TITLE: DETERMINING THE ABSORPTION SPECTRUM OF COLEUS LEAVES:

Question

Introduction
Photosynthesis is a vital process through which plants convert light energy into chemical energy, primarily using pigments such as chlorophyll. Chlorophyll a and b are the primary photosynthetic pigments, capable of absorbing light in the blue and red wavelengths, while secondary pigments such as carotenoids capture additional wavelengths of light, thus enhancing the overall efficiency of photosynthesis (Taiz & Zeiger, 2010). Coleus leaves, known for their vibrant colours due to the presence of multiple pigments, provide an excellent opportunity to study the absorption spectrum related to photosynthesis (Starr et al., 2016). This experiment aimed to determine the absorption spectrum of pigments in Coleus leaves and identify the wavelengths that these pigments most efficiently use.
Aim of the Experiment
The aim of the experiment was to determine the absorption spectrum of various pigments present in Coleus leaves by measuring their absorbance at different wavelengths from 400 nm to 700 nm.
Materials Used
In the experiment, several materials were utilized for the analysis. Coleus leaves were obtained, and the leaves were weighed at 0.3 grams. Ethanol was used to extract the pigments from the leaves, with a total of 30 mL being prepared. The extraction process involved grinding the leaves in a mortar and pestle, followed by filtration through a coffee filter. Equipment used included a balance for measuring the leaf sample, a spectrophotometer for measuring absorbance, cuvettes for sample placement, and syringes for liquid handling. Proper safety equipment was worn, including safety goggles and aprons, to prevent exposure to hazardous materials.
Method Used
The experiment was conducted in several parts, starting with the sample preparation. A Coleus leaf sample weighing 0.3 grams was torn into small pieces and placed into a mortar along with 10 mL of ethanol. The mixture was ground to release the pigments, and an additional 20 mL of ethanol was added before the suspension was filtered into a beaker. The resulting leaf extract was then calibrated using a spectrophotometer set to a wavelength of 640 nm, ensuring the sample was within a 30-40% transmittance range. If transmittance was less than 30%, the sample was diluted with ethanol until an acceptable reading was achieved.
Once the extract was calibrated, the absorption spectrum was determined by measuring absorbance at wavelengths ranging from 400 nm to 700 nm, increasing in increments of 20 nm. Absorbance readings were taken after calibrating the spectrophotometer to a blank cuvette filled with ethanol, and the values were systematically recorded for each wavelength.

Results
Figure 1: The data collected from the experiment was organized into the following table below.
Wavelength (nm)
Absorbance (%)

400 nm
1.10 %

420 nm
1.30 %

440 nm
0.69 %

460 nm
0.85 %

480 nm
0.80 %

500 nm
0.49 %

520 nm
0.40 %

540 nm
0.18 %

560 nm
0.35 %

580 nm
0.35 %

600 nm
0.36 %

620 nm
0.36 %

640 nm
0.37 %

660 nm
0.50 %

680 nm
0.40 %

700 nm
0.24 %

Figure 2: The graph of Absorbance versus Wavelength below.

Discuss the results based on the Data table and the graph of Absorbance vs wave length including citations and reference list...
Make detailed discussion, conclusion and recommendation, check with aim improved or not.

Answer 1

Discussion

The absorption spectrum obtained from the Coleus leaves reveals key insights into the efficiency of light absorption by the various pigments present in the leaves. Based on the data presented in the results, the peak absorbance occurs at 420 nm with 1.30%, indicating that this wavelength is one of the most effectively absorbed by the leaf pigments. This peak absorption aligns with the known absorption characteristics of chlorophyll and carotenoids, which are known to have strong absorbance in the blue region of the spectrum (Taiz & Zeiger, 2010).

As we analyze the data further, it is evident that the absorbance decreases significantly at the wavelengths approaching the red region. For instance, at 640 nm, the absorbance is recorded at 0.37%, and it further diminishes to 0.24% at 700 nm. This gradual decline supports the findings that chlorophyll a and b have optimal absorbance in the blue and red wavelengths, while the effectiveness decreases in the green spectrum (520 nm and 540 nm), where the absorbance is notably low (0.40% and 0.18%, respectively). This correlates well with the green color of the leaves, which indicates that the green wavelengths are being reflected rather than absorbed.

The absorbance levels for other wavelengths provided subtle peaks and troughs. For example, the values at 480 nm and 560 nm show absorbance values of 0.80% and 0.35%, suggesting that some chlorophyll is also absorbing in the cyan and yellow-green wavelengths. Such data supports the hypothesis that while chlorophyll a and b are the primary pigments, other pigments such as carotenoids also play a role in capturing additional light energy, which enhances photosynthetic efficiency (Starr et al., 2016).

Conclusion

The experiment achieved its primary objective of determining the absorption spectrum of pigments in Coleus leaves. The data show that although the pigments effectively absorb light within the blue and red areas of the spectrum, there is less absorption in the green wavelengths. The results validate the presence of chlorophyll as the key pigment involved in photosynthesis, as well as the contribution of additional pigments, such as carotenoids, in expanding the light absorption range for photosynthesis.

Recommendations

1. Replication of Experiment: It is recommended to reproduce this experiment with varying leaf species to compare absorption spectra across different botanical classifications. Such studies may help emphasize the role of pigments in plants adapted to different environments.

2. Use of Advanced Techniques: Consider employing high-performance liquid chromatography (HPLC) to separate and identify individual pigments in Coleus leaves more accurately. HPLC can provide greater insight into the types and concentrations of pigments contributing to photosynthesis.

3. Wider Spectrum Analysis: Future experiments could extend the wavelength range beyond 700 nm to evaluate near-infrared absorption, which may provide additional information on pigment behavior under different light conditions.

4. Experimental Controls: It is beneficial to include controls, such as using leaves treated under different light conditions or subjected to varying nutrient levels, to examine how these factors influence pigment absorbance properties.

References

• Taiz, L., & Zeiger, E. (2010). Plant Physiology. Sinauer Associates, Inc.
• Starr, C., Taggart, R., Evers, C., & Starr, L. (2016). Biology: The Unity and Diversity of Life. Cengage Learning.