Respiration

Energy is Essential to Life

• Reactions of energy transformations in biological systems are collectively referred to as metabolism
  • Metabolic reactions belong to two phases
    • Anabolism which is responsible for the assembly of biological compounds
    • Catabolism which is involved in the breakdown of substances to extract energy

Flow of Energy

• Life depends on the energy delivered to the earth from the sun
• 1,300,000,000,000,000,000,000,000 cal/year
• The amount of solar energy reaching the surface each day is 1.5 billion times the amount of electrical energy generated in the U.S. each year
  

Availability of Solar Energy

• Approximately 1/3 of solar energy reaching earth is reflected into space as light.
• The remaining 2/3 is absorbed by the earth and converted into heat (A process which is vital to the maintenance of life on earth)
• Less that 1% of the solar energy reaching the earths surface is transformed into mechanical and chemical energy through photosynthesis
  

The Flow of Energy

• Evolution of metabolism
  • Energy of early organisms preceded the abundance of oxygen in the atmosphere
    • Chemotrophs represented today by relict species found in harsh environments such thermal pools, oceanic vents and bogs
      • Extracted energy from inorganic substances to produce organic compounds
      • CO₂ + H_{2 CH4} + H₂O + Energy
      • H₂ + S H₂S + Energy

Release of Chemical Energy

• Energy is released when electrons are transferred from a higher electron energy shell to a vacant lower shell
  • This release of energy can occur within an atom, between atoms or during a bond formation
    • The amount of energy released depends on the electronegativity of the atoms involved
    • C-H have more energy than O-H bonds (11 kcal)

How Energy is Transferred in Reactions

• Carbohydrates such as glucose are energy rich, while CO2, the degradation product, is energy poor.
  • During cellular respiration the carbon-hydrogen bonds are replaced by oxygen-hydrogen bonds
    • Transfer of hydrogen atoms from carbons to oxygen atoms is referred to as oxidation
      • Loss of electrons is an oxidation process
    • Conversely the storing of energy is referred to as a reduction
      • Gain of electrons is reductive process

ATP: The currency of energy exchange

• Adenosine triphosphate (ATP) occupies a key role in the energy transformations of living systems
  • ATP is regenerated in the process of phosphorylation from ADP and Pi
  • Hydrolysis of ATP to ADP and Pi releases a useful amount of energy (7.3 kcal/mol)
    • Sufficient for the making and breaking of covalent bonds

How cells harvest energy

• Large amounts of energy are stored in lipids, carbohydrates and proteins
  • These energies are released in a series of small steps catalyzed by specific enzymes.
    • The released energy is conserved in ATP through phosphorylation processes
  • Metabolism of glucose plays a central role in the energy exchange of living systems

Aerobic Respiration

• The complete catabolism of glucose involves 5 stages
  • Glycolysis (Glucose to Pyruvate)
  • Oxidation of pyruvic acid
  • Krebs Cycle (Tricarboxylic acid cycle)
  • Mitochondrial Electron Transport
  • Oxidative Phosphorylation (Chemiosmotic Synthesis)

Stage 1: Glycolysis

• Glycolysis is the anaerobic portion of the process
  • Catabolism of glucose (686 kcal/mol) to pyruvic acid
    • Occurs within the cytoplasm
    • Most ancient of the biochemical pathways
  • Glycolysis occurs as a series of 10 enzyme-mediated steps

End-products of glycolysis

• Pyruvic acid (590 kcal/mol) is the final product of glycolysis
  • Catabolism of glucose to pyruvate represents a yield of 14% of the available potential energy
    • Conserved in a net of 2 ATP and 2 NADH
    • This margin of energy gain is utilized by organisms in anaerobic environment
  • Under anaerobic conditions it is essential to recycle the NAD for continued energy gain
    • Fermentation pathways are utilized in the absence of oxygen

Fermentation

• Occurs in the absence of oxygen
• The NADH generated during glycolysis serves to shuttle high energy electrons from one reaction to another.
  • In the absence of oxygen the cellular pools of NAD quickly become tied up and must be recycled to continue any energy gain.
• This recycling is accomplished by the reduction of pyruvate to either lactic acid (animals) or alcohol (plants and microorganisms)
  

