Essay: Cellular respiration

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Adenosine triphosphate is made of the organic molecule adenosine bonded to a chain of three phosphate groups. ATP is an organic phosphate molecule that is the principal source of energy for cellular works. (Reece, et al., 2011) Energy is released by ATP when bonds between phosphate groups are broken by hydrolysis, ATP thus becoming adenosine diphosphate. Animals and plants produce and store ATP in the process of cellular respiration, but plants also do during photosynthesis. (Reece, et al., 2011) This essay will detail cellular respiration and photosynthesis focussing on oxidative and substrate-level phosphorylation and chemiosmosis processes.
Cellular respiration is the process during which glucose is broken down to provide energy to cells. It happens in both animals and plants, and it can be divided in 3 stages: Glycolysis, Citric acid cycle and Electron transport and chemiosmosis. (Reece, et al., 2011) Glycolysis happens in the cytosol and begins the breaking of glucose into two molecules of pyruvate that are oxidised into acetyl CoA. This compound then enters the citric acid cycle to complete the breakdown of glucose into carbon dioxide. (Reece, et al., 2011) A small amount of ATP is formed in reactions during these stages through substrate-level phosphorylation that occurs when an enzyme transfers a phosphate group from an organic molecule generated during cellular respiration to ADP. The net yield of Glycolysis is of 2 ATP. (Cohen, 2011) During the citric acid cycle, the substrate phosphorylation of GDP form GTP, then converted in ATP (Bhagavan, 2002). During electron transport chains, electron carriers alternate between reduced and oxidized states, passing on electrons to more electronegative neighbours. It begins when an electron carrier, reduced nicotinamide adenine dinucleotide (NADH) passes electrons acquired during the first stages to membrane-bound electron carriers (Cohen, 2011), the first one being flavoprotein, and the last one of the chain being a cytochrome that passes electrons to oxygen that will form water with hydrogen ions from inside and outside the cell. FADH2 can be another source of the electron transport chain but it has a lower energy level than NADH, providing less energy for ATP synthesis. The free energy released during each step is stored in mitochondria to be used in ATP synthesis from ADP and phosphate group. This is done through oxidative phosphorylation, powered by a redox equation. (Reece, et al., 2011) The electron transport chain establishes a proton gradient, as certain electron carriers accept from the mitochondrial matrix and release protons into the intermembrane space. Along the electron transport chain, more protons accumulate outside the membrane which results in a proton gradient. ‘Proton gradients store energy, and they would rapidly cross the membrane to restore equilibrium if allowed. The cell membrane is impermeable to protons, except through protein complexes called ATP synthases’. (Cohen, 2011) ATP synthase is a protein complex with four main parts that contains a proton channel that allows re-entry of protons into the mitochondrial matrix. It is located in the inner membrane of mitochondria and makes ATP from ADP and an inorganic phosphate group. ATP synthases uses energy stored in the form of the proton ion gradient to drive the synthesis of ATP which is called chemiosmosis. Protons move one by one into binding sites on the Rotor, that spins clockwise when H+ ions flow past it down its gradient, so that it catalyses ATP production. (Reece, et al., 2011). Cellular respiration has a net yield of ATP production per molecule of glucose of around 32ATP. (Reece, et al., 2011)
All the stages described above are involved in the process of cellular aerobic respiration that takes place inside mitochondria. It provides most of the energy necessary for cellular processes to animals though fermentation that makes ATP by lactic acid fermentation in muscle cells in the absence of oxygen only by using substrate-level phosphorylation of glycolysis can also provide some energy. (Reece, et al., 2011) For plants, cellular respiration is at the origin of only a small proportion of the energy produced and stored, as they use photosynthesis instead.
