Determining Reactants for Photosynthesis

Photosynthesis Lab

Purpose:

The purpose of this investigation is study is to determine if reactants such as, light, carbon dioxide or chlorophyll, are essential for photosynthesis to take place. This will be determined through the iodine test, the presence of starch can be determined within the leaves of a geranium plant, variegated plant, and two plants with and without carbon dioxide to find out whether or not photosynthesis is taking place.

Hypothesis:

If plants make their own food by combining carbon dioxide gathered from the air with water obtained through the soil, and with sunlight obtained from chlorophyll then the presence of all three reactants is vital to produce its own food as photosynthesis will not work if even one reactant is absent and the plant will be unable to sustain life. The iodine test changes the colour of the leaf to dark blue only if starch is present, and if a reactant is missing, then glucose cannot be produced and then be converted to starch for the rest of the plant, resulting in no change in colour after the iodine test.

Materials:

Procedure:

1. Place the plants in the dark for 48 hours before starting any experiments in order to properly de-starch the plants.
2. Make and record physical observations for a leaf from each plant before conducting each experiment on an observations chart.
3. Wear a lab coat and a pair of safety goggles to prevent any substances from staining cloths or entering the eyes as well as following all safety precautions as instructed
4. Fill up one test tube with 10 mL of alcohol and place it on the test tube rack.

5. Add water to a 500-mL beaker and place it on a hot plate to set up a hot water bath.

Part A: Products of Photosynthesis

1. Use one leaf from a geranium plant and place it on a petri dish.

2. Pick up the leaf of the geranium plant using a pair of tongs, and place it in the beaker on the hot plate.
3. Take the leaf out of the hot water bath using the tongs and put in the test tube of alcohol.
4. Place the test tube with the leaf and alcohol inside the beaker with boiling water.

5. Wait until the leaf has lost all of its pigment before taking it out of the beaker.

6. When the leaf loses all of its pigment, place the test tube on the test tube rack using a pair of tongs and take the leaf out of the test tube and rinse it with cold water.

7. After properly rinsing the leaf, place it onto a petri dish, and using an eye dropper, add five drops of Lugol’s Iodine solution on top of the leaf.
8. Swirl the petri dish around so that the leaf is covered with the solution and record the change in colour of the leaf on an observations chart.

9. Clean the petri dish, the test tube, the beaker, and the pair of tongs to prevent contamination and add the same samples into the corresponding piece of material.

Part B: Do Plants Need Light for Photosynthesis?

1. Use one leaf from a geranium plant and place it on a petri dish.

2. Carefully place the paper cut-out on top of the leaf in the petri dish without damaging the leaf.
3. Turn on a lamp, and place the leaf under the light for 24 hours.

4. After 24 hours, remove the paper cut-out from on top of the leaf on the petri dish.

5. Repeat steps 2-8 from Part A with the new leaf from the geranium plant.

6. Turn off the lamp and clean the petri dish, the test tube, the beaker, and the pair of

tongs to prevent contamination and add the same samples into the corresponding piece of material.

Part C: Do Plants Need Carbon Dioxide for Photosynthesis?

1. Take one plant that has enough carbon dioxide and place it under a lamp.

2. Take one plant that has no available carbon dioxide and place it under a lamp.

3. Add 50 mL of soda lime from the closed 500 mL beaker to the pot of the first plant that has enough carbon dioxide to remove it from the air.
4. Add the dish of 50 mL of sodium hydrogen carbonate to the pot of the second plant that has no available carbon dioxide to add it to the air.
5. Secure a clear plastic bag over both plants and leave it under the light for 24 hours.

6. Use one leaf from both plants and place them into separate petri dishes.

7. Repeat steps 2-8 for the first leaf from the plant that has enough carbon dioxide and for the second leaf that has no available carbon dioxide.

8. Turn off both lamps and clean the petri dish, the test tube, the beaker, and the pair of tongs to prevent contamination and add the same samples into the corresponding

piece of material.

Part D: Do Plants Need Chlorophyll for Photosynthesis?

1. Place a variegated plant under a lamp for 24 hours.

2. Use one leaf from the variegated plant and place it on a petri dish.

3. Repeat steps 2-8 from Part A with the new leaf from the variegated plant.

4. Turn off the lamp and clean the petri dish, the test tube, the beaker, and the pair of tongs.

Observations:

Table 1: Description of Experiment and Observations Before and After

Diagrams and Screen Shots

Discussion:

Part A

1. Why were the plants kept in the dark prior to the experiment? Why is this important?

The plants were kept in the dark prior to the experiment so that any starch that was present in the leafs would be removed, this process is called de-starching. If the plant gets de-starched before the experiment, the plant cannot convert the starch to glucose. The plants already have starch stored, but once they are placed in the dark, the plants must up the rest of the previously stored starch since there is no light to help cause photosynthesis. Removing the starch prior to conducting the experiment allows the results to be more accurate to help answer the question if a factor like light is important for photosynthesis to happen.

