CHAPTER 6
Photosynthesis and the
Chloroplast
Introduction (1)
• The earliest living organisms were heterotrophs,
which survived on nutrients from the environment.
• Autotrophs manufacture organic nutrients from CO2
and H2S.
• Synthesis of complex molecules from CO2 requires a
large input of energy.
– Chemoautotrophs use energy from inorganic molecules.
– Photoautotrophs use radiant energy to make organic
compounds.
Introduction (2)
• Photosynthesis converts energy from sunlight
into chemical energy stored in carbohydrates.
– Low energy electrons are removed from a donor
molecule.
– First photoautotrophs used H2S as electron source
– About 2.7 million years ago, cyanobacteria used
electrons from water to produce oxygen as a
waste product:
light
CO2 + H2O  (CH2) + O2
6.1 Chloroplast Structure and
Function (1)
• Photosynthesis in eukaryotes takes place in
the chloroplast, a cytoplasmic organelle.
• Chloroplasts have a double membrane.
– The outer membrane contains porins and is
permeable to large molecules.
– The inner membrane contains light-absorbing
pigment, electron carriers, and ATP-synthesizing
enzymes.
The functional
organization of a leaf
The internal
structure of a leaf
Chloroplast Structure and Function
(2)
• The inner membrane of a chloroplast is folded
into flattened sacs (thylakoids), arranged in
stacks called grana.
• Chloroplasts are self-replicating organelles
containing their own DNA.
• Thylakoid membranes contain a large
percentage of glycolipids, which make the
membrane highly fluid for diffusion of
proteins complexes.
Thylakoid membranes
6.4 An Overview of Photosynthetic
Metabolism (1)
• Photosynthesis is a redox reaction transferring
an electron from water to carbon dioxide:
6 CO2 + 12 H2O  C6H12O6 + 6 H2O + 6 O2
• Experiments using 18O showed that O2
molecules released from photosyntheiss came
from two molecules of H2O, not from CO2.
An Overview of Photosynthetic
Metabolism (2)
• Photosynthesis oxidizes water to oxygen;
respiration reduces oxygen to form water.
– Respiration removes high energy electrons from
reduced organic substrates to form ATP and
NADH.
– Photosynthesis uses low energy electrons to form
ATP and NADPH, which are then used to reduce
CO2 to carbohydrate.
Overview of the energetic of photosynthesis and
aerobic respiration
An Overview of Photosynthetic
Metabolism (3)
• Photosynthesis occurs in two stages:
– Light-dependent reactions (light reactions)in
which sunlight is absorbed, converting it into ATP
and NADPH.
– Light-independent reactions (dark reactions) use
the energy stored in ATP and NADPH to produce
carbohydrate.
6.3 The Absorption of Light (1)
• Absorption of photons (light “particles”) by a
molecule makes them go from ground state to
excited state.
– Energy in the photon depends on the wavelength
of light.
– Energy required to shift electrons varies for
different molecules.
– Molecules absorb specific wavelengths of light.
The Absorption of Light (2)
• Photosynthetic
Pigments – molecules
that absorb light of
particular wavelengths.
– Chlorophyll contains a
porphyrin ring that
absorbs light and a
hydrophobic tail
embedding it to the
photosyntheic
membrane.
The Absorption of Light (3)
• The alternating single
and double bonds
along the porphyrin
ring form a cloud
making it a
conjugated system.
• Conjugated bond
systems absorb
energy of a range of
wavelengths.
The Absorption of Light (4)
• Besides chlorophyll,
there are accessory
pigments called
carotenoids.
• Carotenoids absorb
light in the blue-green
region of spectrum.
• Various pigments allow
for greater absorption
of incoming photons.
6.4 Photosynthetic Units and
Reaction Centers (1)
• Each photosynthetic unit contains several
hundred chlorophyll molecules.
• One member of the group—the reactioncenter chlorophyll—transfers electrons to an
electron acceptor.
• Pigments that do not participate directly in
the conversion of light energy, they are
responsible for light absorption, and are called
antenna pigments.
The transfer of excitation energy
Photosynthetic Units and Reaction
Centers (2)
• Oxygen Formation: Coordinating the Action of
Two Different Photosynthetic Systems
– Two large pigment-protein complexes called
photosystems act in series to raise electrons from
H2O to NADP+.
• Photosystem II (PSII) boosts electrons from below
energy level of water to a midpoint.
• Photosystem I (PSI) boosts electrons to a level above
NADP+.
Overview of the flow of electrons during the
light-dependent reactions of photosynthesis
Photosynthetic Units and Reaction
Centers (3)
• Oxygen Formation (continued)
– The reaction center of PSII is referred to as P680,
and that of PSI as P700 standing for the
wavelengths where absorption is stronger.
– Electrons are transferred to a primary electron
acceptor.
– The flow of electrons from H2O to NADP+ is
referred to as the Z scheme.
