CELLULAR RESPIRATION

Cellular respiration is the enzymatic breakdown of glucose (C6H12O6) in the presence of oxygen (O2) to produce cellular energy (ATP):

C6H12O6 + 6O2 -->6 CO2 + 6H2O + 38 ATP


THREE STAGES OF CELLULAR RESPIRATION

1.Glycolysis:

  • a ten-step process that occurs in the cytoplasm

  • converts each molecule of glucose to two molecules of pyruvic acid (a 3-carbon molecule)

  • an anaerobic process - proceeds whether or not O2 is present ; O2 is not required

  • net yield of 2 ATP per glucose molecule

  • net yield of 2 NADH per glucose (NADH is nicotine adenine dinucleotide, a co-enzyme that serves as a carrier for H+ ions liberated as glucose is oxidized.)

The pyruvic acid diffuses into the inner compartment of the mitochondrion where a transition reaction occurs that serves to prepare pyruvic acid for entry into the next stage of respiration:

(a) pyruvic acid acetic acid + CO2 (a waste product of cell metabolism) + NADH +

(b) acetic acid + co-enzyme A -> acetyl CoA

2. Citric Acid or Krebs Cycle:

  • occurs in the inner mitochondrial matrix

  • the acetyl group detaches from the co-enzyme A and enters the reaction cycle

  • an aerobic process; will proceed only in the presence of O2
  • net yield of 2 ATP per glucose molecule (per 2 acetyl CoA)

  • net yield of 6 NADH and 2 FADH2 (FAD serves the same purpose as NAD)

 

 

3. Electron Transport System:

  • consists of a series of enzymes on the inner mitochondrial membrane

  • electrons are released from NADH and from FADH2 and as they are passed along the series of enzymes, they give up energy which is used to fuel a process called chemiosmosis by which H+ ions are actively transported across the inner mitochondrial membrane into the outer mitochondrial compartment. The H+ ions then flow back through special pores in the membrane, a process that is thought to drive the process of ATP synthesis.

  • net yield of 34 ATP per glucose molecule

 

  • 6 H2O are formed when the electrons unite with O2* at the end of electron transport chain. [* Note: This is the function of oxygen in living organisms!]