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National 5 Biology – Living Cells – Photosynthesis Photosynthesis Photosynthesis, ATP, ADP, chloroplast, chlorophyll, photolysis, carbon fixation, starch, cellulose Learning Outcomes You will be able to: Describe the two stage process of photosynthesis. Give details of the summary word equation for photosynthesis. Describe what happens during the light reaction. Describe what happens during the carbon fixation. Describe the possible uses of sugar made during photosynthesis. Photosynthesis Photosynthesis is the chemical reaction which takes place in plant cells in order to produce sugar using light energy from the sun. The word equation for this reaction is: In this topic we're going to zoom in on the steps involved in photosynthesis in more detail and consider the effect of different conditions on the rate of photosynthesis. Before we begin however, we're going to be mentioning the molecule ATP in both this topic and the next, so it makes sense to explain what this is a little first. 1|P ag e National 5 Biology – Living Cells – Photosynthesis ATP You don't need to know what ATP stands for, but it might help you understand how it works. ATP stands for Adenosine Triphosphate. So, it's a molecule with three (tri) phosphate groups attached. The best way to imagine ATP is like the charged battery of the cell. If a cell wants to do anything which requires energy (such as active transport) then it needs some energy in the form of ATP. The cell releases the energy by breaking ATP up into ADP (the D standing for Di instead of Tri), which has only two phosphate groups attached, and a separate inorganic phosphate molecule (Pi). To recharge the ADP to ATP, energy is required to add a phosphate molecule. These processes are summarised in the following diagram: Learning Activity 1 1. State the name of the raw materials needed for photosynthesis. 2. State the name of the products of photosynthesis. 3. State what other essential requirements are needed for photosynthesis to take place. 4. Copy the equation for photosynthesis, Circle the raw materials in red, essential requirements in yellow and products in blue produce a key to show this. 5. What is ATP and how is it made? 2|P ag e National 5 Biology – Living Cells – Photosynthesis Chemistry of Photosynthesis Photosynthesis is a series of enzyme controlled chemical reactions occurring in two stages: 1. the light reaction (photolysis) 2. carbon fixation Photolysis: The light reactions of photosynthesis take place in the chloroplasts of the plant cell. Here, the green pigment chlorophyll absorbs light energy from the sun. This energy is used to do two things. Some energy is used to produce ATP as discussed above. This ATP acts as an energy store and can be used in the second stage of photosynthesis. At the same time, light energy is used to split water into hydrogen and oxygen. The hydrogen also goes to the second stage of photosynthesis whereas the majority of the oxygen diffuses out of the cell, and out of the leaf, as a waste product. 3|P ag e National 5 Biology – Living Cells – Photosynthesis Carbon Fixation: The second stage of photosynthesis also takes place in the chloroplast, but does not require chlorophyll as it does not require light energy directly. During Carbon Fixation the ATP and hydrogen from the Light Reactions is used to convert Carbon Dioxide into the sugar glucose. Uses of Sugar by Plants The sugar which is produced by photosynthesis can be used in lots of different ways by the cell. Some of it will be used for respiration. However, some of it will be joined together into long chains to form molecules of starch. This acts as an energy store for the plant, which is why storage organs such as the food store in seeds and tubers (potatoes) are packed full of starch. The plant can also join the sugar molecules together to form a different form of long-chained molecule called cellulose. This is the molecule which makes up the majority of the structure of plant cell walls which helps give the plant structure and support. 4|P ag e National 5 Biology – Living Cells – Photosynthesis Learning Activity 2 1. Describe the role of chlorophyll in the light reaction and the energy change which takes place. 2. Explain what happens to the hydrogen which is released. 3. Insert and complete the ‘Light Reaction diagram’ into your notes. 4. State where carbon fixation takes place. 5. Name the molecule which combines with the CO2. 6. Name the end product which is formed. 7. Insert and complete the ‘Carbon Fixation diagram’ into your notes. 8. Explain the fate of sugar produced during photosynthesis. 