Photosynthesis
The light reactions
Chapter 10
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
Carbon dioxide
Water
C6H12O6 + 6 O2
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
• Carbon (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.
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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 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
(CHOH) + 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 II: Absorbs red light of 680 nm
– Produces a strong oxidant, capable of oxidizing water
– Also produces a weak reductant
• Photosystem I: Absorbs far-red light (700 nm)
– Produces a strong reductant, capable of reducing NADP
– Also produces a weak oxidant
• 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
•
light at 680 nm absorbed by PS-II makes strong oxidant (& weak reductant)
•
Light at 700nm absorbed by PS-I makes strong reductant (& weak oxidant)
•
PS-II oxidizes water
PS-I reduces NADP
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
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 O2-evolution
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).
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 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 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
(CHOH) + O2
ANY
QUESTIONS?