3.5.2 RESPIRATION

• Respiration = Plant and animal cells release energy from glucose. Energy used to power all the biological processes in a cell.

  Respiration produces ATP. (A cell can’t get its energy directly from glucose - in respiration the energy released from glucose is used to make ATP). 
• • Aerobic — using oxygen. Produces carbon dioxide and water, and releases energy. 

  C6H12O6 + 6O2 ---> 6CO2 + 6H2O + Energy

  • Anaerobic — without oxygen. In plants and yeast produces ethanol and carbon dioxide and releases energy.  In humans, produces lactate and releases energy. 

  Both types of respiration produce ATP, but anaerobic respiration produces less. 

  Both types of respiration start with glycolysis.
• 4 stages in aerobic respiration:

  1. Glycolysis
  2. The link reaction.
  3. The Krebs cycle.
  4. Oxidative phosphorylation. (Electron transport chain)

  Glycolysis happens in the cytoplasm of cells and the other three stages take place in the mitochondria. 
• Glycolysis, the link reaction and the Krebs cycle are a series of reactions which produce ATP, reduced NAD, reduced FAD and CO2.

  The reduced coenzymes (NAD and FAD) are then used in oxidative phosphorylation, to produce loads more ATP.
• 1 - Glycolysis

  Anaerobic.

  In cytoplasm.  

  Produces pyruvate. 

  1 glucose (6C) split into 2 pyruvate (3C). 

  Net gain of 2 ATP and 2 reduced NAD.

  • Phosphorylation of glucose to TP using ATP.
  • Oxidation of triose phosphate to pyruvate.
  • Net gain of ATP (4 produced, 2 used).
  • NAD reduced.
• Products of glycolysis:

  • 2 NADH - go to stage 4 (oxidative phosphorylation)
  • 2 pyruvate - Actively transported into the mitochondrial matrix for use in the link reaction
  • 2 ATP (net gain) - Used for energy

  2 pyruvate molecules are made for every glucose molecule that enters glycolysis. This means the link reaction and the Krebs cycle happen twice for every glucose molecule.
• 2 - Link Reaction

  In mitochondria.

  Converts pyruvate to acetyl coenzyme A.

  • 1 carbon atom removed from pyruvate (carbon dioxide).
  • Pyruvate oxidised to form acetate
  • NAD reduced to form reduced NAD.
  • Acetate combined with coenzyme A (CoA) to form acetylcoenzyme A (acetyl CoA).
• Products of 2 link reactions:

  2 acetyl coenzyme A - go to krebs cycle

  2 CO2 - released as waste product

  2 NADH - go to stage 4 (oxidative phosphorylation)
• 3 - The Krebs cycle

  Produces reduced coenzymes and ATP. 

  In matrix. 

  Cycle happens once for every pyruvate molecule.

  • Acetyl CoA + 4C molecule forms 6C molecule. Releases CoA.
  • 6C --> 5C molecule. NAD reduced.
  • 5C --> 4C molecule.
  • 1 molecule of reduced FAD and 2 reduced NAD produced. ATP produced.
• When a phosphate group is directly transferred from one molecule to another = substrate-level phosphorylation
• Products of 1 Krebs Cycle:

  1 CoA - Reused in next link reaction

  4C molecule - Regenerated for use in next Krebs cycle

  2 CO2 - Released as waste product

  1 ATP - Used for energy

  3 NADH - go to stage 4 (oxidative phosphorylation)

  1 FADH - go to stage 4 (oxidative phosphorylation)
• 4 - Oxidative Phosphorylation 

  • Makes ATP using energy from the reduced coenzymes.
  • The process where the energy carried by electrons, from reduced coenzymes (reduced NAD and reduced FAD), is used to make ATP. 

  (The whole point of the previous stages is to make reduced NAD and reduced FAD for the final stage.)

  • Involves the electron transport chain and chemiosmosis.
• Oxidative Phosphorylation 1

  • Hydrogen atoms released from reduced NAD and reduced FAD as they’re oxidised to NAD and FAD. Hydrogen atoms split into protons (H+) and electrons (e–).
  • Electrons move down the e.t.c (made up of electron carriers), losing energy at each carrier.
  • This energy used by electron carriers to pump protons from matrix into the intermembrane space (space between inner and outer mitochondrial membranes).
  • Concentration of protons now higher in the intermembrane space than in the mitochondrial matrix — forms an electrochemical gradient (a concentration gradient of ions).
• Oxidative Phosphorylation 2

  • Protons move down electrochemical gradient, across the inner mitochondrial membrane into the matrix, via ATP synthase (embedded in the inner mitochondrial membrane). Movement drives synthesis of ATP from ADP and Pi.
  • In the mitochondrial matrix, at the end of the transport chain, the protons, electrons and oxygen (from the blood) combine to form water. Oxygen is said to be the final electron acceptor.
• • Process of ATP production driven by the movement of H+ ions across a membrane (due to electrons moving down an electron transport chain) is called chemiosmosis (which is described by the chemiosmotic theory).
• 2.5 ATP are made from each reduced NAD and 1.5 ATP are made from each reduced FAD.

  A cell can make 32 ATP from 1 molecule of glucose in aerobic respiration. (1 molecule of glucose produces 2 pyruvate, so the link reaction and Krebs cycle happen twice.)
• ANAEROBIC RESPIRATION

  • Glycolysis - produces pyruvate (same as aerobic)
  • Converted into ethanol (alcoholic fermentation - plants & yeast) or lactate (lactate fermentation - animals cells & some bacteria) using reduced NAD

  (No link reaction, Krebs cycle or oxidative phosphorylation).
• Anaerobic Respiration - The production of lactate/ethanol regenerates NAD, so glycolysis continues and there is continued production of ATP. 
• The folds (cristae) in the inner membrane of the mitochondria provide a large surface area to maximise respiration.
• A coenzyme is a molecule that aids the function of an enzyme by transferring a chemical group from one molecule to another.

  Coenzymes used in respiration include NAD, coenzyme A and FAD. 

  • NAD and FAD transfer hydrogen from one molecule to another. This means they can reduce (give hydrogen to) or oxidise (take hydrogen from) a molecule. 
  • Coenzyme A transfers acetate between molecules.
• Glucose can be used as a respiratory substrate in both aerobic and anaerobic respiration. 

  It isn't the only respiratory substrate that can be used in aerobic respiration. Some products resulting from the breakdown of other molecules, such as fatty acids from lipids and amino acids from proteins, can be converted into molecules that are able to enter the Krebs cycle.