Final Exam Notes

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University of Toronto Scarborough
Biological Sciences
Mary Olaveson

Photosynthesis (chapter 7, 8, 9) Thylakoids (H20 is oxidized): H20 + light energy O2 + ATP + NADPH Stroma (carbon fixation, C02 is reduced): CO2 + ATP + NADPH (CH2O) = C02 + H20 (CH20) + O2 keq= 10 -500 (light driven redox process) Hill reaction: thylakoids were isolated & given light. O2 was released and reduction of Fe happened 3+ 2+ rather than C02 reduction. 4Fe + 2H2O 4Fe + O2 + 4H+ Artificial electron acceptor showed how photosynthesis & respiration worked Light has electric and magnetic field components (electromagnetic). Energy moves in wave w defined wavelength and frequency. Energy moves as particle in packets of energy called photons (quantum). Energy of photon is inversely related to its wavelength. Electromagnetic spectrum: shows type of radiation , F, E (gamma, xrays) , F, E (radio waves) The visible spectrum can be utilized and absorbed by plants (= 400-700nm) Fig 7.3 solar spectrum and relation to absorption of chlorophyll of light energy. - Top curve is energy of output of sun as function of wavelength - Second curve is energy that strikes Earth surface. It is less bc some energy is absorbed by water vapor in atmosphere. - Third curve is absorption spectrum of chlorophyll which strongly absorbs in blue (430nm) and red (660nm) spectrum portions. When a photon of light is absorbed by chlorophyll pigment, the electron distribution is altered, and some electrons move away from nucleus. The chlorophyll molecule then goesback to ground state. Chl + photon (ground state) Chl* (excited state; altered e- distribution) 4 routes to dissipate energy from lowest excited state: Note: photosynthesis doesnt use 1&2, 3&4 need to occur rapidly to compete w 1&2 1. Heat 2. Fluorescence: Re-emit photon of longer i.e. less energy bc some of energy already lost as heat 3. Resonance transfer: energy transfer to another molecule, usually chlorophyll & its physical energy transfer 4. Photochemistry: electron transfer from donor to acceptor Fig 7.5: light absorption and emission by chlorophyll. Blue light (E photon,) Red light (E photon,) - All absorption of light can lead to at least the lowest excitable state which drives photochemical and resonance transfer. The highest excited state is not very useful so it quickly dissipates to the lowest excited state. Red light is just as good at driving photosynthesis as blue light. Chlorophyll a & b have small structural differences but in general are a porphyrin-like ring structure w central Mg and hydrophobic tail that allows for effective interaction w hydrophobic bilayer of thylakoid membrane. Chlorophyll strongly absorbs in blue (430nm) and red (660nm) absorption spectrum. Carotenoids are orange, accessory pigments for photosynthesis that absorb 400-500nm of light and transfer them to chlorophyll for photosynthesis. Carotenoids use that chlorophyll isnt able to use. They have a photoprotective role to protect organism from damage caused by light. Chlorophyll B are different bc 1 functional group on ring is different- different absorption spectrum. Bacteriochlorophyll a & bilin pigments are found in photosynthetic bacteria. Common to all is the complex ring structure. Fig 7.7 Absorption spectra of photosynthetic pigments dissolved in nonpolar solvents. However, the true absorption spectrum of the pigments is dependent on membranes environment. Pigments shown in this figure are Curve 1= bacteriochlorophyll a, Curve 2=chlorophyll a, Curve 3=chlorophyll b, Curve 4=phycoerythrobilin, Curve 5=B carotene. Fig 7.8 Absorption spectra compared w action spectra Absorption spectra: effectiveness of chlorophyll molecule to absorb different light . Action spectra: effectiveness of different light to drive some biological process(here it is O2 evolution) - There is a very close match btw absorption and action spectra. To measure O2 concentration, light was shined onto the alga (spirogyra)s chloroplast. Aerotactic bacteria aggregated at O2 regions (in spectrum region where chlorophyll absorbed most effectively Blue, Red). It was first indication of effectiveness of light absorbed by accessory pigments in driving photosynthesis. Fig 7.11 Relationship of oxygen production to flash energy Chlorella- short light flashes of different intensity, O2 release was measured, O2 release saturated at high intensity. They thought that 1chlorophyll would yield 1 O2. However, the finding was that 2500 chlorophyll molecules would release 1 O2 at saturating energies. This was done by quantifying chlorophyll amount and O2 amount and forming a ratio. Reasons for 1 O22500 chlorophylls: 1. Hundreds of chlorophylls absorb light and transfer energy via resonance transfer until its all funneled to 1 chlorophyll molecule (reaction center). Electron transfer then happens. 2. Each reaction center needs to operate 4 times to generate 1 O2. 3. 2 different types of reaction centers (photosystems) working in series Fig 7.10 Basic concept of energy transfer during photosynthesis 2 stages: (1) Physical transfer: many chlorophyll pigments collect light and transfer it via resonance transfer to reaction center. (2) chemical reactions store some of energy in reaction center by transferring e- to an acceptor mol. An e- donor then reduces the chlorophyll reaction center again. A. Red drop effect not consistent w findings. Quantum yield = # of photochemical produces total # of quanta absorbed Far red light (> 680nm) is inefficient at driving photosynthesis. Quantum yield of O2 evolution falls off drastically for far-red light of wavelengths > 680n meaning that red light alone isnt enough at driving photosynthesis. The dip near 500nm reflects the lower efficiency of photosynthesis using light absorbed by accessory pigments (carotenoids). B. Emerson enhancement effect: the rate of photosynthesis when red and far-red light are given together is greater than the sum of the rates when they are given apart. The enhancement effect provided proof that photosynthesis is carried out by 2 photochemical systems working in tandem but slightly different optima. 2 photosystems working in series: 2 physically and chemically different, each w own chlorophyll antenna pigments and reaction center. Both are liked by electron transport chain. PS II 690nm absorption maxima, poor absorption of far red (P680 reaction center), in grana lamellae PS I 700nm absorption maxima (P700 reaction center), in stroma lamellae The Z scheme (zig zag scheme) Red light is absorbed by PSII producing strong oxidant and weak reductant. Far red light is absorbed by PSI and produces strong reductant and weak oxidant. The strong oxidant generated by PSII oxidizes water while the strong reductant produced by PSI reduces NADP+. The reductant produced by PSII re-reduces oxidant produced by PSI. Fig 7.16 Schematic picture of overall organization of chloroplast membranes Chloroplast is surrounded by inner and outer membranes (envelopes, lipid bilayer).
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