NOTE: It is expected that you have studied this topic in High School Biology. This subject may not be covered in the lectures, but you are responsible for all of the information in these notes because it is important background for topics in this course, suchas muscle cell physiology (Chapter 7). Please be familiar with this material before we reach those topics in lecture. Pay special attention to bold and underlined terms.

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


1. Glycolysis: (Fig. 18-2)

  • 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 (Fig. 18-3) 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 TCA Cycle: (Fig. 18-3)

  • 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)

  • in this stage of cellular respiration, the oxidation of glucose to CO2 is completed


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!]



The above notes describe the process of carbohydrate (glucose) catabolism for the production of ATP. When glucose is in adequate supply, such as shortly after consumption of a meal, the hormone insulin from the pancreas increases glycogen formation (glycogenesis) in the liver. When glucose levels drop between meals, the hormone glucagon is released from the pancreas and stimulates the conversion of glycogen into glucose (by the process of glycogenolysis). If all glycogen supplies are depleted, then other substances in the body are converted into glucose or intermediate products that can enter the above-outlined cellular respiration pathway. The conversion of fatty acids (from lipids) or amino acids (from proteins) into glucose or intermediate products is called gluconeogenesis (p. 500).



Fats (lipids) are stored in adipose tissue. These stored fat molecules are synthesized in the body from the breakdown products of fat digestion (glycerol and fatty acids), in a process known as lipogenesis (p. 501). When needed as an energy source, the fat reserves are mobilized, moved out of adipose tissue, and broken down into glycerol and fatty acids in the liver by the process of lipolysis. Glycerol is changed into one of the intermediate products of glycolysis, so enters the cell respiration pathway. Fatty acids are changed in a series of reactions called beta-oxidation into acetyl CoA molecules, which enter cell metabolism at the Kreb's Cycle. When fats are being used as the primary energy source such as in starvation, fasting or untreated diabetes, an excess amount of acetyl CoA is produced, and is converted into acetone and ketone bodies. This produces the sweet smell of acetone on the breath, noticeable in a diabetic state.



Proteins are used as an energy source only if protein intake is high, or if glucose and fat sources are depleted, in which case amino acids from protein breakdown are converted into molecules that can enter the TCA Cycle. These molecules are produced by either of two categories of reactions that alter the structure of amino acids. Transamination transfers an amino group (NH2) from one amino acid to another, whereas deamination removes an amino group from an amino acid. As they accumulate, the amino groups removed by the process of deamination are altered to form a harmful waste product (ammonia), so are converted by the liver into urea which is excreted by the kidneys.