Photosynthesis Lecture 7 Fall 2008 Photosynthesis Photosynthesis The process by which light energy from the sun is converted into chemical energy 1 Photosynthesis Inputs CO 2 Gas exchange occurs through stomata Stomata (stoma) small pore CO 2 in, O 2 out H 2 O enters roots from soil Energy from sunlight Also need minerals from soil e.g. potassium (K), nitrogen (N), and phosphorous (P) Also need O 2 from soil 2 The Chloroplast 3 Fig.10.3 The Chloroplast 4 Photosynthetic prokaryotes 5 Fig.10.3 Structure Outer membrane Inner membrane Stroma Thick fluid inside inner membrane Thylakoids Membrane bound sacs Interconnected Photosynthetic pigments embedded in membranes Thylakoid space Interior of the thylakoid Grana (granum) Stacks of thylakoids Large surface area Infoldings of plasma membrane allow for specialized functions Endosymbiosis of photosynthetic prokaryote led to chloroplast
6 Photosynthesis & Cellular Respiration 7 Read Tracking Atoms through Photosynthesis: Scientific Inquiry, pgs. 187-188 Cellular respiration Redox reactions move electrons (and hydrogen) from glucose to oxygen fall of electrons Produces energy in the form of ATP Photosynthesis Redox reactions move electrons (and hydrogen) from water to carbon dioxide to form glucose Electrons moved uphill Requires large initial investment of energy (sunlight) Produces energy in the form of glucose molecules Photosynthesis Overview 8 Photosynthesis Overview 98 Two metabolic stages of photosynthesis Each process occurs in a specific area Light Reactions In the thylakoids Calvin cycle In the stroma Fig. 10.5 Light Reactions Convert solar energy to chemical energy ATP & NADPH Water split Calvin cycle Synthesizes sugar from CO 2 Uses the ATP & NADPH produced in the light reactions Fig. 10.5 The Nature of Sunlight 10 The Nature of Sunlight 11 Electromagnetic energy (electromagnetic radiation) Radiation = emission of energy in the form of electromagnetic waves or photons Wavelength - distance between the crests of two adjacent waves Photon - discrete packet of energy Electromagnetic spectrum Range of wavelengths of electromagnetic energy Gamma rays Short waves High energy Radio waves Long waves Low energy Fig. 10.6 Sunlight radiates the full spectrum Our atmosphere filters out much of the spectrum Visible light Passes through atmosphere Light that humans can see with our eyes (colors) Wavelengths that powers photosynthesis Fig. 10.6
The Nature of Sunlight When light meets matter: Reflected Wavelengths bounce back from matter Transmitted Wavelengths pass through matter Absorbed Wavelengths disappear into matter Pigments Chemical compounds that absorb certain wavelengths of light We only see color of wavelength that is reflected or transmitted If a pigment absorbs all wavelengths, then we see black If a pigment absorbs wavelengths from 380 to 550, what color would we see? 12 Which Wavelengths are Used in Photosynthesis: The Scientific Method at Work Question: Which wavelengths are used in photosynthesis? Observations: Photosynthetic organisms use visible light from the sun Visible light comes in many wavelengths By using a prism, light can be separated into its wavelengths Unicellular algae are photosynthetic organisms Bacteria tend to gather in areas of high oxygen 13 Which Wavelengths are Used in Photosynthesis: The Scientific Method at Work Hypothesis: Algae will photosynthesize when exposed to its ideal wavelengths Predictions: If the algae photosynthesize in response to a particular wavelength, then O 2 will be released in that area If O 2 is produced in an area, then aerobic bacteria will gather in that area 14 Which Wavelengths are Used in Photosynthesis: The Scientific Method at Work Methods: Algae placed in strip on microscope slide Bacteria add to slide Light shown through a prism onto slide 15 Which Wavelengths are Used in Photosynthesis: The Scientific Method at Work 16 Why are Leaves Green? 17 Results? Conclusions? Inside chloroplasts are photosynthetic pigments Pigments chemical compounds that absorb certain wavelengths of light Chlorophyll a absorbs blue-violet and red light Given that info why are leaves green? Fig. 10.9 Fig. 10.7
Photosynthetic Pigments in Chloroplasts 18 Photosynthetic Pigments in Chloroplasts 19 Chlorophyll a Required for photosynthesis Absorbs blue-violet and red light Accessory pigments Pigments other than chlorophyll a Broadens the spectrum of light that can be absorbed & used for photosynthesis Used as a sunscreen protection against UV radiation Coloration attract pollinators to flowers, attract fruit dispersers to fruit Accessory pigments Chlorophyll b Present in plants, some algae Absorbs blue and orange light Carotenoids Present in plants, algae, cyanobacteria Absorb blue-green light Chlorophyll c Present in some algae Why would so many different pigments evolve? Fig. 10.9 The structure of thylakoids and position of pigments critical to function Pigments arranged into photosystems Photosystem reaction center plus light harvesting complexes within the plasma membrane Fig. 7.10 20 Reaction center complex Protein complex with: Special chlorophyll a molecule (2) Primary electron acceptor Light harvesting complex Protein complex with many photopigments (chl a, b, carotenoids) Able to harvest light over broader spectrum How light