Photosynthesis Chapter 8 PDF

CHAPTER 8 PHOTOSYNTHESIS 1 1. Overview of Photosynthesis Learning outcomes: Understand the general equation of photosynthesis Explain how photosynthesis powers the biosphere Describe the chloroplast structure Compare 2 phases of photosynthesis: Light & Dark Reactions...

CHAPTER 8 PHOTOSYNTHESIS 1 1. Overview of Photosynthesis Learning outcomes: Understand the general equation of photosynthesis Explain how photosynthesis powers the biosphere Describe the chloroplast structure Compare 2 phases of photosynthesis: Light & Dark Reactions 2  Trophic organization Heterotroph Must eat food, organic molecules from their environment, to sustain life Autotroph Make organic molecules from inorganic sources Photoautotroph Use light as a source of energy Green plants, algae, cyanobacteria ? ? 3  Photosynthesis The green portions of plants, particularly leaves, carry on photosynthesis. Energy within light is captured and used to synthesize carbohydrates CO2 + H2O + light energy → C6H12O6 + O2 + H2O CO2 is reduced H2O is oxidized Biosphere Regions on the surface of the Earth and in the atmosphere where living organisms exist Largely driven by the photosynthetic power of green plants Cycle where cells use organic molecules for energy and plants replenish those molecules using photosynthesis Plants also produce oxygen 4  Chloroplast Organelles in plants and algae that carry out photosynthesis Chlorophyll: green pigment Majority of photosynthesis occurs in leaves in central mesophyll Stomata: small openings allowing carbon dioxide to enters and oxygen to exits leaf Water moves up from roots into vascular tissue (up stem) 5  Chloroplast anatomy CO2 and water diffuse into chloroplasts. The material within the chloroplast and surrounded by a double membrane is called the stroma where carbon dioxide is first attached to an organic compound and then reduced to a carbohydrate. The stroma contain a concentrated mixture of enzymes. Inner membrane system within the stroma form flattened sacs called thylakoids which are the site of photosynthesis. The thylakoids are arranged in stacks called grana (singular: granum). Chlorophyll and other pigments within thylakoid membranes are capable of absorbing solar energy representing the energy that drives photosynthesis. 6  2 stages of photosynthesis Two sets of reactions (1905 – Blackman) Light Reaction : - Chlorophyll absorbs solar energy and energizes electrons. - Energized electrons move down an electron transport system. - Energy is captured and later used for ATP production. - Energized electrons are also taken up by NADP+, an electron carrier. After NADP+ accept electrons, it becomes NADPH. Solar energy  ATP, NADPH Calvin Cycle Reaction (Dark reaction): - CO2 is taken up in the stroma and reduced to a carbohydrate. - Reduction requires ATP and NADPH. 7 2. Reactions that harness light energy Learning outcomes: Describe general properties of light Explain how pigments absorb light energy Describe the photosystems and their function Describe cyclic photophosphorylation that produces only ATP 8  Light energy Visible light is a part of a larger spectrum of radiation called the electromagnetic spectrum. Electromagnetic radiation travels as waves Short to long wavelengths Also behaves as particles- photons Shorter wavelengths have more energy 9 Photosynthetic pigments absorb some light energy and reflect others Leaves are green because they reflect green wavelengths Absorption boosts electrons to higher energy levels Wavelength of light that a pigment absorbs depends on the amount of energy needed to boost an electron to a higher orbital After an electron absorbs energy, it is an excited state and usually unstable Releases energy as: Heat and Light Excited electrons in pigments can be transferred to another molecule or “captured” Captured light energy can be transferred to other molecules to ultimately produce energy intermediates for cellular work 10  Pigments Photosynthetic Pigments: - Pigments are molecules that absorb wavelengths of light. - Pigments found in chloroplasts (chlorophyll a and b and carotenoids) absorb various portions of visible light. chlorophyll a and b have an important roles in photosynthesis (absorb violet, blue, red) Carotenoids: important in fall (when chlorophyll Chlorophyll a breaks down) and absorb violet-blue-green range. Chlorophyll b - Most pigments absorb only some wavelengths of light and reflect or transmit the other wavelengths. Absorption Spectra