Biochem Photosynthesis PHOTOSYNTHESIS In 1780 Joseph Priestly discovered photosynthesis: “…plants can “restore air which has been injured by the burning of candles.” “…the air would neither extinguish a candle, nor was it all inconvenient to a mouse which I put into it.” Stryer, Biochemistry, 4 th ed.
Biochem Photosynthesis Photosynthesis provides essentially all free energy in biological systems by converting solar energy into chemical energy. Carbohydrates are formed from light- driven reactions that collectively appear deceptively simple: CO 2 + H 2 O + light C n H 2n O n + O 2
Biochem Photosynthesis Photosynthesis occurs in specialized organelles called chloroplasts: Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis TEM of Chloroplast from Corn Biochemistry, Voet & Voet,, 3rd Ed
Biochem Photosynthesis Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Photosynthesis consists of two sets of reactions: the LIGHT and DARK reactions. The LIGHT-driven reactions are the primary events of photosynthesis; these occur in the thylakoid membrane. The DARK reactions occur in the stroma.
Biochem Photosynthesis The light-reactions of photosynthesis generate high-energy electrons that are used to form NADPH. On their way to NADPH, these high- energy electrons flow through a membrane-bound “electron transport” pathway, generating a proton motive potential (Δp) from which ATP is made.
Biochem Photosynthesis Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis
Walker, et.al, Fundamentals of Chemistry, 1st ed.
Biochem Photosynthesis Chlorophyll a: Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis
Sunlight spectrum, Chlorophyll a and Bacteriochlorophyll a
Biochem Photosynthesis Voet & Voet, Biochemistry, 3 rd ed., pg 875
Biochem Photosynthesis Light-harvesting Pigments help absorb wavelengths of light in the region of the spectrum where chlorophyll pigments do not absorb light. Photons absorbed by these pigments also transfer energy to chlorophyll. β-carotene helps protect plants from photochemical reactions, expecially those involving oxygen; (it serves as an “antioxidant.”)
Biochem Photosynthesis
The majority of chlorophyll molecules in a photosynthetic unit do not get involved in photosynthesis directly. Rather, they transfer their absorbed energy to a reaction center complex. C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C
Biochem Photosynthesis Light energy absorbed by antenna chlorophyll molecules and auxillary pigments is transferred to specialized reaction centers. This energy transfer (often called “resonance energy transfer” or “exciton hopping”) is very efficient, approaching 100%!
Biochem Photosynthesis Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Voet & Voet, Biochemistry, 3 rd ed., pg 877
Biochem Photosynthesis Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Voet & Voet, Biochemistry, 3 rd ed., pg 877
Biochem Photosynthesis Voet & Voet, Biochemistry, 3 rd ed., pg 877
Biochem Photosynthesis Garrett & Grisham, Biochemistry, 3 rd ed.
Biochem Photosynthesis Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis X-ray Structure of Photosystem II (S. elongatus) Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Voet & Voet, Biochemistry, 3 rd ed., pg 888 X-ray Structure of Photosystem II (S. elongatus)
Biochem Photosynthesis Mn cluster in the OEC Schematic mechanism of oxygen generation in PSII: Voet & Voet, Biochemistry, 3 rd ed., pg 889
Biochem Photosynthesis Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Voet & Voet, Biochemistry, 3 rd ed., pg 877
Biochem Photosynthesis Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Herbicides that inhibit Photosystem II by blocking the transfer of electrons to QH 2 : Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Voet & Voet, Biochemistry, 3 rd ed., pg 877
Biochem Photosynthesis Electron Flow through PSII:Electron Flow through Cytochrome bf: Plastocyanin: Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Photosystem I: Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Photosystem I: 700nm photon Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Ferredoxin Structure Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis “Z-Scheme” of Photosynthesis: Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Stryer, et.al., Biochemistry, 5 th ed. Andre Jagendorf and Ernest Uribe’s classic experiment, showing the production of ATP from an artificially induced proton gradient across isolated chloroplasts in the absence of any light! (First Reported in 1965)
Biochem Photosynthesis
Stroma Lumen (Thylakoid Space)
Biochem Photosynthesis Stroma Lumen Inner-membrane Space Matrix Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis What about CO 2 ? CO 2 + H 2 O + light C n H 2n O n + O 2
Biochem Photosynthesis Melvin Calvin discovered the CO 2 acceptor by using C-14 radioactive labeled carbon dioxide: He discovered that the reactions of CO 2 fixation are almost identical to reactions of the phosphate shunt: Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis The Calvin Cycle: From light-driven reactions Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Ribulose 1,5-bisphosphate carboxylase/oxygenase [“rubisco”]: Rubisco consitutes more than 16% of all protein in chloroplasts. It is the most abundant protein in the entire biosphere! Stryer, et.al., Biochemistry, 5 th ed.
Biochem Photosynthesis Proposed mechanism for Rubisco: Stryer, et.al., Biochemistry, 5 th ed.
End of Slides For Photosynthesis Credits: Many of the diagrams used in these slides were taken from Stryer, et.al, Biochemistry, 5th Ed., Freeman Press (in our course textbook) and from prior editions of this text.