Photosynthesis & Cellular Respiration - Light Dependent Reaction PDF

Unit c: Photosynthesis & Cellular Respiration ~ BIO 20 ~  Lesson 3: Light Dependent Reaction The following questions will be explored in this lesson: 1. How is solar energy captured and transferred to electrons and splits water? 2. How is ATP and...

Unit c: Photosynthesis & Cellular Respiration ~ BIO 20 ~  Lesson 3: Light Dependent Reaction The following questions will be explored in this lesson: 1. How is solar energy captured and transferred to electrons and splits water? 2. How is ATP and NADPH made?  Photosynthesis Occurs in chloroplasts of plants Combines CO2, H2O, and energy from sun to synthesize C6H12O6. O2 is a by-product of photosynthesis involves two separate sets of reactions: 1)Light-Dependent Reactions (PHOTO) 2)Light-Independent Reactions (SYNTHESIS)  Photosynthesis Overview 2 reactions and 3 stages of Photosynthesis: 1. Light Dependent Reaction (in thylakoid membranes) Stage 1 – capturing solar energy and transferring it to electrons through splitting of H2O Stage 2 – using energy to make ATP and transferring electrons to make NADPH Light Independent Reaction (occurs in stroma) Stage 3 – using energy in ATP and high energy electrons in NADPH to form glucose from CO2 Called Carbon Fixation – occurs through Calvin  Photosynthesis Requirements Products Sunlight ATP LIGHT-DEPENDENT NADPH H 2O O2 ATP CO2 LIGHT-INDEPENDENT G3P NADPH RuBP light-dependent reactions: solar energy trapped by chlorophyll and used to generate two high-energy compounds, ATP and NADPH. light-independent reactions: energy of ATP and reducing power of NADPH are used to reduce carbon dioxide and  Light-Dependent Reactions first step of photosynthesis require energy from the sun to take place Main Purpose: to synthesize ATP and NADPH in stroma of chloroplast reactions also release oxygen as a by-product. energy from ATP & NADPH is used to synthesize glucose via the light-independent reactions. Light Dependent Reaction Occurs in thylakoid membrane!  Light Dependent Reaction Photosystem – clusters of chlorophyll and other pigments embedded in thylakoid membrane. Chloroplast have two different photosystems: Photosystem I (PSI) Photosystem II (PSII) Comes first in light reaction! Light Dependent Reaction Stage 1: Capture Solar Energy (link) Photosystem II: Light strikes chloroplast  photons excite chlorophyll molecules in thylakoid membranes Chlorophyll captures light energy by absorbing photons and passing the energy to electrons Energy is transferred to the reaction center. Light Dependent Reaction Stage 1: Capture Solar Energy - Photolysis Light energy causes photolysis (splits water) Photolysis occurs in the thylakoid lumen, water is divided into hydrogen ions, oxygen and electrons Oxygen is made and released into atmosphere (leaves through the stoma!) H+ ions are formed and remain the lumen Light Dependent Reaction Stage 1: Capture Solar Energy -Electron Excitation Electrons are excited and move from the lumen to chlorophyll molecules in photosystem II, which is found in the thylakoid membrane Electrons are then transferred to photosystem I via the electron transport chain. Recall: How does the electron move? Pigments in photosystem II captures light energy and passes this energy to an electron (that are residing in the main chlorophyll molecule) The electron is then moved down the ETC in a staircase manner (not a hill  doesn’t go fast) Light Dependent Reaction (link) Electron acceptor carrying molecules help the electron move down the ETC between photosystems Electrons are passed from one molecule to the next towards PSI (each molecule has a stronger pull on the electron and can steal it from the previous acceptor What has been produced thus far? Photons have caused: Hydrolysis  Splitting of H2O  creating O2 and H+ ions Which donates electron back to PSI Oxygen is then released to environment H+ accumulate inside thylakoid lumen This creates concentration gradient! more on why this is important next! The excitement of electrons…. …which brings us to stage  Stage 2: Making ATP and NADPH Electrons move from PSII to PSI  Every step down the excited electron takes, it gives up some stored (potential) energy  this energy is used to make ATP (we will come back to this!) Once the electron reaches PSI, it is hit with light again and excited. The electron leaves PSI and moves down the ETC once again, releasing energy with every step Light Dependent Reaction Stage 2: Making NADPH This time, the energy is used for something else… When the electron reaches the bottom of the electron transport chain, the energy released by photosystem l is used to rejoin the high energy electrons with the hydrogen ions and NADP+ to produce NADPH, the final electron acceptor Light Dependent Reaction Stage 2: Making NADPH NADP+ gains electrons to form NADPH  REDUCTION The NADPH carries the electrons, and their energy to the Light Independent Reaction  Stage 3 – the Calvin Benson Cycle (next class!) Stage 2: Making ATP How does the energy released between PSII and PSI make ATP? It is not a direct process, there are many steps that must occur before ATP is produced! Let’s backtrack! Light Dependent Reaction Stage 2: Making ATP SOLAR ENERGY H2O Light Dependent Reaction Stage 2: Making ATP When the electrons are passed down the ETC from PSII to PSI, the energy released is used to pull (against the concentration gradient) H+ in the thylakoid membrane. As H+ ions build up inside the thylakoid lumen, they create a positive charge in the thylakoid lumen and a steep Light Dependent Reaction Stage 2: Making ATP Even though H+ ions would likely diffuse back across, they cannot because the membrane is impermeable to them. So, they are stuck there! How do they get out?  Chemiosmosis! Light Dependent Reaction Stage 2: Making ATP Because the membrane is impermeable to H+, a special structure called ATP synthase, embedded in the thylakoid membrane, provides the only pathway for the H+ to move out. The process for synthesizing ATP using the energy from a H+ gradient and the ATP synthase enzyme is called Light Dependent Reaction Stage 2: Making ATP The movement of H+ through the complexes releases energy which is used to combine ADP and Pi into ATP Energy from light, indirectly, phosphorylates ADP ATP Light Dependent Reaction Summary What is required at beginning? Light energy, water What is produced at the end? Oxygen, NADPH, ATP Video Introduction to Light Dependent Reaction: http://www.youtube.com/watch?v=g78u tcLQrJ4