Photosynthesis Versus Cellular Respiration


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Photosynthesis is the storage of energy in carbon compounds. In photosynthesis, the sun's energy is converted to chemical energy and stored in glucose molecules. In photosynthesis, we see energy associated with electrons or being released from association of electrons. There are two pathways associated with photosynthesis: light and dark reactions. I will concentrate on the non-cyclic light reactions of photosynthesis. Light reactions are driven by light energy. This pathway produces ATP and a reduced electron carrier (NADPH + H ). The overall reaction is solar energy + 6CO + 6H O ® C H O + 6O + heat. Whereas photosynthesis takes place in specific groups of organisms, cellular respiration is done by all cells. In respiration, the energy stored in organic compounds may have been produced by the cell itself or stolen from another organism. Respiration uses oxygen from the environment to create carbon dioxide. The overall reaction for cellular respiration is C H O + 6O ® 6CO + 6H O + energy (heat and ATP).

The electron transport chain of cellular respiration and the non-cyclic light reaction of photosynthesis are somewhat similar to each other. Just as NAD goes hand in hand with metabolic events that take place in cellular respiration, NADP also goes hand in hand with photosynthesis. The only difference in NAD and NADP is an extra phosphate group that binds to the sugar ribose. Photosynthesis and cellular respiration are similar in that they both undergo chemiosmosis. In respiration, high energy electrons are transported from the Krebs cycle to the electron transport chain by the carrier molecule NADH. In photosynthesis, high energy electrons hitch a ride on NADPH. NADPH carries the excited electron (Hydrogen atom) to the dark reactions in the stroma to make glucose. In photosynthesis, electrons go down the chain and produce ATP between PHAEO and Photosystem I. This is the same in cellular respiration. There are very few similarities between photosynthesis and cellular respiration.

There are many differences between the non-cyclic reactions of photosynthesis and the electron transport chain of cellular respiration.

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Photosynthesis requires energy as photons. Cellular respiration supplies energy in the form of ATP. In photosynthesis, electron transport occurs in the membranes of chloroplasts or thylakoid discs. Electron transport in cellular respiration takes place in the membranes of mitochondria or the cristae. In photosynthesis, water breaks down to supply 2 electrons and 2 Hydrogen ions and release ½ O at the beginning of the chain. In cellular respiration, 2 electrons at the end of the chain combine with 2 Hydrogen ions and ½ O to produce water. This is the complete opposite. NADPH + H is produced at the end of the chain in photosynthesis. In respiration, it is fed into the beginning of the chain. In the last step of photosynthesis, 2 x 3C ®C6 (glucose). In respiration, the first step is C6 (glucose)® 2 x 3C (pyruvate). The Calvin cycle requires NADPH plus a Hydrogen ion, produces C3, and takes place in the stroma. The Krebs' cycle produces NADH plus a Hydrogen ion, requires 2C, and takes place in the matrix. Photosynthesis takes in carbon dioxide and water and produces glucose. Cellular respiration is the counterpart. It takes in glucose and produces carbon dioxide and water. The two reactions are quite different from each other. They are total opposites in most cases.



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