Several bacteria like cynobacteria and other purple-sulfer bacteria have capacity to generate energy by photosynthesis without chlorophyll.

1. Microbial physiology
Bacterial Photosynthesis Unit 1

2. Introduction
• In all of the metabolic pathways just discussed, organisms
obtain energy for cellular work by oxidizing organic
compounds.
• But where do organisms obtain these organic compounds?
Some, including animals and many microbes, feed on
matter produced by other organisms. For example, bacteria
may catabolize compounds from dead plants and animals,
or may obtain nourishment from a living host.
• Other organisms synthesize complex organic compounds
from simple inorganic substances. The major mechanism
for such synthesis is a process called photosynthesis. which
is used by plants and many microbes.

3. • Essentially, photosynthesis is the conversion of
light energy from the sun into chemical
energy.
• The chemical energy is then used to convert
CO2 from the atmosphere to more reduced
carbon compounds, primarily sugars.
• This synthesis of sugars by using carbon atoms
from CO2 gas is also called carbon fixation.

5. • Photosynthesis, in bacteria, is defined as “the
synthesis of carbohydrates by the chlorophyll
in the presence of sunlight, CO2 and
reductants taken from air and oxygen do not
evolve as by product, except in cynobacteria.

6. • The most important biological process on
Earth is photosynthesis, the conversion of light
energy to chemical energy.
• Organisms that carry out photosynthesis are
called phototrophs.
1

7. • Most phototrophic organisms are also
autotrophs, capable of growing with CO2 as
the sole carbon source.
• Energy from light is used in the reduction of
CO2 to organic compounds (photoautotrophy).
However, some phototrophs use organic
carbon as their carbon source; this lifestyle is
called photoheterotrophy.

8. • Photoautotrophy requires that two distinct sets of reactions
operate in parallel: (1) ATP production and (2) CO2 reduction to
cell material.
• For autotrophic growth, energy is supplied from ATP, and
electrons for the reduction of CO2 come from NADH (or NADPH).
The latter are produced by the reduction of NAD+ (or NADP+) by
electrons originating from various electron donors.
• Some phototrophic bacteria obtain reducing power from
electron donors in their environment, such asreduced sulfur
sources, for example hydrogen sulfide (H2S), or from hydrogen
(H2).
• By contrast, green plants, algae, and cyanobacteria use electrons
from water (H2O) as reducing power.

9. • The oxidation of H2O produces molecular
oxygen (O2) as a by-product. Because O2 is
produced, photosynthesis in these organisms
is called oxygenic photosynthesis.
• However, in many phototrophic bacteria H2O
is not oxidized and O2 is not produced, and
thus the process is called anoxygenic
photosynthesis.

10. Photosynthetic Microorganisms
• All life can be divided into three domains, Archaea, Bacteria and
Eucarya, which originated from a common ancestor.
• Historically, the term photosynthesis has been applied to organisms
that depend on chlorophyll (or bacteriochlorophyll) for the
conversion of light energy into chemical free energy.
• These include organisms in the domains Bacteria (photosynthetic
bacteria) and Eucarya (algae and higher plants).
• The most primitive domain, Archaea, includes organisms known as
halobacteria, that convert light energy into chemical free energy.
• However, the mechanism by which halobacteria convert light is
fundamentally different from that of higher organisms because
there is no oxidation/reduction chemistry and halobacteria cannot
use CO2 as their carbon source. Consequently some biologists do
not consider halobacteria as photosynthetic.

11. 2

12. 3

13. Classification of
photosynthetic bacteria
• Two broad groups:
1) Anoxygenic photosynthetic bacteria
2) Oxygenic photosynthetic bacteria

14. Anoxygenic photosynthetic bacteria
• Some photosynthetic bacteria can use light
energy to extract electrons from molecules other
than water.
• These organisms are of ancient origin, presumed
to have evolved before oxygenic photosynthetic
organisms.
• Anoxygenic photosynthetic organisms occur in
the domain Bacteria and have representatives in
four phyla – Purple-Sulphur Bacteria, Purple non-
Sulphur Bacteria, Green-Sulfur Bacteria, Green
non-Sulfur Bacteria.

