Aquatic Plants Rate Of Photosynthesis Biology Essay

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Acid rain is a known effect of global warming, which has damaged many aquatic environments and aquatic plants throughout the world. This study investigates the effects of acid rain on an aquatic plant's rate of photosynthesis. The research question is "How does the change in pH of an elodea's habitat due to acid rain affect the plant's rate of photosynthesis?" Samples of elodea were placed into two acidic solutions, 0.01 Molar solution of nitric acid and 0.001 Molar solution of nitric acid, in a photosynthometer. The control group is water in the photosynthometer with a pH of 7. Each trial took 24 hours and the experiment was done in a high school chemistry lab near a window ledge, away from direct sunlight. After each 24 hour period, the amount of oxygen released by the plants is measured to determine the rate of photosynthesis.

As the environment becomes more acidic, the elodea samples' volume of oxygen release decreases. The average volume of oxygen release for the elodea samples in the control water group is 0.57 mL, 0.29 mL for the pH 4.0 group, and 0.15 for the pH 3.5 group. The results from this study suggest that there are big differences among the three groups and that acid rain negatively affects the rate of photosynthesis.

1.0 Introduction

1.1 Rationale of Study

Nowadays, global warming has become a big issue on the forefront of environmental problems. It's not simply the devastation of rising temperatures all over the world but other effects as well which can be much more dangerous and harmful to the human race. One of these effects is the increase in the acidity of precipitation, or acid rain. At present, acid rain affects large parts of the United States and is especially noticeable near large cities. The annual acidity value averages at pH 4 but values as low as pH 2.1 have been observed. It is confirmed that the increased use of natural gas and development of factories have been associated with the increasing acidity of precipitation and thus, efforts had been made to rely on more clean, renewable energy sources and the development of air quality emission standards. However, not all the economic and ecological effects due to the introduction of strong acids into the natural systems are known and therefore, this study looks into one of its many harmful effects, the acidification of freshwater ecosystems.

Studies suggest that no matter how small the change in pH of any aquatic ecosystem is, large amounts of magnesium and calcium would still be lost in the vicinity affected by acid rain. As a result of the loss of these vital elements, the response and recovery of any aquatic ecosystem toward the decrease in acid deposition would be delayed significantly. Because of this, any further reduction in pH of the ecosystem will bring about exponential increases in damage to any living organism within the affected area. Just like how damage to the human immune system would lead to significant, exponentially increasing damage to the human body in the form of diseases, the damage done to the recovery mechanisms of an aquatic ecosystem due to acid rain opens up possibilities for catastrophes that could be of a far worse magnitude.

One such possible disaster is the harming of aquatic plants' rates of photosynthesis. Alongside the endangerment of the population of crustaceans, insects, and fish within the aquatic ecosystem, the damage done to the aquatic plants' rates of photosynthesis such as Elodea Canadensis's, can be one of the worst possible effects of acid rain and global warming. This study is worthwhile in that plants are the basis of the food chain and any damage inflicted upon the process of photosynthesis in general would surely mean that our very own existence is in jeopardy.

1.2 Aim

The aim of this paper is to study the effects of the change in pH of Elodea Canadensis's environment on the plant's rate of photosynthesis. In a broader context, this study investigates the effects of acid rain on aquatic plants.

Hence, the research question is: How does the change in pH of an elodea's freshwater habitat due to acid rain affect the plant's rate of photosynthesis?

The rate of photosynthesis is measured with a photosynthometer in which oxygen released from the elodea samples are collected under differing pH environments. The elodea samples are placed in differing solutions of nitric acid, which are used to simulate elodea living in freshwater habitats affected by acid rain. Because oxygen is a product of photosynthesis and is correlated to the rate of photosynthesis, oxygen is then collected from the various experimental groups. Most experiments that require the measurement of the rate of photosynthesis of a macrophyte determine the changes in oxygen concentration of the system in which the macrophyte is kept in and thus, this study is done in the most popular manner. The volume of oxygen collected from each group would then be analyzed to determine the optimal conditions for an aquatic plant to live in and the effects of acid rain on the rate of photosynthesis.

1.3 Acid Rain

Because the value for unpolluted precipitation is officially set at pH 5.65, the same value as distilled water, acid rain is a term that describes rain with a pH of less than 5.6. Man-made emissions of sulfur and nitrogen pollutants had always been blamed as a major cause of acid rain but a genuine cause-effect relationship has never been determined. However, it is certain that sulfur and nitrogen compounds react with the atmosphere to produce acids that would lower the pH of precipitation. There are also many natural sources of these sulfur and nitrogen compounds. For example, approximately 50 percent of atmospheric nitrogen compounds are produced by lightning discharges, which may bring about acid rain.