2. Why is it important to boil the leaf?

Boiling the leaf is important because it breaks the leaf’s cell wall and membrane, which allows the iodine to penetrate into the leaf to test for the presence of starch. Boiling the leaf prevents any reactions from occurring. This is because if there were to be any other reactions occurring within the leaf, it could possibly use up the starch and change the results.

3. Explain why it was necessary to use boiling alcohol?

It is necessary to use boiling alcohol because this makes it easier to spot any change in colour after conducting the iodine test to test for starch. The alcohol removes the chlorophyll and any pigments in the leaf, this turns the leaf to a yellowish colour. By removing the chlorophyll, light energy is unable to be converted to ATP to produce food and nutrients for the leaf.

4. What is the purpose of the iodine?

The purpose of the iodide is to test for starch in a leaf through the iodine test. After decolourizing a leaf, if it turns dark blue when iodine is added, this means that photosynthesis occurred and starch is present in the leaf. If the leaf undergo this colour change  to a dark blue, then this means that photosynthesis did not occur and no starch is present in the leaf.

Part B

5. What was the purpose of keeping the leaf in the dark and then covering it with a cardboard cut-out?

Keeping the leaf in the dark destarches the leaf, which gives more accurate results. The opaque part of the paper cut-out prevented light from reaching the leaf, and only the cut-out part allows light to reach the leaf. Starch is removed from the parts covered by the opaque portion of the paper cut-out, thus eventually testing negative after the iodine test.

6. What conclusions can you draw from this part of the lab?

In conclusion, light is required for photosynthesis to occur, and without light, photosynthesis cannot take place. From the paper cut-out experiment, only the portion of the leaf that was exposed to light turned dark blue, while the rest of the leaf remained the same colour. This proves that light is a requirement for photosynthesis and the creation of starch.

Part C

7. Why was a variegated leaf used in this experiment?

A variegated leaf was used in this experiment because it has a section where chlorophyll is present and a section on the border where chlorophyll is absent. The middle of the leaf is green, which contains chlorophyll, and the border of the leaf is white, which does not contain chlorophyll. A variegated leaf can prove whether or not chlorophyll is needed for photosynthesis.

8. What conclusions can you draw about starch production in a variegated leaf?

In conclusion, chlorophyll is needed for photosynthesis to occur to produce starch, and without chlorophyll, photosynthesis cannot take place and cannot produce starch. From the experiment using the variegated leaf, only the middle part that contained chlorophyll turned dark blue after being mixed with iodine, but the border of the leaf that does not have chlorophyll did not change colours since starch was not produced.

Part D

9. In this experiment what was the purpose of adding the soda lime?

The purpose of adding soda lime is to eliminate CO₂ (carbon dioxide) from the surrounding air to see if photosynthesis can occur with or without the presence of CO₂. Since soda lime is an alkali, it absorbs carbon dioxide from the surrounding air, causing the plant to lack CO₂

10. Why was a sealed bag placed around each plant?

Placing a sealed bag around each plant prevents any CO₂ from the outside to affect the results. The bag creates a barrier and restricts the air to a smaller portion

allowing the soda lime and the sodium hydrogen carbonate to be more affective.

Placing a bag around each plant gives more accurate results.

11. What happened in the control plants?

The first plant that has soda lime in its pot did not change colours after iodine was mixed with it, meaning photosynthesis did not take place to produce starch due to the lack of CO₂. However, the second plant which has sodium hydrogen carbonate in its pot, changed to a dark blue colour proving that photosynthesis did take place and starch was present.

12. What was the result on photosynthesis?

The final result on photosynthesis showed that carbon dioxide is mandatory for photosynthesis to occur, and without it, photosynthesis cannot take place. The experiment in which soda lime was added to a plant and covered up for 24 hours shows that photosynthesis cannot take place without carbon dioxide, which was eliminated due to the soda lime. The second plant, which tested positive for starch and changed to a dark blue colour with sodium hydrogen carbonate in its pot, also proves that without carbon dioxide, starch cannot be produced.

Conclusion:

