RESPIRACÃO

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RESPIRAÇÃO
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Consists of four processes:
Glycolysis (in cytosol and plastid; yields pyruvate, ATP and NADH)
Pentose phosphate pathway (in cytosol and plastid; yields NADPH)
Citric acid cycle (in matrix of mitochondria; pyruvate is oxidized to CO2, yields NADH, FADH2, ATP)
Electron transport chain/oxidative phosphorylation (in inner membrane of mitochondria; transport of electrons from NADH to O2, yields ATP)
Overview of respiration
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Overview of respiration
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The matrix contains Pyruvate Dehydrogenase, enzymes of Krebs Cycle, and other pathways, e.g., fatty acid oxidation & amino acid metabolism.
The outer membrane contains large channels, similar to bacterial porin channels, making the outer membrane leaky to ions & small molecules. 
Glycolysis occurs in the cytosol of cells. 
Pyruvate enters the mitochondrion to be metabolized further.
Mitochondrial Compartments:
outer 
membrane
inner 
membrane 
matrix
inter-
membrane
space
mitochondrion
cristae
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It contains various transport catalysts, including a carrier protein that allows pyruvate to enter the matrix. 
It is highly convoluted, with infoldings called cristae. 
Embedded in the inner membrane are constituents of the respiratory chain and ATP Synthase. 
The inner membrane is the major permeability barrier of the mitochondrion. 
outer 
membrane
inner 
membrane 
matrix
inter-
membrane
space
mitochondrion
cristae
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Glycolysis: A cytosolic and plastidic process
Energy-conserving phase of glycolysis
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Glycolysis: A cytosolic and plastidic process
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Glycolysis: A cytosolic and plastidic process
Energy-conserving phase of glycolysis
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Glycolysis: A cytosolic and plastidic process
In the absence of O2 (e.g. in plant roots in flooded soil), fermentation regenerates the NAD+ needed for glycolysis.
	- glycolysis can then be the main source of energy
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Reactions of the oxidative pentose phosphate pathway
- Alternate route to glycolytic
 pathway
- runs in cytosol and plastid,
 but pathway in plastids
 predominates
- roles in plant metabolism:
→ NADH supply for biosyn-
 thetic redox reactions
→ NADPH supply for
 respiration
→ supply of biosynthetic
 substrates (precursors
 for DNA, RNA)
→ Generation of Calvin 
 cycle intermediates
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Fig.1 – Via Glicolítica. As reações em que o ATP ou o NADH estão em destaque.
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Pyruvate Dehydrogenase, catalyzes oxidative decarboxylation of pyruvate, to form acetyl-CoA.
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Pyruvate Dehydrogenase
pyruvate acetyl-CoA
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The final electron acceptor 
is NAD+.
In the overall reaction catalyzed by the Pyruvate Dehydrogenase complex, the acetic acid generated is transferred to coenzyme A.
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acetic acid
acetyl-CoA
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2e + H+
NAD+ NADH
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 Ciclo do ácido cítrico (animal)
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 Citric Acid Cycle (vegetal)
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Electron transport chain
Catalyzes a flow of electrons from NADH to O2
Electron transport is coupled with formation of proton gradient → used for ATP synthesis
Consists of 5 complexes:
Complex I (NADH dehydrogenase)
Complex II (succinate dehydrogenase)
Complex III (Cytochrome bc1 complex)
Complex IV (Cytochrome c oxidase)
Complex V (ATP synthase)
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Organization of mitochondrial electron transport chain
Plant mitochondria contain additional enzymes (in green), which do not pump protons.
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Organization of mitochondrial electron transport chain
NADH Dehydrogenase (complex I)
	- oxidizes NADH
	- transfers e- to Ubiquinone (UQ)
	- pumps 1H+ per e-
Succinate Dehydrogenase (complex II)
	- oxidation of succinate (from 
	 citric acid cycle) 
	- e- are transferred via FADH2
	- does not pump protons
Cytochrome bc1 complex (complex III)
	- oxidizes reduced UQ (= ubiquinol) 
	- pumps 1H+ per e-
Cytochrome c oxidase (complex IV)
	- reduces O2 to H2O
	- pumps 1H+ per e-
ATP synthase (complex V)
	- uses electrochemical proton gradient 
	 to synthesize ATP
Oxidized ubiquinone
Reduced ubiquinol
via semiquinone
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Fig.5 – Gradiente de prótons formado na membrana mitocondrial interna como resultado do transporte de elétrons.
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Transmembrane transport in plant mitochondria
ATP
Inorganic 
Phosphate (Pi)
HPO42-, H2PO4_
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ATP yield during respiration
Total ATP molecules from respiration of one molecule sucrose = 
	About 33% of energy is released from one sucrose molecule by oxidation
	about 67% ( heat )
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Mechanisms of plants to lower ATP yield – 
The role of the Alternative Oxidase and the Uncoupling Protein
Alternative oxidase
Energy required for active transport through ATP hydrolysis
Study effect of CN_ on membrane potential
CN_ poisons mitochondria, blocks ATP production, because
CN_ inhibits cytochrome c oxidase
Membrane potential of a pea cell collapses when CN_ is added to the external solution
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Mechanisms of plants to lower ATP yield – 
The role of the Alternative Oxidase and the Uncoupling Protein
Alternative oxidase
- some plants have cyanide-resistant respiration; can be 10-25%, even up to
 100% of uninhibited control rate
- enzyme responsible for this cyanide-resistant oxygen uptake → Alternative
 oxidase
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Mechanisms of plants to lower ATP yield – 
The role of the Alternative Oxidase and the Uncoupling Protein
Alternative oxidase
How can this energetically wasteful process be of importance for plant metabolism?
Example: floral development in some members of the Araceae (arum family), e.g. voodoo lily (Sauromatum guttatum) → Thermogenesis
Spadix
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Alternative oxidase pathway
Only in plant mitochondria
No ATP synthesis, so heat is generated
Role of alternative oxidase pathway
Heat generation
Regulation of ATP synthesis
Regulation of metabolite synthesis
Helps overcome environmental stresses
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Mechanisms of plants to lower ATP yield – 
 Uncoupling Protein
Uncoupling protein
- Increases proton permeability of inner mitochondrial membrane
- acts as an uncoupler → less ATP, more heat is produced
- UCP em animais e PUMP em vegetais (Anibal Vercesi 1995 UNICAMP)
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Glycolysis, pentose phosphate pathway, and citric acid cycle 
contribute 
precursors
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Factors influencing respiration
Plant species, specific organs (roots vs. leaves)
Plant age (older plants respire more)
Plant habitat (tropical plants respire more due to higher night temperatures)
Oxygen (substrate; diffusion of oxygen through aqueous phase in plant tissue could limit plant respiration → low O2 decreases respiration)
Water saturation/low O2 (hydroponic culture, growth of plants in wet/flooded soils)
Temperature (respiration increases with temp.; store fruits and vegetables in cool temperatures)
Carbon dioxide (high CO2 of 3-5% inhibits respiration → store foods at low temp., 2-3% O2 and 3-5% CO2)
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Tabela 01
BALANÇO ENERGÉTICO DA RESPIRAÇÃO 
 
Via
NADH FADH2 ATP Total de ATP
Glicólise 2 0 2 5
Ciclo de Krebs 8 2 2 25
Gasto c/ piruvato 1 
Total de ATP 29
 
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