Lecture5BIO155BB - BIO155 Photosynthesis Dr Jessica Pamment...

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Unformatted text preview: BIO155 Photosynthesis Dr. Jessica Pamment Overview Overview • • • 1. 2. • Significance of photosynthesis Site of photosynthesis, the chloroplast Two stages of photosynthesis: Light reactions Calvin Cycle Alternative methods of carbon fixation Significance of Photosynthesis Significance of Photosynthesis • About 160 billion metric tons of organic material are produced on Earth each year by photosynthesis • All of the food consumed by humans can be traced to photosynthetic plants • Photosynthesis provides materials used for clothing, housing, pharmaceuticals and cooking • The chemistry of photosynthesis affects our global environment Green Energy Green Energy • • • • Wood burning was used by humans to produce heat and light before industrialization Use of fossil fuels took over With problems associated with use of fossil fuels, there is renewed interest in alternative sources, such as biomass Wood burning to produce electricity; renewable, few sulfur compounds, tree crops remove CO2 Energy Plantation Energy Plantation Photosynthesis Photosynthesis • Photosynthesis is the conversion of light energy into chemical energy in the form of sugars and other organic molecules • Plants are photoautotrophs PHOTOSYNTHETIC AUTOTROPHS Plants (mostly on land) Photosynthetic Protists (aquatic) Photosynthetic Bacteria (aquatic) Forest plants Kelp, a large alga Micrograph of cyanobacteria LM Leaf cross section Vein Mesophyll Stomata CO2 O2 Chloroplast Mesophyll cell Chloroplasts: Site of photosynthesis 5 µm Chloroplast Outer membrane Thylakoid Stroma Granum Thylakoid space Intermembrane space Inner membrane 1 µm Equation for Photosynthesis Equation for Photosynthesis Photosynthesis • During cellular respiration there is a ‘fall’ of electrons from food molecules to O2 to produce water and energy • During photosynthesis electrons are boosted ‘uphill’ and added to CO2 to produce sugar Photosynthesis Photosynthesis • • 6CO2 + 12H2O Light energy C6H12O6+6O2+ 6H2O Two processes: 1. The light reaction converts solar to chemical energy 2. Calvin Cycle is responsible for carbon fixation H2O Light NADP+ ADP + Pi Light Reactions Chloroplast H2O Light NADP+ ADP + Pi Light Reactions ATP NADPH Chloroplast O2 H2O CO2 Light NADP+ ADP + Pi Light Reactions ATP NADPH Calvin Cycle Chloroplast O2 H2O CO2 Light NADP+ ADP + Pi Light Reactions ATP NADPH Calvin Cycle Chloroplast O2 [CH2O] (sugar) Light Energy Light Energy • Light from the sun is a type of electromagnetic energy or radiation 750 nm • Visible light wavelength ranges from 380 nm to • Photons are discrete amounts of energy, used to describe amount of energy in light Light Reflected light Chloroplast Absorbed light Transmitted light Granum Investigation: Investigation What wavelengths drive photosynthesis The Process of Science The Process of Science • Observation: Certain bacteria cluster in areas of high O2 concentration • Question: Which wavelengths of light are best for promoting photosynthesis • Hypothesis: O2­seeking bacteria will congregate near regions of algae undergoing most photosynthesis The Process of Science The Process of Science • Experiment: algae on slide with bacteria. Shine spectrum of light on slide. • Results: most bacteria congregate around algae exposed to red­orange and blue­ violet light. Least in area of green light The Process of Science The Process of Science • Conclusion: Light in the violet­blue and red ends of the spectrum are the most effective in driving photosynthesis (this is where most of the bacteria congregated) Chloroplast Pigments • Chlorophyll a, absorbs mainly blue­violet and red light • Chlorophyll b, absorbs mainly blue and orange light • Carotenoids, absorb mainly blue­green light Carotenoid Pigments in Autumn Carotenoid Chlorophyll Chlorophyll • Photosynthesis depends on the structure of chlorophyll • Chlorophyll molecule is excited by a quantum of light (photon) • Excited molecule is unstable and will return to its original, unexcited state Excited state – Light Heat Light (fluorescence) Photon Chlorophyll molecule Ground state (a) Absorption of a photon, results in excited electrons Fig. (b) 7-08b Fluorescence of a glow stick Chlorophyll Chlorophyll 1. Extra energy converted to heat or combination of heat and light (fluorescence) 1. Extra energy transferred, but not the electron, to neighboring chlorophyll electron acceptor 1. High­energy electron transferred to an Photon