In this investigation, the objective was to determine if important reactants like light, carbon dioxide and chlorophyll affect photosynthesis and if it occurs properly. The way to determine if photosynthesis occurred or not is through the iodine test, which will change the colour of a leaf to a dark blue colour if tested positive for starch. To start the lab, the plants were left dark for 48 hours to de-starch, this eliminates any starch stored in the plant, in order to get accurate results. For every experiment, a leaf was boiled in hot water, this was to break the cell wall and membrane to allow the iodine to penetrate the leaf. In addition, the leafs then were boiled in alcohol to eliminate the pigment and chlorophyll to make colour change easily recognizable. Removing the chlorophyll eliminates any other reactants from affecting the results. After being heated in water and alcohol the leaf loses its colour and turs yellowi, then iodine is mixed with the leaf. Iodine is used in this test because it determines, through the change in colour, if photosynthesis took place. If the leaf turns dark blue then that proves that starch is present meaning photosynthesis has occurred, and if the leaf does not change colour it proves that starch was not present and photosynthesis did not occur. For the first experiment, a paper cut-out was used to determine whether or not light is important for the production of starch through photosynthesis. First, a leaf was taken from a geranium plant and covered up by a paper cut-out, only exposing the leaf to light where the paper cut-out was not covering it. The leaf was left under the light for 24 hours, and then the iodine test was performed to test for starch. The only part that tested positive and changed to a dark blue colour was the part that was exposed to the light, the part of the leaf that was covered by the opaque part of the paper cut-out did not change colours, meaning that photosynthesis did not take place due to the lack of light. From this experiment, it can be concluded that light is an important reactant for photosynthesis to take place. In real life, usually plants lean to where light is coming from, for example, if a plant was left beside a window, overtime it will lean towards the window where the sunlight comes through so that it can capture as much light as possible for energy. In the second experiment, soda lime was added to a plant and covered up by a plastic bag, and sodium hydrogen carbonate was added to a second plant which was also covered up by a plastic bag for 24 hours. The purpose of using soda lime is to eliminate any CO2 in the air, and the purpose of using sodium hydrogen carbonate was to add CO2 to the air. Both were covered up by plastic bags to restrict the environment to a smaller space so that the soda lime and sodium hydrogen carbonate can be more affective. After 24 hours and after being heated in alcohol the leaf from the plant that had soda lime tested negative for starch, while the plant that had sodium hydrogen carbonate tested positive for starch. The leaf from the first plant remained the same colour, while the leaf from the second plant changed to a dark blue colour, indicating that photosynthesis has took place and starch has been produced. From this experiment, it can be concluded that carbon dioxide is an important reactant for photosynthesis to take place. In real life, plants are usually stored in places where the environment is rich in carbon dioxide, for example, flower stores sometimes leave their plants outside rather than inside so that the plants are left in an environment filled with carbon dioxide. In the final experiment, a leaf from a variegated plant was used to determine if chlorophyll was necessary for the production of starch through photosynthesis. A variegated plant was used because the middle part of the leaf is green, which has chlorophyll, while the border of the leaf is white, which does not have chlorophyll. After being left in the light for 24 hours, the iodine test was performed on the variegated leaf, and only the middle part where there were chlorophyll cells turned to a dark blue colour while the border did not change. From this experiment, it can be concluded that chlorophyll is an important reactant for photosynthesis to take place. The hypothesis was supported from the results obtained because all experiments proved that photosynthesis cannot take place without the presence of light, carbon dioxide, and chlorophyll. Water was not tested in this experiment because without water the plant would not be able sustain life. One way to extend the lab is by testing if oxygen is an important reactant for photosynthesis to take place or if it limits photosynthesis. For this extension, an aquatic algal plant such as a hydrilla  must be used as photosynthesis still must take place take place underwater. First fill up a beaker with water and place the hydrilla plant inside the water. Use the funnel to cover the hydrilla plant, and then place a test tube on the stem of the funnel. Next, leave the hydrilla plant inside the beaker in the light for 24 hours. Afterwards, slowly lift up the test tube from the funnel and light up a match and place it near the opening of the test tube. Finally, if the test tube glows brightly after the flame has been lit, then this proves that oxygen is an important reactant in photosynthesis. Another extension that may be used is determining if a plant is able to recover after being deprived of some of these reactants necessary for photosynthesis. This may be attained by depriving the leaf of a plant from the reactants for a certain period of time and then bringing them back and observing wheat happens.

Error Analysis:

Various errors may have led to inaccurate results. The errors that may possibly change the results are contamination, overuse of materials, and inaccurate timing. If the petri glass, water, or test tube of alcohol is contaminated with parts of a leaf that tested positive for starch, this could slightly change the results for the other experiments that may or may not test positive for starch. To fix this error, proper use of cleaning equipment like distilled water, soap, and towels would help optimize results and prevent the chance of contamination. Another possible error could be the overuse of certain materials such as, iodine, soda lime, or sodium hydrogen carbonate. This may cause the leafs to react differently if excess material is added or if not enough material is added. The way to prevent this error is by using accurate measurements like an eye dropper or a measured cup to put the exact amount of materials in the plants or on the leafs. Finally, using a timer would eliminate the error of excess exposure to light, or overheating the leaf. If a leaf is overheated it may not react to the iodine the same way, similarly if the plants were left to de-starch for too long or too little. Ensuring everything is precise and clean as possible will help to give the most accurate results.

Works Cited

• Carolina. “Using Aquatic Plants to Demonstrate Photosynthesis.” Carolina Biological Supply: World-Class Support for Science & Math, 2012, www.carolina.com/teacher-resources/Interactive/using-aquatic-plants-to-demonstrate-photosynthesis/tr28607.tr.
• Science and Plants For Schools. “Demonstrating Oxygen Evolution during Photosynthesis Using Pondweed.” Welcome, 2018, www.saps.org.uk/secondary/teaching-resources/190-using-cabomba-to-demonstrate-oxygen-evolution-in-the-process-of-photosynthesis-.