cell walls in plant cells. III. Some is converted into starch and stored in the plant, g in the seeds. IV. Some is converted into sucrose and transported in the phloem. V. Some can be converted into amino acids, y addition of nitrogen. Amino acids are used to make proteins. VI. Some glucose is used to make nectar in flowers which is used to attract insects for pollination. VII. Some can be converted into lipids which are energy sores, especially in seeds. ACTIVITY You are going to use a plant that grows in water for this experiment. This makes it easy to collect the gas that it releases. You will need: the apparatus shown in the diagram. If you do not have any pond water, you can use ordinary tap water. You can use any plant that grows underwater. You can often buy water plants in pet shops, because people like to put them into tanks with their pet fish. Method 1 Set up the apparatus shown in the diagram. 2 Put the apparatus in a place where the water plant will get sunlight. Leave it for at least one day. 3 When plenty of gas has collected in the top of the test tube, put your hand into the beaker of water. Carefully remove the tube from the top of the funnel. Keep the opening of the tube under the water, or the gas will escape! 4 With the tube still upside down, put your thumb over the end of the tube, to keep the water and gas inside. 5 Take the tube out of the water, keeping your thumb over the end. Keep the tube upside down. 6 Very carefully, move your thumb just a little bit, so that the water runs out of the tube but the gas stays inside. Push a stopper into the tube, and then place it – right way up – in a test tube rack. 7 Light a splint, and then blow it out so that it is glowing but not burning. 8 Test the gas with the glowing splint. (Try not to touch the wet sides of the test tube, or the splint will be extinguished!). If the gas contains a lot of oxygen, the splint will relight. 3 Questions 1 Explain why it was best to use a water plant in this experiment. 2 Explain why it was important to leave the apparatus where it would get plenty of sunlight. TESTING A LEAF FOR STARCH. Testing a leaf for photosynthesis NB: Starch is stored in chloroplasts where photosynthesis occurs so testing a leaf for starch is a reliable indicator of which parts of the leaf are photosynthesizing. We use iodine solution to test for starch. If starch is present, the leaf turns blue-black. Leaves can be tested for starch using the following procedure: A leaf is dropped in boiling water to break down the cell membrane so as to allow iodine solution into the enter the cells The leaf is left for 5-10 minutes in hot ethanol in a boiling tube. This removes the chlorophyll so colour changes from iodine can be seen more clearly The leaf is dipped in boiling water to soften it The leaf is spread out on a white tile and covered with iodine solution In a green leaf, the entire leaf will turn blue-black as photosynthesis is occurring in all areas of the leaf 4 5  NB: Care must be taken when carrying out this practical as ethanol is extremely flammable, so at that stage of the experiment the Bunsen burner should be turned off. A video on testing a leaf for starch. https://youtu.be/VeU7ULL9Usw Questions 1 Iodine solution cannot get through cell membranes. Boiling the leaf breaks the cell membranes apart. a In which part of a cell is starch stored? b Explain why it was important to boil the leaf before the starch test could work. 2 Suggest why it is a good idea to remove the green colour from the leaf before you add iodine solution to it. 3 Did the leaf contain starch? If so, explain where the starch came from. 4 If your leaf had some green parts and some white parts, which parts contained starch? Can you explain why? FACTORS THAT AFFECT THE RATE OF PHOTOSYNTHESIS 1) Light intensity 2) Temperature 3) Carbon dioxide concentration 6 EXPERIMENTS I. To show that Chlorophyll is necessary for photosynthesis De-starch a variegated plant Then leave the plan in the light for 6 hours Remove one of the variegated leaves ad test it for starch ( use the steps outlined earlier). Make a record of your observations. Observation: -The green part of the leaf turns blue-black indicating the presence of starch. -The white part of the leaf retains the yellowish-brown colour of iodine, showing the absence of starch. Explanation: The green part of the leaf contains chlorophyll, therefore photosynthesis takes place and glucose is produced hat is converted to starch. The white part doesn’t have chlorophyll therefore no photosynthesis takes place and starch isn’t produced. Conclusion: Chlorophyll is necessary for photosynthesis. II. To show that light is necessary for Photosynthesis De-starch a potted plant Select some leaves and partly cover them with aluminum foil(sa shown in the diagram below). Expose the plant to sunlight for 6 hours Remove the partly covered leaves and test it for starch 7  Make a record of your observations Observation: -The uncovered parts of the leaf turn blue black with iodine solution, showing presence of starch. -The covered parts of the leaf retained the yellowish-brownish colour of iodine solution, indicating absence of starch. Explanation: -The uncovered parts of the leaf receive sunlight and photosynthesis takes place producing glucose, some of which is converted into starch. -The covered parts did not receive light, therefore no photosynthesis takes place and starch is not produced. Conclusion: Light is necessary for photosynthesis. III. To show that carbon dioxide is necessary for photosynthesis De-starch two a potted plants Place soda lime or sodium hydroxide on one pot (soda lime absorbs carbon dioxide) and water on the other pot.. 8  Cover the potted plants with a transparent plastic bag. Leave the plant in light for 6 hours Remove on leaf from the plant and carry out the test for starch Make and record your observation Observation: The leaf from the potted plant with soda lime retained the yellowish brown colour of iodine solution, indicating no starch, hence no photosynthesis. The leaf that contained water turned blue black with iodine solution, indicating presence of starch, hence photosynthesis took place. 