Photosynthesis happens in the chloroplasts of plants that carry out photosynthetic phosphorylation, which is the conversion of light energy into ATP, without the participation of respiration. (Arnon, et al., 1954) There are two stages of photosynthesis, the light reaction that generates ATP and NADH and the Calvin Cycle that produces sugar. In chloroplasts, light reaction occurs in the thylakoids occupied by two photosystems that move electrons across the thylakoid membrane. They are composed of reaction centre complex which allows ‘an energized chlorophyll molecule to lose an electron to an acceptor molecule’, powering electron transport, and an ‘array of around 300 chlorophylls, pigments that absorb light and supply excitation energy to reaction center’. (Allen, 2002) These are Photosystem II (PS II), which functions first and is best at absorbing light with a wavelength of 680 nm, its associated reaction-center chlorophyll a is called P680, and Photosystem I (PS I), best at absorbing a wavelength of 700 nm, its reaction-center chlorophyll a is called P700. (Reece, et al., 2011) During the light reactions, there are two possible routes for electron flow: cyclic and linear electron flow. The flow of electrons through photosystems and other molecular components into the thylakoid membrane is called linear electron flow. Linear electron flow requires photosystem 1 and 2, it produces 1 oxygen molecule around 1ATP and 2NADPH using light energy. (Arnon, et al., 1954) A photon hits a pigment in photosystem 2, and one electron acquires enough energy for it to be raised to a higher energy level. As electron falls back to the ground state, another electron is raised to an excited state. The energy is passed to other pigment molecules until it reaches the P680 in photosystem 2. An electron is excited to a higher energy state in chlorophyll a molecule, then it is transferred from P680 to the primary electron acceptor P680+, a strong oxidizing agent. An enzyme splits water molecule in 2hydrogen ions released into the thykaloid space and electrons reduce P680. Electrons pass from the primary electron acceptor of PS2 to PS1 via an electron transport chain that has the same components, such as cytochrome complex as the cellular respiration chain. (Reece, et al., 2011) The fall of electrons to lower energy levels provide energy for the synthesis of ATP. As electrons pass through the cytochrome complex, H+ diffuse in the thylakoid space from the stroma, contributing to the proton gradient used for chemiosmosis. The diffusion of H+ from the thykaloid space that has a high H+ concentration to the stroma along the proton gradient powers the ATP synthase complex that is very similar to the one in mitochondria that phosphorylate ADP to ATP. (Reece, et al., 2011) P700+ is also an electron acceptor at the end of the electron transport chain from PS2, and participates in the generation of NADPH. In addition to that, Cyclic electron flow uses photosystem 1 but not 2, generating ATP but not NADPH, generating surplus ATP to satisfy the higher demand in the Calvin cycle. Electrons cycle back to the cytochrome complex and then continue on to a P700 in the photosystem 1. It does not require water oxidation and oxygen evolution, and works with light of wavelength beyond that required for complete photosynthesis. (Arnon, et al., 1954)
Respiration and photosynthesis both involve redox reactions. During respiration, energy is released when electrons and hydrogen are transported. Electrons lose potential energy as they go down the electron transport chain towards oxygen to form water, and the energy is then used to synthesize ATP. ‘Photosynthesis reverses the direction of electron flow, electrons are transferred along with hydrogen ions from water to carbon dioxide reducing it to sugar. Electrons increase in potential energy when moving from water to sugar, so the process requires energy’ provided by light, unlike respiration. (Reece, et al., 2011)
Chemiosmosis is an ‘energy coupling mechanism’ that generates ATP using an electron transport chain and a proton gradient that happens in mitochondria during cellular respiration and in the chloroplasts of plants during the process of photosynthesis. However, both of these processes use different sources of energy, mitochondria use chemical energy from food while chloroplasts transform light energy into chemical energy used for the synthesis of ATP. (Reece, et al., 2011) Another difference is that in chloroplasts, protons are pumped across the thylakoid membrane from the from the stroma into the lumen and then back into the stroma through ATP synthase (Allen, 2002), while in mitochondria, protons are pumped to the intermembrane space and powers ATP synthase as they diffuse back into the mitochondrial matrix. (Reece, et al., 2011) Electrons also have a different origin, organic molecules in mitochondria, water in chloroplasts.
In conclusion, animals and plants use processes that are very much alike to produce and store energy in the form of ATP. Cellular respiration is common to both plants and animals, producing ATP by substrate level and oxidative phosphorylation. Similarly to light reactions in plants only, it produces ATP by the mechanisms chemiosmosis that uses ATP synthase complexes to drive the synthesis of ATP.

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