aerobic

• What are the four stages of aerobic respiration?

  1. glycolysis
  2. link reaction
  3. krebs cycle
  4. oxidative phosphorylation
• Key info

  first three stages are a series of reactions

  the products of these are used in the last stage to produce loads of ATP

  first stage takes places in the cytoplasm, the rest take place in the mitochondria

  all cells use glucose to respire, but organisms can also break down complex organic molecules (e.g. fatty acids, amino acids) which can then be respired
• Stage 1: Glycolysis 


  involves splitting 1 molecule of glucose (6C) into 2 smaller molecules of pyruvate (3C)

  happens in the cytoplasm

  happens in anaerobic and aerobic respiration
• Glycolysis - Step 1

  Phosphorylation

  1. glucose is phosphorylated by adding 2 phosphates from 2 molecules of ATP
  2. this creates 1 molecule of hexose bisphosphate and 2 molecules of ADP
  3. then hexose bisphosphate is split up into 2 molecules of of triose phosphate
• Glycolysis - Step 2

  Oxidation

  1. triose phosphate is oxidised (loses hydrogen) forming 2 molecules of pyruvate
  2. NAD collects the hydrogen ions, forming 2 red. NAD
  3. 4 ATP are produced, but 2 were used up in stage one so there's a net gain of 2 ATP
• What happens to the products of glycolysis?

  The 2 molecules of red. NAD go to oxidative phosphorylation

  The 2 molecules of pyruvate are actively transported to matrix of mitochondria for for link reaction
• Stage 2: The Link Reaction

  Takes place in the mitochondrial matrix

  1. Pyruvate is decarboxylated - 1 carbon atom is removed from pyruvate in the form of CO2
  2. NAD is reduced to NADH - it collects hydrogen from pyruvate, changing pyruvate into acetate
  3. Acetate is combined with coenzyme A to form acetyl coenzyme A
  4. No ATP is produced in this reaction
• How many times does the Link Reaction occur?

  Twice

  Because two molecules of pyruvate are made in each glycolysis reaction
• In the Link Reaction what products are formed for each glucose molecule?

  2 molecules of acetyl coenzyme A to go into the Krebs cycle

  2 CO2 molecules are released as a waste product

  2 molecules of reduced NAD are formed and go to oxidative phosphorylation
• Stage 3: The Krebs Cycle

  involves a series of oxidation-reduction reactions

  takes place in the matrix of the mitochondria

  happens once for each pyruvate molecule, so happens twice for each glucose molecule
• Krebs Cycle - Step 1

  The acetyl group from acetyl coenzyme A (formed in link reaction) combines with oxaloacetate to form citrate

  This catalysed by citric synthase

  Coenzyme A goes back to Link Reaction to be used again
• Krebs Cycle - Step 2

  The 6C citrate molecule is converted a 5C molecule

  Decarboxylation occurs where CO2 is removed

  Dehydrogenation also occurs where hydrogen is removed

  The hydrogen is used to produce reduced NAD from NAD
• Krebs Cycle - Step 3

  The 5C molecule is then converted into a 4C molecule

  Decarboxylation and dehydrogenation occur producing 1 molecule of reduced FAD and 2 of reduced NAD

  ATP is produced by the direct transfer of a phosphate group from an intermediate compound to ADP

  When a phosphate is directly transferred from one molecule to another it's called substrate-level phosphorylation

  Citrate has now been converted to oxaloacetate
• Substrate-Level Phosphorylation


  When a phosphate group is directly transferred from one molecule to another
• Krebs Cycle - Products from one cycle

  1 coenzyme A reused in the next Link Reaction

  Oxaloacetate regenerated for next use in the Krebs Cycle

  2 CO2 released as a waste product

  1 ATP used for energy

  3 reduced NAD to oxidative phosphorylation

  1 reduced FAD to oxidative phosphorylation
• Stage 4: Oxidative Phosphorylation

  The process where the energy carried by electrons , from reduced coenzymes (red. NAD/FAD), is used to make ATP

  (whole point of Krebs Cycle to make red. NAD/FAD for this stage)

  Takes place in the inner mitochondrial membrane
• Oxidative Phosphorylation - Steps 1-2

  Steps 1-2

  1. Hydrogen Atoms are released from red. NAD and red. FAD as they are oxidised to NAD and FAD. The H atoms split into protons (H+) and electrons (e-)
  2. The electrons move along the electron transport chain (made up of three electron carriers) losing energy at each carrier
• Oxidative Phosphorylation - Steps 3-4

  Steps 3-4

  1. This energy (lost by electrons) is used by the electron carriers to pump protons from the mitochondrial matrix into the intermembrane space
  2. The concentration of protons is now higher in the intermembrane space than in matrix - forming an electrochemical gradient
• Oxidative Phosphorylation - Step 5-6

  Steps 5-6

  1. Protons move down the electrochemical gradient back into the matrix via ATP synthase. This movement drives the synthesis of ATP from ADP and inorganic phosphate
  2. This process of ATP production driven by the movement of H+ ions across a membrane (due to electrons moving down an ETC) is called chemiosmosis
• Oxidative Phosphorylation - Step 7

  Step 7

  1. In the mitochondrial matrix at the end of the transport chain, the protons, electrons and O2 (from the blood) combine to form water. Oxygen is said to be the final electron acceptor
• How many ATP are made from 1 glucose molecule?

  32