Biology Report

General Biology BI102

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LAB 6: PHOTOSYNTHESIS

How Living Things Trap Energy from the Non-living World Learning objectives (What we expect you to get out of this lab): Concepts

Vocabulary Define, give examples or recognize examples of:

• Plants contain pigments to capture sunlight energy to produce glucose (sugar). cellular respiration • Different colors of light contain different amounts of energy. carbon bonds • Know how plants produce oxygen during photosynthesis. absorption spectrum • Know how plants use carbon dioxide to produce glucose. wavelength • Understand how sugar can be stored in plants as polymer chains such as starch or

cellulose. pigment

pH indicator Introduction All organisms must take in energy because they cannot make energy on their own. The source of energy for most of the living organisms is the sun. Living things can utilize the sun’s energy directly as autotrophs, or indirectly as heterotrophs. Autotrophs (such as plants) take in carbon dioxide and water as a raw material and, with sunlight, assemble these into molecules such as glucose; the energy from the sun is stored in the molecule’s carbon-carbon bonds. Heterotrophs eat these molecules made by plants and release the stored energy by breaking the chemical bonds by a process called cellular respiration. Photosynthesis can be summarized by the following equation:

6CO2 + 6H2O + sunlight (energy) → C6H12O6 + 6O2 Cellular respiration is the opposite process of photosynthesis.

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy Photosynthesis takes place in chloroplasts and it takes place in two phases: a light-requiring phase which captures the sun’s energy (light-dependent Reaction) and an energy-storing phase in which glucose is produced (Calvin cycle). During the light-dependent Reaction phase, light is absorbed and water is split producing oxygen. During the Calvin cycle carbon dioxide is incorporated into glucose molecules using the energy captured in the light-dependent Reaction. In chloroplasts, plant pigments absorb the sun’s energy. Sunlight is composed of different wavelengths of light, each wavelength representing a different color with different amounts of energy. The pigments of plants such as chlorophyll, carotenes and xanthophylls absorb different wavelengths of light. It is these absorbed wavelengths that are used to power photosynthesis and ultimately provide the energy stored in the chemical bonds of glucose.

General Biology BI102

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Part 1: Light absorption by plant pigments 1. Your instructor will provide a demonstration of a spectrograph that shows the variety of pigments present in the plant. A spectrograph records the amount of light absorbed by a substance in each wavelength. 2. Examine the spectrograph. 2a. Draw the wave pattern you see on the spectrograph. 2c. Identify which wavelengths (colors) of light are absorbed the most and which are absorbed the least.

% A

bs or

ba nc

e

400 500 600 700

Wavelength (nanometers)

Spectrograph Most absorbed colors: Least absorbed colors:

General Biology BI102

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Part 2: Effectiveness of different colors (wavelengths) on photosynthesis

1. Fill two test tubes with a cut 4” piece of Elodea in a bicarbonate solution (carbon dioxide source). One test tube is completely covered with foil. The other should remain uncovered. 2. Carefully insert a stopper, which has a long glass tube, into each test tube. If the test tube is full enough, some of the water will rise in the tube. Carefully set both tubes upright in the test tube rack. 3. Place a large beaker of water between the flood light and your test tube in order to absorb excess heat. Place the flood light 2-4 inches from the beaker, and the test tube immediately next to the beaker. Turn on the light. Allow the system to equilibrate for 5 minutes. 4. After the five minute equilibration period, mark the level of water in the glass tube with a Sharpie pen. 5. Let the experiment run 10 minutes. Mark the new water level. 6. Let the experiment run an additional10 minutes. Mark again. 7. Measure the distance the water moved in the first 10 minutes and the second 10 minutes. Average the two and report your results on the class data sheet. Dark Light First 10 minutes: _____ mm _____ mm

Second 10 minutes:______mm _____ mm

Average: _______mm _____ mm

Lab Table Dark Light 1

2

3

4

5

6

General Biology BI102

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Part 3: Carbon dioxide uptake during photosynthesis This part of the lab uses pH as a measure of CO2. This is possible because when it is dissolved in water, CO2 combines with the H and O of water to form Carbonic acid (H2CO3). The chemical equation for this relationship is: CO2 + H2O  H2CO3. Because CO2 enters an equilibrium relationship with carbonic acid, the more CO2 is dissolved in water, the more carbonic acid can form. You should note that the presence (or absence) of O2 does not impact the pH of water in this relationship. 1. Pour a small amount (about 10-20 mL) of bromothymol blue into two small beakers. Put a white piece of paper behind the beaker so that you can better observe color changes. 1a. Bromothymol blue is a pH indicator and changes color when it becomes more acidic. Acidify the

bromothymol blue in both of the beakers by blowing through a straw into the solution. The carbon dioxide in your breath will lower the pH and cause the blue solution to turn yellow.

2. Obtain a 1” section of Elodea that has been sitting under white light from the front of the room. (The exposure to white light for at least 5 minutes will allow photosynthesis to take place). 2a. Add the 1” section of Elodea to one of the beakers containing the yellow solution. 3. Leave the two beakers to sit under white light for at least thirty minutes. 3a. Every ten minutes, examine the two beakers side by side (you may choose to also compare them to a

beaker containing the original bromothymol blue solution). 3b. Make a qualitative description of the color of the solution in each beaker. 4. Explain (using photosynthesis) what caused the color change results that you have observed. Time Started: Color Description 10 minutes:

20 minutes:

30 minutes:

General Biology BI102

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Part 4: Storage of energy as starch 1. On the front counter is a geranium plant that has some leaves partially covered by strips of aluminum foil. The plant was kept in the dark for 24 hours, then the leaves were covered. The plant was then exposed to bright lights for another 24 hours. Use a razor blade to carefully cut one of the partially covered leaves from the plant. 2. Place the leaf in a beaker. Add alcohol to cover the leaf. 2a. Place the beaker in a hot water bath. Chlorophyll is soluble in alcohol and should be extracted in

approximately 5 minutes (once the leaf is pale and flabby). 2b. Remove the beaker from the hot water bath and remove the leaf from the alcohol. Carefully rinse the

leaf in cool water. 3. Spread the leaf out in a Petri dish. 3a. Cover the leaf with Lugol's solution and let sit for about 30 – 60 seconds. 3b. Carefully rinse the leaf under cool water. 3b. Hold it up to the light to show blackish stained areas which indicate the presence of starch. 4. Draw your leaf and indicate which portions have stained with Lugol’s solution. Note which areas show starch as present and which do not.

General Biology BI102

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Questions Name________________________ 1. What organelle do plants need to capture the light energy in sunlight? 2. What raw materials (molecules) do plants need to make glucose? 3. Where in the structure of glucose molecule do plants store the sun’s energy? 4. What color of light does chlorophyll absorb the least? Why? 5. How did you measure the rate of photosynthesis? 6. As you blew into the bromothymol blue solution, did the color change result from lowering or

raising the pH? 7. What happened to the color of the bromothymol blue solution when carbon dioxide was removed

during photosynthesis? 8. Where did the carbon dioxide go in the previous question? 9. Is starch present in the areas of the leaf that were covered with tinfoil or in the areas of the leaf that

were uncovered? Explain your answer. 10. Do plants perform cellular respiration? How would you test this?