What effect does the brightness of light have on the growth rate of a plant - Coursework Example

?Introduction Plants are important in the continuing existence of life, as it provides food for animals. What is interesting about plants is that they make their own food, and factors that may affect this can understandably deplete resources for the other functions of the plant, such as growth. Any change in the growth of plants will thus affect the ecosystem, and thus is important to be studied. In this experiment, pea plants were used to determine whether sunlight has an effect on plant growth. A group of growing seedlings were exposed to the sun, while another set was placed away from light. The heights of the plants were measured repeatedly for 7 days after the seed germination. Results are shown below. Results Table 1. Height of the well-lit (L1-L5) and non-lit (D1-D5) pea plants from day 7 to day 14 after seeding.   Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 8 L1 7.3 7.3 7.5 7.8 8.1 8.4 8.4 8.5 L2 8.3 8.4 8.7 9.2 9.6 9.8 10 10.2 L3 6.5 6.8 7.2 7.7 7.9 8.5 8.7 8.8 L4 7 7.5 8.3 8.8 9.4 9.8 10.4 11.1 L5 7.2 7.5 7.7 7.7 8 8.5 8.8 9.3 Average 7.26 7.5 7.88 8.24 8.6 9 9.26 9.58 Standard deviation 0.5885575 0.5176872 0.5455273 0.6343501 0.7402702 0.6542171 0.7889233 0.9537295 D1 6 6.2 6.3 6.3 6.4 6.5 6.6 6.6 D2 5.6 5.8 5.9 6 6.1 6.1 6.1 6.2 D3 6.3 6.3 6.3 6.3 6.5 6.6 6.6 6.6 D4 5.1 5.1 5.1 5.3 5.4 5.5 5.5 5.5 D5 5.3 5.3 5.3 5.5 5.6 5.6 5.7 5.8 Average 5.66 5.74 5.78 5.88 6 6.06 6.1 6.14 Standard deviation 0.4929503 0.5319774 0.5585696 0.4604346 0.484768 0.5029911 0.5049752 0.4878524 Graph Figure 1: Scatter plot of the average values from Table 1. Standard deviations are indicated as error bars. It can be seen that plants L1-L5 were generally taller than D1-D5. As well, it can be noted that the rate at which the plants grow from day 7 to day 14 are faster in well-lit plants than in non-lit plants, as the slope of the trend line for the former is steeper than that of the latter. Conclusion After the experiment, it was observed that well-lit plants grow longer than the non-lit plants. Sunlight is thus necessary in the growth of plants. Theory Photosynthesis Sunlight, through the photons it bears, drives photosynthesis, the food-making process of plants. Without it, plants do not have a source of nutrition needed to power even the most basic processes such as growth (Campbell and Reece, 2002). Photosynthesis is a collection of reactions that results in the reduction of carbon dioxide (CO2) to carbohydrate and the production of oxygen gas (O2) from water (H2O). Pigment molecules such as chlorophyll and carotenoids in the chloroplast organelle are converted to excited electronic state, called excitons, by the photons. This conversion, no matter how effective, is still inefficient, as some of the excitons are converted back into photons as fluorescence, while some are lost as heat. For those that remain in the chloroplast, they drive the electron transport chain along the membrane proteins of the chloroplast, and produce a proton gradient between the membranes. This gradient, in turn, drives the production of adenosine triphosphate (ATP), which is necessary to drive the other part of photosynthesis that converts CO2 to carbohydrates (Whitmarsh and Govindjee, 1995). Phytochromes Aside from photosynthesis, light influences the plants on its other functions through molecules called phytochromes. The light-sensing portion (chromophore) of phytochromes has two isomeric forms, the red-absorbing (Pr) and the far red-absorbing (Pfr), with the former being the synthesized form and the latter instigating most of the plant developmental processes such as germination. Upon exposure of the seed and plant to light, especially red light, Pr is converted to Pfr (Campbell and Reece, 2002). According to Batty (1989), this mechanism ensures that wherever the seed falls, it will be germinating in a place with sunlight, which is needed for photosynthesis, as described above. In contrast, when it falls in a shaded soil, germination is delayed until, by some mechanical force, it is transferred to an area with more sunlight. Aside from germination, phytochromes also affect what kind of growth the plant will allocate its resources to. Pr induces vertical growth, while Pfr inhibits vertical growth, and stimulates branching instead (Campbell and Reece, 2002). Characteristics of Light The quantity, quality and duration of light all affect plant growth in different ways. Sunlight is composed of a range of wavelengths of light. However, they do not affect plant growth equally. Blue and red light have the greatest effect on plant growth, with the former inducing vegetative growth and the latter encouraging flowering. In contrast, plants do not use green light, and so they are reflected back, giving their usual, characteristic color (http://extension.oregonstate.edu/mg/botany/light.html). Evaluation In this experiment, five replicates were used per set-up to prove that sunlight has a positive effect on the growth of the plants. The number of replicates increased the validity of the study, as a causative effect is much more likely when same results, in this case the increased length, can be obtained from many replicates subjected to the same causative agent, or presence of sunlight. Since the standard deviation values were not too much, the gathered data were precise. This contributed to the validity of the resulting conclusion. However, certain improvements can be made in cases wherein this experiment will be conducted again in the future, so that the present conclusion, the positive effect of sunlight on plant growth, can be further solidified. First, the experimenters can try to further standardize the conditions of the study, so that no factors other than sunlight can affect the differences in growth of plants. Such changes may include standardizing the leaf surface area so that the amount of leaf surface that can be exposed to the sun may be equal among the replicates and the amount of water provided. As well, other data to be gathered may include the weight of the plants, which should be decreased among non-lit plants. References Batty, D., 1989. The Effect of Light on Germination and Seedlings. [online] Available at: [Accessed 1 April 2012] Campbell, N. A. and Reece, J. B., 2002. Biology. 6th ed. San Francisco: Pearson. Whitmarsh, J. and Govindjee. 1995. Photosynthesis. Encyclopedia of Applied Physics. 13. 513-522. Environmental Factors Affecting Growth – Light, Temperature, Water, Nutrition [online] Available at: [Accessed 1 April 2012] Read More