MS2 - Cellular Respiration

• The metabolic pathway involved in respiration can be split into three main parts:

  1. glycolysis - occurs in the cytoplasm
  2. citric acid cycle - occurs in the matrix of the mitochondria.
  3. Electron transport chain - occurs in the inner membrane of the mitochondria.
• Glycolysis is the break down of glucose to pyruvate in the cytoplasm
• ATP provides energy for the phosphorylation (addition of phosphate) of glucose and intermediates during the energy investment phase of glycolysis.
• This leads to the generation of ATP during the energy pay-off stage.
• 2 ATP are used but 4 ATP made resulting in a net gain of 2 ATP
• Dehydrogenase enzymes remove hydrogen ions and electrons and pass them to the coenzyme NAD - converting it into NAdH
• In aerobic conditions (oxygen present) glycolysis is followed by the citric acid cycle.
• The pyruvate leaves the cytoplasm and enters the mitochondria.
• with the removal of carbon dioxide, pyruvate is broken down to an acetyl group.
• This acetyl group combines with coenzymes A forming acetyl coenzyme A.
• The citric acid cycle occurs in the matrix of the mitochondria
• In the citric acid cycle the acetyl group from acetyl coenzyme A combines with oxaloacetate to form citrate
• During a series of enzyme-controlled steps, citrate is gradually converted back into oxaloacetate which can combine with another acetyl group
• ATP is generated and carbon dioxide is released from the citric acid cycle
• Dehydrogenase enzymes remove hydrogen ions and electrons and pass them to the coenzyme NAD, forming NADH
• This occurs in both glycolysis and the citric acid cycle
• The coenzyme NAD carries hydrogen ions and electrons (as NADH) to the electron transport chain
• The electron transport chain is the last stage of the respiration pathway and is the stage that produces the most ATP
• The electron transport chain is a series of carrier proteins attached to the inner mitochondrial membrane
• Electrons are passed along the electron transport chain, releasing energy
• This energy is used to pump hydrogen ions across the inner mitochondrial membrane
• The flow of these hydrogen ions back through the membrane protein ATP synthase results in the production of ATP (ADP + Pi)
• Finally, hydrogen ions and electrons combine with oxygen to form water
• If glucose is not available, other respiratory substrates such as starch, glycogen, proteins (amino acids) and fats can all be broken down into intermediates in glycolisis or the citric acid cycle
• This provides alternative metabolic pathways to make ATP
• In the absence of oxygen, pyruvate undergoes fermentation this takes place in the cytoplasm.
• In animal cells, pyruvate is converted to lactate in a reversible reaction
• In plants and yeast, ethanol and co2 are produced in an irreversible reaction
• Fermentation results in much less ATP being produced than in aerobic respiration
• ATP is the energy carrying molecule used in cells because it can release energy very quickly
• Energy is released from ATP when the end phosphate is removed
• Once ATP has released energy, it becomes ADP which is a low energy molecule
• ADP can be recharged back into ATP by adding a phosphate. This requires energy
• ATP is used to transfer energy to cellular processes which require energy