Part B. Investigation using the chloroplasts  

Part B. Investigation using the chloroplasts  

Time / min

Tube 1

Tube 2

Tube 3

Tube 4

Tube 5

5.0

5.0

5.0

5.0

5.0

4.0

 

5.1

 

3.7

2.2

 

4.9

 

2.3

0.5

 

5.2

 

1.0

0.3

6.2

5.4

4.9

0.3

Table 1: Results from an investigation of chloroplasts activity by following the decolourization of DCPIP over time. The absorbance depth – 665 nm.

The rates of chloroplast activities were determined by the calculations of the best-fit-lines gradients:

, where  and

 

Tube #1 -  = -0.258 ( ).

Tube #2 -  = 0.01

Tube #3 -  = 0.018

Tube #4 -  = 0.005

Tube #5 - = -0.242 ( )

Negative sign indicates the decline in the data, therefore, rate of the change itself must be read by its absolute value ( ).

Discussion

In this laboratory practical, homogenized and centrifuged chloroplasts were investigated in 5 tubes with different conditions.

 

Tube #1 contained leaf extract and DCPIP and was observed under light conditions, showing significant absorbance of light and change in color.

This tube was designed to illustrate the DCPIP reduction in chloroplasts which were present in leaf extract and to serve as a sample showing the link between light and dark reactions of photosynthesis.

 

2nd tube had only isolation medium and DCPIP. Because of the absence of any light-absorbing agent, the data shows an increase after 20 minutes. Chloroplasts are required for the reduction of DCPIP, therefore, color did not change.

 

The 3^rd tube demonstrates that light is essentially required for DCPIP reduction. Instead of placing the tube in front of the 100W bright light, it was located in the darkness to prevent chloroplasts of getting any light energy. Obtained results prove that DCPIP does not reduce in the absence of light, which is essential for the electrons to be excited. Small deviations can be attributed to the presence of daylight in the room,

 

The 4^th tube was designed to prove that alterations in light absorption levels are not associated with changes in the chloroplasts themselves. There was no DCPIP added in it, and although it was placed in front of the light, chloroplasts activity did not change from their original values. Moreover, an addition of distilled water caused bursting of chloroplasts’ envelope membranes owing to osmotic pressure, making them incapable to absorb the light.

 

Tube #5 was not changed in color, despite that it has absorbed the same amount of light as the 1st tube. Possible reason is that supernatant contained some accessory pigments, but due to the absence of chlorophyll a, light energy was only absorbing (Mader and Windelspecht 2016, 119).

 

Conclusion

Several conclusions can be drawn on the basis of the obtained experimental data. First, it can be concluded that the reducing agent, light, and chloroplasts are essential for photosynthesis to take place. Next, different environmental conditions are affecting the rate of chloroplasts’ activity, such as light, osmotic pressure, pH and temperature. There were several limitations which possibly affected the observed results: both DCPIP and leaf extract had the color (blue and green), thus the color change did not go from blue to fully colorless, which could have affected observation results; measuring errors; equipment purity. Moreover, experiment was conducted in the daytime, and the ambient light could lead to minor deviations in the data.

 

Reference list

Karp, Gerald. Cell Biology. Hoboken (NJ): J. Wiley & Sons, 2010.

Mader, Sylvia S., Michael Windelspecht. 2016. “Biology”. New York: 2 Penn Plaza.

Reece, Jane B., Martha R. Taylor, Eric J. Simon, Jean L. Dickey, and Neil A. Cambell. Campbell biology: concepts & connections. 7^th edition. San Francisco, CA.: Benjamin Cummings, 2009.