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Introduction to Photosynthesis (181-200) Life on Earth is SOLAR powered Photosynthesis (Ps) nourishes almost all living organisms Autotrophs - mainly Ps organisms (photoautotrophs) that make their own food (using sun E, CO2, and H2O) Also called producers of the biosphere Exs = green plants and Ps protist groups (fig 10.2) Heterotrophs - get E from organic compounds produced by other organisms Also called consumers of the biosphere Exs = fungi, animals, & many protist groups Photosynthesis converts light E to chemical E of food The Process That Feeds the Biosphere • Photosynthesis – Is the process that converts solar (light) energy into chemical energy • Plants and other autotrophs – Are the producers of the biosphere Plants are photoautotrophs • They use the energy of sunlight to make organic molecules from water and carbon dioxide Figure 10.1 Photosynthesis • Occurs in plants, algae, certain other protists, and some prokaryotes These organisms use light energy to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed not only themselves, but the entire living world. (a) On land, plants are the predominant producers of food. In aquatic environments, photosynthetic organisms include (b) multicellular algae, such as this kelp; (c) some unicellular protists, such as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as these purple sulfur (a) Plants bacteria, which produce sulfur (spherical globules) (c, d, e: LMs). (c) Unicellular protist 10 m (e) Purple sulfur bacteria Figure 10.2 (b) Multicellular algae (d) Cyanobacteria 40 m 1.5 m Heterotrophs • Heterotrophs – Obtain their organic material from other organisms – Are the consumers of the biosphere – Includes fungi, animals, many protist groups and many bacteria Chloroplasts – Sites of Ps within the cell Primarily found in leaves (mesophyll = main part of a leaf) Stomata = regulated holes in leaves where gas exchange occurs (what gases does a plant need to exchange for Ps?) Organelles enclosed by a double-membrane system (endosymbiosis) Stroma = internal fluid-filled cavity Thylakoids = system of interconnected membrane sacs (separates the stroma from the thylakoid space) Grana = stacks of thylakoids Chlorophyll = green pigment that absorbs light E = molecular bridge between sunlight and Ps activity Molecules are embedded in the thylakoid membrane system Chloroplasts: The Sites of Photosynthesis in Plants • The leaves of plants – Are the major sites of photosynthesis Leaf cross section Vein Mesophyll Stomata Figure 10.3 CO2 O2 Chloroplasts • Chloroplasts – Are the organelles in which photosynthesis occurs – Contain thylakoids and grana – Stroma is the fluid in the internal cavity Mesophyll Chloroplast 5 µm Outer membrane Stroma Granum – Thylakoid Thylakoid space Intermembrane space Inner membrane Chlorophyll is imbedded in the thylakoid membranes 1 µm Tracking Atoms Through Photosynthesis: • Photosynthesis is summarized as 6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O OR CO2 + H2O [CH2O] + O2 Overall Ps equation has been known since the 1800s The equation for Ps (fig 10.4) = reverse of respiration But carbohydrates are not made by simply reversing what happens in respiration BOTH processes occur in plant cells! Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Splitting of Water • Chloroplasts split water into – Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules Reactants: Products: Figure 10.4 12 H2O 6 CO2 C6H12O6 6 H2O 6 O2 Photosynthesis as a Redox Process • Photosynthesis is a redox process – Water is oxidized, carbon dioxide is reduced Two Stages of Photosynthesis Two stages of Ps (fig 10.5): 1. Light rxns: depend on light make ATP & NADPH and give off O2 NADPH = very similar in structure to NADH (just add a phosphate group to NADH) = the e- carrier Photophosphorylation = how ATP is generated (using chemiosmosis again) 2. Calvin cycle: use ATP and NADPH to fix C from the atmosphere into organic compounds Carbon fixation = initial incorporation of C into organic compounds The Two Stages of Photosynthesis • Photosynthesis consists of two processes – The light reactions – The Calvin cycle sunlight water carbon dioxide ATP ADP + Pi light-dependent rxns Calvin cycle NADPH NADP+ oxygen P glucose new water The Light Reactions • The light reactions – Occur in the grana – Split water, release oxygen, produce ATP, and form NADPH The Calvin Cycle • The Calvin cycle – Occurs in the stroma – Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power An overview of photosynthesis H2O CO2 Light NADP ADP + P LIGHT REACTIONS CALVIN CYCLE ATP NADPH Chloroplast Figure 10.5 O2 [CH2O] (sugar) Light Reactions • The light reactions convert solar energy to the chemical energy of ATP and NADPH Light • Light = electromagnetic energy, which travels in waves • Wavelength = distance between crests/troughs of waves (nm - km) – Smaller wavelengths = stronger light waves • Electromagnetic spectrum (fig 10.6) = entire range of light – Visible light (380-750 nm) important to