Lesson Two
Photosynthesis
Photosynthesis
• Photosynthesis occurs only in the chlorophyllcontaining cells of green plants
,
algae
,
and certain protists
and bacteria.
Photosynthesis
• Overall, it is a process that converts light energy
into chemical energy that is stored in the form of
molecular bonds.
Carbon
dioxide
Water
Glucose
PHOTOSYNTHESIS
Oxygen
gas
Photosynthesis
• From the point of view of chemistry and
energetics, it is the opposite of cellular respiration.
Photosynthesis
Potential energy of molecules
• Whereas cellular respiration is highly exergonic
and releases energy,
reactants
Amounts of
energy output
products
Photosynthesis
Potential energy of molecules
• ….photosynthesis requires energy and is highly
endergonic.
products
Amount of
energy input
reactants
Photosynthesis
• Photosynthesis starts with CO2 and H2O as
raw materials and proceeds through two sets
of partial reactions. In the first set, called
the light-dependent reactions, water
molecules are split (oxidized), O2 is
released, and ATP and NADPH are formed.
Photosynthesis
• In the second set, called light-independent
reactions, CO2 is reduced (via the addition
of H atoms) to carbohydrates. These
chemical events rely on the electron carrier
NADPH and ATP generated by the first set
of reactions.
3
CO2
1
3 P
P
6
RuBP
3-PGA
6
3 ADP
3
P
Lightindependent
reactions
4
(CALVIN CYCLE)
ATP
ATP
6 ADP + P
6 NADPH
6 NADP+
5
P
6
G3P
P
G3P
3
OUTPUT:
1
P
G3P
Glucose
and other
compounds
Photosynthesis
• Both sets of reactions
take place in
chloroplasts.
Photosynthesis
• Most of the enzymes
and pigments for the
light-dependent
reactions are
embedded in the
thylakoid membrane
of chloroplasts.
Photosynthesis
• The dark reactions
take place in the
stroma.
stroma
How Light Energy Reaches
Photosynthetic Cells
• The energy in light photons in the visible part of
the spectrum can be captured by biological
molecules to do constructive work.
How Light Energy Reaches
Photosynthetic Cells
• The pigment
chlorophyll in plant
cells absorbs photons
within a particular
absorption spectrum –
a statement of the
amount of light
absorbed by
chlorophyll at
different wavelengths.
How Light Energy Reaches
Photosynthetic Cells
• When light is absorbed it
alters the arrangement of
electrons in the absorbing
molecule. The added
energy of the photon
boosts the energy
condition of the molecule
from a stable state to a
less-stable excited state.
How Light Energy Reaches
Photosynthetic Cells
• During the lightdependent reactions of
photosynthesis, as the
absorbing molecule
returns to the ground
state, the “excess”
excitation energy is
transmitted to other
molecules and stored
as chemical energy.
How Light Energy Reaches
Photosynthetic Cells
• All photosynthetic
organisms contain
various classes of
chlorophylls and one
or more carotenoid
(accessory) pigments
that also contribute to
photosynthesis.
How Light Energy Reaches
Photosynthetic Cells
• Groups of pigment
molecules called
antenna complexes are
present on thylakoids.
How Light Energy Reaches
Photosynthetic Cells
• Light striking any one
of the pigment
molecules is funneled
to a special
chlorophyll a
molecule, termed a
reaction-center
chlorophyll, which
directly participates in
photosynthesis.
How Light Energy Reaches
Photosynthetic Cells
• Most photosynthetic organisms possess two
types of reaction-center chlorophylls, P680
and P700, each associated with an electron
acceptor molecule and an electron donor.
These aggregations are known respectively
as photosystem I (P700) and photosystem II
(P680).
How Light Energy Reaches
Photosynthetic Cells
The Light-Dependent Reaction:
Converting Solar Energy into
Chemical-Bond Energy
The Light-Dependent Reaction
• The photosystems of the light-dependent
reactions are responsible for the packaging
of light energy in the chemical compounds
ATP and NADPH. The packaging takes
place through a series of oxidationreduction reactions set in motion when light
strikes the P680 reaction center in
photosystem II.
The Light-Dependent Reaction
• In this initial event water
molecules are cleaved,
oxygen is released, and
electrons are donated.
These electrons are
accepted first by
plastoquinone and then
by a series of carriers as
they descend an electron
transport chain.
The Light-Dependent Reaction
• For each four
electrons that pass
down the chain, two
ATPs are formed. The
last acceptor in the
chain is the P700
reaction center of
photosystem I.
The Light-Dependent Reaction
• At this point incoming
photons boost the
energy of the electrons,
and they are accepted
by ferredoxin.
The Light-Dependent Reaction
• Ferredoxin is then
reoxidized, and the
coenzyme NADP+ is
reduced to the
NADPH. The ATP
generated previously
and the NADPH then
take part in the lightindependent reactions.
The Light –Dependent Reaction
• The production of ATP from the transport of electrons
excited by light energy down an electron transport chain is
termed photophosphorylation. The one-way flow of
electrons through photosystems II and I is called noncyclic
photophosphorylation;
The Light-Dependent Reaction
• …plants also derive additional ATP through cyclic
photophosphorylation, in which some electrons are
shunted back through the electron transport chain between
photosystems II and I.
