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Photosynthesis, Dark Reactions - Coggle Diagram
Photosynthesis
Light reactions
A. Can be transformed to chemical energy
Planck's law
Discrete quanta (photons) whose energy is given by:
E=hv=hc/wavelength
h = Planck's constant
c = speed of light
v is frequency of radiation
Energy difference between the two states must match the energy of absorbed photon.
Several different ways:
Internal conversion
Electronic -- kinetic energy (heat)
Flourescence
Excited molecule decays to its ground state by emitting photon
Longer wavelength than initially absorbed
Light energy is transferred to reaction centres through exciton transfer among antenna pigments
Exciton transfer
Resonance energy transfer
Directly transfers excitation energy to nearby unexcited molecules
Photooxidation
Light excited donor molecule is oxidised through transfer of electron to acceptor molecule
Energy of absorbed photon is chemically transferred to photosynthetic reaction system
B. Electron transport in photosynthetic bacteria follows a circular path
Purple photosynthetic bacteria RC
Transmembrane protein
Arranged with perfect twofold symmetry
H, L and M subunits
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Photon rapidly photooxidizes the special pair
Electrons are returned to the photooxidised special pair via an electron transport chain
C. Two-centre electron transport is a linear pathway
Produces O2 and NADPH
Photosynthesis is a noncyclic process
Uses reducing power of oxidation of H2O to produce NADPH
Involves two reaction centres
Photosystem II
Oxidises H2O
Resembles the PbRC
Photosystem I
Reduces NADP+
Each photosystem independently activated by light, electrons flow from PSII to PSI
Z scheme
Reflects the loci for photo-chemical events required to drive electrons from H20 to NADP+
Components involved in electron transport from H2O to NADPH are largely organised into three thylakoid membrane-bound particles.
Plastiquinone Q
Pastoquinol QH2
plastocyanin PC
transports electrons from cytochrome b6f to PSI
O2 generated by a five-stage water-splitting reaction
D The Proton Gradient drives ATP synthesis
Photophosphorylation
Chloroplasts generate ATP by coupling dissipation of a proton gradient to enzymes synthesis of ATP
Requires a intact thylakoid membrane
Uncoupled from light-driven electron transport by compounds eg. 2,4-dinitrophenol
CF1CF0 complex
Chloroplast version of F1F0 mitochondria complex
CF0 and F0
hydrophobic transmembrane proteins that contian a proton-translocating channel
CF1 and F1
Hydrophilic peripheral membrane proteins
Both ATP synthases inhibited by oligomycin and dicyclohexylcarbodiimide.
Photosynthesis with noncyclic electron transport
Produces 1 ATP per absorbed phton
12 protons enter lumen per molecule of O2 produced by non-cyclic electron transport
Products
O2
NADPH
ATP
Reactants
H20
NADP+
ADP + phosphate
Dark Reactions
C. Calvin Cycle Controlled indirectly by light
At night plants must use nutriotional reserves to generate ATP and NADPH through glycolysis, Ox Phos and PPP
Light-sensitive control mechanism prevents CC from consuming this nightfood wastefully
Stoma contains enzymes of glycolysis, PPP and CC
Control of flux occurs at enzymatic steps far from equillibrium
Flux control are ceations catalysed by
RuBP carboxylase,
Activity responds to 3 light-dependent factors
MG2+
2-carboxyarabinitol-1-phosphate (CA1P)
Transition state analog
Synthesised by many plants
Inhibits RuBP carboxylase in the dark
facilitated by RuBP carboxylase activase
Also catalyses its carbamoylation
pH
Optimum is 8.0
FBPase
SBPase
Activated by increased pH, Mg2+ and NADPH
Thioredoxin
2nd regulatory system which responds to redox potential of stroma
Occurs in many types of cells
Reduced thioredoxin activates FBPase and SBPase by disulfide interchange reaction
Contains a reservibly reducible disulfide group.
Redox level maintained by 2nd disulfide-containing enzyme
ferredozin-thioredoxin reductase
Responds directly to redox state of soluble ferredoxin in stroma
Deactivates PFK.
In plants, light stiumlates CC while deactivating glycolisis
NADPH --> NADP+
ATP --> ADP
Reactants/ products
Reactants
CO2
ATP
NADPH
Products
NADP+
ADP+P
glucose
Photorespiration competes with photosynthesis
illuminated plants consume O2 and evolve CO2 in distinct pathway, not oxphos
Outstrips photosynthetic CO2 fixation
O2 competes with CO2 as a substrate for RuBP carboxylase
Also known as RuBP carboxylase-oxygenase (RuBisCO
O2 reacts with RuBP to form 3PG and 2-phosphoglycolate
Hydrolyzed to glycolate by phosphoglycolate phosphatase
Partially oxidized to yield CO2 through series of enzymatic reactiosn in the peroxisome and mitochondrion
Dissipates ATP and NADPH
Glycolate exported from the chloroplast to peroxisome
Oxidised by glycolate oxidase to glyoxylate and H202.
