Chapter 18

• Stages of aerobic respiration

  • glycolysis (cytoplasm)
  • The link reaction (matrix)
  • The Krebs Cycle (matrix)
  • Oxidative Phosphorylation/ Electron transport chain (cristae)

  
• Stages of anaerobic respiration

  • glycolysis
  • fermentation
• What type of process is glycolysis?


  anaerobic
• Main steps of glycolysis 


  1. phosphorylation
  2. lysis
  3. phosphorylation
  4. dehydrogenation & formation of ATP

  
• Glycolysis - first phosphorylation

  • 2 phosphates (released from ATP) are attached to a glucose molecule
  • forms a hexose bisphosphate

  
• Glycolysis - lysis

  • hexose bisphosphate is destabilised
  • splits into 2 triose phosphate molecules

  
• Glycolysis - second phosphorylation

  • another phosphate group (from free inorganic phosphate ions (Pi) present in cytoplasm) is added to each of the triose phosphates
  • forms 2 triose bisphosphate molecules

  
• Glycolysis - dehydrogenation & formation of ATP

  • the 2 triose bisphosphate molecules are oxidised by the removal of hydrogen atoms (dehydrogenation) - forms 2 pyruvate molecules
  • NAD coenzymes accept removed hydrogens - they are reduced, forming 2 reduced NAD (NADH) molecules
  • 4 ATP molecules are produced using phosphates from the triose bisphosphate

  
• The formation of ATP without involvment of an ETC is an example of....

  • substrate level phosphorylation
  • ATP is formed by the transfer of a phosphate group from a phosphorylated intermediate (triose bisphosphate) to ADP
• Overall net ATP yield from gylocylisis

  • 2 molecules of ATP
  • 2 ATP molecules are used to prime the process at beginning & 4 ATP molecules are produced
• What happens to reduced NAD after glycolysis

  its used in a later stage to synthesise more ATP
• Label molecules in the stages of glycolysis

  A) glucose

  B) ATP

  C) ATP

  D) hexose bisphosphate

  E) triose phosphates

  F) inorganic phosphate ion

  G) inorganic phosphate ion

  H) triose bisphosphates

  I) ATP

  J) ATP

  K) ATP

  L) ATP

  M) reduced NAD

  N) reduced NAD

  O) pyruvates

  
• Label mitochondrion

  A) outer membrane

  B) matrix

  C) intermembrane space

  D) cristae

  E) inner membrane

  
• Outer membrane of mitochondria

  separates contents of mitochondrion from rest of cell - creating cellular compartment w ideal conditions for aerobic respiration
• Inner membrane of mitochondrion

  contains electron transport chains & ATP synthase
• Intermembrane space of mitochondria

  space where proteins are pumped into by the electron transport chain - space is small so concentration builds up quickly
• Matrix of mitochondria

  contains enzymes for the Krebs cycle & the link reaction, also contains mitochondrial DNA
• Cristae of mitochondria

  projections of the inner membrane which increase the surface area available for oxidative phosphorylation
• Link reaction

  • Pyruvate enters mitochondrial matrix by active transport via specific carrier proteins 
  • Pyruvate undergoes oxidative decarboxylation - CO2 is removed along w hydrogen 
  • NAD accepts hydrogen atoms removed - NAD is reduced to form NADH 
  • Resulting 2C acetyl group is bound by coenzyme A forming acetylcoenzyme A (acetyl CoA) 
  • Acetyl CoA delivers the acetyl group to the Krebs cycle - coenzyme A can be reused

  
• Krebs cycle 


  • acetyl CoA delivers an acetyl group to the krebs cycle - the 2C acetyl group combines with 4C oxaloacetate to form 6C citrate
  • citrate molecule undergoes decarboxylation & dehydrogenation - produces one reduced NAD & CO2 - a 5C compound is formed
  • the 5C compound undergoes further decarboxylation & dehydrogenation reactions, eventually regenerating oxaloacetate, so cycle continues
  • more CO2, 2 more reduced NAD & one reduced FAD are produced
  • ATP also produced by substrate-level phosphorylation
  • NADH & FADH2 deliver hydrogen to ETC

  
• Importance of coenzymes


  • Needed to transfer protons, electrons & functional groups between molecules
  • Many of reactions in stages of respiration involve redox reactions 
  • Removal of electrons or hydrogen atoms (H)
  • These hydrogen atoms are transferred to carrier molecules (coenzymes) & moved to where they are needed next 
  • Coenzymes are reduced in this process
• NAD vs FAD

  • NAD is used in all stages of respiration, FAD only used in the Krebs cycle
  • NAD accepts 1 hydrogen, FAD accepts 2 hydrogens
  • NADH is oxidised at start of ETC, FADH2 is oxidised further along the ETC
  • NADH leads to the synthesis of 3 ATP molecules, FADH2 leads to the synthesis of 2 ATP molecules
• Oxidative phosphorylation 


  • hydrogen atoms collected by NAD & FAD are delivered to ETCs in cristae
  • hydrogen atoms dissociate into hydrogen ions & electrons
  • energy is released as the electrons move from carrier to carrier - this energy used to create proton gradient leading to diffusion of protons through ATP synthase resulting in ATP synthesis
  • at end of ETC, electrons combine with hydrogen ions & O2 to form water
  • O2 is final electron acceptor & ETC can't operate unless O2 present

  
• Why can't hydrogens from NAD and FAD directly combine?

  • hydrogens released from NAD & FAD could combine directly with oxygen, releasing energy from the formation of bonds during production of water
  • however, this energy could not be used to synthesise ATP - heat released in the exothermic reaction would simply raise temp of the cell
• Why do the electrons released from reduced FAD lead to the synthesis of less ATP than the electrons released from reduced NAD?

  • Reduced NAD releases electrons to carriers at the start of the ETC whilst reduced FAD releases electrons to carriers after the start of the ETC
  • with FAD electrons are transported a shorter distance so fewer protons are actively transported
• Substrate level phosphorylation


  production of ATP involving the transfer of a phosphate group from a highly reactive intermediate