– needed for:   – movement
– synthesis of complex compounds
– active transport


– provides energy needed by cells
– broken down > heat (body heat, excess removed)
chemical storage form

Cellular Respiration

– is the process by which chemical energy stored in food (usually glucose, but may be protein and lipids is released by cells to be used to carry out life processes (ex: locomotion, cell division, active transport)

– takes place in a series of steps involving many enzymes located in mitochondria

Energy Storage and Release

Two steps involved:

1. breakdown of glucose leads to storage of energy in ATP

2. breakdown of ATP provides cell with energy


  • · release of energy stored in food molecules such as glucose without oxygen
  • · very in efficient process à glucose is only partially broken down
  • · occurs in yeast, certain bacteria, fungi and active muscle cells

A. Glycolysis (splitting of glucose)

  • · through a series of steps, glucose, which is a six-carbon compound (C6H12O6) is converted

into two three-carbon compounds of pyruvic acid.

  • · this results in a net gain of 2 ATP

glucose   ————–>  2 pyruvic acid

(Note: net yield is 4 ATP-2ATP = 2ATP)

B. Next, carbon dioxide and ethanol (ethyl alcohol) or lactic acid are produced without the yield of any additional energy.

pyruvic  ————–> carbon dioxide + ethanol or lactic acid

A great deal of energy is still present in the ethanol or lactic acid molecules.

C. Summary of Alcohol Fermentation

glucose ————–> ethanol + carbon dioxide + 2 ATP

D. Lactic Acid Fermentation

  • · produced in some bacteria and fungi
  • · also occurs in active muscles, where anaerobic respiration occurs.
  • · accumulation of lactic acid results in muscle fatigue and muscle pain

glucose ————–> lactic acid + carbon dioxide + 2 ATP


  • · respiration requiring oxygen
  • ·efficient process —> glucose is completely broken down to carbon dioxide and water
  • · involves two stages:

A. Glycolysis (Anaerobic phase)

B. Kreb’s Cycle or Citric Acid Cycle (Aerobic Phase)

  • · the pyruvic acid molecule from glycolysis undergo a series of chemical reactions which result in the formation of CO2 and H2O.
  • · energy is released in small units and stored in 36 ATP molecules.

C. Net Yield of Energy from One Molecule of Glucose:


i) Glycolysis: 2 ATP (Anaerobic)

ii)      Kreb’s Cycle: 36 ATP (Aerobic)

Total: 38 ATP

D. Word Equation for Aerobic Respiration:

glucose + oxygen    ———-> carbon dioxide + water + 38 ATP

E. Chemical Equation for Aerobic Respiration:

C6H12O6 + 6O2 ———–> 6 CO2 + 12 H2O + 38 ATP

author avatar
William Anderson (Schoolworkhelper Editorial Team)
William completed his Bachelor of Science and Master of Arts in 2013. He current serves as a lecturer, tutor and freelance writer. In his spare time, he enjoys reading, walking his dog and parasailing. Article last reviewed: 2022 | St. Rosemary Institution © 2010-2024 | Creative Commons 4.0


  1. Hi, Richard, I believe you might be a little confused with the concepts. While you are correct that oxygen is not the terminal acceptor in anaerobic respiration, the process is much less efficient than aerobic respiration. Less ATP is generated and fermentation is a prominent example of this pathway in yeast; while humans will experience lactic acid formation.

  2. Hi,

    The information on ANAEROBIC RESPIRATION (Fermentation) is incorrect. What you describe is fermentation, which is different from anaerobic respiration. Anaerobic respiration is basically the same as aerobic respiration, but alternative terminal electron accepts are used (in the absences of oxygen). It is almost as efficient as aerobic respiration.

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