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Plant Biology - Photosynthesis

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    Study Guide Plant Biology – Photosynthesis 1. Details of Photosynthesis in Plants 1.1 Introduction to Photosynthesis Photosynthesis is a series of chemical reactions that help plants make their own food. These reactions happen at specific places inside plant cells and at specific times. Special proteins called enzymes control and speed up all these reactions. Although we usually study photosynthesis in plants, the basic process is similar in algae and cyanobacteria . Many steps happen extremely fast — within nanoseconds — but learning them step by step makes the process easier to understand. Photosynthesis mainly takes place inside chloroplasts , and it can be divided into two major stages: 1. Light reactions (energy - transferring reactions) 2. Carbon fixation reactions (Calvin cycle)

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    Study Guide 1.2 Light Reactions of Photosynthesis (Energy - Transferring Reactions) Photosystems: Light - Catching Units Inside the chloroplast, light energy is absorbed by pigment molecules arranged in the thylakoid membrane . These pigments form groups called photosystems . Each chloroplast has hundreds of photosystems. Each photosystem has: • An antenna complex (200 – 300 pigment molecules) that collects sunlight • A reaction center where energy conversion begins Light energy moves from one pigment to another without loss. This is called resonance energy transfer .

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    Study Guide 1.3 Types of Photosystems There are two photosystems : 1. Photosystem II (PSII) • Reaction center pigment: P680 • Absorbs light best at 680 nm • Starts the light reactions 2. Photosystem I (PSI) • Reaction center pigment: P700 • Absorbs light best at 700 nm Both photosystems are needed because one alone cannot produce enough energy for the plant’s needs. 1.4 Events in Photosystem II When light hits P680 , one of its electrons becomes energized and leaves the chlorophyll molecule. This electron enters an electron transport chain made of molecules that pass electrons step by step. As the electron moves: • Energy is released gradually • This energy is used to pump H ⁺ ions (protons) into the thylakoid space Splitting of Water (Photolysis) When P680 loses an electron, it becomes unstable. It replaces the lost electron by splitting water molecules . From two water molecules: • Oxygen (O ₂ ) is released • 4 protons (H ⁺ ) are produced • 4 electrons replace those lost by PSII This process is the main source of oxygen in Earth’s atmosphere .

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    Study Guide 1.5 ATP Formation (Photophosphorylation) The build - up of protons creates a proton gradient . When protons flow back into the stroma through ATP synthase , energy is released. This energy is used to convert: • ADP + Pi → ATP This process is called photophosphorylation because: • Light energy is used • A phosphate group is added 1.6 Events in Photosystem I Electrons from PSII reach Photosystem I and replace electrons lost by P700 . When P700 absorbs light: • Its electrons become excited • They pass through another electron transport chain Finally, the electrons are transferred to NADP ⁺ , which combines with H ⁺ to form NADPH . NADPH is important because it provides reducing power for making carbohydrates later. 1.7 Cyclic and Non - Cyclic Photophosphorylation Non - Cyclic Photophosphorylation • Uses PSII and PSI • Produces ATP, NADPH, and O ₂ • Electrons do not return to the original chlorophyll Cyclic Photophosphorylation • Uses PSI only • Produces ATP only

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