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Zami Talukder Photosynthesis Introduction Photosynthesis is the process in which plants make high level energy sugars called glucose. Glucose is what supplies a plant for their growth and development. Almost all autotrophic organisms perform photosynthesis to obtain usable energy. The most common autotrophic organisms are plants. The basic equation of photosynthesis is H2O + CO2 + light energy

C6H12O6 + O2 + H2O. Photosynthesis is the way that energy initially enters a food

chain. The glucose that plants make is consumed by herbivores when they eat the plant. Consumers later eat the herbivores and continue the food chain. Since plants are the producers, a food chain would not survive without them. Photosynthesis is also part of the carbon cycle. Photosynthesis takes carbon from the atmosphere to make glucose. When plants die, they become organic compounds which later help other plants grow. The whole process of photosynthesis happens in plant leaves. The process of photosynthesis can be broken down into 2 reactions, the light reaction and the dark reaction. The dark reaction is also known as the Calvin cycle. The light reaction is what initially starts the photosynthesis process. In the light reaction, plants use pigments to capture light energy. The energy of light is stored in photons, which is what the plant pigments actually capture. There are many pigments that do this, but the 4 main pigments that plants use are chlorophyll a, chlorophyll b, carotene and xanthophylls. The pigments sit on the membrane of the thylakoids inside the plant chloroplast. After the energy is captured by the pigment, it must be put in somewhere as a temporary storage. For this, plants use the H2O that they absorb using their roots. The water is broken down into H+, O2, and an

electron, using enzymes. The plant puts the energy of the light into the electron. This results in a highly charged electron. Energy from the electrons is used to transport the H+ into the thylakoid. (Kenneth R. Miller, 2003) After this happens for some time, H+ builds up inside the thylakoid. Because the H+ is positively charged, the outside of the membrane becomes negatively charged. This makes the H+ molecules attracted to the outside of the thylakoid, and so they use the ATP synthase enzyme to escape. As the H+ go through the ATP synthase, they turn ADP molecules into ATP. As the H+ is leaving the thylakoid and entering the stroma, the highly charged electron is also let out into the stroma. Both the H+ and the electron get picked up by a NADP+ which then reduces into NADPH. This whole process of putting light energy into an electron and using a H+ molecule to turn ADP into ATP and putting the H+ and the electron into NADP+ is called the light reaction. (J. Witmarsh, 1995) The next reaction happens in the stroma, and light is not present in this reaction. This reaction is called the Calvin cycle. It starts out with 3 CO2 molecules entering the cycle. Plants get the CO2 from the air using their stomates. The stomates are pores on the leaves which open up and let CO2 into the plant. There are already 3 5-carbon molecules present inside the cycle when 3 more CO2 molecules enter the cycle. Each of the 3 CO2 molecules bond with 1 of the 5carbon molecules to make 3 6-carbon molecules. These 3 6-carbon molecules are very unstable, so they break down into 6 3-carbon molecules. 1 of the 3-carbon molecules get 6 ATP added to it to increase the amount of energy the molecule currently has. The ATP then gets turned back into ADP and goes back to the light reaction. As the cycle moves forward, 6 hydrogen molecules get added to the same 3-carbon molecule. This 3-carbon molecule is now highly charged with

energy and gets left behind as the other 5 3-carbon molecules go back into the cycle. 3 ATP is added to 1 of the 5 3-carbon molecules and turned back into ADP, which also returns to the light reaction. The 5 3-carbon molecules then turn back into 3 5-carbon molecules, and the Calvin cycle starts over. The cycle happens again to create another highly charged 3-carbon molecule. The 3-carbon molecule from before bonds with this new 3-carbon molecule to create 1 6-carbon molecule. This completes the making of 1 glucose molecule. The process keeps going to make more glucose molecules (Carol Both,2011). The question that was asked for these experiments were how can we compare the absorbance of three wavelengths of light by chlorophyll? And how can the use of CO2 during photosynthesis be demonstrated? The hypothesis for the absorbance and wavelength experiment was that if the wavelength of red light was over .5, then red is one of the colors of light that chlorophyll absorbs the best. The hypothesis for the role of CO2 experiment was that if the tube with the elodea plant was exposed to sunlight, then the color of the bromothymol blue would change from yellow back to blue. Reference: Rising CO2 Plants and Biodiversity Article by Carol Both, Tim Low Biology by Kenneth R. Miller and Joseph S. Levine “Photosynthesis� by J. Whitmarsh and Govindjee (1995), published in Encyclopedia of Applied Physics (Vol. 13, pp 513-532) by VCH Publishers, Inc

