Photosynthesis
The light reactions
Photosynthesis
• 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 defense.
• 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
C6H12O6 + 6 O2
Carbon dioxide Water
Carbohydrate
Oxygen
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
spectrum
– Green light is reflected and
gives plants their color.
Photosynthetic pigments
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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 CHO
• 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, co-coordinated
to a central atom.
– 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
photosynthesis
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
nanoseconds
• This energy causes chemical
reactions to occur
• These reactions are the
fastest known to science!!!!
Photosynthesis
• 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
Photosynthesis
• 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 reactions
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 photosynthesis
• 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
reductant)
• 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
protein-synthesizing machinery
– Ribosomes, transfer RNAs,
nucleotides.
– Thought to have evolved
from endosymbiotic bacteria.
– 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 membranes
• 3rd membrane system
– Thylakoids
• Stack of Thylakoids = Granum
• Surrounded by Stroma
– Works like mitochondria
• During photosynthesis, ATP
from stroma provide the energy
for the production of sugar
molecules
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
cartoon.
• 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 molecules
– can dispose of this energy as:
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Heat
Fluorescence
Energy transfer
Photochemistry – the light reactions of photosynthesis
• Absorption of light occurs in the thylakoid
membranes of the chloroplast by chlorophyll a & b
Summary of light reactions
• Plants have two reaction centers:
– PS-II
• 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
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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 damage
• 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
photosynthesis
– 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