Cellular respiration is a set of metabolic reactions that take place within cells to convert nutrients, such as glucose, into energy in the form of adenosine triphosphate (ATP). It occurs in three main stages: glycolysis, the citric acid cycle (also known as the Krebs cycle), and oxidative phosphorylation (including the electron transport chain).

1.Glycolysis:

a) 6 carbon glucose is split into two 3 carbon pyruvates
b) anaerobic – proceeds whether or not O₂ is present ; O₂ is not required
d) net yield of 2 ATP per glucose molecule
e) net yield of 2 NADH per glucose —> sent to the ETC in mitochondria

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, this converts them an acetyl CoA which enters the Kreb’s cycle.

If oxygen is not present, pyruvate is converted to lactic acid in the cytoplasm — anaerobic respiration

2. Citric Acid or Krebs Cycle

a)occurs in the inner mitochondrial matrix
b) an aerobic process; will proceed only in the presence of O2
c) net yield of 2 ATP per glucose molecule
d) net yield of 6 NADH and 2 FADH₂ (NAD+ is reduced to NADH, FAD+ is reduced to FADH)
e) in this stage of cellular respiration, the oxidation of glucose to CO₂ is completed.

See Graphic on Citric Acid Cycle

3. Electron Transport System:

a) consists of a series of enzymes on the inner mitochondrial membrane
b) 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, which drives the process of ATP synthesis using an enzyme called ATPase.
c) net yield of 32 ATP per glucose molecule
d) 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!

Food for Thought

1. What is the purpose of cellular respiration?
2. Where does cellular respiration occur within the cell?
3. What is the waste product of cellular respiration?
4. Comparing photosynthesis to respiration,
   • Where does each occur?
   • What are the products of each?
   • What compounds are needed to start the process?
   • What is the function of the Electron Transport Chain in each process
   • Describe the role of ATPase in photosynthesis and cellular respiration.