chapter 22 photosynthesis mary k. campbell shawn o. farrell paul d. adams university of arkansas
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Chapter 22Photosynthesis
Mary K. CampbellShawn O. Farrellhttp://academic.cengage.com/chemistry/campbell
Paul D. Adams • University of Arkansas
Photosynthesis
• Photosynthetic organisms carry out the reaction
6CO2 + 6H2O → C6H12O6 + 6O2
• The equation represents two processes• Light Reactions: NADPH and ATP are produced• Dark Reactions: ATP and NADPH provide the
energy and reducing power for the fixation of CO2
Where is the Site of Photosynthesis?
• Site of photosynthesis• Prokaryotes: in granules bonded to the plasma
membrane• Eukaryotes: in chloroplasts
• Chloroplast• Inner, outer, and _______________ membranes• _________, which consist of stacks of thylakoid disks
• Trapping of light and production of O2 take place in thylakoid disks
• Light reactions take place in ______________ disks• Dark reactions take place in the ____________
Photosynthesis in Eukaryotes
Chlorophyll
• Structure similar to the heme group of Mb, Hb, and the cytochromes
• Based on tetrapyrrole ring of porphoryns
Chlorophyll
• Absorb red (600-700 nm) & blue (400-500 nm) light• Accessory pigments absorb light and transfer
energy to chlorophylls (Chl) • Chlorophylls arranged in photosynthetic units• Antennae chlorophylls gather light• Harvested light energy passed to specialized Chl
molecules at a reaction center• Several hundred light-harvesting antennae Chl for
each Chl at a reaction center• Chemical reactions of photosynthesis begin at
reaction centers
Visible Spectra of Chlorophylls and Accessory Pigments
Summary
• In eukaryotes, photosynthesis takes place in chloroplasts. The light reactions take place in the thylakoid membrane, a third membrane in chloroplasts in addition to the inner and outer membrane
• The dark reactions of photosynthesis take place in the stroma, in between the thylakoid membrane and the inner membrane of the chloroplast
• The absorption of light by chlorophyll supplies the energy required for the reactions of photosynthesis. All types of chlorophylls have a tetrapyrrole ring structure similar to that of the porphyrins of heme, but they also have differences that affect the wavelength of light they absorb
• This property allows more wavelengths of sunlight to be absorbed than would be the case with a single kind of chlorophyll
Photosynthesis I and II and Light Reactions
• In the light reactions of photosynthesis, H2O is oxidized to O2 and NADP+ is reduced to NADPH
• This series of redox reactions is coupled to the phosphorylation of ADP to ATP in a process called ______________________________________________________________
HH22O + NADPO + NADP++ → NADPH + H → NADPH + H++ + O + O22
ADP + PADP + Pii → ATP → ATP
• Two distinct photosystems of the he light reactions: photosystem I and photosystem II
Light Reactions
• Photosystem I (PSI), reduction of NADP+ to NADPH
• Photosystem II (PSII): oxidation of H2O to O2
• The reaction is endergonic (G˚’=+220 kJ mol-1)
• The reaction is driven by the light energy absorbed by the chlorophylls of the two photosystems
The Z Scheme of Photosynthesis
The Oxygen Evolving Process
• Involves photosystem II through a system of five oxidation states S0-S4
• The net reaction of photosystems I and II is
2H2O + 2NADP+ → O2 + 2NADPH + 2H+
Cyclic Electron Transport in PSI Coupled to ATP Production
The Structure of Photosynthetic Reaction Centers
Most extensively studied is in Rhodopseudomonas• A reaction center contains a pair of bacteriochlorophyll
molecules embedded in a protein complex that is, in turn, an integral part of the photosynthetic membrane
• Absorption of light raises it to a higher energy level• An excited electron is passed to pheophytin, then to
menaquinone, and then to ubiquinone (next screen)• A cytochrome molecule transfers an electron to the
reaction center; the cytochrome molecule now has a positive charge
• The excited electron is passed to menaquinone and then to ubiquinone
• The charge separation represents stored energy
Quinone Electron Acceptors
The Structure of Photosynthetic Reaction Centers
• Accessory pigments have specific positions close to the special pair of chlorophylls
• The first of the accessory pigments is pheophytin
• Pheophytin is structurally similar to chlorophyll
• The next electron acceptor is menaquinone (QA)
• Menaquinone is structurally similar to coenzyme Q
• The last electron acceptor is coenzyme Q (QB)
Summary
Photosynthesis consists of two processes. The light reactions are electron transfer processes, in which water is oxidized to produce oxygen and NADP+ is reduced to produce NADPHThe path of electrons in the light reactions: • The first is the transfer of electrons from water to the reaction-
center chlorophyll of PSII• Next is the transfer of electrons from the excited-state
chlorophyll of PSII to an electron transport chain consisting of accessory pigments and cytochromes, with energy provided by absorption of a photon of light. The components of this electron transport chain resemble those of the miochondrial electron transport chain; they pass the electrons to the reaction-center chlorophyll of PSI
• The third and last part of the path of the electrons is their transfer from the excited-state chlorophyll of PSI to the ultimate electron NADP+, producing NADPH. Again, energy is provided by absorption of a photon of light
Photosynthesis and ATP Production
• A proton gradient across the inner mitochondrial membrane drives ___________________________
• The proton gradient is created• By the splitting of H2O which releases H+ into the
thylakoid space• By electron transport from Photosystem II to
Photosystem I• When Photosystem I reduces NADP+ by using H+ in
the stroma to produce NADPH• The flow of H+ back to the stroma through ATP
synthase provides the energy for ADP + Pi → ATP
Photosynthesis and ATP Production
• Chloroplasts can phosphorylate ADP in the dark if they are provided with ________________________
• The production of ATP does not require _____________; the proton gradient produced by the pH gradient supplies the energy
Summary:• The mechanism of ATP in
chloroplasts closely resembles the process that takes place in mitochondria.