Photosynthetic Units and Reaction
Centers (4)
• PSII Operations: Obtaining Electrons by
Splitting Water
– PSII uses absorbed light energy to remove
electrons and generate a proton gradient.
– Two proteins, D1 and D2, bind the P680
chlorophyll and perform reactions to oxidize H2O.
– Light is harvested by a pigment-protein complex
called light-harvesting complex II (LHCII).
The functional organization of photosystem II
Photosynthetic Units and Reaction
Centers (5)
• The Flow of Electrons from PSII to
Plastoquinone
– Harvested energy is passed from LHCII to innerantenna molecules within PSII.
– Excited P680 (P680*) transfers energy to an
electron acceptor generating P680+ and Pheo-.
– P680+ and Pheo- are transferred to opposite sides
of the thylakoid membrane where Pheo- passes
an electron to plastiquinone (PQ).
Photosynthetic Units and Reaction
Centers (6)
• The Flow of Electrons from PSII to
Plastoquinone (continued)
– PQ passes the electron to another PQ.
– The electron is then moved to the stromal side of
the membrane.
Plastoquinone
Photosynthetic Units and Reaction
Centers (7)
• The Flow of Electrons from Water to PSII
– The redox potential of P680+ pulls electrons from
water (photolysis).
– Formation of O2 requires four electrons from H2O:
2 H2O  4 H+ + O2 + 4 e–
– Four electrons required to form O2 are transferred
in cycles through P680+ to four Mn ions and one
Ca ion that form the oxygen-evolving complex.
Measuring the kinetics of O2 release
Photosynthetic Units and Reaction
Centers (8)
• The Flow of Electrons from Water to PSII
(continued)
– Protons produced in photolysis are retained in the
thylakoid lumen.
– Oxygen produced is a released as a waste product
into the environment.
Photosynthetic Units and Reaction
Centers (9)
• From PSII to PSI
– Production of O2 leads to formation of two
molecules of PQH2.
– Reduced PQH2 then diffuses through thylakoid
membrane and binds cytochrome b6f, and
releases protons the lumen of thylakoid.
– Electrons from cytochrome b6f are passed to
another carrier, plastocyanin.
– Plastocyanin transfers electrons to P700+.
Electron transport between PSII and PSI
Photosynthetic Units and Reaction
Centers (10)
• PSI Operations: The Production of NADPH
– The PSI consists of a reaction core center of 12–14
different polypeptides and a complex of proteinbound pigments called LHCI.
– Photons harvested by antenna pigments in PSI
(LHCI) oxidizes chlorophyll a, forming P700*.
– Absorption of light leads to production of P700+
and Ao–.
– Redox potential of P700+/Ao– reduces NADP.
The functional organization of photosystem I
Photosynthetic Units and Reaction
Centers (11)
• PSI Operations (continued)
– The reduction of NADP+ to NADPH is catalyzed by
ferredoxin-NADP+ reductase.
– Some electrons passed to ferredoxin end up
reducing nitrate, ammonia or sulfate to form
other important biological molecules.
Photosynthetic Units and Reaction
Centers (12)
• An Overview of Photosynthetic Electron
Transport
– For every 8 photons absorbed:
2 H2O + 2 NADP+  O2 + 2 NADPH
– Electron transport also produces a proton gradient
across the thylakoid membrane.
Summary of the light-dependent reactions
Summary of the light-dependent reactions
Photosynthetic Units and Reaction
Centers (13)
• Killing Weeds by Inhibiting Electron Transport
– Common herbicides bind to a core protein of PSII.
– Light reactions serve as targets of herbicides.
– Some herbicide produce oxygen radicals, which
are toxic to the human tissue.
6.5 Photophosphorylation (1)
• The machinery for ATP synthesis in a
chloroplast is similar to that of mitochondrial
enzymes.
• The ATP synthase consists of a head (CF1), and
a base (CF0).
• The CF1 heads project outward into the
stroma, keeping with the orientation of the
proton gradient.
ATP synthase in the chloroplast
Photophosphorylation (2)
• Protons move into the lumen through the CF0
base of the synthase.
• Measurements of chloroplasts during ATP
synthesis show an increase in the ΔpH of more
than 3 units.
• The movement of protons during photosynthesis does not create a significant change
in the membrane potential since other ions
are transported simultaneously.
Photophosphorylation (3)
• The movement of electrons during the
formation of oxygen is called noncyclic
photophosphorylation because ions move in
a linear path.
• Cyclic vs. noncyclic photophosphorylation:
– Cyclic photophosphorylation is carried out by PSI
independently of PSII.
– Cyclic photophosphorylation is thought to provide
additional ATP required for carbohydrate synthesis
Cyclic photophosphorylation
6.6 Carbon Dioxide Fixation and
the Synthesis of Carbohydrate (1)
• The movement of carbon in the cell can be
followed during photosynthesis using [18C]O2
as a tracer.