The Light-Independent Reactions:
Building Carbohydrates
• In the light-independent reactions of
photosynthesis, which are driven by ATP
and NADPH, CO2 is converted to
carbohydrate. The reactions are also known
as the Calvin-Benson cycle.
The Light-Independent Reactions
• Atmospheric CO2 is
fixed as it reacts with
ribulose 1,5bisphosphate (RuBP),
a reaction that is
catalyzed by the
enzyme ribulose 1,5bisphosphate
carboxylase (rubisco).
The Light-Independent Reactions
• The reduction of CO2
to carbohydrate
(fructose 1,6bisphosphate) is
completed via several
more steps of the cycle.
Glyceraldehyde 3-phosphate
The Light-Independent Reactions
• Finally, RuBP is
regenerated so that the
cycle may continue.
Oxygen: An Inhibitor of
Photosynthesis
• High levels of oxygen in
plant cells can disrupt
photosynthesis and can
also cause
photorespiration – an
inefficient form of the
dark reactions in which O2
is fixed rather than CO2
and no carbohydrate is
produced.
rubisco
Reprieve from Photorespiration:
The C4 pathway
• Most plants are C3
plants; they experience
decreased
carbohydrate
production under hot,
dry conditions as a
result of the effects of
photorespiration.
The C4 pathway
• Among C4 plants,
however, special leaf
anatomy and a unique
biochemical pathway
enable the plant to
thrive in arid
conditions.
The C4 pathway
• Thus C4 plants lessen
photorespiration by
carrying out
photosynthesis only in
cells that are insulated
from high levels of
CO2. They also
possess a novel
mechanism for carbon
fixation.
Glossary
•
•
•
•
•
•
Absorption spectrum
Calvin-Benson cycle
Carotenoid
Chlorophyll
Chloroplast
Cyclic
photophosphorylation
• C3 plant
• C4 plant
• Light-dependent reactions
• Light-independent
reactions
• Noncyclic
photophosphorylation
• Photon
• Photophosphorylation
• Photosynthesis
• Photosystem
• Ribulose 1,5-bisphosphate
• thylakoid
Absorption spectrum
• A graphic
representation of the
amount of light energy
absorbed by a
substance plotted
against the wavelength
of the light (From Taiz and
Zeiger, Plant Physiology, 3e).
Calvin-Benson Cycle (Calvin
Cycle)
• The biochemical
pathway for the
reduction of CO2 to
carbohydrate. The
cycle involves three
phases.
Carotenoid
• Linear polyenes arranged as a planar zigzag chain, with the
repeating conjugated double-bond system –CH=CHCCH3=CH-. These orange pigments serve both as antenna
pigments and photoprotective agents.
Chlorophyll
• A group of light
absorbing green
pigments active in
photosynthesis.
Chloroplast
• The organelle that is
the site of
photosynthesis in
eukaryotic
photosynthetic
organisms.
Cyclic photophosphorylation
(cyclic electron flow)
• In photosystem I, flow of
electrons from the electron
acceptors through the
cytochrome b6f complex
and back to P700, coupled
to proton pumping into the
lumen. This electron flow
energizes ATP synthesis
but does not oxidize water
or reduce NADP+.
C3 plant
• Plants in which the
first stable product of
photosynthetic CO2
fixation is a threecarbon compound (i.e.
3-phosphoglycerate).
C4 plant
• Plants in which the first
stable product of CO2
assimilation in mesophyll
cells is a four-carbon
compound that is
immediately transported to
bundle sheath cells and
decarboxylated. The CO2
released enters the Calvin
cycle.
Light-dependent reactions
• photosynthetic
electron transport
• Electrons flow from
light-excited
chlorophyll and the
oxidation of water,
through PSII and PSI,
to the final electron
acceptor NADP+.
Light-independent reactions
• Dark reactions
• Calvin-Benson cycle
• Calvin cycle
Noncyclic photophosphorylation
• Noncyclic electron
flow
• Electrons flow from
light-excited
chlorophyll and the
oxidation of water,
through PSII and PSI,
to the final electron
acceptor NADP+.
Photon
• A discrete physical
unit of radiant energy.
• A particle that has zero
mass or charge and
unit spin, the quantum
of the electromagnetic
field and carrier of the
electromagnetic force
Photophosphorylation
• The formation of ATP
from ADP and
inorganic phosphate
(Pi) using light energy
stored in the proton
gradient across the
thylakoid membrane.
Photorespiration
• Uptake of atmospheric O2 with
a concomitant release of CO2
by illuminated leaves.
Molecular oxygen serves as
substrate for rubisco and the
formed 2-phosphoglycolate
enters the photorespiratory
carbon oxidation cycle. The
activity of the cycle recovers
some of the carbon found in 2phosphoglycolate, but some is
lost to the atmosphere.
Photosynthesis
• The conversion of
light energy to
chemical energy by
photosynthetic
pigments using water
and CO2, and
producing
carbohydrates.
Photosystem
• A functional unit in
the chloroplast that
harvests light energy
to power electron
transfer and to
generate a proton
motive force used to
synthesize ATP.
Ribulose 1,5-bisphosphate
(RuBP)
• A compound with a
backbone of five
carbon atoms that is
required for carbon
fixation in the CalvinBenson cycle of
photosysthesis.
Thylakoid
• The specialized,
internal, chlorophyll
containing membranes
of the chloroplast
where light absorption
and the chemical
reactions of
photosynthesis take
place.