Download Photosynthesis

Photosynthesis: Energy from the Sun
Identifying Photosynthetic Reactants and Products
 Reactants needed for photosynthesis:
 H2O, & CO2,
 Products of photosynthesis:
 carbohydrates and O2
 Energy driving reaction:
 Light
 6 CO2 + 12 H2O  C6H12O6 + 6 O2 + 6 H2O
The Two Pathways of Photosynthesis: An Overview
 Photosynthesis occurs in the chloroplasts of plant cells
 Photosynthesis can be divided into two pathways:
 The light reaction is driven by light energy captured
by chlorophyll
 Light energy transformed to chemical energy
 ATP and NADPH + H+.
 The Calvin–Benson cycle uses ATP, NADPH + H+,
and CO2 to produce sugars.
 Carbon fixation
Chloroplast Structure
Photosynthesis in the Chloroplast
The Electromagnetic Radiation: Wave-Particle Duality
 Electromagnetic radiation
comes in discrete packets
called photons
 Photons behave as particles
and as waves
 Particles – mass and impart
energy through collisions
 Waves – interfere positively
and negatively with each
other
 Photonic energy
 Wavelength ()  1/energy
 Frequency  1/ 
 Frequency  energy
The Interactions of Photons and Molecules
 Transmission
 Photon passes through molecule without interacting
 Absorption
 Photonic energy transferred to molecule
 Molecules absorb photons of discrete energies
(wavelengths) and transmit photons of other energies
 Molecules that absorb visible wavelengths are called
pigments or chromophores
The Interactions of Light and Pigments
 Plotting the absorption by the
compound as a function of
wavelength results in an
absorption spectrum.
 If absorption results in a
measurable activity, plotting the
effectiveness of the light as a
function of wavelength is called
an action spectrum.
Absorption of Photonic Energy
 Electrons in high
enough exited states
can move from
molecule to molecule
 Essentially an electric
current
Light Absorbing Pigments for Photosynthesis
 Primary chromophores
 chlorophyll a and chlorophyll b.
 Absorption max in blue and red wavelengths
 Accessory pigments
 Carotenoids (xanthophylls) &
phycobillins
 Absorption maxima between the
red and blue wavelengths
Figure 8.7 The Molecular Structure of Chlorophyll
The Interactions of Light and Pigments
 molecule enters an excited state when it absorbs a photon.
 excited state is unstable, and the molecule may return to the
ground state.
 When this happens, some of the absorbed energy is given
off as heat and the rest is given off as light energy, or
fluorescence.
 molecule may pass some of the absorbed energy to other
molecules
The Interactions of Light and Pigments
 Pigments in photosynthetic organisms are arranged into
antenna systems.
 The excitation energy is passed to the reaction center of
the antenna complex.
 In plants, the pigment molecule in the reaction center is
always a molecule of chlorophyll a.
Figure 8.8 Energy Transfer and Electron Transport
The Light Reactions: Photophosphorylation
 Excited chlorophyll (Chl*) in the reaction center acts as a
reducing agent and participates in a redox reaction
 Chl* can react with an oxidizing agent in a reaction such as:
Chl* + PQ  Chl+ + PQ–
 PQ- passes the e- to a series of carriers in the thylakoid
membrane
 The e- carriers pump H+ into the thylakoid space
 The e- is ultimately donated to NADP to generate NADPH +
H+
 The H+ gradient is used to synthesize ATP by ATPases in the
thylakoid membrane and is called photophosphorylation
Electron Transport, Reductions, and Photophosphorylation
 There are two different systems for transport of electrons in
photosynthesis.
 Noncyclic electron transport produces NADPH + H+ and
ATP and O2
 e- come in from H2O and leave on NADPH
 Cyclic electron transport produces only ATP
 e- come from chl and are returned to chl
The Light Reactions: Photophosphorylation
 Photosystems
 light-driven molecular units consisting of chlorophylls and
accessory pigments bound to proteins in energy-absorbing
antenna systems
 Photosystem I (PS I)
 Alone carries out cyclic electron transport
 In combo with PS II, - non-cyclic transport
 reaction center chlorophyll a is P700 (max = 700nm)
 Photosystem II (PS II)
 Initiates non-cyclic e- transport
 Splits H2O to produce e-, H+, and O2.
 reaction center chlorophyll a is P680 (max = 680nm)
 To keep noncyclic electron transport going, both
photosystems must constantly be absorbing light
Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems
 Coupled PS II and PS I is the arrangement found in all most
all photosynthetic organisms – cyanobacteria to redwoods
Photosynthetic Machinery
Mn4
PQ- plastoquinone
Fd – ferredoxin
Cyt – cytochrome complex
PC - plastocyanin
Photosynthetic Machinery and Grana
The Calvin–Benson Cycle: When carbon breaks, we fix it
 Calvin-Benson cycle reactions occur in the stroma
 Requires the ATP and NADPH + H+ produced in the light reactions and
these can not be “stockpiled”.
