Respiration

• 4 stages in aerobic respiration:

  • Glycolysis - occurs in cytoplasm
  • Link Reaction - occurs in mitochondria
  • Krebs Cycle - occurs in mitochondria
  • Oxidative Phosphorylation - occurs in mitochondria

  First 3 stages = series of reactions whose products are used in the final stage of oxidative phosphorylation to produce lots of ATP
• All cells use glucose to respire but organisms can also break down other complex organic molecules such as fatty acids and amino acids which can then be respired
• Stage 1: Glycolysis makes pyruvate from glucose

  1. Glycolysis = splitting one molecule of glucose with 6 carbons into 2 smaller pyruvate molecules which are 3 carbon
  2. Occurs in cytoplasm of cell
  3. Glycolysis = first stage of both aerobic and anaerobic respiration and does not need oxygen to take place - it is an anaerobic process
• The 2 stages in glycolysis are:

  • Phosphorylation
  • Oxidation
• In glycolysis, first, ATP used to phosphorylate glucose to triose phosphate which is then oxidised to release ATP.

  Overall there is a net gain of 2 ATP.
• Glycolysis stage 1: Phosphorylation

  1. Glucose is phosphorylated by adding 2 phosphates from 2 molecules of ATP
  2. This creates 1 molecule of hexose biphosphate and 2 molecules of ADP
  3. Then hexose biphosphate is split up into 2 molecules of triose phosphate

  Glycolysis stage 2: Oxidation

  1. Triose phosphate is oxidised (loses hydrogen), forming 2 molecules of pyruvate
  2. NAD collects hydrogen ions, forming 2 reduced NAD
  3. 4 ATP are produced but 2 used up in stage one so there is a net gain of 2 ATP

  
• 2 molecules of reduced NAD produced in glycolysis go to the last stage (oxidative phosphorylation)

  2 molecules of pyruvate produced in glycolysis are actively transported into the matrix of mitochondria for the link reaction
• Stage 2: link reaction occurs in mitochondria and converts pyruvate to acetylene coenzyme A

  1. Pyruvate decarboxylated - one carbon atom is removed from pyruvate in the form of carbon dioxide
  2. NAD reduced to NADH - collects hydrogen from pyruvate, changing pyruvate into acetate
  3. Acetate combined with coenzyme A (CoA) to form acetyl coenzyme A (acetyl CoA)
  4. No ATP produced in this reaction

  
• Link reaction occurs twice for every glucose molecule. 2 pyruvate molecules are made for each glucose molecule that enters glycolysis meaning link reaction and third stage (Krebs cycle) happen twice for every glucose molecule.

  For each glucose molecule:

  • 2 molecules of acetyl CoA go into Krebs cycle
  • 2 carbon dioxide molecules released as a waste product of respiration
  • 2 molecules of reduced NAD formed and go to last stage called oxidative phosphorylation
• What combines with oxaloacetate to form citrate in the Krebs Cycle?

  The acetyl group from acetyl CoA
• What enzyme catalyzes the formation of citrate from acetyl CoA and oxaloacetate?

  Citrate synthase
• What happens to CoA after it combines with the acetyl group?

  It goes back to the link reaction to be reused
• What is the initial transformation of the 6C citrate molecule in the Krebs Cycle?

  It is converted to a 5C molecule
• What processes occur during the conversion of the 6C citrate molecule to the 5C molecule?

  Decarboxylation and dehydrogenation
• What is produced from the hydrogen removed during the conversion of citrate to a 5C molecule?

  Reduced NAD from NAD
• What happens to the 5C molecule in the Krebs Cycle?

  It is converted to a 4C molecule
• What is produced during the conversion of the 5C molecule to a 4C molecule?

  1 molecule of reduced FAD and 2 molecules of reduced NAD
• How is ATP produced in the Krebs Cycle?

  By direct transfer of a phosphate group from an intermediate compound to ADP
• What is substrate-level phosphorylation?

  When a phosphate group is directly transferred from one molecule to another
• What does citrate convert into at the end of the Krebs Cycle?

  Oxaloacetate
• Stage 3: Krebs cycle produces reduced coenzymes and ATP which involves a series of oxidation-reduction reactions, occurring in the mitochondrial matrix. The cycle occurs once for every pyruvate molecule meaning it goes round twice for every glucose molecule.

  
• Some products of the Krebs cycle are used in oxidative phosphorylation. Note that some products are reused, some are released and others are used for the next stage of respiration:

  • 1 CoA - reused in next link reaction
  • Oxaloacetate - regenerated for use in the next Krebs cycle
  • 2 carbon dioxide molecules released- released as a waste product
  • 1 ATP - used for energy
  • 3 reduced NAD (NADH) - to oxidative phosphorylation
  • 1 reduced FAD - to oxidative phosphorylation
• Stage 4 - oxidative phosphorylation produces lots of ATP

  1. Oxidative phosphorylation is the process where energy carried by electrons from reduced coenzymes (NADH and FADH) is used to make ATP
  2. Oxidative phosphorylation takes place in inner mitochondrial membrane as protons are pumped across the inner mitochondrial membrane
• What is oxidative phosphorylation?

  A process that produces ATP using energy from electrons moving down an electron transport chain (ETC)
• What happens to hydrogen atoms during oxidative phosphorylation?

  They are released from reduced NAD and reduced FAD as they are oxidized to NAD and FAD
• What do hydrogen atoms split into during oxidative phosphorylation?

  Protons (H+) and electrons (e-)
• What is the role of the electron transport chain (ETC) in oxidative phosphorylation?

  Electrons move along the ETC and lose energy at each carrier
• What is done with the energy lost by electrons in the ETC?

  It is used to pump protons from the mitochondrial matrix into the intermembrane space
• What is formed as a result of pumping protons into the intermembrane space?

  An electrochemical gradient due to higher proton concentration in the intermembrane space
• How do protons return to the mitochondrial matrix?

  They move down the electrochemical gradient via ATP synthase
• What does the movement of protons through ATP synthase drive?

  The synthesis of ATP from ADP and inorganic phosphate
• What is chemiosmosis?

  The process of ATP production driven by the movement of H+ ions across a membrane due to electrons moving down an ETC
• What happens at the end of the electron transport chain in the mitochondrial matrix?

  Protons, electrons, and oxygen combine to form water
• What is the final electron acceptor in oxidative phosphorylation?

  Oxygen
• The electron transport chain is located in the mitochondrial matrix.
• The mitochondrial matrix is folded into cristae which increases the membrane’s surface area. This maximises respiration.
• Intermembrane space: space between the inner and outer mitochondrial membranes
• Electrochemical gradient: a concentration gradient of ions
• 32 ATP can be made from 1 glucose molecule
• Oxidative phosphorylation makes ATP using energy from the reduced coenzymes - 2.5 ATP are made from reduced NAD and 1.5 ATP are made from each reduced FAD. One molecule of glucose produces 2 pyruvate so the Link Reaction and Krebs Cycle occur twice