biological systems • Different pigments absorb different wavelengths and reflect others (what we see that makes them colored) – What wavelength of light do plants reflect? The Nature of Sunlight • Light – Is a form of electromagnetic energy, which travels in waves • Wavelength – Is the distance between the crests of waves – Determines the type of electromagnetic energy The electromagnetic spectrum • The electromagnetic spectrum – Is the entire range of electromagnetic energy, or radiation 10–5 nm 10–3 nm Gamma rays X-rays UV 1m 106 nm 106 nm 103 nm 1 nm Infrared Microwaves 103 m Radio waves Visible light 380 450 500 Shorter wavelength Figure 10.6 Higher energy 550 600 650 700 Longer wavelength Lower energy 750 nm The visible light spectrum • The visible light spectrum – Includes the colors of light we can see – Includes the wavelengths that drive photosynthesis Photosynthetic Pigments • Photosynthetic pigments absorb specific wavelenths of light • Absorption spectrum = a pigment’s light absorption vs. wavelength • Spectrophotometer = instrument that measures absorbance of specific wavelengths (fig 10.8) • Beam of light sent through solution fraction of light transmitted at each wavelength measured Photosynthetic Pigments: Light Receptors • Photosynthetic Pigments – Are substances that absorb specific wavelengths within the visible light spectrum Pigments – Reflect some light, which include the colors we see Light Reflected Light Chloroplast Absorbed light Granum Transmitted light Figure 10.7 The spectrophotometer • The spectrophotometer – Is a machine that sends light through pigments and measures the fraction of light transmitted at each wavelength Transmitted light is NOT absorbed by that particular pigment An absorption spectrum • An absorption spectrum – Is a graph plotting light absorption versus wavelength Refracting Chlorophyll prism solution White light 2 Photoelectric tube Galvanometer 3 1 0 100 4 Slit moves to Green pass light light of selected wavelength The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. 0 Figure 10.8 Blue light 100 The low transmittance (high absorption) reading chlorophyll absorbs most blue light. Photosynthetic Pigments • Chlorophyll a (fig 10.10) absorption spectrum (fig 10.9a) • Chlorophyll b = accessory pigment similar to chl. a • When chlorophyll pigment absorbs light energy boosts an e- to an orbital of higher energy level (pigment is in its excited state) • If chlorophyll is isolated from chloroplast (fig 10.11) fluoresces (emits light) in red-orange end of spectrum (E given off as heat) • Carotenoids = other accessory pigments (hydrocarbons) reflecting various shades of orange/yellow/red (fig 10.9a) • Most important function = photoprotection (absorb & dissipate excess light E) Pigment Absorption Spectra • The absorption spectra of chloroplast pigments – Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis Absorption spectra of three pigments in chloroplasts Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below. EXPERIMENT RESULTS Absorption of light by chloroplast pigments Chlorophyll a Chlorophyll b Carotenoids Wavelength of light (nm) (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Figure 10.9 The action spectrum for photosynthesis Rate of photosynthesis (measured by O2 release) • Profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis (b) Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids. The action spectrum for photosynthesis • Was first demonstrated by Theodor W. Engelmann Aerobic bacteria Filament of alga 500 600 700 400 (c) Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b. CONCLUSION photosynthesis. Light in the violet-blue and red portions of the spectrum are most effective in driving Types of Chlorophyll • Chlorophyll a – Is the main photosynthetic pigment CH3 in chlorophyll a CHO in chlorophyll b CH2 • Chlorophyll b CH C H3C C H C C C – Is an accessory pigment C C N C N C Mg N C C C H C N C H3C CH3 H CH2 H H C C C O C H C CH3 CH3 Porphyrin ring: Light-absorbing “head” of molecule note magnesium atom at center C O O CH2 C C CH2 C O O CH3 CH2 Figure 10.10 Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown Other Pigments • Other accessory pigments – Absorb different wavelengths of light and pass the energy to chlorophyll a Excitation of Chlorophyll by Light • When a pigment absorbs light – It goes from a ground state to an excited state, which is unstable e– Excited state Heat Photon (fluorescence) Photon Figure 10.11 A Chlorophyll molecule Ground state Chlorophyll absorbs energy • If an isolated solution of chlorophyll is illuminated – It will fluoresce, giving off light and heat – The excited electron drops back to the ground-state orbital. Figure 10.11 B • Tomorrow, we will start with the different types of photosynthetic pigments, and which wavelengths of light each absorbs. • We will also discuss the light reaction portion of photosynthesis. The light reaction produces ATP and NADPH which go to power the fixation and reduction of carbon dioxide into sugar by the Calvin Cycle.