Light driven phosphorylation -
production of ATP via photo-phosphorylation
ADP + P -->
| Cellular process
eukaryotic cells with
Capture of light energy by pigments -
Capture e’s as reducing power in
Reduction of CO2
2 Fundamental Reaction Mechanisms
LIGHT Reactions (photo-chemical reactions)
molecular excitation chlorophyll by light...
hydrolysis of H2O
generation of proton motive force (H+ gradient)
across thylakoid membranes
reduction of NADP to
NADPH via an ETS
DARK Reactions (thermo-chemical reactions)
via (reduction stages - reverse of
+ RuBP --> 2 PGA [ 1C + 5C -->
2 (3C) ]
with NADPH --> PGAL
regeneration of RuBP via HMP
(5C sugar) pathway --> RuBP
6CO2 + 12H2O* -->
+ 6H2O + 6O*2
Evolutionary Basis of Photosynthesis...
origin of chloroplast may have been symbiotic (mcb
1st autotrophic cells probably used
as e- source
as purple-sulfur bacteria of today
H2S --> CH2O
+ H2O + 2S
cyanobacteria - are oxygenic photosynthetic prokaryotes
H2O --> CH2O
+ H2O + O2
van Neil equation (gs atSU)...
Phts is a REDOX reaction
CO2 + H2A --> CH2O + H2O + 2A
Molecular Excitation of Chlorophyll
Absorption of Light Energy...
Visible light: - blue light [440nm] = 71.5 Kc/einstein
- red light [700nm] = 40.9 Kc/einstein
Chlorophyll* Molecule in its Ground State -
e's with opposite spin which =
light absorption moves non-bounded
e's to higher
1st excited singlet state...
2nd excited singlet state...
1st long-lived state...
FATES of Absorbed Light Energy
1. Re-radiated as vibrational heat
2. Re-radiated as
emission of light of longer wavelength
700nm --> 710nm in time frame 10-9sec, less energetic
3. Re-radiated as
emission of light much longer wavelength
700nm --> 720nm in real time (1sec)
Induced resonance* -
e excitation induces similar electronic vibrations
in adjacent molecules causing their excitation,
e's enters into the photochemical reactions of
pass to an acceptor leaving an ionized chl+
Photosynthetic Electron Flow...
what happens to the photoionized e-
4 stages PHTS*
1) light absorption,
transport & PMF, 3)
ATP synthesis, & 4)
- chlorophylls, accessory pigments, reaction center, primary
PS-II - contain
e- flow within PS-II & PS-I is
--> noncyclic electron flow
captures e- into coenzyme NADP+ ---> NADPH
2) releases O2
from splitting of H2O O2-evol*
Karp 6.9 non-cyclic
path of e- flow PS1 -
cyclic electron flow
basis & mechanism of action...
& mcb 12.42* &
some possible locales
cooperation of PSII & PSI
within the complexity of the e- reactions...
Karp fig 6.10 PS-II &
Karp fig 16.14 PSI &
Karp fig 6.13
ATPase makes ATP (just like in mitochondria)...
proton gradient generate a PMF
locations of chemiosmosis (chlp & mito)
Reactions of Photosynthesis...
occur in stroma (chloroplasm)
consume ATP and NADPH made in light reactions
reduces (fixation) CO2 into
Radiocarbon tracing C-atoms during dark reaction
& Calvin "lollipop*"
chromatography was used to
3 different pathways to make sugar
1. CALVIN cycle
CO2 + 5C RuBP ---> (2) 3C sugars (PGA)
(2) 3C sugars combine ---> 1 net glucose
RuBP carboxylase -
- [50% of leaf protein]
but Rubisco is inhibited by O2 -->
Hatch & Slack Pathway...
of Carbon Fixation...
