{"id":9543,"date":"2026-06-01T21:33:48","date_gmt":"2026-06-01T21:33:48","guid":{"rendered":"https:\/\/kapdec.com\/help\/?p=9543"},"modified":"2026-06-01T21:33:48","modified_gmt":"2026-06-01T21:33:48","slug":"photosynthesis","status":"publish","type":"post","link":"https:\/\/kapdec.com\/help\/photosynthesis\/","title":{"rendered":"Photosynthesis"},"content":{"rendered":"

Unit: Cellular energetics<\/strong><\/h2>\n

Chapter: Photosynthesis<\/strong><\/h3>\n

Reference:<\/em><\/strong> Photosynthesis, Chloroplasts, Two predominant types of chlorophylls, Nature of light-Pigment systems, Light reaction, The Electron Transport System, Dark reaction<\/em><\/p>\n

 <\/p>\n

Learning objectives<\/strong><\/p>\n

    \n
  • To understand about light and dark reaction<\/li>\n
  • To learn about the structure of chloroplast, light harvesting complex<\/li>\n<\/ul>\n

    Photosynthesis<\/strong><\/p>\n

    Photosynthesis is the only process on earth by which solar energy is trapped by green plants and converted into food. It is defined as synthesis of carbohydrates from inorganic materials like CO2 and H2O with the help of solar energy trapped by pigments like chlorophyll.<\/p>\n

    Chloroplasts <\/strong>– mainly located in the mesophyll cells of leaves. The CO2 reaches them through the stomata and water reaches them through veins. In higher plants, the chloroplasts are discoid or lens-shaped. Each chloroplast is bounded by double membrane. Inside the membranes is found a ground substance, the stroma. Inside the stroma is found a system of chlorophyll bearing double membrane sacs or lamellae. These are stacked one above the other to form grana. Individual sacs in each granum are known as thylakoids. All the pigments chlorophylls, carotenes and xanthophylls are located in the thylakoid membranes. These pigments absorb light of a specific spectrum in the visible region. The pigments are fat soluble and located in the lipid part of the membrane. With the help of certain enzymes, they participate in the conversion of solar energy into ATP and NADPH. The enzymes of stroma utilize ATP and NADPH to produce carbohydrates.<\/p>\n

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    Two predominant types of chlorophylls:<\/strong><\/p>\n

                 Chlorophyll a and b differ in the nature of groups. Chlorophyll a has a methyl group while chlorophyll b has an aldehyde group. Chemically chlorophyll molecule consists of two parts head of tetrapyrrole the porphyrin ring and a long hydrocarbon tail called phytol attached to the porphyrin group. Carotenoids are lipid compound present universally in almost all the higher plans and several microorganisms. They are usually red, orange, yellow, brown, and are associated with chlorophyll. They are of two types the carotenes and xanthophylls. The carotenes are orange red and xanthophylls contain oxygen. The light energy absorbed by the carotenoids is transferred to chlorophyll a to be utilized in photosynthesis. All photosynthetic plants have these pigments that absorb light between the red and blue region of the spectrum. Carotenoids found mainly in higher plants absorb primarily in the violet to blue regions of the spectrum. They not only absorb light energy and transfer it to chlorophyll but also protect the chlorophyll molecule from photo oxidation.<\/p>\n

    Nature of light-<\/strong> light is a form of energy. Light has different wavelengths having different colours. One can see electromagnetic radiation with wavelengths ranging from 390 nm to 730nm.This part of the spectrum is called visible light. It lies between wavelengths of ultraviolet and infra- red.<\/p>\n

    Pigment systems<\/strong><\/p>\n

    The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within Photosystem I (PS I) and Photosystem II (PS II).<\/p>\n

    These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. The single chlorophyll a molecule forms the reaction centre. The reaction centre is different in both the photosystems. In PS I, the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700, while in PS II it has absorption maxima at 680 nm, and is called P680.<\/p>\n

    Light reaction<\/strong><\/p>\n

    light reactions or the ‘photochemical’ phase include light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH.<\/p>\n

    The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PS I) and Photosystem II (PS II).<\/p>\n

    The LHC are made up of hundreds of pigment molecules bound to proteins.<\/p>\n

    Each photosystem has all the pigments (except one molecule of chlorophyll a) forming a light harvesting system also called antennae.<\/p>\n

    The single chlorophyll a molecule forms the reaction centre.<\/p>\n

    In PS I, the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700, while in PS II it has absorption maxima at 680 nm, and is called P680.<\/p>\n

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    The Electron Transport System<\/strong><\/p>\n

