Chapter 7 Nutrition in Plants Lesson 3 - Experiments testing for photosynthesis
Experiments to test for factors involved in photosynthesis
Photosynthesis Therefore the evidence to show that photosynthesis has taken place is by: Detecting the presence of starch starch stored as sunlight chlorophyll Carbon dioxide + water -> glucose + oxygen + water vapour
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?
To test for starch in a leaf <ul><li>Remove a green leaf from the plant </li></ul>2. Boil the leaf in boiling water 3. Put the boiled leaf in boiling tube containing alcohol (ethanol) 4. Return the leaf to the hot water 5. Remove leaf and perform the iodine test Extract chlorophyll as ethanol dissolves lipids in cell membranes Stop enzymatic reactions within cells in leaf Softens the leaf and allows penetration of iodine
Hypothesising 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… sunlight chlorophyll Carbon dioxide + water -> glucose + oxygen + water vapour starch stored as
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?
(absorbs CO 2 ) (to ensure CO 2 from organisms in soil is contained) (absorbs CO 2 )
Things to note: <ul><li>All experiments are concluded with whether starch is produced (iodine test) </li></ul><ul><li>There has to be a suitable control to compare the results </li></ul>
Test for chlorophyll Qn: Is there a way to remove chlorophyll from the plant? sunlight chlorophyll Carbon dioxide + water -> glucose + oxygen + water vapour starch stored as
Test for chlorophyll Destarch a plant with variegated leaves Expose the plant to strong sunlight for few hours Only certain parts contain chlorophyll (green) 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
Results normal leaf leaf after stained with iodine
Test for sunlight Qn: How are we going to test if sunlight is necessary for photosynthesis? sunlight chlorophyll Carbon dioxide + water -> glucose + oxygen + water vapour starch stored as
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
Results of iodine test whole leaf exposed to light half of leaf covered normal leaf
To find out what gas is given off during photosynthesis (provides CO 2 ) Ensure no air trapped Test for O 2 ?
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?
Results 60 50 40 30 20 No. of bubbles Distance of lamp/cm
Question: <ul><li>Heat emitted from the bulb may affect the experiment as temperature affects enzyme action. Suggest how you would modify the experiment to curb this. </li></ul>
Modification to experiment <ul><li>Place a beaker of water between the plant and light source. </li></ul><ul><li>Water is absorbs the heat from the light source. </li></ul>
To investigate the effect of different temperatures on the rate of photosynthesis <ul><li>Given that the experiment setup is the same as that of different light intensities, what would be a possible variable that is changed? </li></ul>
Experimental methods If starch is present, iodine turns dark blue Treat with iodine 4 Allows penetration of iodine Soften leaf in hot water 3 Extract chlorophyll as ethanol dissolves lipids in cell membranes Boil in ethanol 2 Kill off cells Boil leaf in water 1 Reasoning Method Steps