Biochem 3070 - Photosynthesis

Biochem 3070 - 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, 4th ed.


• 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:

CO2 + H2O + light CnH2nOn + O2


• Photosynthesis occurs in specialized organelles called chloroplasts:

Stryer, et.al., Biochemistry, 5th ed.


TEM of Chloroplast from Corn

Biochemistry, Voet & Voet,, 3rd Ed




• 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.


• 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.





Walker, et.al, Fundamentals of Chemistry, 1st ed.


Chlorophyll a:




Sunlight spectrum, Chlorophyll a and Bacteriochlorophyll a


Voet & Voet, Biochemistry, 3rd ed., pg 875


• 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.”)



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


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%!












Garrett & Grisham, Biochemistry, 3rd ed.




X-ray Structure of Photosystem II (S. elongatus)




X-ray Structure of Photosystem II (S. elongatus)


Mn cluster in the OEC

Schematic mechanism of oxygen generation in PSII:









Herbicides that inhibit Photosystem II by blocking the transfer of electrons to QH2:





Electron Flow through PSII: Electron Flow through Cytochrome bf:

Plastocyanin:



• Photosystem I:



Photosystem I:

700nm photon


Biochem 3070 - PhotosynthesisFerredoxin Structure



“Z-Scheme” of Photosynthesis:










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)



Stroma

Lumen

(Thylakoid Space)


Stroma

Lumen

Inner-membrane Space

Matrix



What about CO2?

CO2 + H2O + light CnH2nOn + O2


Melvin Calvin discovered the CO2 acceptor by using C-14 radioactive labeled carbon dioxide:

He discovered that the reactions of CO2 fixation are almost identical to reactions of the phosphate shunt:



The Calvin Cycle:

From light-driven reactions



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!



Proposed mechanism for Rubisco:


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.

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Biochem 3070 - Photosynthesis