      \n
    • Reaction centre of photosystem II absorbs light of 680 nm in red region and causing electron to become excited. These electrons are picked by an electron acceptor which passes to electron transport system consisting of cytochromes.<\/li>\n
    • Electrons are passed down the electron transport chain and then to the pigment of PS I.<\/li>\n
    • Electron in the PSI also get excited due to light of wavelength 700nm and are transferred to another accepter molecule having a greater redox potential.<\/li>\n
    • When electron passes in downhill direction, energy is released. This is used to reduce the ADP to ATP and NADP+ to NADPH. The whole scheme of transfer of electron is called Z-scheme due to its shape.<\/li>\n
    • Photolysis of water release electrons that provide electron to PS II. Oxygen is also released during this process.<\/li>\n<\/ul>\n

      Dark reaction<\/strong><\/p>\n

      Carbon fixation occurs in the stroma by a series of enzyme catalysed steps. Molecules of ATP and NADPH produced in the thylakoids come in the stroma where carbohydrates are synthesized. The path of carbon fixation in dark reaction through intermediate compounds leading to the formation of sugar and starch was worked out by Calvin, Benson and their co-workers. The various steps in the dark reactions are as follows:<\/p>\n

      1.Carboxylation- CO2 reduction starts with a 5-Carbon sugar, ribulose -1,5 – bisphosphate. It is a 5-carbon sugar with two phosphate groups attached to it. RuBP reacts with CO2 to produce a short lived 6-carbon intermediate in the presence of an enzyme RUBISCO and splits into 3-Phosphoglyceric acid.<\/p>\n

      2.Glycolytic reversal- molecules of 3-PGA form 1,3 – diphosphoglyceric acid utilizing ATP molecules. These are reduced to glyceraldehyde 3-phosphate by NADPH supplied by the light reactions of photosynthesis. Out of each of 12 molecules of 3-phosphoglyceraldehyde ,2 molecules are used for synthesis of one glucose molecule and remaining 10 molecules are used for regeneration of 6 molecules of RuBP.<\/p>\n

      3.Regeneration of RuBP- for every 6 molecules of CO2 and ribulose-1,5- biphosphate used,12 molecules of 3-phosphoglyceraldehyde are produced. Out of these 12 molecules, only two are utilized for the formation of a molecule of glucose; the other 10 molecules are converted into ribulose-1,5-biphosphate which combines with fresh CO2.<\/p>\n

      \"\"<\/p>\n

      Solved examples:<\/strong><\/p>\n

      Example 1<\/strong>. Rubisco is important in which step of photosynthesis<\/p>\n

      a) energizing the electrons in chlorophyll<\/p>\n

      b) carbon fixation when CO2 is connected to RUBP during Calvin cycle<\/p>\n

      c) regeneration phase as RUBP is made during Calvin cycle<\/p>\n

      d) reduction of the carbon chain during the Calvin cycle<\/p>\n

      Solution 1:<\/strong> b. Rubisco is important in the step during carbon fixation<\/p>\n

      Example 2.<\/strong> Photosystems are made of:<\/p>\n

      a) chloroplasts b) chromosomes c) chlorophyll   d) cytoplasm<\/p>\n

      Solution 2:<\/strong> c. Photosystems are made of chlorophyll.<\/p>\n

                                                 Summary<\/strong><\/p>\n

        \n
      • Photosynthesis takes place only in green parts of the plants, mainly the leaves.<\/li>\n
      • Within the leaves, the mesophyll cells have many chloroplasts that are responsible for CO2 fixation.<\/li>\n
      • Photosynthesis has two stages: the light reaction and the carbon fixing reactions.<\/li>\n
      • In the light reaction the light energy is absorbed by the pigments present in the antenna, and funnelled to special chlorophyll a molecule called reaction centre chlorophylls.<\/li>\n
      • In carbon fixation cycle, CO2 is added by the enzyme, RUBISCO, to a 5-carbon compound RuBP that is converted to 2 molecules of 3-carbon PGA.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"

        Unit: Cellular energetics Chapter: Photosynthesis Reference: Photosynthesis, Chloroplasts, Two predominant types of chlorophylls, Nature of light-Pigment systems, Light reaction, The Electron Transport System, Dark reaction   Learning objectives To understand about light and dark reaction To learn about the structure of chloroplast, light harvesting complex Photosynthesis Photosynthesis is the only process on earth by which […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[629],"tags":[],"class_list":["post-9543","post","type-post","status-publish","format-standard","hentry","category-ap-biology"],"_links":{"self":[{"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/posts\/9543","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/comments?post=9543"}],"version-history":[{"count":0,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/posts\/9543\/revisions"}],"wp:attachment":[{"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/media?parent=9543"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/categories?post=9543"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/tags?post=9543"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}