Organic molecules and processes within organisms are chemically adapted to visible light. Carotenoids 11  Absorption vs. action spectrum Absorption spectrum Wavelengths that are absorbed by different pigments in the plant Action spectrum Rate of photosynthesis by whole plant at specific wavelengths 12 13 3. Molecular features of Photosystems (PS) Learning outcomes: Explain how PSII captures light energy and produces oxygen Diagram the variation in the energy of an electron as it moves from PSII to PSI to NADP+ 14  Photosystems Thylakoid membrane Photosystem I (PSI) Photosystem II (PSII) 15  Photosytem II (PSII) 2 main components: Light-harvesting complex or antenna complex in the thylakoid membrane Directly absorbs photons Energy transferred via resonance energy transfer Reaction center P680 →P680* Relatively unstable Transferred to primary electron acceptor Removes electrons from water to replace oxidized P680 Oxidation of water yields oxygen gas 16  Photosystem II (PSII) Electrons accepted by primary electron acceptor enters an electron transport system chain located in the thylakoid membranes and then transferred to a pigment molecule in the reaction center of PSI. Electron releases some of its energy along the way Establishes H+ electrochemical gradient ATP synthesis uses chemiosmotic mechanism similar to mitochondria 17 Key role to make NADPH Photosystem I (PSI) Light striking light-harvesting complex of PSI transfers energy to a reaction center High energy electron removed from P700 and transferred to a primary electron acceptor NADP+ reductase NADP+ + 2 electrons + H + → NADPH Plastoquinone = Q Plastocycnine = pc P700+ replaces its electrons from plastocyanin Ferridoxin = Fd No splitting water, no oxygen gas formed 18  Summary 1. O2 produced in thylakoid lumen by oxidation of H2O by PSII 2 electrons transferred to P680+ 2. ATP produced in stroma by H+ electrochemical gradient 1. Splitting of water places H+ in the lumen 2. High-energy electrons move from PSII to PSI, pumping H+ into the lumen 3. Formation of NADPH consumes H+ in the stroma 3. NADPH produced in the stroma from high-energy electrons that start in PSII and boosted in PSI NADP+ + 2 electrons + H + → NADPH 19  Cyclic and noncyclic electron flow Noncyclic Electrons begin at PSII and eventually transfer to NADPH Linear process produces ATP and NADPH in equal amounts Cyclic photophosphorylation Electron cycling releases energy to transport H+ into lumen driving synthesis of ATP Both pathways produce ATP, but only the noncyclic pathway also produces NADPH. 20  Noncyclic Electron Pathway Electron flow can be traced from water to a molecule of NADPH. This pathway uses two photosystems, PS I and PS II. Photosystem consists of:. Pigment complex ( chlorophyll a, b and the carotenoids). The pigment complex helps gather solar energy. Electron acceptor molecules in the thylakoid membrane. summary of the light reactions  Calvin cycle ATP and NADPH used to make carbohydrates Somewhat similar to citric acid cycle CO2 incorporated into carbohydrates Precursors to all organic molecules Energy storage CO2 incorporation Also called Calvin-Benson cycle Requires massive input of energy For every 6 CO2 incorporated, 18 ATP and 12 NADPH used Glucose is not directly made 23  The three phases of Calvin Cycle 1. Carbon Dioxide Fixation CO2 (x6) is attached to Ribulose BiPhosphate (RuBP). The result is a 6-carbon molecule (x6). The Rubisco Enzyme (Ribulose BiPhosphate carboxylase) speeds up this reaction. 2. Reduction of Carbon Dioxide Each of the resulting 6-carbon molecules (x6) formed by carbon dioxide fixation splits into two 3-carbon molecules (phosphoglycerate [PGA]) (x6). The two molecules of PGA (x6) are reduced to form PGAL (phosphoglyceraldehyde). The PGAL molecules also have three carbon atoms each. This reaction requires energy from ATP and electrons from NADPH. The two molecules of PGA (x6) are reduced to form Mader: Biology PGAL (phosphoglyceraldehyde). The PGAL molecules 8th Ed. – also have three carbon atoms each. This reaction Modified by requires energy from ATP and electrons from NADPH. Taoufik Salah Ksiksi Two of the PGAL molecules are used to form one glucose molecule (C6). Ten PGAL remain. 3. Regeneration of RuBP The remaining 10 PGAL (3 carbons each, total = 30 carbons) can therefore be reassembled into 6 RuBP (5 carbons each, total = 30 carbons). This rearrangement uses 6 ATP. For each six CO2 molecules that enter the cycle one glucose molecule is produced. 26