Chapter Twenty-Two Photosynthesis

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? Prokaryotes: in granules bonded to the plasma membrane Eukaryotes: in chloroplasts Chloroplast Inner, outer, and thylakoid membranes Grana, which consist of stacks of thylakoid disks Trapping of light and production of O2 take place in thylakoid disks Light reactions take place in thylakoid disks Dark reactions take place in the stroma

Photosynthesis in Eukaryotes

Light Dependent and Light-Independent Reactions of Photosynthesis

Chlorophyll Structure similar to the heme group of Mb, Hb, and the cytochromes It is based on tetrapyrrole ring of porphoryns

Chlorophyll (Cont’d) Absorb red (600 - 700 nm) and 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 photophosphorylation H2O + NADP+ ---> NADPH + H+ + O2 ADP + Pi ---> ATP The light reactions are accomplished by two distinct photosystems; photosystem I and photosystem II

Light Reactions Photosystem I (PSI) is the reduction of NADP+ to NADPH Photosystem II (PSII) is the 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 can be Coupled to ATP Production

The Structure of Photosynthetic Reaction Centers Most Extensively studied is from 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 (Cont’d) 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 NADPH The path of electrons in the light reactions of photosynthesis can be considered to have three parts: 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 phosphorylation of ADP 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 the synthesis of ATP from ADP and Pi

Photosynthesis and ATP Production (Cont’d) Chloroplasts can phosphorylate ADP in the dark if they are provided with a pH gradient, ADP, and Pi The Production of ATP does not require light, 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 in the chloroplasts and mitochondria are similar

Components of the Electron Transport Chain 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 (Cont’d)

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 too 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 stroma The actual reaction pathway has features in common with glycolysis and some in common with the pentose phosphate pathway The overall reaction is called the Calvin cycle after Melvin Calvin, Nobel Prize for Chemistry in 1961

The Calvin Cycle

The Calvin Cycle (Cont’d) 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 (twelve total) Two molecules of 3-phosphoglycerate are converted to glucose Ten 3-phosphoglycerates are used to regenerate six molecules of ribulose-1,5-bisphosphate

The Calvin Cycle (Cont’d) The reaction of ribulose-1,5-bisphosphate with CO2 produces the 3-phosphoglycerate

The Calvin Cycle (Cont’d) The cycle can be divided into four stages: preparation, reshuffling, isomerization, and phosphorylation: Preparation: production of glyceraldehyde-3-phosphate and dihydroxyacetone phosphate Reshuffling: many reshuffling reactions are like those of the pentose phosphate pathway and involve transaldolase and transketolase reactions Isomerization: conversion of both ribose 5-phosphate and xylulose-5-phosphate to ribose-5-phosphate Phosphorylation: 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 ribulose-1,5-bisphosphate 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 and Pi 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