Hill reaction

The evolution of oxygen during the light-dependent steps in photosynthesis (Hill reaction) was proposed and proven by the British biochemist Robin Hill. He demonstrated that isolated chloroplasts would make oxygen (O2) but not fix carbon dioxide (CO2). This is evidence that the light and dark reactions happen in different places within the cell. The evolution of oxygen during the light-dependent steps in photosynthesis (Hill reaction) was proposed and proven by the British biochemist Robin Hill. He demonstrated that isolated chloroplasts would make oxygen (O2) but not fix carbon dioxide (CO2). This is evidence that the light and dark reactions happen in different places within the cell. The Hill reaction of photosynthesis was discovered by Robin Hill (1937). He found that isolated chloroplasts from plants can release oxygen when they are illuminated by sunlight in the presence of a suitable electron acceptor such as ferricyanide. To demonstrate the Hill reaction in the laboratory dichlorophenolindophenol (DCPIP) was used as the terminal electron acceptor, replacing NADP which was not available due to the extraction of the chloroplasts from the plant cellular environment. The ferrocyanide is reduced (just as NADP is when the chloroplast is in vivo) while water (H2O) is oxidized into oxygen (O2) and hydrogen cations during the reaction. By using this technique to observe the reaction, Hill revealed a variety of valuable facts about photosynthesis. The Hill reaction confirms that oxygen (O2) is produced by a reaction that is separate from carbon dioxide (CO2) fixation. The reaction in which oxygen is released requires light; therefore the Hill reaction can be described as the light dependent reaction of photosynthesis. The reaction in which oxygen is released takes place within the chloroplast of plants and the Hill reaction demonstrated that the release of oxygen (O2) is only a partial reaction which requires one step of photosynthesis. The Hill reaction implies that the light dependent reaction of photosynthesis is a result of a series of redox reactions and a suitable terminal electron acceptor is required for that reaction to occur. Plants have natural electron acceptors such as NADP, that play a major role in the oxidation of water. The Hill reaction also shows that the natural electron acceptors of the reaction can be substituted by an artificial electron acceptor such as DCPIP, again allowing for the discharge of oxygen. The technique of replacing the natural electron acceptors with artificial electron acceptors can be used in the laboratory to provide a means to measure phosphorylation in chloroplasts and observe its relation to the discharge of oxygen in this reaction. Hill's finding was that the origin of oxygen in photosynthesis is water (H2O) not carbon dioxide (CO2) as previously believed. Hill's reaction also demonstrates that the light dependent redox reaction is the first reaction to take place in photosynthesis. The electrons thus freed by the splitting of water provides the electrons needed by the antenna system of photosystem II where they are boosted in energy and then sent along the electron transport system (ETS). Those high energy electrons are able to move against a chemical potential gradient. Hence, solar energy is converted to chemical energy by the reduction of NADP to NADPH. Photosynthesis is the process in which light energy is absorbed and converted to chemical energy. This chemical energy is eventually used in the conversion of carbon dioxide (CO2) to sugar (CH2O) in plants. During the process of photosynthesis, a natural electron acceptor, nicotinamide adenine dinucleotide phosphate (NADP), is reduced in chloroplasts to NADPH. Overall within a chloroplast, the following equilibrium reaction takes place.