Overview of Photosynthesis

Overview of photosynthesis
Cell type Prokaryotes Eukaryotes
Organism Nonoxygenic prokaryotes Oxygenic prokaryotes Algae Plants
Light-reaction photophosphorylation cyclic noncyclic noncyclic noncyclic
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 cycleCalvin 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



CH3 H Farnesyl ester CH3
cg C-OH-CH3 CH3 C2H5





H Stearyl alcohol CH3
d C-OH-CH3 CH3 C2H5





H Farnesyl ester H
e C-OH-N CH3 C2H5





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


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

. . . since 10/06/06