Photosynthesis Lesson 3_1P91 Student 1 PDF

PHOTOSYNTHE SIS Dr. Szuroczki Chapter 8 Key Concepts 1. Overview of Photosynthesis 2. Reactions That Harness Light Energy 3. Synthesizing Carbohydrates via the Calvin Cycle 4. Variations in Photosynthesis Photosynthesis Is the process by which some organisms capture the energy from the...

PHOTOSYNTHE SIS Dr. Szuroczki Chapter 8 Key Concepts 1. Overview of Photosynthesis 2. Reactions That Harness Light Energy 3. Synthesizing Carbohydrates via the Calvin Cycle 4. Variations in Photosynthesis Photosynthesis Is the process by which some organisms capture the energy from the sun (solar) and transform it into energy (chemical) that can be used by living things Producer – an organism that makes its own energy-rich food compounds using the Sun’s energy On land, major producers are green plants: contain chlorophyll, which captures light energy Autotrophs Organisms that make their own food are called autotrophs Phototrophs: use solar energy (photosynthesis) to get energy Convert H2O and CO2 into sugar and O2 Chemotrophs: use different chemical processes to get energy To Photosynthesize or not to Photosynthesize that is the question Many organisms cannot photosynthesize (done by plants) they are called consumers Consumers: an organism that obtains its energy from consuming other organisms To obtain usable energy from food, consumers undergo cellular respiration….. THEREFORE….. Heterotrophs Organisms that must take in food to meet their energy needs are called heterotrophs Consume autotrophs (herbivores), other heterotrophs (carnivores) or both (omnivores) for their energy needs Complex chemicals are broken down and reassembled into chemicals and structures needed by Dinosaur Extinctio n A HUGE asteroid is thought to have hit Earth near Mexico which sent up so much dust that it actually blocked the Sun for many years No Sun = No Food = No more dinosaurs! Photosynthesis powers the biosphere Biosphere: regions on the surface of the Earth and atmosphere where living organisms exist Largely driven by the photosynthetic power of plants, algae, and cyanobacteria Energy cycle: cells use organic molecules for energy and plants replenish those molecules using photosynthesis In the process plants also produce oxygen  Energy within light is captured and used to synthesize glucose and other organic molecules Photosy Two stages: 1. Light reactions: light nthesis energy is absorbed by chlorophyll and converted to ATP and NADPH 2. Dark reactions (Calvin Cycle): ATP and NADPH used to drive the synthesis of carbohydrates  Photosynthesis and Cellular RespirationPhotosynthesi slight energy carbon dioxide  water    sugar  oxygen sugar  oxygen  carbon dioxide  water  energy Cellular Respiration What is created in one reaction is used up in the other reaction! Chloroplast Organelle in plants and algae that carries out photosynthesis Green pigment is chlorophyll Majority of photosynthesis occurs internally in leaves, in the mesophyll Mesophyll cells must receive light, water, and carbon dioxide Carbon dioxide enters and oxygen exits leaf through pores called stomata Chloroplast anatomy Outer and inner membrane separated by intermembrane space A third membrane, the thylakoid membrane contains pigment molecules Membrane forms thylakoids Enclose thylakoid lumen Granum: stack of thylakoids Fluid filled region between thylakoid membrane and inner membrane is the stroma Reactions That Harness Light Energy During photosynthesis energy in the form of light is transferred from the sun to a pigment molecule in a plant Light is a type of electromagnetic radiation Travels as waves: Short to long wavelengths – distance between the peaks in a wave pattern Electromagnetic spectrum encompasses all possible wavelengths of electromagnetic radiation Reactions That Harness Light Energy Light also behaves as particles called photons: massless particles traveling in a wavelike pattern and moving at the speed of light Shorter wavelength radiation carries more energy per unit time than longer wavelength radiation Molecules can absorb the energy of visible light in a way that does not cause damage Photosynthetic Pigments Pigments absorb some light energy and reflect others: Leaves are green because they absorb red and violet, and 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 Having different pigments allows plants to absorb light at many different wavelengths Photosynthetic Pigments After an electron absorbs energy, it is an excited state and usually unstable Releases energy as heat or light Excited electrons in pigments can be transferred to another molecule or “captured” Structure of pigment molecules In plants different pigment molecules absorb the light energy used in photosynthesis Chlorophylls a and b contain a porphyrin ring A magnesium ion is bound to the porphyrin ring Carotenoids are another pigment in chloroplasts; often the major pigments in flowers and fruit Absorption versus action spectrum Absorption spectrum: Plots a pigment’s light absorption as a function of the light’s wavelength Chlorophylls absorb light strongly in the red and violet parts of the spectrum and reflect green Carotenoids absorb blue and blue-green visible light; reflect yellow and red Absorption versus action spectrum Action spectrum: Plots the rate of photosynthesis as a function of the wavelength of light Different pigments allow plants to absorb light at many different wavelengths The highest rates of photosynthesis in green plants correlate with the wavelengths that are strongly absorbed by chlorophylls and carotenoids Algal Photobioreactors Two stages of photosynthesis 1. Light reactions Use light energy Take place in thylakoid membranes Produce A T P, N A D P H and O2 2. Dark reactions (Calvin cycle) Occurs in stroma Uses A T P and N A D P H