15. • Anoxygenic photosynthesis depends on electron
donors such as reduced sulphur compounds,
molecular hydrogen or organic compounds.
• They are found in fresh water, brackish water,
marine and hypersaline water.
• Anoxygenic photosynthetic bacteria have been
divided into three groups on the basis of
pigmentation: purple bacteria, green bacteria and
heliobacteria.

16. Purple Bacteria
• The anoxygenic phototrophs grow under
anaerobic conditions in the presence of light
and do not use water as electron donor as
higher plants.
• They grow autotrophically with CO2 and
hydrogen or reduced sulphur compounds act
as electron donor.
• The pigment synthesis is repressed by O2.

17. • Purple bacteria contain Bchl a and b as photosynthetic
pigment.
• The colour of purple bacteria shows brown, pink brown-
red, purple-violet based on carotenoid contents.
• The photosynthetic pigments are innfluenced by light
intensity. At high intensity, photo-apparatus is inhibited.
• Carotenoids give rise to purple colour; mutants lack
carotenoids are blue green reflecting the actual colour of
BChl a.
• Purple Bacteria are of two types: purple-suphur bacteria
and purple non-sulphur bacteria.

18. Purple-sulphur bacteria
• Family: Chromatiaceae
• They are gram negative bacteria which contain
BChl a and b and grow chemolithotrophically
in dark with thiosulphate as electron donor.
• They are also chemoorganotrophs, utilize
acetate, pyruvate and few other compounds.
• The mole % of G+C varies from 46-70.

19. • The cells of purple-sulphur bacteria are larger
than green bacteria and packed with
intracellular sulfide deposition.
• They are found in anoxic zone of lakes and
sulphur springs.
• They are photolithotrophs and motile in
nature e.g. Ectothiorhodospira, Chromatiium,
Thiocapsa, Thiospirillum, Thiodictyon,
Thiopedia etc.

20. Purple non-sulphur Bacteria
• Family: Ectothiorhodospiraceae
• They also contain BChl a and b and use low
concentration of sulphide.
• The concn of sulphide utilized by purple sulphur
bacteria proved toxic to this category of bacteria.
• Earlier, scientists thought that these bacteria are
unable to use sulphide as ele donor for reduction
of CO2 to cell material, thus named them non-
sulphur.

21. • Some non-sulphur bacteria grow anaerobically in the dark
using fermentative metabolism, while others can grow
anaerobically in the dark by respiration in which ele donor
may be an organic/inorganic compound as H2.
• This group is most versatile energetically due to broad
requirements and are photoorganotrophs i.e. use organic
acids, amino acids, benzoate and ethanol.
• They also grow as chemoorganotrophs and require
vitamins.
• The DNA base composition is 61-73 mole % (G+C) and the
sulphur granules are formed outside the cell.
• Eg, Rhodomicrobium, Rhodopseudomonmas,
Rhodospirillum, Rhodocyclus, etc.

22. Green Bacteria
• Instead of green in colour, these are brown due
to the presence of carotenoids components.
• They are gram-negative.
• They contain BChl c, d and e plus small amount of
Bchl a.
• The photosyntheic apparatus is chlorosomes.
• They do not require vitamins for their growth.
• Green bacteria are of two types: green sulphur
bacteria and green non-sulphur bacteria.

23. Green Sulphur Bacteria
• Family: Chlorobiaceae
• They are non-motile, rods, spiral and cocci.
• Chlorosomes are present in the cell.
• They are strictly anaerobic and obligate
phototroph.
• Deposit sulphur extracellularly.
• Mol % G+C is 45-58.
• Eg; Chlorobium, Prostheochloris, Pelodictyon,
Chloroherpeton

24. Green non-Sulphur Bacteria
• The Green non-Sulphur Bacteria are filamentous,
gliding bacteria and thermophilic in nature.
• The pigments are Bchl a, Bchl c, and carotenes.
• Chlorosomes are present when grown
anaerobically.
• They are photoheterotrophic and
photoautotrophic and show gliding movement.
• They do not deposit sulphur.
• The mol % G+C vary 53-55.
• Eg, Chloroflexus

25. Heliobacteria
• Based on 16S rRNA sequencing and other
morphological and biochemical characters,
helicobacter are quite different with other
anoxygenic photosynthetic bacteria.
• They are gram-positive, rod shaped, motile
either by gliding or by means of flagella.
• The mol % G+C is between 50-55.
• Eg, Heliobacterium, Helophilum, Heliobacillus

26. • Most of them produce endospores and grow
up to 42°C.
• The heliobacteria are green in colour.
• Most of the heliobacteria are found in tropical
soils of paddy fields.
• They contain BChl g.