H202 (potentially harmful) converted to H2O and O2 by catalase (contains heme)
Can be converted to glycine in a transamination reaction and exported to mitochondrion
In mitochondrion, 2 molecules of glycine converted to 1 molecule of serine and 1 of CO2
Serine moved back to peroxisome
Transamination converts to hydroxypyruvate
Reduced to glycerate and phosphorylated in cytosol to 3PG
Reenters chloroplast and is reconverted to RuBP in calvin cycle
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Origin of CO2 generated by photorespiration
No known metabolic function
But RuBP carboxylases all exhibit oxygenase activity
Might confer a selective advantage by protecting photosynthetic apparatus from photooxidative damaage from insufficient CO2
When chloroplasts or leaf cells are brightly illuminated in absence of CO2 and O2, photosynthetic capacity is rapidly lost
C4 plants concentrate CO2.
On hot days photosynthesis has depleted CO2 chloroplast levels, raised O2
Rate of photorespiration approaches rate of photosynthesis
Major limitation on plant growth
Some species like sugarcane and corn have metabolic cycle
Concentrates CO2 in photosynthetic cells
Almost prevents photorespiration
Known as the C4 cycle
mesophyll cells lack RuBP carboxylase
Take up atmospheric CO2 by condensing it as HCO3- with PEP to yield oxaloacetate
Oxaloacetate reduced by NADPH to malate
Exported to the bundle-sheath cells
Oxidatively decarboxylated by NADP to form CO2, pyruvate and NADPH
CO2 enters calvin cycle, pyruvate returned to mesophyll cells to regenerate PEP
Mediated by enzyme: pyruvate-phosphate dikinase
Concentrates CO2 in bundle-sheath cells for 2 ATP equivalents
Thus, photosynthesis in C4 plants consumes 5ATP per CO2 fixed vs 3 ATP required by Calvin cycle alone.
These plants have characteristic anatomy
Fine veins
Concentrically surrounded by single layer of bundle-sheath cells
surrounded by layer of mesophyll cells
generally occur in tropics as they grow faster in hot, sunny conditions than C3 plants
CAM plants store CO2 through C4 cycle variant
Occurs in succulents
If they opened stomata by day to acquire CO2, they would lose huge amounts of water to evaporation
CAM allows plants to carry out photosynthesis with minimal water loss.
Crassulacean acid metabolism
PEP needed to store day's supply of CO2 obtained from breakdown of starch via glycolysis
During the day, malate broken down to CO2 and pyruvate
A. The calvin cycle fixes CO2
carboxylation reaction involves a pentose derived from ribulose-5-phosphate Ru5P
C6 product results, splits into two C3 compounds
3-phosphoglycerate (3PG)
Calvin cycle generates GAP from CO2 in two stages
Production phase
3 molcules of Ru5P react with three molecules of CO2 to yield six molecules of GAP
Equivalent to synthesis of 1 GAP from 3 CO2 molecules
at expense of 9 ATP and 6 NADPH molecules
3Ru5P +3CO2 = 6GAP
Recovery phase
carbon atoms of remaining 5 GAPs shuffled into PPP-like reactions
C3+C3--C6
C3+C6->C5+C4
C3+C4->c7
c3+c7->c5+c5
Overal stochiometry:
5 C3-> 3 C5
Stages
Phosphoribulokinase phosphorylates Ru5P
Forms ribulose-1,5-bisphosphate (RuBP)
RuBP carboxylase catalyses CO2 fixation
Most important enzyme
Accounts for 50% of leaf proteins
8 large L subunits encoded by chloropast DNA :
8 small S subunits specified by nuclear gene
Requires Mg2+
Stabilises developing negative charges during catalysis
3PG converted to 1,3-bisphosphoglycerate (BPG) and GAP
Isomerization of GAP to dihydroxyacetone phosphate DHAP
Two paths
6
7
8
9
10
11
Aldolase-catalyzed aldol condensations
phosphate hydrolysis reactions
Calvin cycle products converted to
Starch
Alpha-amylose synthesised
in chloroplast stroma as a temporary storage depot
as long-term storage molecule elsewhere in the plant
G1P reacts with ATP to form ADP-glucose
Catalysed by ADP-glucose pyrophosphorylase
Reaction driven by exergonic hydrolysis of PPi released in formation of ADP-glucose
Similar to glycogen synthesis and UDP-glucose
Starch synthase transfers glucose residue to nonreducing end of amylose
Sucrose
disaccharide of glucose and fructose
Delivers carbohydrates to nonphotosynthesising cells
Major photsynthetic product of green leaves.
Synthesised in cytosol
GAP OR DHAP transported out of chloroplast by antiporter
Exchanges phosphate for triose phosphate
Two trioses combine to form F6P and G1P
Activated by UTP to form UDP-glucose
Sucrose-6-phosphate produced by catalysis of sucrose-phosphate synthase
Cellulose
Overall stochiometry
3 CO2 + 9 ATP + 6NADPH
-> GAP + 9 ADP + 8 Pi + 6 NAPD+
GAP
Used inside and outside chloroplast
Converted to fructose-6-phosphate
Then to glucose-1-phosphate by phosphoglucose isomerase and phosphoglucomutase
G1P is the precurse of higher order carbohyrdates in plants
Calvin cycle
6CO2
Converts to
2G3P
1 glucose
6NADP+
18ADP
12 NADPH
18 ATP
occur in stroma