{"id":304130,"date":"2026-06-21T00:31:22","date_gmt":"2026-06-20T19:01:22","guid":{"rendered":"https:\/\/www.aakash.ac.in\/blog\/?p=304130"},"modified":"2026-06-21T00:31:22","modified_gmt":"2026-06-20T19:01:22","slug":"photosynthesis-revision-notes","status":"publish","type":"post","link":"https:\/\/www.aakash.ac.in\/blog\/photosynthesis-revision-notes\/","title":{"rendered":"Photosynthesis Revision for Re-NEET 2026: Light Reaction Explained"},"content":{"rendered":"<p><span style=\"font-weight: 400;\">Photosynthesis in Higher Plants is one of those chapters that NEET aspirants either love or dread. It has enough theory to confuse you, enough diagrams to overwhelm you, and enough direct questions to make or break your biology score. If you are preparing for <\/span><b>Re-NEET 2026<\/b><span style=\"font-weight: 400;\">, this photosynthesis revision is exactly what you need before your exam.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Let&#8217;s walk through the most critical part of this chapter, the <\/span><b>Light Reaction<\/b><span style=\"font-weight: 400;\">, in a clear, exam-focused way.<\/span><br \/>\nWatch this video:<\/p>\n<div class=\"jeg_video_container jeg_video_content\"><iframe loading=\"lazy\" title=\"What is Light Reaction? \u26a1 Complete Photosynthesis Revision for Re-NEET 2026\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/_hirqgo6FUc?feature=oembed\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" referrerpolicy=\"strict-origin-when-cross-origin\" allowfullscreen><\/iframe><\/div>\n<h2><b>Why Photosynthesis Matters for NEET 2026<\/b><\/h2>\n<p><span style=\"font-weight: 400;\">Photosynthesis in Higher Plants is a high-weightage topic in NEET Biology. Questions appear every year from the light reaction, the Calvin cycle, photosystems, and Z-scheme. Skipping this chapter or leaving it half-revised is a costly mistake.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">This <\/span><b>photosynthesis quick revision<\/b><span style=\"font-weight: 400;\"> focuses on the light-dependent reaction, the foundation that everything else in the chapter builds upon.<\/span><\/p>\n<h2><b>What Is the Light Reaction?<\/b><\/h2>\n<p><span style=\"font-weight: 400;\">The process of photosynthesis takes place in two broad stages. The very first stage is the <\/span><b>Light Reaction<\/b><span style=\"font-weight: 400;\">, which is, as the name tells you, completely light-dependent.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">During the light reaction, four key events take place:<\/span><\/p>\n<ol>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Absorption of light<\/b><span style=\"font-weight: 400;\"> by photosynthetic pigments<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Splitting of water<\/b><span style=\"font-weight: 400;\"> (photolysis)<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Release of oxygen<\/b><span style=\"font-weight: 400;\"> as a byproduct<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Synthesis of ATP and NADPH<\/b><span style=\"font-weight: 400;\">, the energy currencies used in the next stage<\/span><\/li>\n<\/ol>\n<p><span style=\"font-weight: 400;\">These four tasks are the backbone of the light reaction. Every MCQ or short-answer question on this topic revolves around one or more of these four events.<\/span><\/p>\n<p><b>Location:<\/b><span style=\"font-weight: 400;\"> The light reaction takes place in the membrane system inside the <\/span><b>grana<\/b><span style=\"font-weight: 400;\"> and <\/span><b>stroma lamellae<\/b><span style=\"font-weight: 400;\"> of the chloroplast. This is a direct NEET fact, note it clearly in your <\/span><b>photosynthesis short notes<\/b><span style=\"font-weight: 400;\">.<\/span><\/p>\n<h2><b>Photosystems: The Molecular Machinery of Light Reaction<\/b><\/h2>\n<p><span style=\"font-weight: 400;\">Inside the grana and stroma lamellae, we find structures called <\/span><b>photosystems<\/b><span style=\"font-weight: 400;\">. Each photosystem has two major components:<\/span><\/p>\n<ol>\n<li><b> Electron Acceptor<\/b><span style=\"font-weight: 400;\"> This molecule accepts the high-energy electrons that are released during the reaction.