27. Oxygenic Photosynthetic Bacteria
• The Oxygenic Photosynthetic Bacteria are
unicellular or multicellular and possess
bacteriochlorophyll a and carry out oxygenic
photosynthesis.
• They are mostly represented by gram-negative
cynobacteria.
• Carboxysomes and gas vesicles are present and
also show gliding movement.
• Photosynthesis is oxygenic and autotrophic.
• Photosynthates get accumulated in the form of
glycogen.

28. Photosynthetic Pigments
• Because of presence of carotenoids in all
photosynthetic tissues their role is anticipated
in photosynthesis.
• The cells rich in carotenoids devoid of
clorophyll do not photosynthesize.
• Light energy absorbed by carotenoids appears
to be transferred to chlorophyll a or bacterio
chlorophyll a and utilized in photosynthesis.

29. Chlorophylls and
Bacteriochlorophylls
• Phototrophic organisms contain some form of
chlorophyll (oxygenic phototrophs) or
bacteriochlorophyll (anoxygenic hototrophs).
• Chlorophyll a is green in color because it absorbs
red and blue light preferentially and transmits
green light.
• The absorption spectrum of cells containing
chlorophyll a shows strong absorption of red
light(maximum absorption at a wavelength of 680
nm) and blue light (maximum at 430 nm).

30. • There are a number of different chlorophylls and
bacteriochlorophylls, and each is distinguished by
its unique absorption spectrum.
• Chlorophyll b, for instance, absorbs maximally at
660 nm rather than the 680-nm absorbance
maximum of chlorophyll a.
• All plants contain chlorophylls a and b. Some
prokaryotes contain chlorophyll d, while
chlorophyll c is found only in certain eukaryotic
phototrophs.

31. • Among prokaryotes, cyanobacteria produce
chlorophyll a and prochlorophytes produce
chlorophylls a and b.
• Anoxygenic phototrophs, such as the
phototrophic purple and green bacteria,
produce one or more bacteriochlorophylls.
• Bacteriochlorophyll a, present in most purple
bacteria absorbs maximally between 800 and
925 nm, depending on the species.

32. • Why do different phototrophs have different forms of
chlorophyll or bacteriochlorophyll that absorb light of
different wavelengths?
• This allows phototrophs to make better use of the available
energy in the electromagnetic spectrum.
• Only light energy that is absorbed is useful for energy
conservation. By having different pigments with different
absorption properties, different phototrophs can coexist in
the same habitat, each organism using wavelengths of light
that others are not using.
• Thus, pigment diversity has ecological significance for the
successful coexistence of different phototrophs in the same
habitat.

33. • Prokaryotes do not contain chloroplasts. Their
photosynthetic pigments are integrated into
internal membrane systems. These systems arise
(1) from invagination of the cytoplasmic
membrane (purple bacteria); (2) from the
cytoplasmic membrane itself (heliobacteria); (3)
in both the cytoplasmic membrane and
specialized structures enclosed in a nonunit
membrane, called chlorosomes (green bacteria)
or (4) in thylakoid membranes (cyanobacteria)

34. The chlorosome of green sulfur and
green nonsulfur
bacteria. (a) Transmission electron
micrograph of a cross-section of
a cell of the green sulfur bacterium
Chlorobaculum tepidum. Note the
chlorosomes (arrows). (b) Model of
chlorosome structure. The chlorosome
(green) lies appressed to the inside
surface of the cytoplasmic membrane.
Antenna bacteriochlorophyll (Bchl)
molecules are arranged in tubelike
arrays inside the chlorosome, and
energy is transferred from these to
reaction center (RC) Bchl a in the
cytoplasmic membrane (blue) through
a protein called FMO. Base plate (BP)
proteins function as connectors
between the chlorosome and the
cytoplasmic membrane.4

35. Carotenoids and Phycobilins
• Although chlorophyll or bacteriochlorophyll is
required for photosynthesis, phototrophic
organisms contain an assortment of accessory
pigments as well.
• These include, in particular, the carotenoids and
phycobilins. Carotenoids primarily play a
photoprotective role in both anoxygenic and
oxygenic phototrophs, while phycobilins function
in energy metabolism as the major light-
harvesting pigments in cyanobacteria.

