Overview of Photosynthesis

Overview of photosynthesis

Cell type	Prokaryotes		Eukaryotes
Organism	Nonoxygenic prokaryotes	Oxygenic prokaryotes	Algae	Plants
Light-reaction photophosphorylation	cyclic	noncyclic	noncyclic	noncyclic
Photosystems	PSI	PSI & PSII	PSI & PSII	PSI & PSII
Location of photosystems	chlorosome	thylakoid membrane	thylakoid membrane of chloroplast	thylakoid membrane of chloroplast
Metabolism	Calvin cycle, C-3 in most prokaryotes, or reductive tricarboxylic acid cycle, or reductive acetyl coenzyme A pathway, or 3-hydroxypropionate cycle	Calvin cycle	Calvin cycle	Calvin cycle (C-3) C-4 and C-3 CAM and C-3

Structure of bacteriochlorophylls

bc	R1	R2	R3	R4	R5	R6	R7
a	C=O(CH3)	CH3	CH2-CH3	CH3	C=O(OCH3)	Geranylgeraniol ester	H
b	C=O(CH3)	CH3	=CH-CH3	CH3	C=O(OCH3)	Phytyl ester	H
c	C=O-CH3	CH3	C2H5 C3H7 C4H9	CH3	H	Farnesyl ester	CH3
cg	C-OH-CH3	CH3	C2H5 C3H7 C4H9	C2H5 CH3	H	Stearyl alcohol	CH3
d	C-OH-CH3	CH3	C2H5 C3H7 C4H9	C2H5 CH3	H	Farnesyl ester	H
e	C-OH-N	CH3	C2H5 C3H7 C4H9	C2H5 CH3	H	Farnesyl ester	CH3
g	C=CH2	CH3	C2H5	CH3	C=O-O-CH3	Farnesyl ester	H

: molecular diagrams chlorophyll proteins : R groups in table refer to image at left - click will change image to the structure of the reaction center of the purple bacterium Rhodopseudo-monas viridis, in which quinones and the cytochrome participate in electron transfer reactions with the bacteriochlorophylls.

Comparison of Photosynthesis and Respiration

Photosynthesis	Respiration
produces food stores energy consumes H2O consumes CO2 releases O2 light-dependent	consumes food releases energy produces H2O produces CO2 consumes O2 light-independent

Comparison of Plant and Bacterial Photosynthesis

	Plant photosynthesis	Bacterial photosynthesis
Organisms	Plants, Algae, Cyanobacteria	Purple and green bacteria
Produces O2	yes	no
Type of chlorophyll	chlorophyll a absorbs 650-750nm	bacteriochlorophylls absorbs 800-1000nm
Photosystem I (cyclic photophosphorylation)	present	present
Photosystem II (noncyclic photophosphorylation)	present	absent
Photosynthetic electron donor	H2O	H2S, other sulfur compounds, or certain organic compounds

Image Photosystem I & 3D view in Chime, Jmol or Webmol : protein map PSI : Image Photosystem II : Interactive diagram PS II :

Comparison of C-3, C-4, CAM plants

	C-3	C-4	CAM
Initial CO2 fixing enzyme	ribulose 1,5 bisphosphate carboxylase/oxygenase	phosphoenolpyruvate carboxylase	phosphoenolpyruvate carboxylase
Form of C in intermediates	2 PGA (phosphoglycerate) molecules (3C)	4C molecules	4C molecules
C stored in	Starch after C-3	4-C molecules before C-3 Starch after C-3	4-C molecules in vacuoles before C-3 Starch after C-3
Plant type	all plants	grasses in warm climates	plants in hot, arid regions – succulents, cacti, agaves, some orchids, and bromeliads (im)
Location of process	Chloroplasts of all phototrophs starch-rich chloroplasts lacking grana in bundle sheath cells of C4 plants	mesophyll of Kranz-Crown/Halo plants (im, diag, im)	Chloroplasts nocturnal
Evolution	earliest, conserved	later, convergent	later, convergent
Separation from C-3	none	spatial	temporal
CO2 levels		high in bundle sheath cells
Advantages	more efficient in cool, moist environments with moderate light intensity	more efficient in warm, dry environments with high light intensity	more efficient in hot, dry environments with high light intensity

Z-scheme of noncyclic photophosphorylation

Left - diagram of the Z-scheme of noncyclic photophosporylation (click to enlarge).

A photon is absorbed by Photosystem II and the two resultant excited electrons are passed to Photosystem I, which employs a second photon to further boost their energy for the overall reaction: NADP+ + H+ + 2e- → NADPH