5.Describe a suitable control for this experiment ………………………………………………………………………………………………….. …………………………………………………………………………………………………..... NB: Soda lime removes carbon dioxide; hence no photosynthesis as there is no starch. Conclusion: Carbon dioxide is necessary for photosynthesis. Control experiments 9  These are experiments set for comparison; they show the effect of the factor being investigated. These experiments have all conditions needed for a process to take place. Investigating how light intensity affects the rate of photosynthesis Think like a scientist Investigating how light intensity affects the rate of photosynthesis Like the previous experiment, this experiment uses a water plant. You are going to change the amount of light that falls onto the plant and see how this affects the number of bubbles it produces in one minute. You will need: a test tube containing water, a piece of water plant, a lamp, a beaker in which you can stand the test tube, a ruler for measuring the distance between the lamp and the test tube, a timer Safety It is very important to keep electrical wires, plugs and the lamp away from the water. Method 1 Set up your equipment. Place the lamp close to the test tube. Leave the equipment like that for about 5 minutes, to give the plant time to settle down and start to photosynthesise. 10 While you are waiting, read the rest of these instructions and then draw a results chart. When you can see bubbles coming from the cut end of the plant stem, you can start your experiment. 2 Measure the distance between the lamp and the test tube, and write it down in your results chart. 3 Start the timer. Count how many bubbles the plant produces in one minute, and write this down. 4 Repeat step 3 two more times. 5 Now move the lamp a little bit further away from the test tube. Measure the new distance. 6 Repeats steps 3 and 4. 7 Repeat steps 5 and 6 for at least two more distances of the lamp, further from the test tube. Questions 1 Calculate the mean number of bubbles for each distance of the lamp from the test tube. 2 Plot a graph of your results. Put distance of lamp from the test tube on the x-axis. Remember to include units. Put mean number of bubbles per minute on the y-axis. Plot your points as small, neat crosses. Draw a line to show the trend in your results. 3 Light intensity means the amount of light. Copy and complete this sentence: As the distance of the lamp from the test tube increases, the light intensity................. 4 Now write a conclusion for your experiment, by copying and completing this sentence: As the light intensity increases, the rate of photosynthesis.................. 11 Why is photosynthesis important? First, photosynthesis provides energy, in the form of chemical energy in nutrients, for most other organisms. Plants use the energy in sunlight to make glucose and other carbohydrates. These carbohydrates contain some of the energy that was originally in the sunlight. Think about what you have learnt about food chains. When animals eat food, they get some of the energy that was captured by plants. Most of the energy in all the food in the world comes from plants. A food chain shows us how this energy is passed along from one organism to another. The second reason that photosynthesis is so important is that it provides oxygen for the Earth’s atmosphere. Animals and plants, of course, need oxygen for respiration. Oxygen is a waste product of photosynthesis. It is released from the leaves of plants and mixes with the other gases in the atmosphere. About 20% of the air around us is oxygen. When the Earth was first formed, about 4.6 billion years ago, there was almost no oxygen in the Earth’s atmosphere. Scientists think that oxygen first began to collect in the air when bacteria first began to photosynthesise. (There were no plants at that time – they did not appear on Earth until about 4.7 million years ago.) If photosynthesis had never begun on Earth, then no animals would ever have been able to live here. 12 QN: WHERE DOES PHOTOSYNTHESIS TAKE PLACE? ANS: IN THE CHLOROPLSTS INSIDE THE LEAF. Chloroplasts and chlorophyll In the previous topic, you saw that chlorophyll is essential for photosynthesis. Chlorophyll captures energy from sunlight. The energy helps water and carbon dioxide to react together. Chlorophyll is kept inside chloroplasts, inside plant cells. This means that photosynthesis happens inside chloroplasts. Not all plant cells have chloroplasts, so not all of them can photosynthesise. In most plants, the cells in the leaves have the most chloroplasts. Inside the cells in a leaf, carbon dioxide and water are made to react, to produce carbohydrates and oxygen. On warm, sunny days, plants can make more carbohydrates than they need to use immediately. So they store some it for use later on – perhaps at night, or at a time of year when there is less sunlight. Plants store carbohydrates as starch. They store the starch inside the chloroplasts in their cells. One way to check whether a leaf has been photosynthesing is to test it for starch. 13 Inside a leaf Photosynthesis happens inside chloroplasts, which are inside some of the cells in a leaf. The diagram opposite shows a magnified view of the inside of a leaf. On the diagram, you can see that chloroplasts are mostly inside the cells in the middle layers of the leaf. Leaves are very thin, so it is easy for sunlight to reach these cells. Chloroplasts also need plenty of water and carbon dioxide, because these are used in photosynthesis. Water is brought to the cells in the leaf along the veins. You can read more about this in Unit 4. Carbon dioxide diff uses into the leaf from the air. If you look at the diagram, you can see that there are tiny holes in the leaf, which allow gases to diff use in and out. These holes are called stomata (singular: stoma). The gases can easily diff use through the air spaces between the cells inside the leaf. 14 ACTIVITY Which surface of a leaf has most stomata? Work with a partner for this activity. Put some warm (not hot) water into a beaker or glass. Take a fresh leaf, and push it down under the water. Can you see any bubbles coming out of the leaf? Which surface of the leaf do most of the bubbles come from? Discussion questions What do you think the bubbles contain? If most bubbles came from one surface of the leaf, why do you think this happened? Think about what you know about the effect of increased temperature on gases. Can you suggest why the bubbles came out of the leaf when you put it into warm water? Questions 1 The cells inside a plant leaf use up carbon dioxide when they photosynthesise. Use what you have learnt about diffusion to explain how carbon dioxide diffuses into the leaf. 2 The cells inside a plant leaf produce oxygen when they photosynthesise. Suggest what happens to this oxygen. 15