• The structures of ATP synthase in the chloroplasts and mitochondria are similar Fig 22.12 ATP is synthesized in
the dark by a proton gradient
Components of the ETC of the Thylakoid Membrane
Evolutionary Implications of Photosynthesis
• Photosynthetic prokaryotes other than cyanobacteria have only one photosystem and do not produce oxygen
• Anaerobic photosynthesis is not as efficient as photosynthesis linked to oxygen, but the anaerobic version of the process appears to be evolutionary
• The ultimate source of electrons for these organisms is not H2O, but some more easily oxidized substance,one of which is H2S
• The H-acceptor may also be NO2- or NO3
- --> NH3 • There are two possible pathways for electron transport to
occur in an anaerobic organism:• Cyclic• Noncyclic
Evolutionary Implications of Photosynthesis
Summary:
• When photosynthesis first evolved, it was most likely to have been carried out by organisms that used compounds other than water as the primary electron source.
• Cyanobacteria were the first organisms to use water as the source of electrons, giving rise to the present oxygen-containing atmosphere
The Dark Reaction of Photosynthesis Fix CO2
• CO2 fixation takes place in the _______________
• The actual reaction pathway has features in common with __________________ and some in common with the _____________ ______________ ______________
• The overall reaction is called the Calvin cycle after Melvin Calvin, Nobel Prize for Chemistry in 1961
The
Cal
vin
Cyc
le
The Calvin Cycle
• The first reaction is the carboxylation of six molecules of ribulose-1,5-bisphosphate• This carboxylation is the actual fixation step• Each carboxylation product splits to give two
molecules of 3-phosphoglycerate (12 total)• Two molecules of 3-phosphoglycerate are converted
to glucose• Ten 3-phosphoglycerates are used to regenerate six
molecules of ______________________________
The Calvin Cycle
• The reaction of ___________________________ with CO2 produces the 3-phosphoglycerate
The Calvin Cycle
The cycle can be divided into four stages:• _______________: production of glyceraldehyde-3-
phosphate and dihydroxyacetone phosphate• _______________: many reshuffling reactions are like
those of the pentose phosphate pathway and involve transaldolase and transketolase reactions
• _______________: conversion of both ribose 5-phosphate and xylulose-5-phosphate to ribose-5-phosphate
• _______________: in the final step, ribulose-5-phosphate is phosphorylated to ribulose-1,5-bisphosphate
The Calvin Cycle Series of Reactions
Summary
• In the dark reactions of photosynthesis, the fixation of CO2 takes place when the key intermediate ____________________________ reacts with CO2 to produce two molecules of 3-phosphoglycerate. The reaction is catalyzed by RUBISCO.
• The remainder of the dark reaction is the regeneration of ribulose-1,5-bisphosphate in the Calvin Cycle
CO2 Fixation in Tropical Plants
• An alternative pathway for CO2 fixation in tropical plants, known as the Hatch-Slack pathway, also called a C4 pathway
• CO2 enters the outer (mesophyll) cells and reacts with phosphoenolpyruvate to give oxaloacetate & P i
• Oxaloacetate is reduced to malate• Malate is transported to inner (bundle-sheath) cells
where it is oxidized and decarboxylated to pyruvate
• CO2 is then passed to the Calvin cycle where it reacts with ribulose-1,5-bisphosphate
The C4 Pathway
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