Energy Harvesting Pathways
Photosynthesis
photosynthesis
• reverses the oxidation of glycolysis/respiration
C6H12O6 +6 O2 => 6 CO2 +6 H2O + energy
energy +6 CO2 +12 H2O =>6 O2 +C6H12O6 +6 H2O
photosynthesis
• reverses the oxidation of glycolysis/respiration
• reduces highly oxidized carbon
– stores energy in hydrocarbon bonds
• utilizes “free” resources
– water from soil reservoir
– CO2 from atmospheric reservoir
– energy from diurnal light source
• releases O2 as a byproduct
reactants and products
of
photosynthesis
Figure 8.1
photosynthesis
• occurs in chloroplasts
– “light reactions” on thylakoid membranes
– “dark reactions” in aqueous stroma
• two interconnected pathways
– light-driven electron transport generates
reductant & energy
– Calvin-Benson cycle reduces CO2 &
assembles carbohydrates
photosynthesis:
overview
Figure 8.3
visible
light
occupies
a
narrow band of
the
electromagnetic
spectrum
Figure 8.5
photosynthetic light reactions
• light is the visible portion of the
electromagnetic radiation spectrum
– between ultraviolet and infrared
• light travels in wave-like fashion
– wavelength & frequency are inversely
related
• shorter wavelength : higher frequency
• longer wavelength : lower frequency
photosynthetic light reactions
• light energy occurs in discrete units: photons
– energy of a photon is inversely proportional
to wavelength
• shorter wavelength : higher energy
• longer wavelength : lower energy
• intensity measures number of photons striking
a unit area per unit time (e.g. µE·m-2·s-1)
green light
is
transmitted
(and reflected)
as
blue and red
are
absorbed
photosynthetic light reactions
• molecules absorb electromagnetic radiation
– pigments absorb visible light of certain
wavelengths
– photon-pigment interactions
• reflection
• transmission
• absorption - pigment is excited by photon
–excited state - ground state = energy of
photon
absorption
of a photon
excites a
molecule
Figure 8.4
absorption
and
action
spectra
Figure 8.6
Chlorophyll a:
√ tetrapyrrole ring
√ coordinated Mg
√ hydrophobic tail
Figure 8.7
photosynthetic light reactions
• molecules absorb electromagnetic radiation
– a pigment absorbs only certain wavelengths
• an absorption spectrum is a molecular
fingerprint
• an action spectrum plots effectiveness vs.
wavelength
• eukaryotic photosynthesis uses chlorophyll a
as the central pigment
– accessory pigments transfer energy to Chl a
• in plants: Chl b, carotenoids
photosynthetic
electron
transport
mutants
fluoresce…
photosynthetic light reactions
• possible fates of absorbed energy
– loss as heat
– loss as fluorescence
– intermolecular transfer
• direct transfer
• electron transport
fates of
energy
Figure
8.8
photosynthetic light reactions
• excited reaction center chlorophyll a is a good
reducing agent
– PSII Chl a* drives electron transport
through carriers in the thylakoid membrane
– PSI reaction center chlorophyll is reduced
by electrons transported from PSII
– PSI Chl a reduces NADP+ => NADPH
– PSII Chl a+ is reduced with e- from H2O
• O2 is released as a byproduct
Figure 8.9
thylakoids are flat sacks
that reside in the
chloroplast
Figure 8.11
transfers of absorbed energy
Figure 8.11
photosynthetic light reactions
• noncyclic electron transport produces ATP
and NADPH
• cyclic electron transport produces ATP,
but not NADPH
Cyclic electron transport
Figure 8.10
the light reactions of photosynthesis
Figure 8.11
the light reactions of photosynthesis
• electrons flow from water to NADP+
– NADPH is produced
• a proton gradient is formed
– ATP is produced
light and “dark” reactions
are coupled by
ATP &
NADPH
Figure 8.3
carbon fixation reactions
• How does the plant incorporate CO2 into the
existing “carbon pool”?
– CO2 must be attached to one or more
existing molecules - which one(s)?
– …feed a plant CO2 and watch where it
goes…
Calvin, Benson,
et al.
photosynthesis
in Chlorella
with 14CO2
Figure 8.12
carbon fixation reactions
• 3-phosphoglycerate is the first product of
carbon fixation
• other molecules were labeled over time
CalvinBenson
Cycle model
of carbon
fixation
Figure 8.13
12 3PG
carbon fixation reactions
• the acceptor is not a 2-carbon molecule
• it’s ribulose 1,5-bisphosphate
• a 5-C sugar
• the first product is not 3PG
• it’s an unstable 6-C intermediate
3PG is the first stable product
Figure 8.14
CalvinBenson
Cycle model
of carbon
fixation
Figure 8.13
CalvinBenson
Cycle model
of carbon
fixation
Figure 8.13
carbon fixation reactions
• Calvin-Benson cycle accomplishes three tasks
• carbon fixation - by rubisco
• reduction of fixed C into carbohydrate
• 3-phosphoglyceric acid =>
• glyceraldehyde 3-phosphate
• requires reductant & energy
• formation of more RuBP (hence, cycle)
• requires multiple enzymes & ATP
CalvinBenson
Cycle model
of carbon
fixation
Figure 8.13
Product of Calvin-Benson Cycle
• G3P is the reduced product of the CalvinBenson cycle
Product of Calvin-Benson Cycle
• G3P is the reduced product of the CalvinBenson cycle
Product of Calvin-Benson Cycle
• G3P is the reduced product of the CalvinBenson cycle
– 1/6 of G3P is product; 5/6 are reaction
intermediates
– “excess” G3P is used to make
monosaccharides
• 1/3 of G3P is stored in the chloroplast as
starch
• 2/3 of G3P is transported elsewhere as
sucrose
Figure 8.3
carbon fixation reactions
• ribulose bisphosphate carboxylase/oxygenase
• Rubisco
• most abundant protein in the world,
but…
photorespiration
• ribulose bisphosphate carboxylase/oxygenase
is very ineffective
• rubisco adds CO2 to RuBP or adds O2 to
RuBP
• 5C + 1C => 2 · 3C
• 5C + 0C => 2C + 3C
• costs ATPs to regenerate RuBP
chloroplast, peroxisome,
mitochondrion
Figure 8.15
photorespiration
• ribulose bisphosphate carboxylase/oxygenase
• carboxylase & oxygenase activities compete
• rubisco CO2 affinity is low
• stomata must be open for efficient PS
• easy access for 20% O2 & 0.035% CO2
• up to 30% of fixed carbon is lost to
photorespiration in important crops plants
• some plants don’t suffer so much from
photorespiration
photorespiration - solution
• C3 plants and C4 plants
• 3PG is the first detectable product of C
fixation in C3 plants
• C4 plants produce a 4-C product first
PEP + CO2 ======> oxaloacetate
3C 1C PEP C’ase 4C
• PEP carboxylase
• has high CO2 affinity
• is never an oxygenase
photorespiration - solution
• but…
• C4 plants use rubisco, just like C3 plants
• PEP carboxylase and rubisco are separated
into different compartments
C3 & C4 leaf anatomies
Figure 8.16
mesophyll cells and
bundle sheath cells
communicate in C4 plants
Figure 8.17
photorespiration - solution
• C4 bundle sheath cells are enriched in CO2
relative to O2
• rubisco fixes O much less often
spatial separation of initial C fixation
and Calvin Benson cycle
Table 8.1