Download Unique plant respiration

Plant Respiration
Respiration overview
• Retrieval of energy captured in photosynthesis; oxidation of carbohydrates
C6H12O6 + 6O2
6CO2 + 6H2O + Energy
Overall pathway
• 1) Glycolysis
– Glucose converted to pyruvate
• 2) Citric Acid Cycle (aka Krebs or TCA cycle)
– Pyruvate converted to CO2 and electrons
• 3) Electron transport chain
– Electrons reduce O2 to H20 and create ATP
Energy storage
• Plants store energy in the form of the carbohydrates sucrose and starch
• In stroma of chloroplast, enzymes such as -amylase and
-amylase break
complex sugars down
• Products exported from chloroplast to cytosol through transport proteins in
plastid envelope
Glycolysis
• Conversion of hexoses to pyruvate
• Anaerobic process that occurs in the cytosol
Glycolysis
Glucose + 2NAD+ + 2ADP + 2P
2 Pyruvate + 2NADH + 2H+ 2H2O
Fate of pyruvate
• If oxygen is present (i.e., conditions are aerobic), pyruvate will cross the
mitochondrial membrane and enter the mitochondria
• If oxygen is absent (i.e. conditions are anaerobic), pyruvate will undergo
fermentation
Fermentation
• Under anaerobic conditions, pyruvate + NADH converted to lactate or primarily
ethanol
• Recycles NAD needed to oxidize Glyceraldehyde 3 Phosphate during
glycolysis
• Less efficient energy production than respiration (2 ATP per glucose)
The citric acid cycle
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AKA Krebs or Tricarboxylic Acid (TCA) cycle
In mitochondrial matrix...
Complete oxidation of pyruvate to CO2 and NADH (8 enzymatic steps)
The citric acid cycle
•
Begins with oxidative decarboxylation of pyruvate (3C) to acetyl CoA (2C),
catalyzed by pyruvate dehydrogenase
Citric acid cycle
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Acetyl CoA (2C) condenses with oxaloacetate (4C) to form citrate (6C)
catalyzed by citrate synthase
Successive oxidation and decarboxylation of citrate to regenerate oxaloacetate
(OAA)
Citric acid cycle
OAA + Acetyl CoA + 3 H2O + ADP + P +3NAD- + FAD
OAA + 2CO2 + CoA + ATP + 3NADH + 3H+ + FADH2
• Two entire turns of the cycle to metabolize the equivalent of one hexose
(glucose) yields
Electron transport chain and oxidative phosphorylation
• What happens to NADH?
• Across mitochondrial inner membrane...
• Electrons move through series of proteins in the mitochondrial membrane
• Electrons move from higher energy level to lower energy level, releasing
energy in the process
• O2 is final electron acceptor
Electron transport chain
• Proton gradient used to produce ATP through ATP synthase
Electron transport chain
• Net 3 ATP per NADH; 2 ATP per FADH2
– Yield 22 ATP
• 18 from 6 NADH
• 4 from 2 FADH2
Unique features of plant respiration
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Plants have alternate membrane proteins in mitochondria
Unique plant respiration
• External NAD(P)H dehydrogenase -alternate source of electrons used to
reduce CoQ
• Complex I not involved, thus only 2 ATP can be produced
• Internal rotenone-insensitive dehydrogenase - oxidizes internal membrane
NADH only
Plant respiration
• Complex IV inhibited by CN, but much plant respiration continues with CN
• Cyanide (CN)-insensitive respiration - insensitive to many respiratory
inhibitors, uses alternative oxidase
• Energy converted to heat, rather than ATP
Unique plant respiration
• Role in heat generation (thermogenesis in skunk cabbage)
• Energy overflow metabolism when cytochrome pathway is saturated