to incorporate CO2 into carbohydrate Light Reactions: Photosystems I and II Captured light energy can be transferred to other molecules to produce energy- intermediate molecules for cellular work Thylakoid membranes of chloroplast contain two distinct complexes of molecules: Photosystem I (P S I) – discovered first Photosystem II (P S I I) – first step in photosynthesis Light excites pigment molecules in both P S I I and P S I PSI PSII Photosystem II & Electron Transport Chain The initial step in photosynthesis 1. Light excites electrons in pigment molecules within the light-harvesting complex of P SII 2. Energy is transferred to P680 pigment molecule 3. Oxidizes water, generating O2 and H+ 4. Electrons exit PSII and enter an electron transport chain (ETC) 5. Energy used to make H+ electrochemical gradient Photosystem I Primary role to make N A D P H Light hits light-harvesting complex of P S I; high energy-electron is removed from P700 pigment molecule and transferred to a primary electron acceptor Ferredoxin then accepts two high-energy electron and transfers electrons to NADP+ reductase Electrons are transferred to NADP+ which accepts a H+ to produce NADPH Linear electron flow: electrons moved linearly from P S I I to P S I and reduce NADP+ to Formation of ATP in chloroplasts A TP synthesis in chloroplasts Achieved by chemiosmotic mechanism called photophosphorylation Driven by flow of H+ from thylakoid lumen into stroma via A T P synthase H+ gradient generated three ways: Increase H+ in thylakoid lumen by splitting of water Increase H+ by E T C pumping H+ into the lumen Increase H+ in stroma from formation of N A D P H Z scheme Zigzag shape of energy curve: Photosynthesis involves increases and decreases in the energy of an electron as it moves from P S I I through P S I to N A D P H Electron on a nonexcited pigment molecule in P S I I starts with the lowest energy Light excites the electron in P S I I Photosystem I boosts the electron to an even higher energy level Electron releases a little energy before it is transferred to NADP+ Three chemical products Generated 1. Oxygen: produced in thylakoid lumen by oxidation of water by PSII Two electrons are transferred to P680+ molecules Three chemical products Generated 2. NADPH: Produced in the stroma from high- energy electrons that start in PSII and are NADP  2 electron  H NADPH   boosted in PSI Three chemical products Generated 3. ATP: Produced in stroma by A T P synthase using the H+ electrochemical gradient Noncyclic and cyclic electron flow Noncyclic Electrons begin at PSII and eventually transfer to NA DPH, a linear process Produces both ATP and NADPH in equal amounts Noncyclic and cyclic electron flow Cyclic photophosphorylation (cyclic electron flow) Electron cycling releases energy to transport H+ into lumen driving ATP synthesis Produces only ATP PSI electrons excited, release energy and eventually return to PSI Favored when NADP+ is low and N ADP H is high Dark reactions: Synthesizing Carbohydrates via the Calvin Cycle Calvin Cycle (aka Calvin-Benson Cycle) CO2 incorporated into carbohydrates Precursors to other organic molecules Energy storage Requires massive input of energy: For every 6 CO2 incorporated, 18 ATP and 12 NADPH must be used Product is glyceraldehyde-3-phosphate (G3P) Glucose is later made from G3P in separate process Calvin cycle: 3 phases Phase 1: Carbon fixation CO2 incorporated into RuBP using RuB P carboxylase/oxygen ase (rubisco) Reaction product is a six-carbon intermediate that splits into two 3- phosphoglycerate molecules (3PG) RuBisCO Ribulose-1,5-bisphosphate carboxylase/oxygenase Enzyme involved in first major step of carbon fixation CO2 Glucose Photorespiration Rubisco functions as a carboxylase R u B P + CO2 → 2 3P G C3 plants make 3P G; 90% of plants on Earth are C3 plants Rubisco can also be an oxygenase Adds O2 to RuBP eventually releasing CO2 This is called photorespiration Using O2 and liberating CO2 is wasteful More likely in hot and dry environments Favored when CO2 low and O2 high Calvin cycle: 3 phases Phase 2: Reduction and carbohydrate production ATP is used to convert 3P G into 1,3- bisphosphoglycerate (1,3- BPG) NADPH electrons reduce it to glyceraldehyde-3- phosphate (G3P) 6 CO2 → 12 G3P Only 2 G3P molecules used for carbohydrates 10 G3P molecules must be used for regeneration of RuBP Calvin cycle: 3 phases Phase 3 – Regeneration of RuBP 10 G3P are converted into 6 RuBP using 6 A TP Calvin cycle beings by using carbon from an inorganic source and ends with organic molecules that will be used to make other molecules Variations in Photosynthesis Environmental conditions can influence both the efficiency and way the Calvin cycle works Light intensity Temperature Water availability Both depend on regulation of the stomata: tiny openings or pores in plant tissue that allow for gas exchange C4 plants Evolved a mechanism to minimize photorespiration C4 plants make oxaloacetate (4 carbon molecule) in the first step of carbon fixation Leaves have two-cell layer organization: Mesophyll cells: CO2 enters via stomata and 4 carbon compound formed (P EP carboxylase does not promote photorespiration) Bundle-sheath cells: 4 carbon molecule transferred that releases steady supply of CO2, minimizing photorespiration Which is better – C3 or C4? It depends on the environment In warm dry climates C4 plants conserve water and prevent photorespiration In cooler climates, C3 plants use less energy to fix CO2 90% of plants are C3 CAM plants (Crassulacean Acid Metabolism) Some C4 plants separate processes using time CAM plants open their stomata at night CO2 enters and is converted to malate Stomata close during the day to conserve water Oxaloacetate converted to malate Malate broken down into CO2 to drive Calvin cycle during the day Review Video