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Compare Photosynthesis and Cellular Respiration

Both photosynthesis and cellular respiration are the main pathways of energy transportation in organisms. However, the reactants and the products are exact opposites in photosynthesis and in cellular respiration. In photosynthesis, cells take in carbon dioxide (CO2) and water (H2O) by absorbing energy from the sun, and then the cells release oxygen (O2) and store glucose (C6H12O6). The formula of photosynthesis is: Light energy CO2+6H2O > C6H12O6+6O2 In cellular respiration, glucose and oxygen are taken into the cells, then they are converted to carbon dioxide, water and ATP energy and some other energy. Some of the ATP energy is used in photosynthesis; a large amount of energy is used up as heat, only a little energy is stored in the cells. The formula of cellular respiration is: C6H12O6+6O2 > 6CO2+6H2O+ATP Both processes take place inside the cells. In contrast, The site of photosynthesis is chloroplast, while the sites of cellular respiration are cytoplasm and mitochondrion.

The chloroplast, which has two membranes around a central aqueous space, is found mainly in mesophyll cells forming the tissues in the interior of the leaf. The chloroplast is made of grana, thylakoid, and stroma. Chlorophyll, which is a green pigment, is located in the chloroplasts. The mitochondrion also has two membranes, the outer membrane and the inner membrane. Inside the inner membrane is the matrix, folded by cristae. Both chloroplast and mitochondrion also contain DNA and RNA, and they are found only in eukaryotic cells. Photosynthesis is a redox reaction, which reverses the direction of electrons flow in respiration.

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Through photosynthesis, water is split and electrons are transported with H? from the water, into carbon dioxide, then reducing it to sugar. There are two main stages of photosynthesis, which are the light reactions (photo) and the Calvin cycle (synthesis). In the light reactions, light energy, which is absorbed by chlorophyll in the thylakoids, drives the transfer of electrons and hydrogen from water, into NADP? (nicotinamide adenine dinucleotide phosphate) to form NADPH. The light reactions also generate ATP by using chemiosmosis through a process called photosphorylation.

The light energy is converted into chemical energy in the form of two compounds, which are NADPH and ATP. The Calvin cycle occurs with the incorporation of carbon dioxide into organic molecules in carbon fixation. In this process, the fixed carbon is reduced with electrons provided by NADPH. The Calvin cycle takes place during daylight hours, in which the NADPH and ATP can be provided. The Calvin cycle occurs in the stroma, while the light reactions occur in the thylakoids. In contrast, there are four metabolic stages happened in cellular respiration, which are the glycolysis, the citric acid cycle, and the oxidative phosphorylation.

Glycolysis occurs in the cytoplasm, in which catabolism is begun by breaking down glucose into two molecules of pyruvate. Two molecules of ATP are produced too. Some of they either enter the citric acid cycle (Krebs cycle) or the electron transport chain, or go into lactic acid cycle if there is not enough oxygen, which produces lactic acid. The citric acid cycle occurs in the mitochondrial matrix, which completes the breakdown of glucose by oxidizing a derivative of pyruvate into carbon dioxide. The citric acid cycle produced some more ATPs and other molecules called NADPH and FADPH.

After this, electrons are passed to the electron transport chain through proteins called cytochromes. In the electron transport chain, the electrons move from molecule to molecule until they combine with molecular oxygen and hydrogen ions to form water. When transporting electrons through cytochromes, protons (H? ions)are produced, which create a pH gradient between the sides of the cristae, and then go through the ATP synthase, which is also a protein. During this, ATP is made from ADP and inorganic phosphate. The electron acceptor in photosynthesis is NAD? , while NADH is the electron acceptor in cellular respiration.


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