Energy Harvesting Pathways Photosynthesis

photosynthesis reverses the oxidation of glycolysis/respiration C 6 H 12 O 6 +6 O 2 => 6 CO 2 +6 H 2 O + energy energy +6 CO H 2 O =>6 O 2 +C 6 H 12 O 6 +6 H 2 O

photosynthesis reverses the oxidation of glycolysis/respiration reduces highly oxidized carbon –stores energy in hydrocarbon bonds utilizes “free” resources –water from soil reservoir –CO 2 from atmospheric reservoir –energy from diurnal light source releases O 2 as a byproduct

reactants and products of photosynthesis Figure 8.1

photosynthesis occurs in chloroplasts –“light reactions” on thylakoid membranes –“dark reactions” in aqueous stroma two interconnected pathways –light-driven electron transport generates reductant & energy –Calvin-Benson cycle reduces CO 2 & assembles carbohydrates

photosynthesis: overview Figure 8.3

visible light occupies a narrow band of the electromagnetic spectrum Figure 8.5

photosynthetic light reactions light is the visible portion of the electromagnetic radiation spectrum –between ultraviolet and infrared light travels in wave-like fashion –wavelength & frequency are inversely related shorter wavelength : higher frequency longer wavelength : lower frequency

photosynthetic light reactions light energy occurs in discrete units: photons –energy of a photon is inversely proportional to wavelength shorter wavelength : higher energy longer wavelength : lower energy intensity measures number of photons striking a unit area per unit time (e.g. µE·m -2 ·s -1 )

green light is transmitted (and reflected) as blue and red are absorbed

photosynthetic light reactions molecules absorb electromagnetic radiation –pigments absorb visible light of certain wavelengths –photon-pigment interactions reflection transmission absorption - pigment is excited by photon –excited state - ground state = energy of photon

absorption of a photon excites a molecule Figure 8.4

absorption and action spectra Figure 8.6

Chlorophyll a: √ tetrapyrrole ring √ coordinated Mg √ hydrophobic tail Figure 8.7

photosynthetic light reactions molecules absorb electromagnetic radiation –a pigment absorbs only certain wavelengths an absorption spectrum is a molecular fingerprint an action spectrum plots effectiveness vs. wavelength eukaryotic photosynthesis uses chlorophyll a as the central pigment –accessory pigments transfer energy to Chl a in plants: Chl b, carotenoids

photosynthetic electron transport mutants fluoresce…

photosynthetic light reactions possible fates of absorbed energy –loss as heat –loss as fluorescence –intermolecular transfer direct transfer electron transport

fates of energy Figure 8.8

photosynthetic light reactions excited reaction center chlorophyll a is a good reducing agent –PSII Chl a* drives electron transport through carriers in the thylakoid membrane –PSI reaction center chlorophyll is reduced by electrons transported from PSII –PSI Chl a reduces NADP + => NADPH –PSII Chl a + is reduced with e - from H 2 O O 2 is released as a byproduct

Figure 8.9

thylakoids are flat sacks that reside in the chloroplast Figure 8.11

transfers of absorbed energy Figure 8.11

photosynthetic light reactions noncyclic electron transport produces ATP and NADPH cyclic electron transport produces ATP, but not NADPH

Cyclic electron transport Figure 8.10

the light reactions of photosynthesis Figure 8.11

the light reactions of photosynthesis electrons flow from water to NADP + –NADPH is produced a proton gradient is formed –ATP is produced

light and “dark” reactions are coupled by ATP & NADPH Figure 8.3

carbon fixation reactions How does the plant incorporate CO 2 into the existing “carbon pool”? –CO 2 must be attached to one or more existing molecules - which one(s)? –…feed a plant CO 2 and watch where it goes…

Calvin, Benson, et al. photosynthesis in Chlorella with 14 CO 2 Figure 8.12

carbon fixation reactions 3-phosphoglycerate is the first product of carbon fixation other molecules were labeled over time

Calvin- Benson Cycle model of carbon fixation Figure PG

carbon fixation reactions the acceptor is not a 2-carbon molecule it’s ribulose 1,5-bisphosphate a 5-C sugar the first product is not 3PG it’s an unstable 6-C intermediate

3PG is the first stable product Figure 8.14

Calvin- Benson Cycle model of carbon fixation Figure 8.13

carbon fixation reactions Calvin-Benson cycle accomplishes three tasks carbon fixation - by rubisco reduction of fixed C into carbohydrate 3-phosphoglyceric acid => glyceraldehyde 3-phosphate requires reductant & energy formation of more RuBP (hence, cycle) requires multiple enzymes & ATP

Calvin- Benson Cycle model of carbon fixation Figure 8.13

Product of Calvin-Benson Cycle G3P is the reduced product of the Calvin- Benson cycle

Product of Calvin-Benson Cycle G3P is the reduced product of the Calvin- Benson cycle

Product of Calvin-Benson Cycle G3P is the reduced product of the Calvin- Benson cycle –1/6 of G3P is product; 5/6 are reaction intermediates –“excess” G3P is used to make monosaccharides 1/3 of G3P is stored in the chloroplast as starch 2/3 of G3P is transported elsewhere as sucrose

Figure 8.3

carbon fixation reactions ribulose bisphosphate carboxylase/oxygenase Rubisco most abundant protein in the world, but…

photorespiration ribulose bisphosphate carboxylase/oxygenase is very ineffective rubisco adds CO 2 to RuBP or adds O 2 to RuBP 5C + 1C => 2 · 3C 5C + 0C => 2C + 3C costs ATPs to regenerate RuBP

chloroplast, peroxisome, mitochondrion Figure 8.15

photorespiration ribulose bisphosphate carboxylase/oxygenase carboxylase & oxygenase activities compete rubisco CO 2 affinity is low stomata must be open for efficient PS easy access for 20% O 2 & 0.035% CO 2 up to 30% of fixed carbon is lost to photorespiration in important crops plants some plants don’t suffer so much from photorespiration

photorespiration - solution C 3 plants and C 4 plants 3PG is the first detectable product of C fixation in C 3 plants C 4 plants produce a 4-C product first PEP + CO 2 ======> oxaloacetate 3C 1C PEP C’ase4C PEP carboxylase has high CO 2 affinity is never an oxygenase

photorespiration - solution but… C 4 plants use rubisco, just like C 3 plants PEP carboxylase and rubisco are separated into different compartments

C 3 & C 4 leaf anatomies Figure 8.16

mesophyll cells and bundle sheath cells communicate in C 4 plants Figure 8.17

photorespiration - solution C 4 bundle sheath cells are enriched in CO 2 relative to O 2 rubisco fixes O much less often spatial separation of initial C fixation and Calvin Benson cycle

Table 8.1