The Krebs Cycle and

							The Krebs Cycle and


      Electron Transport
Introduction

• At end of glycolysis, 90 % of chemical
  energy that was available in glucose is still
  unused, stored in high-energy electrons of
  pyruvic acid (pyruvate)
• To remove or get out that energy, the cell
  requires oxygen for final stages of cellular
  respiration – this makes the reactions
  aerobic or oxygen requiring
The Krebs Cycle

• In presence of oxygen, pyruvic acid
  produced in glycolysis passes to second
  stage of cellular respiration, the Krebs cycle
• During Krebs cycle, pyruvic acid is broken
  down into carbon dioxide in a series of
  energy-extracting reactions
• Krebs cycle is also known as citric acid
  since this is first compound formed in this
  series of reactions
Krebs Cycle cont’d
• Krebs cycle begins when pyruvic acid
  produced by glycolysis enters the
  mitochondrion
• Where the 3 carbons of pyruvic acid go
  when broken down
  – One carbon atom becomes part of molecule of
    carbon dioxide
  – Two carbon atoms are joined to a compound
    called coenzyme A to form acetyl-CoA
Krebs Cycle cont’d

• Carbon dioxide gas produced in breaking
  down of pyruvic acid is released into the air
• Citric acid is produced when acetyl-CoA
  adds the 2-carbon acetyl group to a
  4-carbon molecule
• Two (2) molecules of carbon dioxide (CO2)
  are released during the energy extraction
  part of the Krebs cycle
  Krebs Cycle cont’d
• Energy tally from one molecule of pyruvic acid:
  4 NADH molecules, 1 FADH2 molecule, and
  1 ATP molecule during the Krebs cycle
• NADH and FADH2 are formed when electrons
  join NAD+ and FAD
• The 4-carbon compound generated in
  breakdown of citric acid is the only permanent
  compound regenerated at the end of each
  complete turn of the cycle
Krebs cycle cont’d
Electron Transport
Chain or ETC
• is a series of proteins in the inner membrane
  of mitochondria
• uses high-energy electrons from Krebs
  cycle to convert ADP into ATP
• in eukaryotes the chain is composed of a
  series of proteins that are located in inner
  membrane of the mitochondrion and in
  prokaryotes the chain is in cell membrane
Electron Transport Chain cont’d

• High-energy electrons for ETC are
  produced in Krebs cycle from NADH and
  FADH2
• Oxygen serves as the final electron acceptor
  of ETC – oxygen is essential for getting rid
  of low-energy electrons and hydrogen ions,
  which form water (H2O), one of the waste
  products of cellular respiration
Electron Transport cont’d
• Energy of high-energy electrons is used to
  transport hydrogen ions across membrane
  every time 2 high-energy electrons move
  down the ETC
• During electron transport, H+ ions build up
  in the intermembrane space, making it
  positively charged. The other side of the
  membrane, from which those H+ ions have
  been taken, is now negatively charged. The
  charge differences that build up cause the
  ions to move.
Electron Transport cont’d

• Three (3) ATP molecules are produced as
  each pair of high-energy electrons moves
  down the ETC
Electron Transport Flowchart
• High-energy electrons from NADH and
  FADH2, are passed into and along the
  – electron transport chain
• The energy from the electrons moving
  down the chain is used to move H+ ions
  across the
  – inner membrane
• H+ ions build up in the intermembrane
  space, making it positively charged and
  making the matrix negatively charged.
Electron Transport Flowchart
cont’d
• H+ ions move through channels of ATP
  synthase in the inner membrane.
• The ATP synthase uses the energy from the
  moving ions to combine ADP and
  phosphate, forming high-energy ATP
The Totals
• Total number of ATP molecules formed
  during cellular respiration is 36
• When oxygen is not available, the Krebs
  cycle and electron transport cannot proceed,
  and glycolysis produces just 2 ATP
  molecules per glucose molecule. Under
  aerobic conditions, the Krebs cycle and
  electron transport enable the cell to produce
  34 more ATP molecules per glucose
  molecule
The Totals cont’d

• Sixty-two percent of the total energy of
  glucose that is not used to make ATP
  molecules is released as heat.
• Final waste products of cellular respiration
  are water and carbon dioxide (H2O & CO2)
Energy and Exercise
• ATP sources for a human body uses at the
  beginning of a race
   – ATP already in the muscles
   – new ATP made by lactic acid fermentation
   – cellular respiration
• A runner for a short race (90 sec) gets supply of
  ATP from lactic acid fermentation
• A sprinter may have an oxygen debt because lactic
  acid fermentation produces lactic acid as a
  byproduct. The only way to get rid of the lactic
  acid is in a chemical pathway that requires extra
  oxygen (plenty of deep breathing afterwards)
Energy and Exercise cont’d
• Cellular respiration is the only way to generate a
  continuing supply of ATP, for example during a
  longer race.
• Benefits of exercise for weight control – Storage
  units of glycogen used in cellular respiration
  usually last for 15-20 minutes. After that, the
  body begins to break down other stored molecules,
  including fats, for energy – aerobic forms of
  exercise (dancing, swimming, running) are good
  for losing weight
Comparing Photosynthesis and Cellular Respiration


• Photosynthesis “deposits” energy for cells,
  and cellular respiration “withdraws” energy
• Photosynthesis removes carbon dioxide
  from the atmosphere, and cellular
  respiration puts it back
• Photosynthesis releases oxygen into the
  atmosphere, and cellular respiration uses the
  oxygen to release energy from food, so they
  are opposite processes to each other.
• http://www.phschool.com/science/biology_
  place/biocoach/cellresp/quiz.html