Photosynthesis The light reactions

The light reactions
• One of the most important biochemical
process in plants.
– Let’s not forget cell wall biosynthesis and
adaptation during plant development, growth,
interaction with the environment, and disease
• Among the most expensive biochemical
processes in plant in terms of investment
• The biochemical process that has driven
plant form and function
General overall reaction
6 CO2 + 6 H2O
Carbon dioxide
C6H12O6 + 6 O2
Photosynthetic organisms use solar energy to synthesize
carbon compounds that cannot be formed without the input
of energy.
More specifically, light energy drives the synthesis of
carbohydrates from carbon dioxide and water with the
generation of oxygen.
Overall Perspective
• Light reactions:
– Harvest light energy
– Convert light energy to
chemical energy
• Dark Reactions:
– Expend chemical energy
– Fix Carbon [convert CO2 to
organic form]
The sun emits a tremendous amount of
energy, only some of which is useable
But only a small fraction of the sun’s
energy reaches the surface
• Solar spectrum and its
relation to the absorption
spectrum of chlorophyll
• Very little of the Sun’s
energy gets to the ground
(red line).
– gets absorbed by water
vapor in the atmosphere
• The absorbance spectra
of chlorophyll (green line).
– Absorbs strongly in the
blue and red portion of the
– Green light is reflected
and gives plants their
Photosynthetic pigments
Two types in plants:
Chlorophyll- a
Chlorophyll –b
Structure almost identical,
– Differ in the composition of
a sidechain
– In a it is -CH3, in b it is
• The different sidegroups 'tune'
the absorption spectrum to
slightly different wavelengths
– light that is not significantly
absorbed by chlorophyll a,
will instead be captured by
chlorophyll b
Photosynthetic pigments
• Chlorophyll has a complex
ring structure
– The basic structure is a
porphyrin ring, cocoordinated to a central
– This is very similar to the
heme group of hemoglobin
• Ring contains loosely
bound electrons
– It is the part of the
molecule involved in
electron transitions and
redox reactions of
Photosynthetic pigments
• When chlorophyll absorbs a
light particle (Proton)
– Enters a higher excitation state
– Becomes unstable, gives up
energy as heat
– Enters lower excited state
• can be stable for a few
• This energy causes chemical
reactions to occur
• These reactions are the
fastest known to science!!!!
• Takes place in complexes
containing light-harvesting
antennas & photochemical
reaction centers
• Antenna complex
– Chemical oxidation & reduction
reactions leading to long term
energy storage take place
• Antenna collects light and
transfers its energy to the
reaction center
• Chemical reactions store some
of the energy by transferring
electrons from chlorophyll to an
electron acceptor molecule
• An electron donor then reduces
the chlorophyll again
• The transfer of energy to the
antenna is a purely physical
phenomenon and involves no
chemical changes
• Even in bright sunlight, a
chlorophyll molecule absorbs
only a few photons each second
– Therefore, many chlorophyll
molecules send energy into a
common reaction center
– The whole system is kept active
most of the time
The chemical reaction of
photosynthesis is driven by light
• The initial reaction of photosynthesis is:
– CO2 +H2O
(CH2O) + O2
– Under optimal conditions (red light at 680 nm), the
photochemical yield is almost 100 %
– However, the efficiency of converting light energy to
chemical energy is about 27 %
• Very high for an energy conversion system
– Quantum efficiency: Measure of the fraction of
absorbed photons that take part in photosynthesis
– Energy efficiency: Measure of how much energy in the
absorbed photons is stored as chemical products
• ¼ energy from photons stored – the rest is converted to heat
Light drives the reduction of
NADP & the formation of ATP
• Overall process of photosynthesis is a redox
chemical reaction
– Electrons are removed from one chemical species
(oxidation) and added to another (reduction)
• Light reduces NADP, which serves as the reducing
agent for carbon fixation during the dark reactions
– ATP also formed during electron flow from water to
NADH and is also used during carbon fixation
• Thylakoid reactions: water oxidized to oxygen,
NADP reduced and ATP is formed
• Stroma reactions: carbon fixation and reduction
Oxygen-evolving organisms have two
photosystems that operate in series
• Two photochemical complexes operate in series to
carry out the early energy storage reactions of
• Photosystem I: Absorbs far-red light (greater
than 680 nm)
– Produces a strong reductant, capable of reducing NADP
– Also produces a weak oxidant
• Photosystem II: Absorbs red light of 680 nm
– Produces a strong oxidant, capable of oxidizing water
– Also produces a weak reductant
• These two photosystems are linked by an
electron transport chain
Oxygen-evolving organisms have two
photosystems (PS) that operate in series
• Z (zigzag) scheme – the basis of understanding O2-evolution
• Far-Red light absorbed by PS-I makes strong oxidant (& weak
• Red light absorbed by PS-II makes strong reductant (& weak oxidant)
• PS-II oxidizes water and PS-I reduces NADP - P680 and P700 refers to
wavelength of max absorption of reaction center chlorophylls
The chloroplast
A quick recap
The Chloroplast
• Contain their own DNA and
– Ribosomes, transfer
RNAs, nucleotides.