 Thus, the Calvin-Benson reactions require light indirectly but take place
only in the presence of light.
Figure 8.12 Tracing the Pathway of CO2
3 sec reaction
30 sec reaction
The Calvin–Benson Cycle: A fixation with carbon
 Initial reaction adds one CO2 to ribulose 1,5-bisphosphate (RuBP; a
pentose)
 The intermediate hexose is unstable and breaks down to form two
molecules of 3-phosphoglycerate (a triose)
 fixation of CO2 is catalyzed by ribulose bisphosphate
carboxylase/oxygenase - a.k.a. rubisco.
 Rubisco is the most abundant protein in the world.
The Calvin–Benson Cycle:
 Fixation of CO2,
 Conversion of fixed CO2 into Gyceraldehyde-3P
 Uses ATP and NADPH
 Regeneration of the CO2 acceptor RuBP
 Uses ATP
Regeneration of RuBP in the Calvin-Bensen Cycle
Figure 8.13 The Calvin-Benson Cycle
The Calvin–Benson Cycle
 The end product of the cycle is glyceraldehyde 3phosphate, G3P.
 There are two fates for the G3P:
 One-third ends up as starch, which is stored in the
chloroplast and serves as a source of glucose.
 Two-thirds is converted to the disaccharide sucrose, which
is transported to other organs.
Importance of The Calvin–Benson Cycle
 The products are the energy yield from sunlight converted to
carbohydrates
 Most of the energy is released by glycolysis and cellular
respiration by the plant itself.
 Some of the carbon of glucose becomes part of amino
acids, lipids, and nucleic acids.
 Some of the stored energy is consumed by heterotrophs,
where glycolysis and respiration release the stored energy.
Photorespiration
 Rubisco as a carboxylase,
 adds CO2 to RuBP.
 Rubisco as an oxygenase
 Adds O2 to RuBP.
 These two reactions compete with each other.
 Reaction with O2, reduces the rate of CO2 fixation
 Oxygenase reaction occurs when CO2 levels are very low
and the O2 levels are very high
 Rubisco binds CO2 with a
 O2 levels become very high when stomata are closed to
prevent water loss (when the weather is hot and dry).
Reaction Pathways
Compensating for
Photorespiration
 RuBP + O2 
phosphoglycolate + 3PG
 glycolate transported into
 glycolate converted to
glycine in peroxisome
 glycine converted to serine
in mitochondria
 serine converted to glycerate
in peroxisome
 glycerate reenters C-B cycle
in chloroplast
Figure 8.15 Organelles of Photorespiration
P
C
M
Overcoming Photorespiration
 C3 plants have a layer of mesophyll cells below the leaf
surface.
 Mesophyll cells are full of chloroplasts and rubisco.
 On hot days the stomata close, O2 builds up, and
photorespiration occurs.
Overcoming Photorespiration
 C4 plants have two enzymes for CO2 fixation in different chloroplasts, in
different locations in the leaf.
 PEP carboxylase is present in the mesophyll cells. It fixes CO2 to 3-C
phosphoenolpyruvate (PEP) to form 4-C oxaloacetate.
 PEP carboxylase does not have oxygenase activity. It fixes CO2 even
when the level of CO2 is extremely low.
 The oxaloacetate diffuses into the bundle sheath cells in the interior of
the leaf which contain abundant rubisco.
 The oxaloacetate loses one C, forming CO2 and regenerating the PEP.
 The process pumps up the concentration around rubisco to start the
Calvin-Benson cycle.

Figure 8.16 Leaf Anatomy of C3 and C4 Plants
Figure 8.17 (b) The Anatomy and Biochemistry of C4 Carbon Fixation
OAA
Pyruvate
Figure 8.17 (a) The Anatomy and Biochemistry of C4 Carbon Fixation
Photorespiration and Its Consequences
 CAM plants use PEP carboxylase to fix and accumulate CO2
while their stomata are closed.
 These plants conserve water by keeping stomata closed
during the daylight hours and opening them at night.
 In CAM plants, CO2 is fixed in the mesophyll cells to form
oxaloacetate, which is then converted to malic acid.
 The fixation occurs during the night, when less water is lost
through the open stomata.
 During the day, the malic acid moves to the chloroplast,
where decarboxylation supplies CO2 for the Calvin–Benson
cycle.
Figure 8.18 Metabolic Interactions in a Plant Cell