The real importance of studying acid rain though is to study its effects on the natural ecosystems. One such ecosystem that is affected greatly by acid rain is the aquatic ecosystem. The chemical composition of lakes is heavily influenced by precipitation and many studies have suggested that acid rain has caused lake acidification. Most importantly, the changes in pH of these ecosystems due to acid rain appeared to have damaged aquatic plants' metabolism, causing a decline in primary productivity. Because these aquatic plant communities are primary producers, any damage done to their metabolism mechanisms (photosynthesis) can drastically reduce the food supply and energy flow within the affected ecosystem. Thus, acid rain has the potential to reduce the supply of minerals and nutrients and endanger the existence of all organisms within an ecosystem, especially aquatic ecosystems.

1.4 Marine Photosynthesis

The metabolism of plants is commonly referred to as photosynthesis. Photosynthesis involves two kinds of processes, photochemical and enzymatical, meaning that the rate of photosynthesis is a function of irradiance and enzyme activity. No enzymes are involved in the photochemical process in which the plant absorbs light in the range of 350 and 700 nm in wavelength. In this process, chlorophyll molecules absorb light and excites electrons, which go through the electron transport and end up producing ATP and NADPH. As its name implies, the photochemical process involves light and is purely chemistry.

The other process is the light-independent enzymatical process of the Calvin cycle. This process occurs after the light-dependent reaction for it requires the ATP and NADPH to reduce CO2 to carbohydrate. At the start of this process, six carbon dioxide molecules attach to six 5-carbon ribulose biphosphate (RuBP) molecules to create six molecules of a 6-carbon compound. Each of these 6-carbon compounds splits into two 3-carbon molecules called phosphoglycerate (PGA). This results in 12 PGA molecules. Energy from ATP and electrons from NADPH are then needed to reduce each of these PGA molecules into twelve G3P (glyceraldehyde 3-phosphate) molecules. Finally, two of these G3P molecules are used to form one glucose molecule and the remaining ten G3P are reassembled into RuBP molecules.

Marine Photosynthesis also requires CO2 to start and this CO2 is acquired when CO2 is dissolved in water. This process is represented by the following formulas:

CO2 + H2O ßà H2CO3

CO2 + OH- ßà HCO3-

The dissolved CO2 in the water can either make the water increase or decrease in pH depending on the pH, temperature, and salinity of the environment. The concentrations of carbonic acid (H2CO3) and bicarbonate (HCO3-) in the aquatic environment form a complex equilibrium, which is needed to sustain optimal living conditions for its inhabitants; the two compounds play a vital biochemical role in the pH buffering system, which strongly affects photosynthetic organisms.

1.5 Elodea Canadensis

Elodea Canadensis is an aquatic vascular plant that spends its entire life cycle under the surface of a body of water. It is a perennial with a flexible branches stem and fibrous roots. Its leaves do not have petioles and they are always in groups of three to seven spread out evenly along the entire length of the stem. The species of Elodea Canadensis is commonly known as waterweed and is abundant in North and South America. However, there are 17 species of the genus Elodea and these plants are common throughout the world with use as an aquarium plant. Its use in science experiments is fairly common as well due to its strongly photosynthetic, dense chloroplast structure. When exposed to a strong light source, the oxygen bubbles given off by the plant is clearly visible. Another reason for its use in science experiments is that it is able to live enough after being cut into smaller strands to be experimented on.

2.0 Variables

2.1 Independent Variable

The elodea plants are placed in 2 different nitric acid solutions of varying pH and molarity. Strands of elodea with 10 leaves each are subjected to either a 0.001 molar solution of nitric acid with a pH of 4.0 or a .01 molar solution of nitric acid with a pH of 3.5. The solution and elodea are placed into the barrel of the syringe in the photosynthometer. Litmus paper is used to measure the pH of the acid solutions.

2.2 Dependent Variable

The rate of photosynthesis of the elodea samples are affected by the varying pH of the solutions they are subjected to. The rate of photosynthesis is indicated by the volume of oxygen given off by each 10 leaf elodea strand and collected in the photosynthometer over a 24 hour experiment period.

2.3 Control Variable

The control variable is tap water with a pH of 7.0, a neutral solution, in the photosynthometer. It is used to determine whether or not the acidic solutions the elodea strands are tested in actually have an effect on the plants' rates of photosynthesis as compared to a neutral aquatic environment.

2.4 Constants

All trials are done in the same chemistry laboratory next to a window ledge, away from direct sunlight. The room and the solutions inside the syringe of the photosynthometer are kept at a constant 26.4° Celsius. Each elodea sample is a 10 leaf strand. The same volume of solution is used for every trial in the photosynthometer.

