Background Theory Photosynthesis: Plants require water from the soil, sunlight and carbon dioxide found in the atmosphere for growth and development. In the process of photosynthesis, carbon dioxide and water – in the presence of chlorophyll and light energy – are converted into sugar and oxygen, which is given off as a by-product. Generally, as sunlight increases in intensity, the rate of photosynthesis also increases [1]. This means greater food production within the plant. Many garden vegetables, like tomatoes, respond best with maximum sunlight exposure.

Production is drastically cut when plants are grown in the shade; this is because the rate of photosynthesis in the plant has been drastically reduced [4]. Photosynthesis is the process in which the photoautotrophs (plants that use light energy) convert sunlight energy into chemical energy and stores it in the form of ATP. Photosynthesis begins with the carbon dioxide in the atmosphere when it enters the stomates; tiny epidermal pores that lie within the leaves and stem of a plant that assist the process of transferring various passes and water vapours.

This is followed by water entering the leaf, usually through the roots, which were created to draw water from the ground, and transport waters through the stem to the leaves [6]. The sunlight then shines onto the leaves, the present chlorophyll’s from the leaves trap the energy from the sunlight. Chlorophyll present in the leaves create the green colour on the leaves; it also helps the plant absorb all sunlight, except the colour green which it reflects.

The hydrogen and oxygen are then produced as the products of photosynthesis; the hydrogen is then combined with carbon oxide to create glucose for the plant, while the oxygen is let out through the stomates on the leaves [5]. The chemical reaction within the cell between the electrons from the chlorophyll molecules and the protons from the water molecules produce ATP (adenosine triphosphate), which provides energy for the cellular reactions, and NADP (nicotinamide adenine dinucleotide diphosphate), which is essential in the plants metabolism.

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The whole process of photosynthesis can be explained through a very simple formula [1]: 6CO2 + 12H2O + Light -> C6H12O6 + 6O2+ 6H2O Light: Visible light is only one small part of the electromagnetic spectrum, which also includes (but not limited to) microwaves, X-rays and Gamma rays. The colour of the visible light is determined by its wavelength, which is defined as “the distance from peak to peak (or trough to trough)”. The energy of the light is inversely proportional to the wavelength: longer wavelengths have less energy than shorter wavelengths, which is why violet light has a much higher energy than red light [2].

White light is actually electromagnetic radiation of various wavelengths, and can be separated by passing it through a prism. Violet light has the shortest wavelength visible to humans, while red has the longest. Wavelengths longer than red are referred to as infrared, while those shorter than violet are ultraviolet [3]. Figure 1 [7] Effect of Coloured light on plants: A pigment is any substance that absorbs light. Chlorophyll, the green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects to be detected by our eyes so leaves appear green.

Black pigments absorb all of the wavelengths that strike them, and white pigments/lighter colours reflect all or almost all of the energy striking them (as seen in Figure 2, below) [3]. Figure 2 [7] The action spectrum of photosynthesis (as seen in figure 3, below) shows the relative effectiveness of different wavelengths of light at generating energy. If a pigment absorbs light energy, one of three things will occur: the energy is dissipated as heat, the energy may be emitted immediately as a longer wavelength (a phenomenon known as fluorescence) or the energy may trigger a chemical reaction, as in photosynthesis [6].

Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a membrane (as in a chloroplast) or the membrane infoldings found in photosynthetic prokaryotes such as cyanobacteria and prochlorobacteria [4]. The figure below shows that plants photosynthesise best using light of a wavelength between ~400-450nm, and lest well using (green) light with a wavelength ~550nm. Figure 3 [7] References: 1. Campbell, N (1999) Biology, Benjamin-Cummings Publ. Co. California [http://biology. clc. uc. edu/courses/bio104/photosyn. htm] 2. Davidson, S. (2007) Electromagnetic Radiation, Maricopia Education, New York, http://www. nationalelectrical. com/clncmpd. htm [13/03/2010] 3. Eates, P. J. (2004) Electromagnetic Spectrum, The Share Guide, Sydney, [http://www. lbl. gov/] 4. Farabee, M (2007) Photosynthesis and Light, Maricopia Education, New York [http://www. emc. maricopa. edu/faculty/farabee/biobk/biobookps. htmlhttp://www. emc. maricopa. du/faculty/farabee/biobk/biobookps. html] 5. Marchuk, W. (1992) A Life Science, Brown Publishers, Iowa. [http://www-saps. plantsci. cam. ac. uk/articles/broad_light. htm] 6. Reece, J. (2001) Biology: Concepts and Connections, Benjamin-Cummings Publ. Co. , California [http://www. ftexploring. com/photosyn/photosynth. html] 7. Purves, F (2000) Effect of Coloured Light on rate of Photosynthesis, Macquarie Educational Publishings, Melbourne [http://library. thinkquest. org/3715/photo3. html]


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