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The Effects of Light Intensity on the Rate of the Light-Dependent Reactions of Photosynthesis in Chloroplasts Extracts of Spinach Leaves Introduction Photosynthesis is the process that converts energy in sunlight to chemical forms of energy that can be used by biological systems Figure 1 Many different organisms ranging from plants to bacteria carry out photosynthesis The photosynthetic process in plants and algae occurs in small organelles known as chloroplasts that are located inside cells The photosynthetic reactions are traditionally divided into two stages - the light reactions which consist of electron and proton transfer reactions and the dark reactions which consist of the biosynthesis of carbohydrates from carbon dioxide The light reactions occur in a complex membrane system that is made up of protein complexes electron carriers and lipid molecules The photosynthetic membrane is surrounded by water and can be thought of as a two-dimensional surface that defines a closed space with an inner and outer water phase A molecule or ion must pass through the photosynthetic membrane to go from the inner space to the outer space Stern 2000 The protein complexes embedded in the photosynthetic membrane have a unique orientation with respect to the inner and outer phase The asymmetrical arrangement of the protein complexes allows some of the energy released during electron transport to create an electrochemical gradient of protons across the photosynthetic membrane Photosynthetic electron transport consists of a series of individual electron transfer steps from one electron carrier to another The electron carriers are metal ion complexes and aromatic groups The reduced forms of plastoquinone or ubiquinone and nicotinamide adenine dinucleotide phosphate NADPH or NADH act as mobile electron carriers Stern 2000 Robin Hill showed that an inorganic substance ferricyanide was reduced to ferrocyanide when extracellular preparations made from plant cells were illuminated At the same time water was oxidized to oxygen Stern 2000 Hills results meant that isolated chloroplasts and chloroplast fragments could release
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