<\/span><\/li>\n<li><b> Light Harvesting System (LHC)<\/b><span style=\"font-weight: 400;\"> The light harvesting system itself has two sub-components:<\/span><\/li>\n<\/ol>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Light Harvesting Complex (LHC):<\/b><span style=\"font-weight: 400;\"> Contains hundreds of pigment molecules (chlorophylls, carotenoids) bound to proteins. Their job is to capture photons and funnel the energy inward.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Reaction Centre:<\/b><span style=\"font-weight: 400;\"> The specific chlorophyll molecule where the actual photochemistry happens. All the energy collected by the LHC molecules gets transferred here.<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">When photons strike the LHC pigment molecules, they do not directly drive the reaction. Instead, the energy is passed from one pigment molecule to the next until it reaches the reaction centre. Once the reaction centre chlorophyll receives enough energy, its electrons gain sufficient energy to be ejected from the molecule. These ejected electrons are then picked up by the <\/span><b>primary electron acceptor molecule<\/b><span style=\"font-weight: 400;\">.<\/span><\/p>\n<h2><b>Two Types of Photosystems: PS I and PS II<\/b><\/h2>\n<p><span style=\"font-weight: 400;\">There are two photosystems in plants, and they differ in their reaction centres. This is one of the most tested <\/span><b>photosynthesis important concepts<\/b><span style=\"font-weight: 400;\"> in NEET.<\/span><\/p>\n<p><b>Photosystem II (PS II):<\/b><span style=\"font-weight: 400;\"> The reaction centre of PS II is <\/span><b>P680<\/b><span style=\"font-weight: 400;\">. This name means that the chlorophyll molecule present in this reaction centre shows maximum light absorption at a wavelength of <\/span><b>680 nanometres<\/b><span style=\"font-weight: 400;\">.<\/span><\/p>\n<p><b>Photosystem I (PS I):<\/b><span style=\"font-weight: 400;\"> The reaction centre of PS I is <\/span><b>P700<\/b><span style=\"font-weight: 400;\">. Here, the same chlorophyll molecule shows maximum absorption at <\/span><b>700 nanometres<\/b><span style=\"font-weight: 400;\">.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">The difference in absorption wavelength is what distinguishes PS I from PS II at the molecular level. In NEET MCQs, P680 and P700 are directly asked, make sure this is locked in your memory.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">For a detailed visual walkthrough of how these photosystems function together, this<\/span><a href=\"https:\/\/youtu.be\/_hirqgo6FUc?si=c8MPJjGE7kPp64Bm\"> <span style=\"font-weight: 400;\">photosynthesis revision video for Re-NEET 2026<\/span><\/a><span style=\"font-weight: 400;\"> explains the light reaction step by step with diagrams.<\/span><\/p>\n<h2><b>Photolysis of Water: Why Oxygen Is Released<\/b><\/h2>\n<p><span style=\"font-weight: 400;\">PS II is directly responsible for the <\/span><b>splitting of water<\/b><span style=\"font-weight: 400;\">, known as photolysis. When water molecules are split using light energy, the reaction produces:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Electrons<\/b><span style=\"font-weight: 400;\"> (which replace the ones lost from the PS II reaction centre)<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Protons (H\u207a ions)<\/b><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><b>Oxygen<\/b><span style=\"font-weight: 400;\"> (released as a byproduct into the atmosphere)<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400;\">This is the source of all the oxygen released during photosynthesis. Every breath of oxygen you take is a product of photolysis. For NEET, remember that water splitting is associated with PS II, not PS I.