36. Carotenoids
• The most widespread accessory pigments in
phototrophs are the carotenoids.
• Carotenoids are hydrophobic light-sensitive pigments
that are firmly embedded in the photosynthetic
membrane.
• Carotenoids are typically yellow, red, brown, or green
in color and absorb light in the blue region of the
spectrum.
• Carotenoids are closely associated with
bacteriochlorophyll in photosynthetic complexes, and
energy absorbed by carotenoids can be transferred to
the reaction center.

37. • Nevertheless, carotenoids primarily function
in phototrophic organisms as photoprotective
agents.
• Bright light can be harmful to cells;
Carotenoids quench toxic oxygen species by
absorbing much of this harmful light and
prevent these dangerous photooxidations.

38. Phycobiliproteins and Phycobilisomes
• Cyanobacteria and the chloroplasts of red algae contain
phycobiliproteins, which are the main light-harvesting
systems of these phototrophs.
• The red phycobiliprotein, called phycoerythrin, absorbs
most strongly at wavelengths around 550 nm, whereas the
blue phycobiliprotein, phycocyanin, absorbs most strongly
at 620 nm.
• Phycobiliproteins assemble into aggregates called
phycobilisomes.
• In a fashion similar to how light-harvesting systems
function in anoxygenic phototrophs, phycobilisomes
facilitate energy transfer to allow cyanobacteria to grow at
fairly low light intensities.

39. Photosynthetic Electron
Transport System
• In the photosynthetic light reactions, electrons travel
through a series of electron carriers arranged in a
photosynthetic membrane in order of their increasingly
more electropositive reduction potential (E0).
• This generates a proton motive force that drives ATP
synthesis.
• Anoxygenic photosynthesis occurs in at least five phyla
of Bacteria: the proteobacteria (purple bacteria); green
sulfur bacteria; green nonsulfur bacteria; the gram-
positive bacteria (heliobacteria); and the acidobacteria.

40. Photosynthetic Reaction Centers
• The photosynthetic apparatus of purple
bacteria has been best studied.
• Membrane vesicles, sometimes called
chromatophores, or membrane stacks called
lamellae are common membrane
morphologies.

41. Membranes in anoxygenic
phototrophs. (a) Chromatophores.
Section through a cell of the purple
bacterium Rhodobacter
showing vesicular photosynthetic
membranes. The vesicles are
continuous
with and arise by invagination of the
cytoplasmic membrane. A cell is
about 1 micro m wide. (b) Lamellar
membranes in the purple bacterium
Ectothiorhodospira. A cell is about 1.5
micro m wide. These membranes are
also continuous with and arise from
invagination of the cytoplasmic
membrane, but instead of forming
vesicles, they form membrane stacks.
5

42. Photosynthetic Electron Flow in
Purple Bacteria
• Pohotosynthetic reaction centers are surrounded by antenna
pigments that function to funnel light energy to the reaction
center.
• The energy of light is transferred from the antenna to the
reaction center in packets called excitons.
• The light reactions begin when exciton energy strikes the special
pair of bacteriochlorophyll a molecules.
• The absorption of energy excites the special pair, converting it
from a relatively weak to a very strong electron donor.
• Once this strong donor has been produced, the remaining steps
in electron flow simply conserve the energy released when
electrons flow through a membrane from carriers of low E0 to
those of high E0, generating a proton motive force.

43. • Before excitation, the purple bacterial reaction
center, which is called P870, has an E0 of about
+0.5 V; after excitation, it has a potential of about
-1.0 V
• The excited electron within P870 proceeds to
reduce a molecule of bacteriochlorophyll a within
the reaction center.
• This transition takes place incredibly fast, taking
only about three-trillionths (3X10-12) of a second.

44. • Once reduced, bacteriochlorophyll a reduces
bacteriopheophytin a and the latter reduces
quinone molecules within the membrane.
• These transitions are also very fast, taking less
than one-billionth of a second.
• Relative to what has happened in the reaction
center, further electron transport reactions
proceed rather slowly, on the order of
microseconds to milliseconds.

