TAMU BIOL 101 - week-06b-photo-resp-tx-gene
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Page 1Page 2Page 3Page 4Page 5Page 6Page 7Page 8Page 9Page 10Page 11Page 12Page 13Page 14Page 15Week 6 - Page - 1-BIOL 101 - Fall 2007- Week 6 - Photosynthesis - Energy - RespirationGene Structure - TranscriptionPlease note - all lecture notes are© Timothy C. Hall, Biology Dept., Texas A&M UniversityPHOTOSYNTHESISHISTORICAL1577 - 1644 Jan-BaptistaVan Helmont Showed that soil contributed only a little to the increase in weightof a potted willow plant.1733-18042Joseph Priestley Showed (1772) rejuvenation of air by (O ) from mint sprig.1742-18092Jean Senebier Showed (1782) that plants need CO .1730-1799Jan Ingen-Housz Showed (1778) that light and green parts of plants are needed for2O generation and (1796) that C goes into the plant.1767-184522Nicholas de Saussure Showed (1804) that equal volumes of CO and O are exchanged2and also showed that H O was needed.1843 - 1909T.W. Engelmann Action spectrum experiment (1883) showing red and blue lightefficacy for photosynthesis.1843 - 1909Julius Sachs Showed that photosynthesis occurs only in light 2221934 C.B. van Niel Theorized that O is evolved from H O, not CO in photosynthesis.22 2 2Sulphur bacteria: CO + 2H S + light 6 CH O + H O + 2S22 2 2General equation: CO + 2H X + light 6 CH O + H O + 2X22 2 2Green plants: CO + 2H O + light 6 CH O + H O + 2O1930s Robin Hill Showed that light drives the splitting of water. Hill showed that22isolated chloroplasts can release O in the absence of CO if given2a suitable electron acceptor for electrons removed from H O.Samuel Ruben;2Martin Kamen Proved (1941) above in experiment using O , a non-radioactive 182isotope of O .The nature of light1660 Sir Isaac Newton Separated light into the spectrum.1905Albert Einstein Proposed that light consists of photons, packets of energy. Lightintensity depends on the number of photons absorbed per unit oftime. The amount of energy carried by each photon is determinedby its vibration. The distance moved during a complete vibration isWeek 6 - Page - 2-the wavelength. The energy of a photon is called a quantum andis inversely proportional to the wavelength: the longer thewavelength, the less energy per photon.Ultraviolet radiation Has too much energy for most organisms. Its ionizingradiation breaks weak bonds.Infrared radiation Has too little energy for living systems but does warm themup.Visible light Contains just the right amount of energy. But it must beabsorbed by pigments.PIGMENTSPigments absorb visible light and changes the configuration of electrons in the photons, resultingin transfer of energy to heat, fluorescence or chemical forms.chlorophyll a absorbs red, violet and blue; reflects green.Accessory pigments broaden the action spectrum by passing energy to chlorophyll a. Aretypically embedded in the thylakoid membrane.chlorophyll b 25% of chlorophyll in leaves.chlorophyll C found in brown and other algae.bacteriochlorophyll purple photosynthetic bacteria.chlorobium chlorophyll green sulfur bacteria.Carotenoidscarotene red, orange, yellow; fat-soluble. $-carotene is vitamin A.xanthophylls zeaxanthinphycobilinswater soluble pigments found in red algae and cyanobacteria.LIGHT ( PHOTOCHEMICAL) REACTIONSSee Fig. 10.5, p. 177 of the Stern (9 edn.) textthEach photosystem (photosynthetic unit) includes an assembly of >200 pigment molecules. Thepigment participate in two closely linked complexes:(1) The reaction center-protein complex.Although all pigments can absorb photons, only one pair of chlorophyll molecules perphotosytem can use the energy in the photochemical reaction.(2) The antenna protein complex.This acts as an antenna for gathering light and transferring energy to the reaction center complex.Week 6 - Page - 3-PHOTOSYSTEM IIThis is described first because it is the dramatic step where light energy entering the system is2used for photolysis, splitting the water molecule and releasing O . Photosystems I and II usuallywork together simultaneously and continuously.680Photosystem II contains a special chlorophyll, P . When four photons of light enter, theirelectrons are transferred to pheophytin (quinone) and water is split into protons and oxygen gas(Mn is essential for this photolysis). This reaction occurs on the inside of the thylakoid2+membrane. The electrons then pass down a proton gradient, driving photophosphorylation.PHOTOSYSTEM I700Four photons of light boost electrons from a P molecule to an electron acceptor (ferredoxin).The electrons then pass downhill to NADP , reducing it to NADPH.+In the light there is a continuous flow of electrons from water from photosystem II tophotosystem I (the "Z scheme"). This is noncyclic electron flow. A total of eight photons arerequired to boost two electrons to form three molecules of ATP and two of NADPH.Cyclic Photophosphorylation700Photosystem I can work independently of Photosystem II. In light, electrons are boosted from Pto ferredoxin, but are shunted to plastoquinone (cytochrome f) in the photon gradient used toconnect the two photosystems.2This cyclic photophosphorylation results in the synthesis of ATP, but: no water is split; no O isevolved and no NADPH is formed. It is thought to occur when cells have sufficient NADPH butneed more ATP.Week 6 - Page - 4-DARK (BIOCHEMICAL) REACTIONS OF PHOTOSYNTHESIS2The second important stage of photosynthesis, in which carbon is fixed from CO , is independentof light.2For algae and cyanobacteria, CO is available in solution in the surrounding water. For most plants,2CO must enter the plant through stomata.There are two major pathways for carbon fixation, named for the researchers who discovered themechanisms: Melvin Calvin; M.D. Hatch and C.R. Slack. The Calvin cycle is often referred to as3the “C " pathway because the first stable product is a 3-carbon sugar and the Hatch-Slack pathway42is termed the “C " pathway because carbon from atmospheric CO is first fixed into a C-4 molecule,oxaloacetic acid (which is rapidly converted to malic or aspartic acid).3The C , CALVIN CYCLE Pathway5This is the commonest pathway in higher plants. Ribulose 1,5 bisphosphate (RuBP) a C molecule,26serves as an acceptor for CO to form a transient C molecule that is immediately split into3molecules of the C compound phosphoglycerate (PGA). This reaction is catalyzed by the enzymeRuBP carboxylase / oxygenase (rubisco).6SIX turns of the Calvin cycle are required to yield one molecule of C sugar. The reactions areessentially the reverse of glycolysis


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