14CO2 + 3C PGA --> 4C-acid
[OAA] --> malate in mesophyll cells
4C-acid ---> 3C (PYR) +
is reduced in Calvin Cycle of bundle sheath cells to glucose.
pathway (mcb 12.46b*)
leaf anatomy differences (mcb
of pathways -
karp fig 6.21
Rubisco's - Km(CO2)
10mM in C3
plants, but about 30mM
in C4 plants
Several groups of angiosperms
sugarcane) as well as certain
& halophytes such as Atriplex (saltbush)
have developed the
adaptations which allow
CO2 uptake & formation of a 4-carbon
molecule [OAA] instead
of the two 3-carbon
PGA's of the Calvin cycle. Hence
these plants are called
of C-4 pathway. Plants that have only the Calvin cycle are thus
plants but don't separate C4 &
C3 pathways in different parts
of leaf (spatially),
3. CAM Pathway (C4-
Crassulacean Acid Metabolism)
- CAM plants...
rather separates them in time. CAM was 1st studied in the plant family
CAM plants take in
CO2 through open stomata
at night, when the succulents are in a cool
CO2 joins with
PEP to form the 4-carbon
oxaloacetic acid. OAA is converted
to 4-carbon malic acid that accumulates during the night in the
In the morning,
stomata close (thus conserving moisture as well as reducing
the inward diffusion of
Accumulated malic acid leaves the vacuole and is broken down
to release CO2
that is taken up into the Calvin (C3)
These temporal and anatomical adaptations also enable these plants to thrive in
(1) high daytime temperatures,
(2) intense sunlight, &
(3) low soil moisture.
The CAM path occurs in a wide variety of plant species, mainly in
arid and tropical regions...
Some examples of CAM plants: Crassulaceae (Sedum,
epiphytic bromeliads including Spanish moss,
Orchids, the century plant,
yucca, and the "ice plant" that grows
parts of the scrub forest biome.
CAM plants represent only about 10% of
the world's flora (C3 are 85% &
Same pathway & carboxylase
enzymes as in C4, but within the same cell..
shows temporal not spatial differences
Comparative figure C4 v. CAM*
RUBISCO catalyzes two
to ribulose bisphosphate - a carboxylase
to ribulose bisphosphate - an
Which one predominates depends on the relative concentrations of
O2 & CO2
: low O2 favoring the
: low CO2 favoring the
light reactions liberate O2 & more
in the cytosol of the cell at higher temps. Thus,
intensities & high temps (above ~ 30°C) favor the
oxygenase second reaction. The uptake
by RUBISCO forms two
3-carbon molecules: [reaction]
1) one is 3-phosphoglyceric acid
[3PGA] just as in the
2) the other is
Cellular basis -
Karp fig 6.22 (p239)... the glycolate enters
where it uses O2 to form intermediates that enter mitochondria where they are broken down to
so this process uses O2
and liberates CO2
just as cellular
respiration does, thus it's called
photorespiration. It undoes the anabolic work of photosynthesis,
reducing the net productivity of a plant. For this reason, much effort so
far largely unsuccessful - has gone into attempts to alter crop plants so that
they carry on less photorespiration.
Speculation: the problem may solve itself. If
concentrations continue to rise, perhaps it will enhance the net
productivity of the world's crops by reducing losses to photorespiration.
of the C4 cycle:
2 diffuses into a
mesophyll cell. Being close to the leaf
surface, these cells are exposed to high levels of
but these cells have little RUBISCO, whose Km is
less efficient here than in bundle sheath cells, so cannot start
photorespiration or the Calvin cycle dark reactions.
is inserted into a 3-carbon compound (C3)
called phosphoenolpyruvic acid (PEP) forming the
4-carbon compound oxaloacetic acid (C4).
is converted into malic acid or
aspartic acid (both have 4 carbons), which is
transported (by plasmodesmata) into a bundle sheath cell.
Bundle sheath cells
are deep in the leaf interior so atmospheric oxygen cannot diffuse easily to
them; often have thylakoids with reduced
photosystem II complexes (the one that produces O2). Both of
these features keep oxygen levels low.
Here the 4-carbon compound (malic/aspartic
acids) is broken down into CO2
carbon dioxide, which enters the Calvin cycle to form sugars and starch.
Pyruvic acid (C3) - which is transported back to a mesophyll
cell where it is converted back into PEP.
- After entering through stomata,
plants represent only 5% of world's species,
but fix 18% of world's organic carbon
plants are well adapted to (and likely to be found in) habitats with high
temperatures and intense sunlight... including: crabgrass,
corn (maize), sugarcane,