3.0 Procedures

3.1 Preparation before experimentation

3.1.1 Test Trials

Before any definite procedure of experimentation is made, test trials needed to be done first in order to see which acid solutions would not kill elodea in a 24 hour period. Strands of elodea are placed in test tubes with 0.001 molar, 0.01 molar, and 0.1 molar nitric acid solutions and are labeled. By the end of the 24 hour period, the elodea in the test tubes with the 0.1 molar nitric acid solution died since the leaves lost all of their green color and oxygen bubbles were not released from the leaves even before the 24 hour period. This meant that the plant could not perform photosynthesis anymore and was dead. The other two elodea samples were alive and thus, the 0.001 molar and 0.01 molar nitric acid solutions were used for experimentation to mimic the effect of acid rain on an aquatic plant's rate of photosynthesis.

3.1.2 Nitric Acid Solutions Preparation

The 0.01 Molar solution of nitric acid is made by mixing 1 mL of a 1.0 Molar solution of nitric acid with 99 mL of tap water in a graduated cylinder. The tap water is measured with the graduated cylinder and a pipette is used to hold 1 mL of the 1.0 Molar solution of nitric acid.

1 Liter of a 0.001 Molar solution of nitric acid is prepared by mixing 1 mL of a 1.0 Molar solution of nitric acid with 999 mL of tap water in a liter plastic laboratory bottle. 1 Liter of this solution is made since it is more convenient to create a large volume of an acid solution with a low concentration than it is to create a small volume of a highly diluted acid solution.

3.1.3 Apparatus Preparation

The photosynthometer is assembled by connecting a syringe to a graduated 1-cm3 pipette with a short length of rubber tubing. The length of the rubber tubing is arbitrary as long as it is tight enough to secure the syringe to the pipette, preventing any liquid from coming out of either the syringe or pipette.

The apparatus is fixed in a vertical position with the test tube clamp and ring stand (Figure 1), using the test tube clamp on the syringe and connecting that to the ring stand.

3.2 Method for Experimentation with the Photosynthometer

3.2.1 Application of Elodea Sample and Solutions into the Photosynthometer

Before experimentation, a sample of elodea is taken by cutting a strand of elodea with 10 leaves. The mass of the elodea is recorded and measured in order to look for patterns after experimentation. The plunger of the syringe is then removed and the elodea sample is placed into the barrel of the syringe. Since any liquid placed in the syringe with the plunger off will fall straight through and out the apparatus, the elodea sample is placed in the apparatus first before anything. 30 mL of the 0.001 Molar solution of nitric acid is then poured into the barrel of the syringe and the barrel is immediately sealed with the plunger to prevent any more liquid from leaving the apparatus. No matter what, some of the solution would still leave the apparatus with the plunger off. Therefore, 30 mL of the solution is used in the barrel so that any excess amount of the nitric acid solution could be expelled by pushing down on the plunger until 15 mL of the solution is left in the barrel of the syringe. With the apparatus removed from the test tube clamp and the open end of the pippette pointing upwards, any trapped air inside the syringe and pipette is expelled by slowly and softyly pushing the plunger into the barrel until all of the trapped air expelled, making sure not to have any of the solution leave the pipette.

3.2.2 Maintaining Constants

When the apparatus is placed back onto the test tube clamp and ring stand, the temperature of the nitric acid solution inside the barrel of the syringe is measured with an infrared thermometer and recorded. The temperature is measured to make sure that the temperature remains constant for all trials since temperature does affect the rate of photosynthesis. To maintain constant temperatures and weather conditions as well, experimentation is done in one room for all trials and begins at the same time of day. In my case, experimentation was done in the school's chemistry laboratory, which was kept at a constant 26.4° Celsius, at 16:00 US central time.

3.2.3 Data Collection

The volume (the location of the meniscus) of the nitric acid solution in the pipette of the apparatus is measured and recorded. The time is measured and recorded as well. The elodea sample is left in the apparatus for 24 hours. After that time, the amount of oxygen the sample of elodea gave off is measured and recorded by looking at the location of the meniscus of the acid solution in the pipette. All experimental procedures are then repeated with the 0.01 Molar nitric acid solution and tap water instead of the 0.001 Molar nitric acid solution.

Figure 1: Photosynthometer Setup

4.0 Data Collection

4.1 Raw Data: Volume of Oxygen given off by Elodea in Different pH Environments


Volume of Oxygen Released (mL)

Trial 1

Trial 2

Trial 3

Trial 4
















Table 1

4.2 Raw Data: Mass of each Elodea Sample


Mass of Elodea Sample (g)

Trial 1

Trial 2

Trial 3

Trial 4
















Table 2

4.3 Data Processing: Comparing Mean Volume of Oxygen Release of Elodea

Graph 1

4.4 Data Processing: Trendline for Mean Volume of Oxygen Release VS. pH of Environment

Graph 2

4.5 Data Processing: Comparing Percent Difference of the Average Volumes of Oxygen Release of the Experimental Groups to the Control Group's Average Volume of Oxygen Release


Percent Difference





Table 3

4.6 Data Processing: Comparing Standard Deviations of Volume of Oxygen Release of Elodea


Standard Deviations







Table 4

4.7 ANOVA test

The ANOVA (Analysis of Variance) test is also used to further verify the difference of the results among the experimental groups. The result of this ANOVA test indicates whether the experimental variable (pH of the elodea's environment) causes significant difference on the elodeas' rates of photosynthesis.

