Photosynthesis: the Spinach Disks

The spinach discs were tested in different conditions where the number floating is what is observed. The number of floating discs is dependent on whether or not photosynthesis occurred in the discs. The different conditions or solutions tested all had unique effects on the photosynthetic rate. It is known that the presence of CO2 and light promote photosynthesis, which yields an increase of O2 as a product. The higher the production of the gases, the more that will be trapped in the mesophyll, or spongy layer, of the cell, which makes the spinach discs less dense than the water causing them to float to the top of the solution. Our experimental design was set up in order to indirectly measure the rate of photosynthesis by counting the number of discs floating that is a result of oxygen gas being released which is a byproduct of photosynthesis.

CO2 has an imperative role in the dark reactions of photosynthesis. Leaves take in CO2 through the stomata (pores at the bottom of the leaves) and introduce it into the Calvin Cycle. Without CO2 Calvin Cycle cannot run and therefore cannot produce sugar (Campbell, Reece 193-195). In our experiment, one the control cups was without the addition of CO2 and was supposed to be used as a control to show that no discs floated or no photosynthesis occurred in the absence of CO2 which would reinforce the necessity we know it is. However we observed that there were discs rising in this cup and at the end of the time period there were 4 when we knew there should be 0 like in our control with no light. This result is due to human error in the experiment. There are two possibilities for this error which are that we did not completely suction the air out of all the discs or that the same syringe was used for this cup as our other two control cups which had the CO2 in the solution. Either one of these errors could have led to these skewed and incorrect results.

Since there was a mistake made with one cup, we cannot trust the validity of the results from our other cups making our experiment invalid. However, if our experimental controls had worked we still would expect the more acidic environments to be better suited for photosynthesis to occur. We can see that by 25 minutes all 10 of the discs had risen in our two acidic solutions and only 7 in the solution that contains CO2 and light which functions as the baseline control and this suggests our hypothesis was correct and that lowering the pH increases photosynthetic rate. However this is very preliminary and subject to change as one of our controls failed and we need to repeat the experiment to validate results. If the trend we see here continued in another trial of this experiment, our hypothesis could then be proved correct.

During the light reactions, the electrons in the photosystems need to capture light energy with the primary electron acceptor. When an electron is captured in photosystem II, the electron transport chain begins and results in the production of ATP. When an electron is then taken in by photosystem I, it is transferred to a NADP molecule and produces NADPH. Both ATP and NADPH are required in the dark reactions of photosynthesis and without them the Calvin Cycle cannot synthesize sugar. Thus, light is needed in to carry out the light and dark reactions, and in the absence of light, photosynthesis cannot occur. This is the reason the cup not exposed to any light had no discs floating for the entire time.

The level of acidity refers to the relative concentration of H+ ions in a solution: the stronger the acid, the higher the concentration of H+ ions is and the lower the pH. The stronger and weak acids provide more H+ ions to the solution than any of the controls that have a more basic pH. H+ ions have an imperative role in producing ATP that will be consumed in the Calvin Cycle. This ATP is produced by chemiosmosis that is run by the proton-motive force that is formed by the electron transport chain between photosystem II and photosystem I. After the primary acceptor in photosystem II captures an electron, it transfers it to the electron transport chain. As the electron proceeds toward the most electronegative molecule, it passes from one molecule to another; the electron reduces and oxidizes each molecule as it continues. When the electron is transferred to a more electronegative molecule, energy is released. This energy that is freed is then converted into the proton-motive force that increases with a higher concentration of H+ ions.

The energy released in the electron transport causes the H+ ions to rush inside the thylakoid space creating a H+ ion concentration gradient. The H+ ions accumulated in the thylakoid space then start passing through ATP synthase because of the tendency to move from a more concentrated solution to a less concentrated one. When it passes through ATP synthase, ATP is produced that is going to participate in the Calvin Cycle. An acidic solution, provides more H+ ions to the surrounding of the leaf discs which can help the leaves take in more H+ ions through the stomata than the leaves in the basic solution due to availability. This allows the H+ ion concentration in the thylakoid space to increase more rapidly and can drive the rush of H+ ions through the ATP synthase can produce more ATP faster. In other words, the H+ ions in the surrounding solution contribute to increasing the rate of photosynthesis. If our experiment was valid, the results would support the notion of an increased H+ ion concentration, or lower pH, increasing the rate of photosynthesis and would yield more discs floating in the cups with acidic solutions than in the control cups.

Hydrochloric acid and sodium bicarbonate (base) interaction results with the end products of CO2 (carbon dioxide), H2O(water), and NaCl (table salt) as the equation below shows.

2 HCl (aq) + NaHCO3 (aq), 2 NaCl(aq) + CO2 (g) + H2O (l)

This reaction provides more CO2 in addition to the CO2 that is already provided by the sodium bicarbonate (New Media Press). Since a higher concentration of CO2 would increase the rate of photosynthesis, leaves surrounded with the mixture of HCl and NaHCO3 would have a higher rate of photosynthesis. Since the addition of If our experiment was valid, we would expect a higher rate of photosynthesis in the cups with HCl acid.

