Week 11 Biology: Photosynthesis PDF

Biology Topic D: Photosynthesis By: Assoc. Prof. Rasha M. Abu El Khair Associate professor of Pharmacognosy and Vice Dean for Education affairs, College of Pharmacy, AASTMT The Light Reactions The Light Reactions Occur in Association with the Thylakoid Membranes      The light reaction...

Biology Topic D: Photosynthesis By: Assoc. Prof. Rasha M. Abu El Khair Associate professor of Pharmacognosy and Vice Dean for Education affairs, College of Pharmacy, AASTMT The Light Reactions The Light Reactions Occur in Association with the Thylakoid Membranes      The light reactions occur in and on the thylakoid membranes. These membranes contain many photosystems, each consisting of a cluster of chlorophyll and accessory pigment molecules surrounded by various proteins. There are two types of photosystems—photosystem II and photosystem I—that work together during the light reactions. The photosystems are named according to the order in which they were discovered, but the light reactions start with photosystem II and then proceed to photosystem I. Adjacent to each photosystem is an electron transport chain (ETC) consisting of a series of electron-carrier molecules embedded in the thylakoid membrane. Photosystem II and Its Electron Transport Chain Capture Light Energy, Create a Hydrogen Ion Gradient, and Split Water  The light reactions begin when photons of light are absorbed by pigment molecules clustered in photosystem II 1 .  The energy hops from one pigment molecule to the next until it is funneled into the photosystem II reaction center 2 .  The reaction center of each photosystem consists of a pair of specialized chlorophyll a molecules and a primary electron acceptor molecule embedded in a complex of proteins.  When the energy from light reaches the reaction center, it boosts an electron from one of the reaction center chlorophylls to the primary electron acceptor, which captures the energized electron 3 .  For photosynthesis to continue, the electrons that were boosted out of the reaction center of photosystem II must be replaced.  The replacement electrons come from water.  Water molecules are split by an enzyme associated with photosystem II, liberating electrons that will replace those lost by the reaction center chlorophyll molecules.  Splitting water also releases two hydrogen ions, and for every two water molecules split, one molecule of O2 is produced     Once the primary electron acceptor in photosystem II captures the electron, it passes the electron to the first molecule of the adjacent ETC in the thylakoid membrane 4 . The electron then travels from one electron carrier molecule to the next, releasing energy as it goes. Some of this energy is harnessed to pump H+ across the thylakoid membrane and into the thylakoid space, where it contributes to the H+ gradient that generates ATP Finally, the energy-depleted electron leaves the ETC and enters the reaction center of photosystem I, where it replaces the electron ejected when light strikes photosystem I 6 . The Hydrogen Ion Gradient Generates ATP by Chemiosmosis  As an energized electron travels along the ETC associated with photosystem II, it releases energy in steps.  Some of this energy is harnessed to pump H+ across the thylakoid membrane and into the thylakoid space .  This creates a high concentration of H+ inside the space and a low concentration in the surrounding stroma.  During chemiosmosis, H+ flows back down its concentration gradient through a special type of channel called ATP synthase that spans the thylakoid membrane.  ATP synthase produces ATP using ADP and phosphate dissolved in the stroma 3  It takes the energy from about three H+ passing through ATP synthase to synthesize one ATP molecule. Photosystem I and Its Electron Transport Chain Generate NADPH  light has also been striking the pigment molecules of photosystem I.  This light energy is passed to a chlorophyll a molecule in the reaction center 6 .  Here, it energizes an electron that is absorbed by the primary electron acceptor of photosystem I 7 .  (This energized electron is immediately replaced by an energydepleted electron from the first electron transport chain.)  From the primary electron acceptor of photosystem I, the energized electron is passed to a second ETC adjacent to photosystem I in the thylakoid membrane  Here, the final electron carrier is an enzyme that catalyzes the synthesis of NADPH.  To form NADPH, the enzyme combines NADP+ and H+ (both dissolved in the stroma) with two energetic electrons from the ETC 9 Cyclic photophosphorylation  cyclic photophosphorylation: the production of ATP using energy from light, involving only photosystem I Non-cyclic photophosphorylation  non-cyclic photophosphorylation: (Z- scheme) the production of ATP using energy from light, involving both photosystem I and photosystem II  this process also produces reduced NADP Photolysis of water     Photosystem II includes a water-splitting enzyme that catalyses the breakdown of water. This enzyme is sometimes known as the oxygen-evolving complex, or the water-splitting complex. It splits a water molecule into hydrogen ions (protons), electrons and oxygen Oxygen is a waste product of this process. It diffuses out of the chloroplast and is eventually lost from the cell or used in a mitochondrion for aerobic respiration. The hydrogen ions combine with electrons from photosystem I and the coenzyme molecule NADP to give reduced NADP Dark Reactions, The Calvin Cycle: How is Chemical Energy Stored in Sugar Molecules?  The ATP and NADPH synthesized during the light reactions are dissolved in the fluid stroma that surrounds the thylakoids.  There, these energy carriers power the synthesis of the threecarbon sugar glyceraldehyde-3-phosphate (G3P) from CO2 in the Calvin cycle.  This metabolic pathway is described as a “cycle” because it begins and ends with the same five-carbon molecule, ribulose bisphosphate (RuBP)  The Calvin cycle is best understood if we divide it into three parts:  (1) carbon fixation,  (2) the synthesis of G3P  (3) the regeneration of RuBP that allows the cycle to continue Carbon fixation     The first step is the reaction of carbon dioxide with a fivecarbon (5C) compound called ribulose bisphosphate (RuBP). This reaction is catalysed by an enzyme called ribulose bisphosphate carboxylase, or rubisco. The enzyme rubisco combines three CO2 molecules with three RuBP molecules to produce three unstable six-carbon molecules that immediately split in half, forming six molecules of phosphoglyceric acid (PGA, a three-carbon molecule) Because carbon fixation generates this three-carbon PGA molecule, the Calvin cycle is often referred to as the C3 pathway. The synthesis of G3P  The synthesis of the simple three-carbon sugar G3P occurs via a series of reactions using energy donated by ATP and NADPH.  During these reactions, six three-carbon PGA molecules are rearranged to form six three-carbon G3P molecules The regeneration of RuBP  Five of the six G3P molecules are used to regenerate three five-carbon RuBP molecules, using ATP generated during the light reactions  The single remaining G3P molecule exits the Calvin cycle Photorespiration  Carbon fixation, the first step in the Calvin cycle, can be disrupted by O2.  The enzyme rubisco that fixes carbon is not completely selective, and it will allow O2 instead of CO2 to combine with RuBP.  When O2 replaces CO2, the result is a wasteful process called photorespiration, which reduces the rate of carbon fixation by roughly 33%. Carbon Fixed During the Calvin Cycle Is Used to Synthesize Glucose  In reactions that occur outside of the Calvin cycle, two three-carbon G3P sugar molecules can be combined to form one six-carbon glucose molecule  Glucose can then be used to synthesize sucrose (table sugar), a disaccharide storage molecule consisting of a glucose linked to a fructose.  Glucose molecules can also be linked together in long chains to form starch (another storage molecule) or cellulose (a major component of plant cell walls).  Some plants convert glucose into lipids for storage.  Glucose is also broken down during cellular respiration to provide energy for the plant’s cells