The effects of Light Quality on the Rate of Photosynthesis Measure through Floating Spinach Disks

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This study was undertaken to determine the relationship of different wavelengths of light and the rate of photosynthesis in spinach leafs. The rate of photosynthesis was measured every five min under light colors of white, green, red, blue and yellow under a light intensity of 2000 lux. The rate of photosynthesis was measured by the spinach disk method in which we replaced the air from the disks with sodium bicarbonate using a vacuum. Under photosynthesis, oxygen, a product of photosynthesis, replaced the bicarbonate solution, made the disks less dense and rise to the top. The rate of photosynthesis was the greatest under white light, followed by red, blue, yellow and green, which produced no detectable photosynthesis signifying that chlorophyll reflects, rather than absorbs, green light. White light was also expected to have the highest rate of photosynthesis as it gives the chlorophyll all the colors of light to absorb. In general, filtered light reduced the rate of photosynthesis because the chlorophyll didn't receive all the different colors of the spectrum it desired as it did in white light to absorb.


Photosynthesis is the ability to convert light energy into chemical energy in the form of sugar (Freeman 2002). The photosynthetic reactions are divided into two sections: light dependent and light independent reactions. In this experiment, we will be dealing with the light dependent reactions. The actual chemistry of the light dependent reaction is with the addition of light: H2O + ADP + inorganic phosphate + NADP+ ¨ 1/2O2 + ATP + NADPH + H.

Light is a necessary input for photosynthesis to take place. Light's electromagnetic radiation carried in photons are absorbed by the photosynthetic pigments in plants. These photons can cause electron excitation to a higher energy level, which can be added to NADP+ form NADPH or the excited electrons can also be passed down an electron transport chain producing ATP. These processes occur in photosystem I and photosystem II respectively. The splitting of water into electrons and oxygen occurs in photosystem II. We can measure the rate of photosynthesis by measuring the amount of oxygen, because oxygen is a product of photosynthesis. The reduced molecules produced from the light dependent reactions are used in the light independent reactions to produce one of the final products of photosynthesis, glucose.

Photosynthesis occurs in the leaf of plants in their chlorophyll. The pigment chlorophyll absorbs red and blue light and transmits green light, it is responsible for the green color in plants (Freeman 2002). There are also other pigments in plants such as carotenes and xanthophylls that help photosynthesis by increasing the range of wavelengths or amount of light absorbed. This is beneficial for the plant because the more absorption of photons, the more electrons can be excited, and thus photosynthesis can occur at a faster rate. Green light is reflected by plant leafs and not absorbed and so it is my hypothesis that the spinach disks receiving green light will have no photosynthetic reactions take place, thus no spinach disks rising to the top. This is because the chlorophyll cannot use the green light to excite electrons because there is no step difference--no difference in possible energy states--that correspond to the amount of energy in a green photon (Freeman 2002). It is also my hypothesis that the blue and red light will stimulate photosynthesis because chlorophyll absorbs those colors and uses those photons to activate photosynthetic reactions by electron excitation. Because the pigments of chlorophyll, chlorophyll a and chlorophyll b, absorb more blue light than red light, it is also my hypothesis that the rate of photosynthesis will be higher with the blue light than the red light. As for the yellow light, because chlorophyll doesn't absorb much yellow light, my hypothesis is that with the yellow filter only a small amount of spinach...
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