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The process by which green plants use light, water, and carbon dioxide to make their food and release oxygen as a byproduct is known as photosynthesis. Photosynthesis process occurs in an organelle called chloroplast which contains the green coloring matter, chlorophyll that absorbs light for photosynthesis. The chloroplast contains pigments that absorb light energy which is later converted into chemical energy. Light is the primary element for the photosynthesis process, and thus it is used to split water molecules to produce H+ and OH– ions. However, photosynthesis process occurs maximum in the blue and red light and little in green and yellow light in both light and dark reaction. There are various factors that affect the rate of photosynthesis such as water, light, temperature and carbon dioxide.
1.1 What is Photosynthesis?
Photosynthesis refers to the method through which green plants combine water and carbon dioxide to synthesize their food and oxygen is released as a byproduct as stated by Shiwnarain, (2017). The method of photosynthesis requires water, carbon dioxide, and light energy. During photosynthesis, carbon dioxide is reduced to carbohydrates while light energy splits water to release oxygen which is referred to as photolysis of water. Thus, photosynthesis can be presented through a chemical equation:
1.2 Where Photosynthesis Occurs
Green plants possess the green coloring matter, chlorophyll, which facilitates the photosynthesis process. Therefore, photosynthesis occurs in greener parts such as leaves, stems and floral buds. Since photosynthesis occurs in leaves, the leaves contain specialized cells known as mesophyll. The mesophyll cell cells contain the chloroplast pigment that produces organelle which is the actual site for photosynthesis. The figure below illustrates the structure of chloroplast and how the process of photosynthesis occurs.
2.0 Photosynthetic Pigments
Chloroplast contains pigments whose function is to absorb light of diverse wavelength then later carry out the photosynthesis photochemical reaction. Light energy absorbed by the pigments is converted into chemical energy. Chloroplast pigments are sited on the thylakoid tissues. The chloroplast pigments are continually placed inside the cells given that the layers are at right angles to increase the rate of light concentration. Higher plants have photosynthetic colorings divided into two classes; the chlorophyll and carotenoids. Chlorophyll is the principal photosynthetic pigment that is mostly involved in photosynthesis as it grips light energy and transforms it into chemical energy. In that case, it is referred to as the reaction center in green plants. Chlorophyll pigment makes leaves to appear green in color as it absorbs light in the violet-blue and red region of the visible spectrum. Alternatively, carotenoids absorb the light in the areas not absorbed by chlorophyll.
Chlorophyll b and carotenoids pigments also referred to harvesting center are used to transport the absorbed energy to chlorophyll a molecule (Tazzini, 2018). The reaction center (chlorophyll) and harvesting center (chl. b and carotenoids) are bundled into useful groups called photosystems. There are two kinds of photosystems, PSI and PSII which are constituted with about 400 pigment molecules each. Photosystem I and II have different forms of chlorophyll with maximum absorption of 700 nm and 680 nm respectively. Hence, the key function of photosystem I and II is to entrap the sun energy and change it into chemical energy (ATP).
Difference between PSI and PSII
3.0 Role of Sunlight in Photosynthesis
Light is the primary element needed in the photosynthesis process and is made up of small energy particles called photons. A single photon is thus referred to as a quantum. The light energy absorbed is conveyed into an excited form thus losing an electron to the external path. The energized chlorophyll molecule is then taken to a low energy state to free extra energy. In the photosynthesis process, the absorbed energy is used to split water molecules to produce H+ and OH–
3.1 Absorption and Action Spectra
An absorption spectrum refers to a graph that illustrates the effective pigment absorbance of different light wavelength. The grid is used to indicate how the rate of photosynthesis is more and less in some areas since not all wavelengths of light are useful. The action spectrum also used to note that photosynthesis is maximum in the blue and red light. On the other hand, in green and yellow light, photosynthesis is little since light rays are reflected from the leaves.
4.0 Photochemical and Biosynthetic Phase
4.1 Light Dependent Phase
The photosynthesis process occurs in an organelle called chloroplast. The chloroplast is structured in a way that the light reaction (light-dependent) and dark reaction(light independent) occur in different parts of the chloroplast. In light reaction, thylakoids which contain pigments absorb light and relocate electrons from PSI and PSII to the excitation site to gain excitation energy. The excited atoms then leave reaction centers having converted light energy into chemical energy and travel in an oxidation-reduction series forming ATP. Also, NADP is
lessen to NADPH2 and combines with ATP to form reducing power products of the light reaction. The products are then transferred from thylakoid into the stroma of the chloroplast.
4.2 Light Independent Phase
This is the second process known as the dark reaction (Buchanan, 2015) and occurs in the stroma. This process occurs either in the presence of light or the absence of sunlight. In this photosynthetic process, CO2 is reduced by the reducing power and produce carbohydrates through the fixation process. The carbon fixation processes yields sugar in the plants, and it is later exported to other plant tissues. The byproduct is enhanced and used as an equal source of living and energy for development and breakdown reactions. The structure below illustrates the chloroplast organelle in which both light and dark reactions occur.
5.0 Factors Affecting Rate of Photosynthesis
The various internal factors include chlorophyll and demand for photosynthate. The higher the amount of chlorophyll present in a leaf, the higher the rate of photosynthesis reaction. On the other hand, demand increases with a higher growth rate of plants. The external factors include light, temperature, CO2,
and water. The higher the rate of photosynthesis the higher the amount of CO2
and light. Extreme temperatures will alter the rate of photosynthesis while a higher amount of water declines the rate of photosynthesis due to the closure of stomata.
Buchanan, B. (2015). The carbon (formerly dark) reactions of photosynthesis. Photosynthesis Research, 128(2), 215-217.
Shiwnarain, M. (2017). What Does Photosynthesis Produce?. Science Trends.
Tazzini, D. (2018). Functions of carotenoids in plants and foods. Retrieved from http://www.tuscany-diet.net/2013/12/12/functions-carotenoids-plants-foods/
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