is harvested Photon absorbed by a pigment molecule Energy transferred from one pigment molecule to another Energy ultimately passed to chl a in reaction center Fig. 10.12 21 Light Energy 22 23 What happens when energy from photon arrives at the reaction center? Electron from chl a is excited What is an excited electron? Electron receives energy and move to an excited state Higher orbital more potential energy Unstable position, so electron falls back down to ground state Process releases energy: Heat Light - florescence If there is a molecule to receive the electron, it retains its high energy and does not fall to the ground state Fig. 10.11 What happens when energy arrives at the reaction center? Electron from chl a is excited Electron passed to the primary electron acceptor Redox reaction Two paths for electrons, depending on photosystem type: Creates NADPH Gets passed to electron transport chain
24 PS1 (NADPH-producing photosystem) 25 Two types: Photosystem 2 or PSII Water-splitting photosystem Photosystem 1 or PS1 NADPH-producing photosystem Electrons from reaction center chl a excited P700 Passed to primary electron acceptor Primary electron acceptor passes electrons ferredoxin (FD) FD transfers electron to NADP+ NADP+ reduced to NADPH Requires NADP+ reductase 2 electrons NADPH will take electrons to the Calvin cycle Energy to produce sugar PS1 (NADPH-producing photosystem) Problem: If the electrons from the reaction center chl a get passed on to an electron acceptor, how do they get replaced? 26 Which is the stronger electron acceptor? 1) the reaction center chl a in NADPH-producing photosystem (PS1) Or 2) the primary electron acceptor in the water-splitting photosystem (PS2) 27 28 PS2 - Water-splitting photosystem 29 Electrons from water- splitting photosystem (PS2) pulled down electron transport chain by the reaction center chl a in the NADPH- producing photosystem (PS1) Electrons from reaction center chl a excited P680 Passed to primary electron acceptor How do the electrons get replaced? Take electrons from H 2 O Water-splitting step Requires enzyme O 2 forms & 4H+ What is the strongest electron acceptor (oxidizing agent) in these photosystems?
PS2 - Water-splitting photosystem Primary electron acceptor passes electron to electron transport chain Replaces electron in P700 chl Entire process (PS2 PS1) called linear electron flow 30 Electron Transport Chain ETC composed of many protein complexes embedded in the thylakoid membrane Plastoquinone (Pq) Cytochrome complex Plastocyanin (Pc) What benefit is gained from being in the thylakoid membrane? 31 Fig. 10.17 Electron Transport Chain 32 Electron Transport Chain 33 Electrons provide by primary electron acceptor of PSII Electrons fall down chain Pulled by P700 chl Produces energy at each step Transfer of electrons activates transfer of H+ H+ moved from stroma, across the thylakoid membrane, and into the thylakoid space Creates a concentration gradient of H+ across the thylakoid membrane Fig. 10.17 Fig. 10.17 ATP Production 34 ATP Production 35 ATP synthase Complex of proteins built into the inner membrane of the thylakoid Chemiosmosis Concentration gradient of H+ harnessed to do cellular work Proton-motive force The thylakoid membranes are not freely permeable to H+ Path down concentration gradient is through ATP synthase As H+ travels through ATP synthase, it causes turbine-like structures to turn, activating enzymes Enzymes generate ATP from ADP + P = phosphorylation Photophosphorylation From light Fig. 9.14 ATP synthase H+ ions enter through half- channel on stator Enter binding sites on rotor Changes conformation, rotor spins One rotation, H+ exits through halchannel Rotor spin causes rod to spin Spinning rod activates catalytic sites on knob ATP produced from ADP + P Read Fig. 9.15 Inquiry Fig. 9.14
Cyclic Electron Flow 36 Light Reactions Summary 37 FD passes electron to cytochrome complex Produces ATP Light Reactions Convert solar energy to chemical energy ATP & NADPH Requires H 2 O Produces O 2 as waste product Fig. 10.15 Fig. 10.5 38 39 Calvin cycle Synthesizes sugar from CO 2 Anabolic Inputs ATP NADPH CO 2 Output Organic compound - G3P (glyceraldehyde 3- phosphate) Used to make glucose and other compounds Fig. 10.5 3 Phases: Carbon Fixation Reduction Regeneration of RUBP 40 41 Carbon fixation CO 2 enters one at a time Attached to ribulose bisphosphate (5-carbon sugar) to become 6-carbon intermediary Enzyme: rubisco (RuBP carboxylase) Most abundant enzyme Splits into 2 molecules of 3-phosphoglycerate Reduction 3- phosphoglycerate phosphorylated by ATP 1,3 bisphosphoglycerate 1,3 bisphosphoglycerate reduced by NADPH G3P (glceraldehyd-3- phosphate) High potential energy One molecule of G3P is output Fig. 10.18 Fig. 10.18
Regeneration of RuBP 5 molecules of G3P rearranged into 3 molecules of RuBP Requires ATP 42 After the Calvin Cycle What happens to the G3P produced in photosynthesis? Transport to other cells 1. Produces glucose & fructose 2. They combine to form sucrose 3. Sucrose transported to other cells If growing cell Sucrose broken down to glucose & fructose & used in cellular respiration & growth If storage cell Sucrose converted to starch Starch production in photosynthetic cell Starch broken down overnight to supply cellular respiration 43