Photosystem Light-harvesting complexes STROMA Primary electron acceptor Reaction-center complex Thylakoid membrane e– Special pair of chlorophyll a molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Photosystems Photosystems • These are multiprotein complexes that catalyze the conversion of the light energy captured in the excited chlorophyll molecules to useful forms Two components: Photochemical reaction centre Antenna complex/ light harvesting complex • i. ii. A Photosystem A Photosystem The Light Reaction of Photosynthesis The Analogy for Light Reactions Reactions Light Reactions Light Reactions • • Light reactions use solar energy to generate ATP and NADPH ATP and NADPH provide chemical energy and reducing power for the Calvin Cycle Thylakoid membrane: site of conversion from light to chemical energy light Light Reactions Light Reactions • Mechanism used for ATP production in thylakoids is similar to that used for cellular respiration in mitochondria Electron transport chain pumps hydrogen across membrane ATP synthase uses energy stored by H+ gradient to make ATP • • High energy Mitochondrion electrons come from food High energy electrons created using light Chloroplast MITOCHONDRION STRUCTURE Intermembrane space Inner membrane Electron transport chain ATP synthase Key CHLOROPLAST STRUCTURE H+ Diffusion Thylakoid space Thylakoid membrane Matrix ADP + P i H+ Stroma ATP Higher [H+] Lower [H+] Summary of Light Reactions Summary of Light Reactions • Take place in the thylakoid membrane • Light energy is captured by photosystems and used to transfer electrons from water to the top of the transport chain and ATP • The end result of the light reaction is NADPH The Calvin Cycle The Calvin Cycle • Similar to the Citric Acid cycle in that starting material is generated at the end of the cycle is making organic compounds, instead of breaking them down leaves in the form of sugar • Different from the Citric Acid cycle in that it • Carbon enters in the form of CO2 and Calvin Cycle: Making sugar from Carbon Dioxide ATP NADPH Calvin cycle ADP P NADP+ G3P P CO Glucose and other compounds Summary of The Calvin Cycle Summary of The Calvin Cycle • Doesn’t require light, but uses ATP and NADPH made during light reactions • Takes place in the stroma using electrons from NADPH and energy from ATP CO2 molecules fixed molecules • One molecule of G3P exits the cycle per three of • G3P is converted to glucose and other organic Alternative Methods of Carbon Alternative Methods of Carbon Fixation • C3 plants not well adapted to hot, arid climates • 1. 2. Two most important adaptations: C4 plants CAM plants C4 Plant C4 Plant C4 Plants C4 Plants • Forms a 4C compound as first product • Unique leaf anatomy: two cell types • Cell type #1 is the site of C4 pathway • Cell type #2 is the site of Calvin Cycle • Mechanism to concentrate CO2 in cell type #2 CAM Plant CAM Plant CAM Plants CAM Plants • Open stomata at night and close them during the day • Prevents water loss by evaporation, but also prevents entry of CO2 • Organic acids made during night • During the day, light reactions provide ATP and NADPH for Calvin Cycle. CO2 is derived from stored organic acids Evolution Connection • C4 and CAM pathways are evolutionary adaptations to allow photosynthesis to take place with partially or completely closed stomata on hot, dry days • The distribution of C4 and CAM plants on earth demonstrate this evolutionary history Environmental Impact of Environmental Impact of Photosynthesis The products of photosynthesis ultimately supports all life Photosynthesis removes CO2 and provides O2 to the atmosphere Deforestation contributes to increases in CO2 in atmosphere­ Effect on global warming Orchids growing in a Greenhouse Orchids Atmosphere, CO2 and Global Warming Atmosphere, Summary Summary • Photosynthesis takes place in the thylakoids within chloroplasts Cycle • Two stages, the light reaction and the Calvin • Light reactions capture solar energy and use it to make ATP and to transfer electrons from water to NADP+ to make NADPH Summary • Calvin Cycle uses NADPH and ATP to make sugar from CO2 • The sugar made supplies the entire plant with chemical energy and carbon skeletons for synthesis of organic compounds respiration by mitochondria • 50% organic material is used for Review Review ...
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This note was uploaded on 04/25/2011 for the course BIO 155 taught by Professor Skoubis during the Fall '10 term at DePaul.

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