A Review On Photosynthesis Biology Essay

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Photosynthesis, a process carried out just by plants, consists of a chemical process which transforms CO2 into organic compounds which such as sugar (glucose) and oxygen, for this process to occur chemical energy is needed, or in other words sunlight. Moreover, this process is taken out in the chloroplast of the plant cell, located on the plants leaves, photosynthesis can also occur in some algae.

In this experiment I am going to compare the different colors that bromotimol blue changes to and see if it is related to the photosynthetic activity of the algae (Elodea) used during the experiment.

The reason of this experiment is to determine if bromotimol blue contributes to determine if an aquatic plant is carrying out photosynthesis or not, due to the fact that bromotimol blue changes colors depending on the ph that the substance have: if the substance is acidic (pH range 6.0 or below) the bromotimol blue should be yellow, if the substance is neutral (pH range 6.1 - 7.9) it should be green, and if the substance is a base (pH range 8.0 or higher) it should be blue. Due to these changes of color depending on the pH of the substance, bromotimol blue should be helpful to determine when an aquatic plant is during the process of photosynthesis.

The expected results are that the bromotimol blue will be useful as it changes colors depending of the pH that it is exposed to, because due to this characteristic it can determine whether the substance have a high concentration of CO2 or not, this is because CO2 have a neutral ph which would turn the bromotimol blue to green. If the bromotimol blue changes from green to blue while the aquatic plant is submerged into it, it would mean that the aquatic plant is carrying out photosynthesis as the aquatic plant should process the CO2, as part of the process of photosynthesis, reducing its amount and also changing the color of the bromotimol blue to blue (its original color) as the CO2 concentration have been reduced.

2. Research question

Does the fixation of CO2 in a bromotimol blue solution, change the color evidencing the photosynthetic activity performed by a plant?

3. Hypothesis

The changes of color of the bromotimol blue solution will evidence when photosynthetic activity is carried out by an aquatic plant, as bromotimol blue solution changes color depending on the pH of the substance.

4. Variables

The variables identified in this experiment were:

Dependant variable: the color of the bromotimol blue solution, as it depends on the photosynthetic activity of a plant.

Independent variable: the photosynthetic activity, due to the fact that it is not affected by any of the variables during the experiment.

Controlled variable:

Distance of the light bulb in the experiment

Concentration of bromotimol blue (1%)

Size of the plant: Elodea (6,5cm ± 0,05cm)

5. Materials and Methods

The materials used for this experiment were:

Spectophotometer (A ±0,0005A)

Light bulb (100 W)

Bromotimol blue solution 1%

1 Elodea plant sample

25ml graduated cylinder (ml ± 0,05ml)???

15ml test tube (ml ± 0,05ml)


2 beakers of 25ml

2 spectrophotometer cuvettes


Towel paper

Chronometer (s ±0,0005s)

Test tube rack

The method used in this experiment was:

First, 20 drops of bromotimol blue were mixed with 10ml of water in a beaker of 50ml.

The prepared solution was passed into a clean beaker so a straw would be placed inside the solution to blow in it by 30 seconds.

The solution was passed into a spectrophotometer cuvette, the absorbance was read at 430nm.

The prepared solution was placed into a small test tube, so a small part of Elodea (aprox 6.5cm ± 0,05cm) was also placed inside. The light bulb was turned and kept a distance away (aprox. 10cm ± 0,05cm) from the test tube that contained the Elodea for 15 minutes.

The absorbance of the solution with the Elodea was read at 430nm.

Process #4 was repeated with the previous solution that was read, but instead of 15 minutes just 5 minutes were waited.

Finally, process #6 was repeated again; with these results a great variety of absorbances can be compared.

6. Data gathering

Been realized the procedure, this is the raw data gathered:

Qualitative data:

While a straw was used to blow measure N°1, it could be appreciated that the color of the bromotimol blue changed from blue to green.

The step in which the Elodea was inside the substance with some artificial energy, little bubbles emerging from the Elodea could be seen.

The substance changed to a light blue color when the Elodea plant was submerged in it carrying out photosynthetic activity.

