Cell and Molecular Biology Fourth Edition Chapter 6: Photosynthesis and the Chloroplast Copyright © 2005 by John Wiley & Sons, Inc. Gerald Karp

Heterotrophs (CO 2, H 2 S), Autotrophs, Chemoautotrophs: NH 3, H 2 SO 3 - (only for bacteria) Photoautotrophs: P.S. plant, algae, flagellated protists, purple, green bacteria, cyanobacteria. 2 billions years, H 2 S is the e- source CO H 2 S → (CH 2 O) + H 2 O + 2S (sulfur spring and deep sea) CO 2 + H 2 O → (CH 2 O) + O 2 need light (cyanobacteria)

1881 German biologist T. Engelmann found the bacteria surround the algae in order to absorp the O2 released by bacteria

6.2 An overview of photosynthetic metabolism Light dependent reaction 2H 2 0 → O 2 + 4H + + 4e- → NADPH, ATP Light independent reaction CO2 → CH 2 O (600x10 12 /year Kg), O 2 (400x10 12 /year Kg)

6.3 The absorption of light 1 mole of 680 nm photon contains 42 Kcal/mole (1.8V of redox potential) In vitro: In vivo:

1. Photosynthetic pigments Chlorophyll: A. porphyrin ring : absorption of light B. hydrophobic phytol tail: in thylakoid membrane Accessory pigments: carotenoids (capture energy and removal excited energy)

6.4 Photosynthetic units and reaction centers 1932: Emerson and Arnold of the CIT. About three hundred chlorophyll molecules absorbed to produce one molecule of oxygen Photosynthetic unit Only one member of the group ”the reaction-center chlorophyll” actually transfer electrons to an electron acceptor.

1. Oxygen formation: coordinating the action of two different photosynthetic systems PSII operations: obtain electrons from water a. The flow of electrons from PSII to plastoquinone b. The flow of electrons from water to PSII From PSII to PSI PSI operations: The production of NADPH

*Electron carriers, *proton gradient, *ATPsynthase Z-scheme PC ferredoxin

From PSII to PSI

pH:5 pH:8

PS1 operations: The production of NADPH 2001, 3D structure of PS1 from a cyanobacterium was discovered at 2.5 A resolution. 12 polypeptides, 96 chlorophylls, 22 carotenoids and other elements

An overview of photosynthetic electron transport Summary of the light-dependent reactions

Killing weeds by inhibiting electron transport 1. Inhibitors of electron carries (herbicides) 2. Binding to a core protein of PSII ( Q B ).

6.5 Photophosphorylation Noncyclic photophosphorylation H2O → e - → NADP + → NADPH, ATP CO 2 → CH 2 O need 3 ATP and 2 NADPH Cyclic photophosphorylation PS1 → A 0 → A 1 → ferredoxin →NADPH → cytb 6 f → PC → PSI

*Electron carriers, *proton gradient, *ATPsynthase Z-scheme PC ferredoxin

6.6 carbon dioxide fixation and the synthesis of carbohydrate 1. Carbohydrate synthesis in C3 plants Photorespiration and peroxisomes 2. Carbohydrate synthesis in C4 plants 3. Carbohydrate synthesis in CAM plants

Chromatogram showing the results of an experiment in which algal cells were incubated for 5 secs in ( 14 C)O 2 prior to immersion in alcohol. 3- phosphoglycerate Rubisco, only fix 3 molecules CO 2 /sec, 10 kg for every human

Redox control of the Calvin cycle Several key enzymes of the Calvin cycle are only active in the light when ATP and NADPH are being produced by PS In the light, a fraction of electrons are transferred to the small protein thioredoxin, which reduces the disulfide groups of certain Calvin cycle enzymes which become ative.