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Introduction to Photosynthesis . Photosynthetic process converts light energy into chemical energy. Plants, algae and some types of bacteria use photosynthesis to make organic compounds.

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introduction to photosynthesis
Introduction to Photosynthesis

Photosynthetic process converts light energy into chemical energy. Plants, algae and some types of bacteria use photosynthesis to make organic compounds.

The photosynthetic process is carried out by a set of complex protein molecules that are located in and around a membrane. Energy drives a series of reactions transforming light energy into chemical energy.

Oxygenic photosynthesis produces O2, but not anoxygenic photosynthesis.

Photosynthetic organisms remove 1e15 grams of C y-1 (or 4e18 kJ) (Houghton and Woodwell, 1990).



In: "Concepts in Photobiology: Photosynthesis and Photomorphogenesis", Edited by GS Singhal, G Renger, SK Sopory, K-D Irrgang and Govindjee, Narosa Publishers/New Delhi; and Kluwer Academic/Dordrecht, pp. 11-51.

John WhitmarshPhotosynthesis Research Unit, Agricultural Research Service/USDADepartment of Plant Biology and Center of Biophysics and Computational Biology, Uiversity of Illinois at Urbana-Champaign

GovindjeeDepartment of Plant Biology and Center of Biophysics and Computational Biology, University of Illinois at Urbana-Champaign


photosynthetic studies
Photosynthetic studies

1640: Jan van Helmont - a Belgian philosopher, chemist and physician – grew a 5 lb willow tree in 200 lbs of soil, and watered regularly with rain water. Within 5 years the tree weighed 169 lbs and the soil 199 lbs. Plants got nutrients from water and somewhere.

1772: Joseph Priestly, a british scientist showed plant shoots produce oxygen. Plants restore oxygen. A few years later, Jan Ingenhousz demonstrated that light is required to produce oxygen. Jean Senebier, a Swiss botanist and naturalist, discovered that CO2 is required for photosynthetic growth and Nicolas- Théodore de Saussure, a Swiss chemist and plant physiologist, showed that water is required.

1800s: F.F. Blackman showed a two-step process for photosynthesis.

photosynthetic studies cont
Photosynthetic studies – cont.

1845 that Julius Robert von Mayer, a German physician and physicist, proposed that photosynthetic organisms convert light energy into chemical energy.

Middle of 19th century: A chemical reaction was proposed, CO2 + 2H2O + Light Energy  [CH2O] + O2 + H2O

Where does O2 comes from, CO2 or H2O?

Hill and Scarisbrick (1940) demonstrated oxygen evolution in the absence of CO2 in illuminated chloroplasts and by Ruben et al. (1941) who used 18O enriched water

photosynthetic studies cont1
Photosynthetic studies – cont.

Carbon reduction can occur in the dark and involves a series of biochemical reactions that were elucidated by Melvin Calvin, Andrew Benson and James Bassham in the late 1940s and 1950s. Using the radioisotope 14C, most of the intermediate steps that result in the production of carbohydrate were identified. Calvin was awarded the Nobel Prize for Chemistry in 1961 for this work.


CHO in Chlorophyll b


Chlorophyll, a pigment, absorbs sun light.

Antenna contains chlorophyll molecules to collect light.

Antenna absorb quanta and transfer the reaction center.


Energy in light reactions

Excite electrons of antenna chlorophyll anchored to proteins

Electron transfer chemical reactions in reaction center

Proton and electron transfer (redox) reactions (Photosystems I and II)

Produce high-energy molecules NADP+ + e  NADPHADP –electrochemical rxn ATP

Reduction of CO2

photosystem ii
Photosystem II

PSII is composed of polypeptides P680 and redox components (chlorophyll, pheophytin, plastoquinone, tyrosine, Mn, Fe, cytochrome b559, carotenoid and histidine)

light-induced electron transfer drive the oxidation of water and the reduction of plastoquinone PQ

Cytochrome b559 must be present, but its involvement in reaction is unknown.

photo energy in psii
Photo-energy in PSII

The energy aspect of the various intermediate species in photosynthesis and the products in PSII

O2 from H2O

the cytochrome bf complex
The Cytochrome bf Complex

The cytochrome bf complex removes the electrons from reduced plastoquinone (PQ or PQH2) and facilitates the release of the protons into the inner aqueous space.

The electrons are eventually transferred to the PS I reaction center.

Electron transfer from the cytochrome bf complex to photosystem I is mediated by a small Cu-protein, plastocyanin (PC).

photosystem i
Photosystem I

PS I is composed of a heterodimer of proteins that act as ligands for most of the electron carriers

Antenna system (200 mainly chlorophyll a P700 molecules) serve PS I.

PS I catalyzes the oxidation of plastocyanin, a small soluble Cu- protein, and the reduction of ferredoxin, a small FeS protein

energy aspects in ps i
Energy aspects in PS I

PS I reduces, Fd, ferredoxin, a small FeS protein

the atp synthesis
The ATP synthesis

ATP Synthase complex is composed CF0 and CF1

CF0 is a channel for H+

CF1 has several protein subunits for the reaction ADP + Pi + H+ ATP+ H2O

ATP is the energy molecule of life.




The protons released into the inner aqueous solution results in a difference in pH and electrochemical potential across the membrane.

light and dark reactions
Light and Dark Reactions









Final exam date

Chem218 Dec. 15, 19-22 pm, MC 4040