45. • From the quinone, electrons are transported in
the membrane through a series of iron–sulfur
proteins and cytochromes, eventually returning
to the reaction center.
• Key electron transport proteins include
cytochrome bc1 and cytochrome c2.
• Cytochrome c2 is a periplasmic cytochrome that
functions as an electron shuttle between the
membrane-bound bc1 complex and the reaction
center.

46. Electron flow in anoxygenic
photosynthesis in a purple
bacterium. Only a single light
reaction occurs. Note how light
energy
converts a weak electron donor,
P870, into a very strong electron
donor,
P870*. Bph, bacteriopheophytin;
QA, QB, intermediate quinones; Q
pool,
quinone pool in membrane; Cyt,
cytochrome.
6

47. Arrangement of protein complexes in the purple
bacterium reaction center. The light-generated proton gradient is
used in the synthesis of ATP by the ATP synthase (ATPase). LH, lightharvesting
bacteriochlorophyll complexes; RC, reaction center; Bph,
bacteriopheophytin; Q, quinone; FeS, iron–sulfur protein; bc1, cytochrome
bc1 complex; c2, cytochrome c2.
7

48. Photophosphorylation
• ATP is synthesized during photosynthetic electron flow
from the activity of ATPase that couples the proton motive
force to ATP formation.
• Electron flow is completed when cytochrome c2 donates an
electron to the special pair.
• This returns these bacteriochlorophyll molecules to their
original ground-state potential (E0= +0.5 V).
• The reaction center is then capable of absorbing new
energy and repeating the process.
• This mechanism of ATP synthesis is called
photophosphorylation, specifically cyclic
photophosphorylation, because electrons move within a
closed loop.

49. Autotrophy in Purple Bacteria:
Electron Donors and Reverse Electron
Flow
• For a purple bacterium to grow as a photoautotroph,
the formation of ATP is not enough.
• Reducing power (NADH or NADPH) is also necessary so
that CO2 can be reduced to the redox level of cell
material.
• As previously mentioned, reducing power for purple
sulfur bacteria comes from hydrogen sulfide (H2S),
although sulfur (S0), thiosulfate (S2O3
2-), ferrous iron
(Fe2+), nitrite (NO2-), and arsenite (AsO3
2-) can also be
used by one or another species.
• When H2S is the electron donor in purple sulfur
bacteria, globules of S0 are stored inside the cells.

50. Phototrophic purple and green sulfur bacteria. (a) Purple
bacterium, Chromatium okenii. Notice the sulfur granules deposited
inside the cell (arrows). (b) Green bacterium, Chlorobium limicola. The
refractile bodies are sulfur granules deposited outside the cell (arrows). In
both cases the sulfur granules arise from the oxidation of H2S to obtain
reducing power. Cells of C. okenii are about 5 m in diameter, and cells
of C. limicola are about 0.9 m in diameter. Both micrographs are brightfield
images.
8

51. • Reduced substances used as photosynthetic electron donors are
oxidized and electrons eventually end up in the “quinone pool”
of the photosynthetic membrane.
• However, the E0 of quinone (about 0 volts) is insufficiently
electronegative to reduce NAD+ (- 0.32 V) directly.
• Instead, electrons from the quinone pool travel backwards
against the thermodynamic gradient to eventually reduce
NAD(P)+ to NAD(P)H.
• This energy-requiring process, called reverse electron transport,
is driven by the energy of the proton motive force
• If NADPH is needed as a reductant instead of NADH, it can be
produced from NADH by enzymes called transhydrogenases.

52. Photosynthesis in Other
Anoxygenic Phototrophs
• Our discussion of photosynthetic electron flow has
thus far focused on the process as it occurs in purple
bacteria.
• Although similar membrane reactions drive
photophosphorylation in other anoxygenic
phototrophs, there are differences in certain details.
• The reaction centers of green nonsulfur bacteria and
purple bacteria are structurally quite similar; however,
the reaction centers of green sulfur bacteria and
heliobacteria differ significantly from those of purple
and green nonsulfur bacteria.