• Extracts of cells are then analyzed by
autoradiography by identifying radiolabeled
compounds compared to known standards.
Chromatogram after incubation with [18C]O2
Carbon Dioxide Fixation and the
Synthesis of Carbohydrate (2)
• Carbohydrate Synthesis in C3 Plants
– C3 plants are those that produce a three-carbon
intermediate (3-phosphoglycerate, PGA) as the
first compound to be identified during carbon
dioxide fixation.
– CO2 is condensed with a five-carbon compound,
ribulose 1,5-bisphosphate (RuBP), to form a sixcarbon molecule which then splits into two
molecules of PGA.
Converting CO2 into carbohydrate
Converting CO2 into carbohydrate
Carbon Dioxide Fixation and the
Synthesis of Carbohydrate (3)
• The condensation of RuBP and the splitting of
the six-carbon molecule are catalyzed by one
enzyme, ribulose bisphosphate carboxylase
(Rubisco).
• Rubisco is the most abundant protein on
Earth, and has a very low turnover number.
• Rubisco fixes ~3 molecules of CO2 per second.
Carbon Dioxide Fixation and the
Synthesis of Carbohydrate (4)
• Carbohydrate Synthesis in C3 Plants
– The C3 pathway is known as the Calvin cycle and
includes:
• Carboxylation of RuBP to form PGA.
• Reduction of PGA to glyceraldehyde 3-phosphate (GAP)
using NADPH and ATP from light reactions.
• Regeneration of RuBP.
Carbon Dioxide Fixation and the
Synthesis of Carbohydrate (5)
• Carbohydrate Synthesis in C3 Plants
– The GAP molecules can be exported into the
cytosol in exchange for phosphate ions and used
to synthesize sucrose.
– GAP can also remain in the chloroplast where it is
converted to starch.
Overview of various stages of photosynthesis
Carbon Dioxide Fixation and the
Synthesis of Carbohydrate (6)
• Carbohydrate Synthesis in C3 Plants
– It is an expensive process.
– Conversion of 6 molecules of CO2 to 1 six-carbon
sugar molecules requires 12 molecules of NADPH
and 18 molecules of ATP.
– Expenditure due to CO2 being the most highly
oxidized form of carbon.
Carbon Dioxide Fixation and the
Synthesis of Carbohydrate (7)
• Redox Control is light-dependent.
– Key enzymes of Calvin cycle are only active when
ATP and NADP are produced by photosynthesis.
– Some electrons used to reduce NADP+ are
transferred to thioredoxin, which are then
accepted to reduce disulfide bridges (-S-S-) in
selected Calvin cycle enzymes.
– In the dark, thioredoxin reduction ceases and
enzymes go back to oxidized state (-S-S-) and are
inactivated.
Redox control of the Calvin cycle
Carbon Dioxide Fixation and the
Synthesis of Carbohydrate (8)
• Photorespiration – uptake of O2 and release
of CO2.
– Rubisco also catalyzes the attachment of O2 to
RuBP to produce 2-phosphoglycolate.
– Glycolate is then transferred to the peroxisome
and leads to release of CO2.
– It accounts for the loss of up to 50% of fixed CO2.
– Rate of photorespiration depends on the CO2/O2
ratio.
The reactions of photorespiration
Carbon Dioxide Fixation and the
Synthesis of Carbohydrate (9)
• Peroxisomes and Photorespiration
– Glycolate produced during photorespiration is
shuttled to the peroxisome.
– Peroxisomal enzymes convert glycolate to
glyoxylate and then glycine, resulting in the loss of
CO2.
The cellular basis of photorespiration
Carbon Dioxide Fixation and the
Synthesis of Carbohydrate (10)
• Carbohydrate Synthesis in C4 Plants
– The C4 pathway involves the production of
phosphoenolpyruvate (PEP), which then combines
with CO2 to produce 4-carbon compounds
oxaloacetate or malate.
– Plants utilizing this pathway are C4 plants, usually
tropical grasses.
Structure and function in C4 plants
Carbon Dioxide Fixation and the
Synthesis of Carbohydrate (11)
• C4 Plants (continued)
– In a hot, dry environment C4 plants get enough
CO2 for photosynthesis while keeping their
stomata partially closed to prevent water loss.
– C4 plants have anatomical adaptations to
transport C4 products into the bundle sheath cells,
where fixed CO2 can be split from the 4-carbon
carrier producing a high CO2 level suitable for
fixation by Rubisco.
Carbon Dioxide Fixation and the
Synthesis of Carbohydrate (12)
• Carbohydrate Synthesis in CAM Plants
– CAM plants carry out light reactions and CO2
fixation at different times of the day using the
enzyme PEP carboxylase.
– CAM (crassulacean acid metabolism) plants keep
their stomata closed during the day to reduce
water loss.