Before the ANOVA test could be carried out, three assumptions are made:

Observations are independent (the value of one observation is not correlated with the value of another observation).

Observations in each group are normally distributed.

Homogeneity of variances (the variance of each group is equal to the variance of any other group).

The null hypothesis of this test is: there is no difference between the means of the different groups (pH 7.0, 4.0, and 3.5). Then, the statistic test is carried out to find the F ratio.

F Ratio = Mean square between groups

Mean square within groups

If the computed F ratio is greater than the F critical value at the significance level of 0.05, the null hypothesis is rejected.


F Ratio

F Critical Value (5%)


7.0 vs. 4.0



Significant Difference

7.0 vs. 3.5



Significant Difference

4.0 vs. 3.5



No Significant Difference

Table 5

5.0 Conclusion

Graph 1 shows that the mean volumes of oxygen release among all the groups had significant differences. The average volume of oxygen release for the pH 4.0 group is 0.29 mL, which is about half of the mean volume of oygen release for the control group, pH 7.0, of 0.57 mL. The mean volume of oxygen release for the pH 3.5 group is 0.15 which is about half of the mean volume of oxygen release of the pH 4.0 group as well. As shown in Graph 2, this trend is shown to be of an exponential decline in mean volume of oxygen release as pH increases; as the environment becomes more acidic, the mean volume of oxygen release declines more sharply. According to Table 3, the average volume of oxygen release of the pH 4.0 group differs from the control pH 7.0 group by 49%. The average volume of oxygen release for the pH 3.5 group differs from the control pH 7.0 group by 74%. These values are large and again emphasize the significant difference of the results of the experimental groups to those of the control group. According to Table 5, ANOVA test results, there is a significant difference between the mean volumes of oxygen release between the pH 7.0 and pH 4.0 groups, as well as the pH 7.0 and pH 4.0 groups. However, there is no significant difference between the results of the pH 4.0 and pH 3.5 groups. From the ANOVA test results, the hypothesis that the elodea's rate of photosynthesis would be harmed in more acidic environments is supported. A change from an environment of pH 7.0 to pH 4.0 would greatly reduce an elodea plant's rate of photosynthesis while a change from an environment of pH 4.0 to pH 3.5 would not bring about a significant reduction in an elodea plant's rate of photosynthesis. The standard devation values from table 4 state on average how far the data varies from the mean. For each group, the standard devation is relatively low comparedto the averages of each trial and thus, the data collected and the methods used to collect the data is very precise.

Increasing the acidity of an environment results in a number of physical, chemical, and biological changes. A chemical change that could occur is the change in the availability of carbon. With the pH of the environment at slightly acidic levels, the amount of dissolved HCO3- in the water drops. This dissolved HCO3- in the water is the plants' source of carbon used for photosynthesis and it is proven that aquatic plants have the best rate of photosynthesis in slightly alkaline environments due to the availability of carbon in the dissolved HCO3-. The lower pH levels in the environment also affect the enzymes in the cells of the plant. If pH levels drop low enough, enzymes such as RuBP used in the Calvin Cycle would shut off and would no longer carry out the chemical reactions needed for photosynthesis. Not only would the acid in the environment kill the enzymes but the acid would also destroy the plant proteins, lipids, and membranes, causing plant cells to malfunction and a major reduction in the rate of photosynthesis. Specifically, the lowered pH of the environment causes alterations in the chlorophyll molecules, which are highly essential to the process of photosynthesis.

6.0 Evaluation and Suggestions

Possible random errors include the inaccuracy of the solution molarity stated, mass stated, and volume of oxygen release stated. These random errors may be caused by the inaccuracy of the measuring equipment. The equipment such as graduated cylinders and pipettes are fairly accurate though to an extent.

One possible systematic error could be the different masses of each elodea sample. The mass of each strand may affect the volume of oxygen released for mass may determine the amount of chloroplast in each sample. Since each elodea sample was cut from a larger strand, this cutting may also cut each sample's life. With a sample's life cut short, the volume of oxygen collected from this experimentation may not truly reflect how plants act outside of these laboratory conditions. Also, the worst source of error in measuring the rate of photosynthesis with a gas collection method may be the gas storage within the leaves. If some oxygen is stored in the leaves, the oxygen collected in the photosynthometer may not fully represent the samples' true rates of photosynthesis in the tested environments.

Some measures that could be taken to prevent these errors could be to use more accurate equipment and using plant samples of similar mass as well as number of leaves.

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