Aerobic Respiration

• In eucaryotic organisms the most efficient energy extraction occurs in the presence of O2 in the mitochondria
  • In aerobic respiration molecular O2 functions as an electron acceptor
• Mitochondrial enzymes progressively move the electrons through a series of redox reactions slowly releasing the energy which they posses
• O₂ +2 NADH 2H₂O + 2 NAD (DG =-52 kcal/mol)

Stage 2: Oxidation of Pyruvate

• The net effect of aerobic respiration is to break the 3 C pyruvate into a CO2 and a 2 C acetyl-CoA molecule
• This decarboxylation process occurs during transport of pyruvate from the cytoplasm across the inner membrane of the mitochondria
• Decarboxylation results in the reduction of 1 NAD for each pyruvate molecule

Stage 3: Krebs Cycle (Tricarboxylic Acid Cycle)

• Acetyl CoA (2C) is added to an oxaloacetic acid (4C) to produce a citric acid molecule (6C)
• Citric acid is enzymatically converted into a ketoglutaric acid (5C) with the release of CO2 and capture of energy in the form of an NADH
• a-Ketoglutaric acid is again decarboxylated to a succinic acid with the release additional energy, harvested as NADH
• Succinic acid is rearranged to reform Oxaloacetic acid with the formation of an ATP, NADH and FADH2

Stage 4: Electron Transport Chain

• Elaborate internal structure of the mitochondrion plays a crucial role in further energy harvest.
  • The folded internal membrane (cristae) divides the mitochondrial compartment into two very different chemical environments
    • Serves as a scaffolding for gates, pumps and enzyme arrays
    • Cristae is a highly selective membrane, impermeable to most substances including hydrogen ions
      

Electron Transport Chain

• The redox proteins of the electron transport chain are embedded or associated with the cristae
  • Three large protein complexes (NADH-Q reductase, Cytochrome reductase and Cytochrome oxidase); one peripheral protein (Cytochrome C); and a lipid soluble ubiquinone
• The function of these redox proteins is harvest the energy stored in the reduced electron carriers (NADH and FADH₂) generated during the earlier stages of glucose catabolism
  

Transformation of the Electron Energy

• As the electrons are moved through the ETS, they are passed vectorially back and forth across the cristae.
  • During the passage of electrons from the matrix to the exterior surface of the cristae membrane, the electrons are accompanied by a H+
    • Upon reaching the exterior membrane surface the H+ and the electrons are separated
      • Electrons are moved to the next carrier in the transport chain, while the H+ are dropped off into the inner membrane space

Strategy of Electron Transport (ETS)

• Electrons are passed from the reduced carriers through a series of proteins that progressively release small portions of their energy.
• The final step in the ETS is pass the electron which are of a lower energy state to O₂, reducing it to H₂O

Generation of an Electrochemical Gradient

• The separation of the H+ at the external membrane face results in the creation of high concentration of hydrogen ions and positive charge.
• Since the hydrogen ions were moved from the matrix without being replaced, the matrix becomes negatively charged
• This concentration of H+ and positive charge relative to the matrix created an electrochemical gradient that contains a relatively high amount of potential energy
• Cristae membrane separates and maintains this energy rich gradient

Harvest of the Electrochemical Gradient

• Chemiosmotic gradient contains large amount of potential energy
  • Energy sufficent to drive the endergonic phosphorylation of ADP to ATP
  • Energy of the chemisomtic gradient is coupled to the phosphoryaltion process through the ATPase protein
    • ATPase is capable of allowing hydrogen ion to move back into the matrix

Overall Efficiency

• Glucose contains 686 kcal/mol
• Aerobic respiration yields
  • 36 ATP (7.3 kcal/mol) 262 kcal/mol
  • Percent of energy harvested is 38%
    • Heat generated 62%
    • Wasted heat can be used to maintain an internal body temperature

Catabolism of Proteins and Fats

• Catabolism of fats begins with their hydrolysis to glycerol and fatty acids
  • Glycerol is converted into phosphoglyceraldehyde and metabolized through glycolysis
  • Fatty acids are broken down into 2 C compounds that are converted into acetyl CoA and then metabolized in the respiratory pathway
• Proteins are hydrolyzed into amino acids, the amino groups are removed (deamination) then the remainder of the compound is converted into either acetyl CoA , pyruvic acid or some citric acid cycle intermediate for further catabolism
• Respiratory pathway serves as central catabolism pathway for the catabolism of organic compounds

• This page is maintained by James C. Pushnik: jpushnik@ecst.csuchico.edu