– Thought to have evolved
from endosymbiotic
– Divide by fusion
– The DNA is in the form
of circular chromosomes,
like bacteria
– DNA replication is
independent from DNA
replication in the nucleus
The Chloroplast
• Membranes contain
chlophyll and it’s
associated proteins
– Site of photosynthesis
• Have inner & outer
• 3rd membrane system
– Thylakoids
• Stack of Thylakoids =
• Surrounded by Stroma
– Works like mitochondria
• During photosynthesis,
ATP from stroma provide
the energy for the
production of sugar
The Chloroplast
PS-I and PS-II are spatially separated
in the thylakoid membrane
• The PS-II reaction center is
located mostly in Granum.
– Stack of Thylakoids
• The PS-I reaction center is
located in the Stroma & the
edges of the Granum.
– There is a cytochrome b6f
complex that connects the two
photosystems that is evenly
distributed between Granum
and Stroma
• One or more of the electron
carriers that function
between the photosystems
diffuses from the from the
Granum to the Stroma.
Electron transfer
A step by step look
Oxygen-evolving organisms have two
photosystems (PS) that operate in series
What is an electron transport chain?
A series of coupled oxidation/reduction reactions where
electrons are passed from one membrane-bound
protein/enzyme to another before being finally attached to a
terminal electron acceptor (usually oxygen or NADPH).
Mechanisms of electron transfer
• The energy changes of
electrons as they flow
through the light reactions
are analogous to the
• The light reactions use
solar power to generate
ATP and NADPH which
provide chemical energy
and reducing power to the
sugar making reactions
The transport chain
• Z (zigzag) scheme – the
basis of understanding O2evolution
• Far-red light absorbed by
PS-I makes strong oxidant (&
weak reductant)
• Red light absorbed by PS-II
makes strong reductant (&
weak oxidant)
• PS-II oxidizes water and PSI reduces NADP - P680 and
P700 refers to wavelength of
max absorption of reaction
center chlorophylls
The transport chain
• PS-II oxidizes water to O2 in the thylakoid lumen
– Releases protons into the lumen
• Cytochrome b6f receives electrons from PS-II & delivers
them to PS-I
– Also transports additional protons into lumen from stroma
The transport chain
• PS-I reduces NADP to NADPH in the stroma
– Uses the action of:
– Ferredoxin (Fd)
– Ferredoxin-NADP reductase (FNR)
• ATP synthesis produces ATP as protons diffuse
back through it from the lumen into the stroma
Summary of light reactions
• Photosynthesis (light reactions):
– Storage of solar energy carried out by plants
• Absorbed photons excite chlorophyll
– can dispose of this energy as:
Energy transfer
Photochemistry – the light reactions of
• Absorption of light occurs in the thylakoid
membranes of the chloroplast by chlorophyll
Summary of light reactions
• Plants have two reaction centers:
• Absorbs Red light – 680mn
• makes strong reductant (& weak oxidant)
• oxidizes 2 H2O molecules to 4 electrons, 4 protons & 1
O2 molecule
• Mostly found in Granum
– PS-I
Absorbs Far-Red light – 700nm
strong oxidant (& weak reductant)
PS-I reduces NADP to NADPH
Mostly found in Stroma
Summary of light reactions
• Excess light energy can damage
photosynthetic systems
– Several mechanisms occur to minimize such
• Some proteins made in chloroplasts act as
photoprotective agents to control excited state of
chlorophyll molecules
• Chloroplasts contain DNA and encode and
synthesize most of the proteins essential for
– Others encoded by nuclear DNA
Summary of light reactions
• Chlorophylls made in a biosynthetic pathway
involving more than 12 steps.
• Once synthesized, pigment proteins are
assembled in the thylakoid membrane.
• Lastly:
• The initial reaction of the light reaction is:
– CO2 +H2O
(CH2O) + O2