53. A comparison of electron flow in purple bacteria, green sulfur bacteria, and heliobacteria.
Note that reverse electron flow in
purple bacteria is necessary to produce NADH because the primary acceptor (quinone, Q) is
more positive in potential than the NAD+/NADH couple. In green and heliobacteria, ferredoxin
(Fd), whose E0 is more negative than that of NADH, is produced by light-driven reactions for
reducing power needs. Bchl, Bacteriochlorophyll; BPh, bacteriopheophytin. P870 and P840 are
reaction centers of purple and green bacteria, respectively, and consist of Bchl a. The reaction
center of heliobacteria (P798) contains Bchl g, and the reaction center of Chloroflexus is of the
purple bacterial type. Note that forms of chlorophyll a are in the reaction centers of green
bacteria and heliobacteria.
9

54. • In green bacteria and heliobacteria the excited state of the
reaction center bacteriochlorophylls resides at a significantly
more electronegative E0 than in purple bacteria and that actual
chlorophyll a (green bacteria) or a structurally modified form of
chlorophyll a, called hydroxychlorophyll a (heliobacteria), is
present in the reaction center.
• Thus, unlike in purple bacteria, where the first stable acceptor
molecule (quinone) has an E0 of about 0 V, the acceptors in
green bacteria and heliobacteria iron–sulfur (FeS) proteins have
a much more electronegative E0 than does NADH.
• In green bacteria a protein called ferredoxin (reduced by the
FeS protein) is the direct electron donor for CO2 fixation.
• This has a major effect on reducing power synthesis in these
organisms, as reverse electron flow, necessary in purple
bacteria, is not required in green sulfur bacteria or heliobacteria.

55. Oxygenic Photosynthesis
• In contrast to electron flow in anoxygenic phototrophs,
electron flow in oxygenic phototrophs proceeds through
two distinct but interconnected series of light reactions.
• The two light systems are called photosystem I and
photosystem II, each photosystem having a spectrally
distinct form of reaction center chlorophyll a.
• Photosystem I (PSI) chlorophyll, called P700, absorbs light
at long wavelengths (far red light), whereas PSII chlorophyll,
called P680, absorbs light at shorter wavelengths (near red
light).
• Oxygenic phototrophs use light to generate both ATP and
NADPH, the electrons for the latter arising from the
splitting of water into oxygen and electrons.

56. Electron Flow in
Oxygenic Photosynthesis
• The path of electron flow in oxygenic
phototrophs resembles the letter Z turned on its
side, and Figure outlines this so-called “Z
scheme” of photosynthesis.
• The reduction potential of the P680 chlorophyll a
molecule in PSII is very electropositive, even
more positive than that of the O2/H2O couple.
• This facilitates the first step in oxygenic electron
flow, the splitting of water into oxygen and
electrons.

57. • Light energy converts P680 into a strong reductant
which reduces pheophytin a (chlorophyll a minus its
magnesium atom), a molecule with an E0 of about -0.5
V.
• An electron from water is then donated to the oxidized
P680 molecule to return it to its ground-state reduction
potential.
• From the pheophytin the electron travels through
several membrane carriers of increasingly more
positive E0 that include quinones, cytochromes, and a
copper-containing protein called plastocyanin; the
latter donates the electron to PSI.

58. • The electron is accepted by the reaction
center chlorophyll of PSI, P700, which has
previously absorbed light energy and donated
an electron that will eventually lead to the
reduction of NADP+.
• Electrons transferred through several
intermediates terminating with the reduction
of NADP+ to NADPH.

59. ATP Synthesis in
Oxygenic Photosynthesis
• Besides the net synthesis of reducing power (that
is, NADPH), other important events take place
while electrons flow in the photosynthetic
membrane from PSII to PSI. Electron transport
generates a proton motive force from which ATP
can be produced by ATPase. This mechanism for
ATP synthesis is called noncyclic
photophosphorylation because electrons do not
cycle back to reduce the oxidized P680, but
instead are used in the reduction of NADP+.