<\/span><\/p>\n<h2><b>ATP and NADPH: The Energy Products<\/b><\/h2>\n<p><span style=\"font-weight: 400;\">The light reaction&#8217;s ultimate output is chemical energy stored in two molecules:<\/span><\/p>\n<p><b>ATP<\/b><span style=\"font-weight: 400;\"> is produced through a process called <\/span><b>photophosphorylation<\/b><span style=\"font-weight: 400;\">, the addition of a phosphate group to ADP, powered by the proton gradient generated during electron transport.<\/span><\/p>\n<p><b>NADPH<\/b><span style=\"font-weight: 400;\"> is produced when NADP\u207a accepts electrons and protons at the end of the electron transport chain in PS I. This molecule carries reducing power into the Calvin cycle (dark reaction) where CO\u2082 is fixed.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Both ATP and NADPH are essential inputs for the <\/span><b>dark reaction (Calvin cycle)<\/b><span style=\"font-weight: 400;\">. Without the light reaction supplying these, carbon fixation cannot proceed.<\/span><\/p>\n<h2><b>FAQs<\/b><\/h2>\n\t\t<div class=\"wp-faq-schema-wrap\">\n\t\t\t\t\t\t<div class=\"wp-faq-schema-items\">\n\t\t\t\t\t\t\t\t\t<h3>Q1. What is the difference between PS I and PS II in photosynthesis?<\/h3>\n\t\t\t\t\t<div class=\"\">\n\t\t\t\t\t\t<p>PS I and PS II differ in their reaction centres. PS II has the reaction centre P680, which absorbs light maximally at 680 nm. PS I has the reaction centre P700, absorbing maximally at 700 nm. Functionally, PS II is involved in water splitting and oxygen release, while PS I is primarily responsible for producing NADPH.<\/p>\n\t\t\t\t\t<\/div>\n\t\t\t\t\t\t\t\t\t<h3>Q2. Where exactly does the light reaction take place in the chloroplast?<\/h3>\n\t\t\t\t\t<div class=\"\">\n\t\t\t\t\t\t<p>The light reaction takes place in the membrane system of the chloroplast, specifically in the thylakoid membranes found in the grana and stroma lamellae. The dark reaction (Calvin cycle), in contrast, takes place in the stroma of the chloroplast.<\/p>\n\t\t\t\t\t<\/div>\n\t\t\t\t\t\t\t\t\t<h3>Q3. What are the four main events of the light reaction in photosynthesis?<\/h3>\n\t\t\t\t\t<div class=\"\">\n\t\t\t\t\t\t<p>The four events are: absorption of light by photosynthetic pigments, splitting of water (photolysis), release of oxygen as a byproduct, and synthesis of ATP and NADPH. These four processes must be memorised in order, as NEET questions often ask which event happens first or which photosystem is responsible for a particular step.<\/p>\n\t\t\t\t\t<\/div>\n\t\t\t\t\t\t\t\t\t<h3>Q4. What is the role of the Light Harvesting Complex (LHC) in a photosystem?<\/h3>\n\t\t\t\t\t<div class=\"\">\n\t\t\t\t\t\t<p>The LHC contains hundreds of pigment molecules (including chlorophyll a, chlorophyll b, and carotenoids) bound to proteins. These pigments capture photons and transfer the energy to the reaction centre through resonance energy transfer. The LHC does not directly drive photochemistry, it acts as an antenna system that maximises light capture and delivers concentrated energy to the reaction centre.<\/p>\n\t\t\t\t\t<\/div>\n\t\t\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\n","protected":false},"excerpt":{"rendered":"<p>Photosynthesis in Higher Plants is one of those chapters that NEET aspirants either love or dread. It has enough theory to confuse you, enough diagrams to overwhelm you, and enough direct questions to make or break your biology score. If you are preparing for Re-NEET 2026, this photosynthesis revision is exactly what you need before [&hellip;]<\/p>\n","protected":false},"author":64,"featured_media":304138,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3716],"tags":[30198,30199,30195,30196,30194,30193,30197],"class_list":["post-304130","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-neet","tag-atp-and-nadph","tag-light-reaction-of-photosynthesis","tag-neet-biology-revision-notes","tag-photosynthesis-important-concepts","tag-photosynthesis-in-higher-plants","tag-photosynthesis-revision-for-re-neet-2026","tag-photosynthesis-short-notes-for-neet"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v26.0 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>Photosynthesis Revision for Re-NEET 2026: Light Reaction Notes &amp; MCQs<\/title>\n<meta name=\"description\" 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