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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
PQ- plastoquinoneFd – ferredoxinCyt – cytochrome complexPC - plastocyanin
Mn4
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 3-phosphate, 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
CM
P
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