60. Electron flow in
oxygenic
photosynthesis, the
“Z” scheme.
Electrons flow
through two
photosystems, PSI
and PSII. Ph,
pheophytin; Q,
quinone; Chl,
chlorophyll; Cyt,
cytochrome; PC,
plastocyanin;
FeS, nonheme iron–
sulfur protein; Fd,
ferredoxin; Fp,
flavoprotein; P680
and P700 are the
reaction
center chlorophylls of
PSII and PSI,
respectively.10

61. The Calvin-Benson cycle
• The light-independent reactions of the Calvin cycle can
be organized into three basic stages: fixation, reduction,
and regeneration.
• The Calvin cycle has three stages. In stage 1, the enzyme
RuBisCO incorporates carbon dioxide into an organic
molecule, 3-PGA. In stage 2, the organic molecule is
reduced using electrons supplied by NADPH. In stage 3,
RuBP, the molecule that starts the cycle, is regenerated
so that the cycle can continue. Only one carbon dioxide
molecule is incorporated at a time, so the cycle must be
completed three times to produce a single three-carbon
GA3P molecule, and six times to produce a six-carbon
glucose molecule.

62. 11

63. • Stage 1: Fixation
• in addition to CO2,two other components are present to initiate the
light-independent reactions: an enzyme called ribulose
bisphosphate carboxylase (RuBisCO) and three molecules of
ribulose bisphosphate (RuBP). RuBP has five atoms of carbon,
flanked by two phosphates. RuBisCO catalyzes a reaction between
CO2 and RuBP. For each CO2 molecule that reacts with one RuBP,
two molecules of 3-phosphoglyceric acid (3-PGA) form. 3-PGA has
three carbons and one phosphate. Each turn of the cycle involves
only one RuBP and one carbon dioxide and forms two molecules of
3-PGA. The number of carbon atoms remains the same, as the
atoms move to form new bonds during the reactions (3 atoms from
3CO2 + 15 atoms from 3RuBP = 18 atoms in 3 molecule of 3-PGA).
This process is called carbon fixation because CO2 is "fixed" from an
inorganic form into organic molecules.

64. • Stage 2: Reduction
• ATP and NADPH are used to convert the six molecules
of 3-PGA into six molecules of a chemical called
glyceraldehyde 3-phosphate (G3P). This is a reduction
reaction because it involves the gain of electrons by 3-
PGA. Recall that a reduction is the gain of an electron
by an atom or molecule. Six molecules of both ATP and
NADPH are used. For ATP, energy is released with the
loss of the terminal phosphate atom, converting it to
ADP; for NADPH, both energy and a hydrogen atom are
lost, converting it into NADP+. Both of these molecules
return to the nearby light-dependent reactions to be
reused and reenergized.

65. • Stage 3: Regeneration
• At this point, only one of the G3P molecules leaves the
Calvin cycle and is sent to the cytoplasm to contribute
to the formation of other compounds needed by the
plant. Because the G3P exported from the chloroplast
has three carbon atoms, it takes three "turns" of the
Calvin cycle to fix enough net carbon to export one
G3P. But each turn makes two G3Ps, thus three turns
make six G3Ps. One is exported while the remaining
five G3P molecules remain in the cycle and are used to
regenerate RuBP, which enables the system to prepare
for more CO2 to be fixed. Three more molecules of ATP
are used in these regeneration reactions.

66. Conclusion
• The Calvin cycle refers to the light-independent reactions in
photosynthesis that take place in three key steps.
• Although the Calvin Cycle is not directly dependent on light, it is indirectly
dependent on light since the necessary energy carriers (ATP and NADH)
are products of light-dependent reactions.
• In fixation, the first stage of the Calvin cycle, light-independent reactions
are initiated; CO2 is fixed from an inorganic to an organic molecule.
• In the second stage, ATP and NADPH are used to reduce 3-PGA into G3P;
then ATP and NADH are converted to ADP and NADP+, respectively.
• In the last stage of the Calvin Cycle, RuBP is regenerated, which enables
the system to prepare for more CO2to be fixed.
• For each molecule of carbon dioxide that is fixed, two molecules of NADPH
and three molecules of ATP from the light reactions are required. The
overall reaction can be represented as follows:

67. 12

68. References
• Reading
• Brock biology of
microorgamism (13th
edition) by Madigan,
Martinko, Stahl, Clark
• Microbiology (10th
edition) by Tortora,
Funke and Case
• Microbiology (5th
edition) by Prescott
• Images
• 1-12: Brock biology of
microorgamism (13th
edition) by Madigan,
Martinko, Stahl, Clark