Biol221 24a metabolism
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Transcript of Biol221 24a metabolism
Fundamentals of cellular energetics
I. Principles of energeticsA. Reaction coupling and ATP B. Cellular use of energyC. Redox
II. Oxidation of glucose to CO2
III. ATP synthesisIV. Photosynthesis
Principles of cellular energetics
• Life requires work (organisms perform work to live, grow, and replicate)
• All organisms use energy for biological work
• Energy transformations in cells follow the laws of thermodynamics
Reaction coupling review
• An energetically unfavorable reaction (∆G˚´>0) can proceed forward if it is coupled to an energetically favorable (∆G˚´<0) reaction
• The hydrolysis of ATP is usually used to drive reactions forward
ATPADP + Pi
∆G˚´ = –7.3 kcal/mol (–30.5 kJ/mol)
Energy currency of the cell
ATP as energy currency
• Cells obtain free energy from either chemical oxidation or sunlight
• Cells use energy to synthesize ATP• Cells spend ATP on unfavorable processes
What types of unfavorable processes do cells spend their hard-earned ATP on?
Cellular use of energy
• Biosynthesis (making biomolecules and biological structures)
• Transport across membranes (against concentration gradient)
• Movement
Redox review (again)
• Oxidation: loss of an electron• Reduction: gain of an electron
Redox (oxidation-reduction) reactions always occur in pairs
Redox and cellular energetics• Electrons are transferred and energy is
transduced as chemicals are oxidized• Energy from oxidation does biological work• Source of electrons are reduced molecules
like glucose (a sugar) and fatty acids• Final electron acceptor is O2
• Electron flow produces proton gradient across a membrane, which is used to synthesize ATP
Electron carriers
NAD+ + 2 e– (+ H+) –> NADHFAD + 2 e– (+ 2H+) –> FADH2
Cofactors in redox reactions
Overview of cellular oxidation
Glycolysis
Citric acidcycle
Oxidativephosphorylation
Glucose
NADHFADH2
CO2
CO2
ADPNAD+
ATPNADH
NAD+
FAD
NADHFADH2
ADPO2
NAD+
FADATPH2O
Fundamentals of cellular energetics
I. Principles of energeticsII. Oxidation of glucose to CO2
A. GlycolysisB. Pyruvate fateC. Citric acid cycle
III. ATP synthesisIV. Photosynthesis
Glycolysis
• Universal pathway conserved throughout evolution for producing energy
• Releases chemical energy in glucose• Forms 2 ATP, 2 NADH, and 2 pyruvate• Ten enzyme-catalyzed reactions• Occurs in cytosol
Fate of pyruvate• Oxidation to enter citric acid cycle
Results in complete oxidation of glucose to CO2
Requires O2 as final electron acceptor
• Lactate fermentation (anaerobic)Muscle during sprintYogurt and cheese production by bacteria
• Alcohol fermentation (anaerobic)Yield ethanol and CO2
Beer, wine, and bread production by yeast
Citric acid cycle• Central pathway in oxidation of fuels• Eight enzyme-catalyzed reactions• Product is starting material: cycle• Each turn yields 2 CO2, 3 NADH, and 1
FADH2 (two turns per glucose)• Occurs in mitochondria• (Also called tricarboxylic acid cycle or Krebs
cycle)
Energetic accountingOne glucose yields:• 2 ATP in glycolysis• 2 NADH in glycolysis• 2 NADH as pyruvate enters citric acid cycle• 2 ATP in citric acid cycle• 6 NADH in citric acid cycle• 2 FADH2 in citric acid cycle
Energetic yield• Each NADH can produce ~3 ATP and each
FADH2 can produce ~2 ATP when O2 is present as final electron acceptor
• One glucose completely oxidized to CO2 yields up to ~38 ATP
Fundamentals of cellular energetics
I. Principles of energeticsII. Oxidation of glucose to CO2
III. ATP synthesisA. MitochondriaB. Electron transportC. Oxidative phosphorylation
IV. Photosynthesis
Mitochondria
• Power plants of eukaryotic cells
• Site of citric acid cycle, electron transport, and ATP synthesis
Electron transport• Electrons from NADH and FADH2 are passed
to O2
– Regenerate NAD+ and FAD– Form H2O– Release energy
• Electron flow is coupled to pumping of protons (H+) across inner mitochondrial membrane (10 H+ per NADH)
• Creates proton gradient across membrane
Oxidative phosphorylation
• Proton gradient is used to synthesize ATP (concentration gradients across a membrane are a form of energy that can be converted to chemical energy)
• Catalyzed by ATP synthase (also called F1F0-ATPase)
ATP synthase
• Catalyzes ATP synthesis as protons flow down their concentration gradient across the inner mitochondrial membrane
• Two functional components:– F0 is a transmembrane proton channel
– F1 catalyzes ATP synthesis
• Mechanism involves conformational changes (H+ translocation powers rotation)