5|P ag e National 5 Biology – Living Cells – Photosynthesis Limiting Factors of Photosynthesis Carbon dioxide, light intensity, temperature, limiting factor Learning Outcomes You will be able to: Describe the limiting factors of photosynthesis. Explain the impact of limiting factors on photosynthesis and growth. Limiting Factors The presence or absence of various factors can limit the rate of photosynthesis. As the products of photosynthesis are used in cell growth, any limit in the rate of photosynthesis will affect the rate of growth also. Possible Limiting Factors in Photosynthesis include: - Carbon dioxide Light intensity Temperature 6|P ag e National 5 Biology – Living Cells – Photosynthesis Carbon Dioxide: As you know, carbon dioxide is a crucial molecule in photosynthesis. It is the raw material in the carbon fixation process. If the Light Reactions are proceeding at their maximum rate but there is a low concentration of carbon dioxide in and around the leaf, then the rate of photosynthesis will be limited, as will the rate of growth of the cell and the plant. Light Intensity: Light intensity is obviously crucial for the rate of photosynthesis as it provides the energy source for the Light Reactions, which in turn transfer the energy to Carbon Fixation through Hydrogen and ATP. If all other raw materials are present in abundance but there is a low level of light intensity, then the rate of photosynthesis will be limited as will growth. 7|P ag e National 5 Biology – Living Cells – Photosynthesis Temperature: Photosynthesis is a series of enzyme-catalysed chemical reactions, and just like any series of enzyme-catalysed reactions it can be affected by temperature. Low temperatures can result in a reduced rate of photosynthesis and growth, however high temperature can also greatly limit the rate of photosynthesis and growth. Why? Because photosynthesis is catalysed by enzymes and so high temperatures will result in denaturation. 8|P ag e National 5 Biology – Living Cells – Photosynthesis So, let's bring it all together now...can you interpret the following graph now? What is the limiting factor at points A, B & C on the graph and why? A: This is the easy one remember. The line is going up, which means the rate of photosynthesis increased when the light intensity (on the horizontal axis) was increased. Therefore, the limiting factor at point A is light intensity. B: The graph has levelled off here, so a higher light intensity doesn't result in a higher rate. Something other than light intensity must be limiting. On this graph it also shows that the experiment was repeated at two carbon dioxide concentrations. At point B the rate increased when the carbon dioxide concentration increased, therefore carbon dioxide concentration was limiting. C: It is more difficult to decide what is limiting at point C. It is definitely not light intensity as the graph has levelled off meaning a higher light intensity didn't result in a higher rate. It could be carbon dioxide concentration again, or it could be temperature. 9|P ag e National 5 Biology – Living Cells – Photosynthesis Learning Activity 1 1. What is meant by a ‘limiting factor’? 2. Name the possible limiting factors of photosynthesis. 3. Sketch the graph shown above into your notes. 4. Label the part of the graph where light is acting as a limiting factor, label the part of the graph where light is no longer a limiting factor. 5. State the relationship between light intensity and rate of photosynthesis. 6. Use the data in the table below to plot a line graph of results. 10 | P a g e National 5 Biology – Living Cells – Photosynthesis 7. Label the part of the graph where carbon dioxide is the limiting factor, and the part of the graph where carbon dioxide is not the limiting factor. 8. State the relationship between carbon dioxide concentration and the rate of photosynthesis. 9. Use the data in the table below to plot a line graph of results. 10. Describe the effect of increasing temperature on the rate of photosynthesis for the first part of the graph. 11. Why does this happen? 12. Photosynthesis cannot occur at all, at very high temperatures. Explain this? 13. Add a label to your graph to show where temperature is not acting as a limiting factor. Learning Activity 2 Collect the experiment card ‘The Effect of Varying Light Intensity’. Carefully follow the instructions to complete the experiment. 11 | P a g e National 5 Biology – Living Cells – Photosynthesis Extension Task Use any resources available to you to find out about producing early crops in horticulture or agriculture. Explain how early crops can be produced in horticulture, with reference to: - supplementary lighting - carbon dioxide enrichment - heating. 12 | P a g e