--- title: "Reaction Center in General Biology I" description: "Reaction center in General Biology I is the pigment-protein core of a photosystem that absorbs light, excites electrons, and starts electron flow." canonical: "https://fiveable.me/college-bio/key-terms/reaction-center" type: "key-term" subject: "General Biology I" unit: "Unit 8" --- # Reaction Center in General Biology I ## Definition A reaction center is the pigment-protein core of a photosystem where light energy first excites electrons. In General Biology I, it sits in the thylakoid membrane and starts the light-dependent reactions of photosynthesis. ## What It Is A reaction center is the part of a photosystem in General Biology I where light energy is turned into electron energy. It is a small pigment-protein complex embedded in the thylakoid membrane, and it contains a special chlorophyll a molecule that can actually pass an excited electron to another molecule. In Photosystem II, the reaction center is called P680 because it absorbs best around 680 nm. In Photosystem I, it is P700, with a best absorption around 700 nm. Those numbers are easy to mix up, but the main idea is that each photosystem has its own reaction center tuned to a slightly different wavelength of light. Here is the basic sequence. Light hits the antenna pigments around the photosystem, energy gets funneled to the reaction center, and the special chlorophyll a there becomes excited. That excited electron does not stay put. It is transferred to the primary electron acceptor, which is the first molecule that can hold onto it and send it along the next steps of electron transport. That electron transfer is the point of the reaction center. Without it, light would be absorbed, but the cell would not capture that energy in a usable form. Once the electron leaves the reaction center, the photosystem can keep moving electrons through the light-dependent reactions, which eventually helps build ATP and, in noncyclic photophosphorylation, NADPH as well. The reaction center is not the whole photosystem, and it is not the same thing as the antenna pigments. The antenna pigments collect light over a wider range, while the reaction center does the chemistry that starts electron flow. That difference matters because photosynthesis is not just about absorbing photons. It is about moving electrons in a controlled path inside the thylakoid membrane. ## Why It Matters The reaction center is the first place where light energy becomes chemical potential in photosynthesis. In General Biology I, that makes it one of the core steps linking sunlight to the production of ATP and NADPH, the molecules cells use to power later stages of carbon fixation. If you understand the reaction center, the rest of the light-dependent reactions make more sense. You can trace why electrons leave chlorophyll, why the primary electron acceptor matters, and why the photosystems have to pass electrons through an electron transport chain instead of keeping them in one place. That flow is what eventually supports proton pumping and ATP synthase activity across the thylakoid membrane. It also helps you distinguish between the two photosystems. PSII and PSI both have reaction centers, but they do different jobs in the overall light reactions. When a question asks where light first triggers electron movement, or which part of the photosystem starts the chain, the reaction center is the answer you want. ## Connections ### Photosystem II PSII contains the P680 reaction center and is usually the first photosystem to act in the light-dependent reactions. When light excites P680, the electron leaves PSII and enters the electron transport chain. If you are tracing the path of electrons in a diagram, PSII is usually where that path begins. ### Photosystem I PSI also has a reaction center, called P700, and it re-excites electrons later in the light reactions. PSI is where electrons get boosted again so they can help reduce NADP+ to NADPH. It is easy to think PSI comes first because of the name, but in the sequence of electron flow, it acts after PSII. ### [primary electron acceptor](/college-bio/key-terms/primary-electron-acceptor) The primary electron acceptor is the first molecule that receives the excited electron from the reaction center. It keeps the electron moving instead of letting the energy fall back into chlorophyll. On diagrams and exam questions, this is the immediate next step after the reaction center absorbs light. ### Thylakoid Membrane Reaction centers sit in the thylakoid membrane, where the light-dependent reactions are organized. That location matters because the membrane holds the photosystems, electron transport chain, and proton gradient in the same system. Without the membrane, the electron flow that powers ATP production would not be spatially coordinated. ## On the AP Exam A quiz or lab question might point to a chloroplast diagram and ask you to identify the spot where light energy first becomes electron energy. That is the reaction center, not the whole photosystem and not the antenna pigments around it. You may also be asked to trace what happens after a photon hits P680 or P700: the electron gets excited, moves to the primary electron acceptor, and then enters electron transport. In a passage analysis or short-answer question, use the term when explaining why photosynthesis can keep moving forward after light absorption. If the question shows cyclic or noncyclic photophosphorylation, the reaction center is where you start describing electron flow. On lab worksheets, it often shows up when you label the parts of PSII or PSI and explain which molecule absorbs light directly. ## reaction center vs antenna pigments Antenna pigments absorb light and funnel that energy to the reaction center, but they do not pass the excited electron into the electron transport chain. The reaction center is the part that actually launches electron transfer. If a question asks which part starts the chemical step of photosynthesis, pick the reaction center, not the antenna pigments. ## Key Takeaways - A reaction center is the pigment-protein core of a photosystem where light energy starts electron transfer. - In General Biology I, PSII has the P680 reaction center and PSI has the P700 reaction center. - The reaction center hands an excited electron to the primary electron acceptor, which begins the electron transport chain. - Antenna pigments collect light, but the reaction center does the reaction that converts that light into usable chemical energy. - If you can trace the electron from the reaction center through the thylakoid membrane, you can follow most of the light-dependent reactions. ## FAQs ### What is a reaction center in General Biology I? It is the part of a photosystem where a special chlorophyll a molecule absorbs energy and passes an excited electron to the primary electron acceptor. That is the step that starts electron flow in the light-dependent reactions. In plants, this happens in the thylakoid membrane of chloroplasts. ### How is the reaction center different from antenna pigments? Antenna pigments capture light and funnel that energy inward, but they do not perform the electron transfer step. The reaction center is the site where the excited electron actually leaves chlorophyll and enters the electron transport chain. If you are labeling a diagram, the antenna pigments are the outer light-gathering part and the reaction center is the core. ### What are P680 and P700? P680 is the reaction center chlorophyll in Photosystem II, and P700 is the reaction center chlorophyll in Photosystem I. The numbers refer to the wavelength of light each one absorbs best. They are not different molecules from chlorophyll a, just special chlorophyll a molecules in each photosystem. ### Why does the reaction center matter in photosynthesis? Without the reaction center, light would be absorbed but not converted into directed electron movement. That electron movement is what leads to ATP and NADPH production in the light-dependent reactions. In other words, the reaction center is where sunlight starts becoming usable chemical energy. ## Related Study Guides - [8.2 The Light-Dependent Reactions of Photosynthesis](/college-bio/unit-8/2-light-dependent-reactions-photosynthesis/study-guide/oyfC6aiT0XiaWeiB) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/college-bio/key-terms/reaction-center#resource","name":"Reaction Center in General Biology I","url":"https://fiveable.me/college-bio/key-terms/reaction-center","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/college-bio/key-terms/reaction-center#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:21:10.328Z","isPartOf":{"@type":"Collection","name":"General Biology I Key Terms","url":"https://fiveable.me/college-bio/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/college-bio/key-terms/reaction-center#term","name":"reaction center","description":"A reaction center is the pigment-protein core of a photosystem where light energy first excites electrons. 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