Impact of Light Intensity on Photosynthesis Essay Example

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Impact of Light Intensity on Photosynthesis


Photosynthesis is the process through which green plants synthesis food (carbohydrates) from water and carbon dioxide in the presence of sunlight. Chlorophyll, the green pigment, plays a vital role in photosynthesis because it absorbs light that is then used in photosynthesis. Chlorophyll, which gives green plants their green color, is found in the chloroplast. It is a photoreceptor, which means that it is able to trap light energy (Goldberg 2004). Therefore, light is a crucial requirement of photosynthesis. In fact, the process cannot take place in the absence of light, which means that without light, green plants will simply die.

There are two sets of photosynthetic reactions. The first set is known as light-dependent reactions because light must be available for it to occur. Water and carbon dioxide are synthesized into carbohydrates with oxygen being the main byproduct. Therefore, green plants take up carbon dioxide and release oxygen, which is the reason behind trees being considered as taking crucial part in controlling global warming through the absorption of carbon dioxide from the atmosphere (Allott 2001). The main products of light-dependent reactions are nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), both of which are energy molecules. Therefore, this set of reactions is also known as energy-releasing reactions because they release energy. The second set is known as light-independent reactions because they take place in the absence of light. In this step, the incorporation of carbon dioxide into an organic molecule, a process known as synthesis, occurs (Allott 2001). Therefore, these processes are also called synthesis reactions, which use the energy released in the first step.

It is apparent that light is extremely important in photosynthesis. Therefore, light intensity is a crucial factor affecting the rate of photosynthesis and hence the growth and production of green plants. Research into the impact of light intensity on the photochemical reactions of photosynthesis shows that there is a positive and linear relationship between the two until an optimum amount of the former is reached. Beyond this point, increase in light intensity results to a reduction in photosynthetic reactions (Kirk 2011). This experiment sought to determine the impact of light intensity on photosynthesis. A redox solution, 2, 6-dichlorophenolindophenol (DCPIP), is often used in photosynthesis experiments. A redox solution means that it can be oxidized and reduced as shown below.

2H2O + 2DCPIP — 2DCPIPH2 + O2

In its oxidized state, DCPIP is blue in color. Reduction is accompanied by loss of the blue color. DCPIP is a hydrogen acceptor capable of capturing light energy; its illumination results to oxygen generation and hence loss of the blue color (Kirk 2011). Therefore, it is used to simulate the process of photosynthesis whereby the rate of loss of the blue color presents photochemical reactions of photosynthesis.

A test tube was then placed in a plastic container filled with ice. This assembly was then placed in the dark, DCPIP added and its absorbance read using the spectrophotometer. A source of light, a lamp, was placed on the bench and its wavelength adjusted to 620nm. Lights were turned off to ensure that the lab was in dark after which the container with the test tube was placed 20cm from the light source, which was then switched ON and illuminated for 60 seconds. The lamp was then switched off and the absorbance of the dye read and recorded. The entire procedure was repeated (using new DCPIP) at distances of 40, 60, 80 and 100 cm from the light source. Absorbance before and after illumination were recorded.

Distance from the light (cm)

Optical density at time zero

Optical density after 60 seconds

Rate of dye reduction

Negative control


A graph of rate of dye reduction against distance from the light source (figure 1) shows that increase in distance was accompanied with reduction in the rate of dye reduction until a given point (about 46cm) when further increase in distance from the source resulted to an increase in rate of dye reduction. Increase in the distance from the light source is equal to reduction in light intensity. Therefore, increase in light intensity is seen to lead to increase in the rate of dye reduction.

Impact of Light Intensity on Photosynthesis

The inverse of the distance from the light source would be interpreted as increase in light intensity. Figure 2 shows a graph of the rate of dye reduction against the inverse of the distance from the source presented as light intensity. From the graph, it is seen that increase in light intensity leads to an increase in dye reduction. However, at very low light intensities, increase in light intensity resulted to reduction in dye reduction. This should not be the case because theoretically, increase in light intensities (at low values) was supposed to lead to increase in rate of dye reduction. Low light intensities may not have illuminated the DCPIP properly so that accurate spectrophotometer readings were not obtained. Other errors such as improper illumination timing might have affected the results.

Impact of Light Intensity on Photosynthesis 1

This experiment shows that there is a positive relationship between light intensity and the rate of photochemical reactions of photosynthesis. It is important to also determine the optimum light intensity giving the maximum rate of photosynthesis. Such an experiment would help farming specialists such as designers of greenhouse covers to design for the optimum light intensity to reach crops.


Allot, A. 2001, Biology for the IB Diploma: Standard and Higher Level. Oxford University Press, New York.

Goldberg, D. T. 2007, AP Biology (2nd ed.). Barron’s Educational Series, Inc., New York.

Kirk, J. T. 2011, Light and Photosynthesis in Aquatic Ecosystems (3rd ed.). Cambridge University Press, Cambridge, UK.