Materials and Methods For the first 2 parts of the whole experiment, chlorophyll was extracted from a large spinach leaf. This was done by putting the leaf into a 100 mL breaker with 10 mL of ethanol. The beaker was then put on a hot plate set to the heat of 2. The beaker was left to heat up as chlorophyll exited the leaf and entered the ethanol solution. The spinach leaf was then disposed of. A cuvette was later filled up to 2/3 of the capacity with the ethanol solution and the chlorophyll. Another cuvette was also filled up to 2/3 of its capacity with only ethanol solution. The colorimeter was then used to calibrate the absorbance of different wavelengths by the chlorophyll. The first wavelength was 565 nanometers, which was the color green. The second wavelength was 635 nm which was the color red. The last wavelength was 470 nm which was the color blue. All the data that was received from the calibration was recorded in a chart. For the last part of the experiment, 15 mL of bromthymol blue was put into a beaker. The solution was then exhaled on, until it turned yellow, with the use of a straw. The yellow bromthymol was then put into 3 test tubes of 5 mL of the solution in each test tube. An elodea plant was put in test tube 1. No plants were added to test tube 2. Another elodea plant of the same size as the one in test tube 1 was placed in test tube 3. Test tube 3 was then covered with aluminum foil to block the light. Rubber stoppers were then put on each test tube. Each test tube was then exposed to light for about 40 minutes. The results were then recorded in a chart.


Figure 1 showed that the wavelength of 470 nm had an absorbance of 1.338. The wavelength of 565 nm had an absorbance of .933, while the wavelength of 635 nm had an absorbance of 1.733 nm. Figure 2:

Wavelength of Light (nm) Green (565 nm) Red (635 nm) Blue (470 nm)

Absorbance .933 1.733 1.338

Figure 2 showed that the absorbance of green light was .933. The absorbance of red light wave was 1.733, while the absorbance of blue light waves was 1.338. Figure 3:

Test tube with Yellow Bromthymol 1 – Elodea plant 2 – no plant 3. Elodea plant – no light

Color Change Light No color change, yellow Lighter

Figure 3 showed that the Elodea plant in test tube 1 changed to a light color. The substance in test tube 2 had no color change, and the elodea plant in test tube 3 changed to a lighter color.

Discussion The results of part B was that the green light had the lowest absorbance while the red and blue had a high absorbance. This was expected because chlorophyll normally reflects green light and absorbs all other light waves except green. The results of part C was that test tube 1 had a light color change compared to test tube 3. Test tube 2 was the control, so no color change was expected from it. However, test tube 1 was expected to show the greatest color change, instead, it was test tube 3 that showed the most color change. This may have been because the aluminum foil was not put on right. The elodea plants could have also had a big difference in age which may explain why the elodea plant in test tube 3 worked faster than the elodea plant on test tube 1. Chlorophyll is green because it is a pigment that absorbs all light waves except green. Since it does not absorb the light waves of the green color, it reflects it into our eyes, which is why we see the pigment to be green. These results make sense because the experiment tested the absorbance of different light waves by chlorophyll. Since chlorophyll was not expected to absorb green light, the results also were not expected to show that chlorophyll absorbs a large amount of green light compared to red and blue light. The other pigments in plants are carotene and xanthophylls. This is an advantage to plants because the plants will have chances to absorb some green light, even if chlorophyll cannot do it. Plants may grow if only a green light source is given. This is because there are 2 other pigments that were mentioned above, carotene and

xanthophylls, which absorb green light. From this, the plant would still have a supply of energy and it would be able to carry out photosynthesis to make glucose and grow. Bromthymol blue is a pH indicator. When an acidic substance is added to it, it turns yellow. When it is around a neutral pH level, it will turn blue. The bromthymol turned green/blue for test tube 1, which only consisted of the elodea plant, because the elodea plant could perform photosynthesis to remove the CO2 that was added to the bromthymol blue from exhaling on it. The plant was also given sufficient amount of light to let it continue with its photosynthesis process. Test tube 2 did not have any plants in it, therefore, no reaction occurred. Test tube 3 did have an elodea plant, but lacked a source of light energy for the plant to successfully go through the process of photosynthesis. The CO2 that was in the bromthymol solution was probably turned into parts of glucose molecules and became a little part of the plant. The control in this experiment was the test tube with no elodea plant and the test tube with the elodea plant and aluminum foil. In test tube 2, there was no place for the CO2 to escape to, since the test tube was capped with a rubber stopper. That is why the bromthymol stayed yellow in that test tube. In test tube 3, the elodea plant did not have light energy to start its process of photosynthesis. This would not allow the latter steps of photosynthesis to occur, and therefore the CO2 from the bromthymol would not be used. Possible sources of errors in this experiment would be if one forgot to cap the test tubes, or place about the same size of elodea plant in each test tube with an elodea plant. This experiment showed how CO2 is a reactant in the process of photosynthesis. Plants use this process in their daily lives in nature. A question that arises from this experiment would be how can 2 cycles of the Calvin cycle to create 1 glucose molecule be demonstrated?


how photosynthesis happens

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