Fundamentals of cellular energetics

I. Principles of energetics

A. Reaction coupling and ATP

B. Cellular use of energy

C. Redox

II. Oxidation of glucose to CO2

III. ATP synthesis

IV. Photosynthesis

Reading and questions (24)

• Chapter 16, pp. 616-648(Browse 648-671)

• Questions– Concepts: 1 and 3– MCAT/GRE-Style: 6, 7, 8, 9

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+) –> NADH

FAD + 2 e– (+ 2H+) –> FADH2

Cofactors in redox reactions

Overview of cellular oxidation

Glycolysis

Citric acidcycle

Oxidative

phosphorylation

Glucose

NADHFADH2

CO2

CO2

ADPNAD+

ATPNADH

NAD+

FAD

NADHFADH2

ADPO2

NAD+

FADATPH2O

Fundamentals of cellular energetics

I. Principles of energetics

II. Oxidation of glucose to CO2

A. Glycolysis

B. Pyruvate fate

C. Citric acid cycle

III. ATP synthesis

IV. 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 sprint

Yogurt 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 accounting

One 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 energetics

II. Oxidation of glucose to CO2

III. ATP synthesis

A. Mitochondria

B. Electron transport

C. 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)

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