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Nutrition in Plants (part 3): Experiments testing for photosynthesis

At the end of the lesson, students will be able to understand:<br>The tests for factors involved in photosynthesis<br>- Test for starch in leaves<br>- To test if sunlight is necessary for photosynthesis<br>- To test if carbon dioxide is necessary for<br>photosynthesis<br>- To test if chlorophyll is necessary for<br>photosynthesis<br>- To test for the gas given out during photosynthesis<br>- To investigate the effect of different light intensities<br>on the rate of photosynthesis<br>- To investigate the effect of different temperatures<br>on the rate of photosynthesis

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Nutrition in Plants (part 3): Experiments testing for photosynthesis

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  1. Experiments to test for factors involved in photosynthesis

  2. Photosynthesis sunlight Carbon dioxide + water → glucose + oxygen + water vapour chlorophyll stored as starch Therefore the evidence to show that photosynthesis has taken place is by: Detecting the presence of starch

  3. Starch grains from bean embryo

  4. Pre-experimental knowledge The test and control experiments use de-starched plants Plants are de-starched by keeping them in the dark for 2 days Plants are tested for the presence of starch at the end of the experiment Is it absolute that no starch will be present?

  5. To test for starch in a leaf 1. Remove a green leaf from the plant 2. Boil the leaf in boiling water Stop enzymatic reactions within cells in leaf 3. Put the boiled leaf in boiling tube containing alcohol (ethanol) Extract chlorophyll as ethanol dissolves lipids in cell membranes 4. Return the leaf to the hot water Softens the leaf and allows penetration of iodine 5. Remove leaf and perform the iodine test

  6. Hypothesising sunlight Carbon dioxide + water → glucose + oxygen + water vapour chlorophyll stored as starch Qn: If we were to remove carbon dioxide would there be starch produced? Qn: What if we remove chlorophyll, would starch still be formed? Let us test our hypothesis…

  7. Test for Carbon dioxide You are given two pots of plants One pot is provided with all the conditions necessary for photosynthesis to occur The other pot is given all the conditions necessary for photosynthesis, except….? How do we remove carbon dioxide?

  8. (absorbs CO2) (to ensure CO2 from organisms in soil is contained) (absorbs CO2)

  9. Things to note: ? All experiments are concluded with whether starch is produced (iodine test) ? There has to be a suitable control to compare the results

  10. Results Without CO2 With CO2

  11. Test for chlorophyll sunlight Carbon dioxide + water → glucose + oxygen + water vapour chlorophyll stored as starch Qn: Is there a way to remove chlorophyll from the plant?

  12. Test for chlorophyll Destarch a plant with variegated leaves Only certain parts contain chlorophyll (green) Expose the plant to strong sunlight for few hours Remove a leaf and make a drawing to show the distribution of the green parts Kill the leaf, decolourise it and test for starch Make a drawing of the leaf to show the distribution of the blue-black colour

  13. Results normal leaf leaf after stained with iodine

  14. Test for sunlight sunlight Carbon dioxide + water → glucose + oxygen + water vapour chlorophyll stored as starch Qn: How are we going to test if sunlight is necessary for photosynthesis?

  15. Test for sunlight

  16. Design the experiment for the test 1. Destarch a plant 2. Remove a leaf and test for starch 3. Sandwich a leaf between 2 pieces of black paper; each paper has a pattern cut on it 4. After a few hours, remove the leaf and test it for starch 5. Make a drawing of the leaf to show the region stained blue black

  17. Results of iodine test normal leaf whole leaf exposed to light half of leaf covered

  18. To find out what gas is given off during photosynthesis Ensure no air trapped (provides CO2) Test for O2?

  19. To investigate the effect of different light intensities on the rate of photosynthesis Do you think that we will be able to control the rate of photosynthesis by varying light intensity?

  20. Results Distance of lamp/cm No. of bubbles 20 30 40 50 60

  21. Question: Heat emitted from the bulb may affect the experiment as temperature affects enzyme action. Suggest how you would modify the experiment to curb this.

  22. Modification to experiment ? Place a beaker of water between the plant and light source. ? Water is absorbs the heat from the light source.

  23. To investigate the effect of different temperatures on the rate of photosynthesis Given that the experiment setup is the same as that of different light intensities, what would be a possible variable that is changed?

  24. Experimental methods Steps Method Reasoning 1 Boil leaf in water Kill off cells 2 Boil in ethanol Extract chlorophyll as ethanol dissolves lipids in cell membranes 3 Soften leaf in hot water Allows penetration of iodine 4 Treat with iodine If starch is present, iodine turns dark blue

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