Table N°1

Absorbance measured in different moments

Measure number

Minutes waited (s ±0,0005s)

Absorbance (A ±0,0005A)

N°1 (no changes)















In Table N°1 it can be seen the different number of measures and how much time it was waited between measures, in the first two measures time wasn´t taken, due to the fact that at those measures the Elodea wasn´t submerged in the substance, the measure of time started when the Elodea was submerged in the substance.

Image N°1

Bromotimol blue color in different measures



Measure N°1 (no changes)


N°3, 4, 5

In Image N°1 it can be seen that Measure N°1, the one that wasn´t exposed to CO2, have a strong blue color (original color). While Measure N°2 have a green color, after it have been exposed to CO2. In case of Measure N°3, 4 and 5, these measures were indicated in one of the cuvettes due to the fact that the color light blue is present on those 3 measures, the only difference between them was that some measures were more clear between than other, as the number of measures increased the substance became more clear, because as the measures increased the more time the Elodea was submerged in the substance, clearing it more and more due to photosynthesis.

7. Data analysis

From Table N°1, it can be seen that the absorbance of the first substance, in other words the original, is the lowest one from all of the absorbances found during the experiment. One reason for this to have happened is because the first substance didn't had contact with anything that would alter its concentration while the other substances were affected when CO2 was added directly to them when a straw was used to blow the substance. The reason for measure N°2 to have such an elevated absorbance in comparison with #1 is because at the time CO2 was added its concentration increased and because concentration and absorbance are depend from each other, if the concentration of the substance increased, in this case of C02, its absorbance would increase too.

From measure N°2 to measure N°5 it can be observed of how the absorbance is constantly decreasing, at this stage an Elodea plant have been put inside the test tube where the substance is for it to start photosynthesis; so the reason for the absorbance to keep decreasing means that the concentration is decreasing as well and the only way for the concentration to decrease is through the reduction of the concentration of C02 and the only way available now for doing such a thing is through the photosynthesis the Elodea is carrying out from the substance N°3 to N°5. Also, it can be seen from the table that when more time is waited the absorbance reduces even more, this can be seen in measure N°3 in which the absorbance reduces 0,202 ± 0,0005 of absorbance by waiting 15 minutes, while the measure N°4, in which have been waited 5 minutes, just reduced 0,25 ± 0,0005 of absorbance; from this information it can be inferred that by more time the Elodea plant carry outs photosynthesis there would be less absorbance; in other words while time passes the absorbance will keep decreasing, making them indirectly proportional, more time, less absorbance.

This affirmation can be summarized in a graphic:

Graphic N°1

In Graphic N°1 it is showed how absorbance keeps decreasing but at one point it will stop which is at the original absorbance because it would be impossible for the substance, that has been added concentration to have less absorbance than the substance who had never been in contact with other type of molecules.

From the qualitative data observed before the change of color of the bromotimol blue to green is because the ph of the substance is now neutral which can be explained as CO2, with a neutral ph, was added to the substance. And the other qualitative data the bubbles from the Elodea was a sign that photosynthesis was taking place because most of sea weed plants produce oxygen while doing photosynthesis, and this oxygen produced can be seen from the bubbles that appear from the Elodea.

8. Conclusion

The change of color in bromotimol blue determines that the Elodea has performed photosynthesis; because it has already been proved that the concentration of C02 keeps decreasing making the absorbance decrease as an effect of it, and the only way available of decreasing the concentration of CO2 in the absorbance could be just through the photosynthesis of the Elodea, which could be seen when some bubbles started to appear evidencing that photosynthesis was occurring; all of this factors would made the green bromotimol blue to change to its original color.

During the process of this experiment a limitation was found, this limitation was that artificial light was used during the experiment, which may have affected the experiment as it is not certain to say that photosynthesis would be faster or slower at sunlight, so it would have been a more specific data if sunlight was used, due to the fact that it is the main source which plants use for photosynthesis; it would have made a result closer to reality.

Some suggestions for this experiment are: to test with different light intensities to compare which light intensity does plants tend to do photosynthesis faster and so determine if it affects the results, this way more information is gathered, making the experiment more accurate, and to wait more minutes for the plant to carry out photosynthesis, in that way more data can be obtained, being more probable for accurate results.