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

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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