However assuming that higher acidic environments always increase photosynthetic rate is a limited and false way of looking at the results. If our experiment was valid, then we could trust our recordations that there was a higher rate of photosynthesis in the cup with the weak acid than the cup with the strong acid. Though acids provide a higher concentration of H+ ions and proportionally increase photosynthetic rate, they also decrease it simultaneously. The explanation to this phenomenon lies on the H+ ion inhibition of the light reactions. According to a study made by Teena Tongra and many other researchers, increased pH value inhibits the electron donor and recipient sites of the photosystem II. An inhibition at the beginning of the photosystem process reflects to the rest of photosynthesis and decreases the rate of photosynthesis. If our experiment was valid, the inhibition of the photosystem II would explain the lower rate of photosynthesis that we recorded in the cup with the stronger acid (Tongra).

If the control group that had failed worked, meaning our experiment would have not failed, we would have expected a denaturation of the enzymes involved in photosynthesis due to high acidity. The pH of the stroma in chloroplasts is around 8. Many proteins are involved during photosynthesis: rubisco is one of the most prominent enzymes that take role in photosynthesis. However, every protein has an optimal pH value in which they can function and for the enzymes functioning during photosynthesis, this values is around 8. Rubisco's optimal pH is between 7.8 and 8. As the pH increases, the enzyme will change structurally and won't be able to perform its function, which is binding to CO2 in carbon fixation. The dysfunctionment of rubisco will prevent sugar synthesis because CO2 will not be able to enter the Calvin Cycle. Many other proteins will undergo the same problem if the pH deviates from their optimal pH values. For this reason, if our experiment was valid, it would support the fact that acids denature enzymes and lower the rate of photosynthesis. Because a more acidic solution deviates more from the optimal pH of the enzymes that function in the chloroplast (around pH 8), the photosynthesis rate in the cup with a pH value of 4 would have been shown to be less than that of the cap with a pH of 6, this is something we cannot conclude because of the human error that occured.

Research done by Sware Semesi, Juma Kangwe, and Mats  is titled, Alterations in seawater pH and CO2 affect calcification and photosynthesis in the tropical coralline alga, Hydrolithon sp. (Rhodophyta). The experiment was began by obtaining the Hydrolithon sp., an algae, which were collected in Chwaka Bay, Zanzibar. The algae were contained in a 5L transparent plastic container and bubbled with air for 2 weeks prior to experimentation. The experimental conditions were set at different pH values. The pH values were then investigated in terms of their photosynthesis and calcification activities.

The experiment was repeated 4 times at each pH value. The photosynthetic and calcification rates were measured in a temperature-controlled 300mL chamber where O2 and the pH values were kept track of continuously using an O2 electrode. This research showed that Hydrolithon sp., an algae found growing in seagrass beds in shallow lagoons had a decrease in photosynthetic rate when the pH was raised to the higher pHs of the 7 pHs tested, making it more basic. They stated that when the pH was above 8.6 the algae had a higher photosynthetic rate. This evidence supports that lowering pH, making the solution more acidic, will cause a higher photosynthetic rate. This evidence showed that the raising of pH decreased the photosynthetic rate (Semesi). We hoped that this evidence would support our findings in the experiment we conducted, but since one of the control groups failed, we can not say that this evidence supports our findings. However, this evidence does support our hypothesis.

Research done by An Long, Jiang Zhang, Lin-Tong Yang, Xin Ye, Ning-Wei Lai, Ling-Ling Tan, Dan Lin, and Li-Song Chen is titled, Effects of Low pH on Photosynthesis, Related Physiological Parameters, and Nutrient Profiles of Citrus. The experiment observed what low pH does to affect Xuegan (Citrus sinesis) and Sour pummelo (Citrus grandis) seeds. They looked at many factors to see how the lower pH would influence the photosynthesis and how the lower pH would lower CO2 assimilation. The experiment was set up at a University called Fujian Agriculture and Forestry University in Fuzhou, China. Two seeds, Xuegan (Citrus sinesis) and Sour pummelo (Citrus grandis), were grown in plastic trays and then later the seeds with single stems were put into six pots, two seeds in each pot. Then they were each given a nutrient solution pH of 2.5, 3, 4, 5, or 6. There were forty seeds looked at. It is said in this experiment that pH 5 was considered the control group because the growth of the seeds and other growth factors will hit the maximum at a pH of 5. If our experiment hadn't failed, this would support our hypothesis because it shows how a lower pH is favored and used as the stable pH for optimal growth, which in turn gives optimal photosynthetic rate.

After many different measurements were taken of the seeds, it was shown that a pH of 5 had the most optimal growth. It was also shown that a pH of 4 did not change the growth of the seeds. The pHs of 2.5 and 3 hurt the growth of the seeds more, with 2.5 hindering the growth the most. This experiment showed how the nutrient solution with a pH of 5, an acidic pH, had optimal growth compared to its very acidic counterparts (Long). If our experiment hadn't failed, it could have supported the results, those of which are invalid. This experiment showed that a pH of 5 and 4, which are around the pH values we used for our experiment, provided optimal seed growth and unchanged seed growth, not hindering the growth. It also showed that when the solution becomes too acidic it hinders the growth as well, something that we hoped to be concluded in this experiment, but we cannot conclude this because our experiment failed. These two experiments proved what we hoped would be the result of our experiment, but since one control group failed, the results are invalid.

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