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Give account of the reaction of the photosynthetic reaction.

Last updated date: 14th Jul 2024
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Hint: Photosynthesis is the process by which green plants and certain other organisms transform light energy from the sun into chemical energy. During photosynthesis in green plants, light energy is used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds.

Complete Answer:
Photosynthesis occurs through cellular respiration. The resulting chemical energy is stored in carbohydrate molecules, such as sugars. These are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek phōs, "light" and synthesis, "putting together". Oxygen is also released as a waste product. Most plants, algae, and cyanobacteria perform photosynthesis and are called photoautotrophs. Photosynthesis is responsible for producing and maintaining the oxygen content of the Earth's atmosphere and also supplies most of the energy necessary for life on Earth.
In the light-dependent reactions, one molecule of chlorophyll absorbs one photon from sunlight and loses one electron. This detached electron is passed to a modified form of chlorophyll and pheophytin, which passes the electron to a quinone molecule. Thus, the flow of electrons down an electron transport chain has started and leads to an ultimate reduction of NADP to NADPH. A proton gradient across the chloroplast membrane is also created, which is used by ATP synthase in the synthesis of ATP. The chlorophyll molecule ultimately regains the lost electron when a water molecule is split in the process called photolysis, which leads to the release of a dioxygen ($O_2$) molecule as a waste product. The equation for light-dependent reactions under the conditions of non-cyclic electron flow in green plants is:
${2}{H}_{2}{O}+{2}{NADP}+{3}{ADP}+{3}{Pi}+{light} → {2}{NADPH}+{2}{H}+{3}{ATP}+{O}_{2}$
The photosynthetic action spectrum depends on the type of accessory pigments present. For instance, in green plants, the action spectrum resembles the absorption spectrum for chlorophylls and carotenoids with absorption peaks in violet-blue and red light. In red algae, the action spectrum is blue-green light. This enables the red algae to use the blue end of the spectrum to grow in deep waters by filtering out the wavelength of red light used by the above-ground green plants. The non-absorbed (i.e., the reflected) part of the light spectrum gives photosynthetic organisms their colour.
In the light-independent (or "dark") reactions, the enzyme RuBisCO captures $CO_2$ from the atmosphere and uses the newly formed NADPH to release three-carbon sugars. This process is called the Calvin cycle. The three-carbon sugars are later combined to form sucrose and starch. The equation for light-independent reactions in green plants is:
${3}{C}{O}_{2}+{9}{ATP}+{6}{NADPH}+{6}{H} →{C}_{3}{H}_{6}{O}_{3}{phosphate}+{9}{ADP}+{8}{Pi}+{6}{NADP}+{3}{H}_{2}{O}$

Carbon fixation is the process that produces the intermediate three-carbon sugar product. The simple carbon sugars produced by photosynthesis are then used to form organic compounds, cellulose, precursors for lipid and amino acid biosynthesis, or as a fuel in cellular respiration. The latter occurs not only in plants but also in animals when the energy from plants is passed through a food chain.